Toward Best Management Practices for Ecological Corridors

land

Review

Andrew Gregory

1,2,3,

*, Emma Spence

, Paul Beier

2,3

and Emily Garding



 

Citation: Gregory, A.; Spence, E.;

Beier, P.; Garding, E. Toward Best

Management Practices for Ecological

Corridors. Land 2021, 10, 140.

https://doi.org/10.3390/land

10020140

Received: 23 November 2020

Accepted: 14 January 2021

Published: 1 February 2021

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4.0/).

Department of Biology, University of North Texas, Denton, TX 76203, USA

Center for Large Landscape Conservation, Bozeman, MT 59771, USA; emma@largelandscapes.org (E.S.);

[email protected] (P.B.)

School of Forestry, Northern Arizona University, Flagstaff, AZ 86011, USA; emily[email protected]

* Correspondence: Andrew[email protected]; Tel.: +1-989-400-3492

Abstract:

Ecological corridors are one of the best, and possibly only viable, management tools to

maintain biodiversity at large scales and to allow species, and ecological processes, to track climate

change. This document has been assembled as a summary of the best available information about

managing these systems. Our aim with this paper is to provide managers with a convenient guidance

document and tool to assist in applying scientiﬁc management principles to management of corridors.

We do not cover issues related to corridor design or political buy in, but focus on how a corridor

should be managed once it has been established. The ﬁrst part of our paper outlines the history

and value of ecological corridors. We next describe our methodologies for developing this guidance

document. We then summarize the information about the impacts of linear features on corridors

and strategies for dealing with them—speciﬁcally, we focus on the effects of roads, canals, security

fences, and transmission lines. Following the description of effects, we provide a summary of the

best practices for managing the impacts of linear barriers. Globally, many corridors are established

in the ﬂood plains of stream and rivers and occur in riparian areas associated with surface waters.

Therefore, we next provide guidance on how to manage corridors that occur in riparian areas. We

then segue into corridors and the urban/suburban environment, and summarize strategies for

dealing with urban development within corridors. The ﬁnal major anthropic land use that may affect

corridor management is cultivation and grazing agriculture. We end this review by identifying gaps

in knowledge pertaining to how best to manage corridors.

Keywords:

ecological corridors; conservation corridors; wildlife; management; conservation biology;

urban/agro-ecology

1. Introduction

An ecological corridor or linkage is a swath of natural land, or stepping stones of

natural land, that is conserved to enhance the ability of plants and wildlife to move among

larger habitat patches [

–

]. The term linkage is commonly used to refer to a connectivity

area with multiple strands, whereas the term corridor suggests a single conduit. In this

paper, we use both terms interchangeably except when one is clearly more appropriate.

Previous reviews have summarized design of connectivity areas [

–

], and strategies

to encourage local land use, transportation, and open space planning agencies to adopt

connectivity conservation plans [

]. In this paper, we will not cover issues related to design

and agency buy in, but will focus on how a corridor should be managed once a design has

been implemented. Each linkage design is simply a hypothesis that conserved swaths of

natural or semi-natural land will promote wildlife and plant movement at a sufﬁcient level

to allow plants and wildlife to maintain genetic and population vigor, recolonize connected

habitat blocks after local extinction, and shift ranges in response to climate change [

]. It

will take decades to collect strong evidence to test these hypotheses [

], and we suspect

the answers will be “it depends on how the linkage was managed.” Our goal in this paper

is to recommend management practices that will most likely result in successful corridors.

Land 2021, 10, 140. https://doi.org/10.3390/land10020140 https://www.mdpi.com/journal/land

Land 2021, 10, 140 2 of 25

In deriving our recommendations, we draw upon studies of animal behavior, gene ﬂow,

and ecological response to human alterations of the landscape.

The history of conservation linkages is rooted in the Theory of Island Biogeogra-

phy [

], which ﬁrst noted that there was a linear relationship between the number and

diversity of species on the islands of Hispaniola and island size and proximity to the

mainland. Diamond [

] applied this species–area relationship to nature reserve areas on

terrestrial landscapes, hypothesizing that corridors connecting otherwise isolated nature

reserves in terrestrial landscapes would help conserve biodiversity. As conﬁrmatory exper-

iments and observations accumulated [

], over 300 plans to conserve land for ecological

corridors have been developed and at least partially implemented around the world (Kee-

ley et al. 2019). However, there has been little guidance on how those corridors should be

managed, hence the reason for this compendium on best practices in corridor management.

Conserving a linkage is rarely as simple as acquiring title or easement to the land

and prohibiting further land transformation. For example, 100% of 27 linkage designs

in Arizona [

] and California’s south coast [

] are crossed by linear barriers such

as highways, canals, and international borders that must be mitigated. Similarly, most

linkage designs include areas used for livestock grazing, water extraction, agriculture, rural

residences, and timber harvest, and all are used for human recreation. Linkages typically

have a high edge to area ratio, often due to islands of urban land in between or within the

linkage (e.g., Figure 1). Despite these modiﬁcations and edge effects, these land uses must

be realized and managed to sustain connectivity [18].

Land 2021, 10, x FOR PEER REVIEW 2 of 26

in successful corridors. In deriving our recommendations, we draw upon studies of ani-

mal behavior, gene flow, and ecological response to human alterations of the landscape.

The history of conservation linkages is rooted in the Theory of Island Biogeography

[13], which first noted that there was a linear relationship between the number and diver-

sity of species on the islands of Hispaniola and island size and proximity to the mainland.

Diamond [14] applied this species–area relationship to nature reserve areas on terrestrial

landscapes, hypothesizing that corridors connecting otherwise isolated nature reserves in

terrestrial landscapes would help conserve biodiversity. As confirmatory experiments

and observations accumulated [15], over 300 plans to conserve land for ecological corri-

dors have been developed and at least partially implemented around the world (Keeley

et al. 2019). However, there has been little guidance on how those corridors should be

managed, hence the reason for this compendium on best practices in corridor manage-

ment.

Conserving a linkage is rarely as simple as acquiring title or easement to the land and

prohibiting further land transformation. For example, 100% of 27 linkage designs in Ari-

zona [15,16] and California’s south coast [17] are crossed by linear barriers such as high-

ways, canals, and international borders that must be mitigated. Similarly, most linkage

designs include areas used for livestock grazing, water extraction, agriculture, rural resi-

dences, and timber harvest, and all are used for human recreation. Linkages typically have

a high edge to area ratio, often due to islands of urban land in between or within the

linkage (e.g., Figure 1). Despite these modifications and edge effects, these land uses must

be realized and managed to sustain connectivity [18].

Figure 1. Three stranded corridors (light tan areas) between the Tortolita Mountains and the Santa

Catalina Mountains protected areas (dark green areas). This linkage will maintain connectivity

between the protected wildland blocks while rapid urbanization north of Tucson, Arizona occurs;

virtually all of the gentle terrain between the two mountain ranges is open to residential and ur-

ban development, which will abut all strands of the corridor within 30–40 years, given current

expansion rates. The design was made to meet the needs of nine focal species. The narrow north–

Figure 1.

Three stranded corridors (light tan areas) between the Tortolita Mountains and the Santa

Catalina Mountains protected areas (dark green areas). This linkage will maintain connectivity

between the protected wildland blocks while rapid urbanization north of Tucson, Arizona occurs;

virtually all of the gentle terrain between the two mountain ranges is open to residential and urban

development, which will abut all strands of the corridor within 30–40 years, given current expansion

rates. The design was made to meet the needs of nine focal species. The narrow north–south corridor

bands provide riparian connectivity. Like almost every corridor, a highway crosses this one.

Land 2021, 10, 140 3 of 25

In this paper, we summarize management practices that, based on scientiﬁc evidence,

are likely to conserve or enhance movement and gene ﬂow for plants and animals inhabiting

designated ecological corridors (Table 1). We organize our recommendations around four

themes, namely mitigation of human-created linear barriers such as roads and canals

(Section 3); management of streams, rivers, and riparian areas (Section 4); management of

urban, suburban, and industrial land uses in or near corridors (Section 5); and management

of agricultural lands in or near corridors (Section 6). Our primary objective is to provide

managers, planners, and landowners with the best information available (Sections 3–7).

Our second objective is to highlight issues for which additional empirical evidence is

needed to inform corridor management (Section 7).

Table 1. Summary of best management practices for conservation linkage management.

Section 3. Recommendations for Human-Created Linear Barriers Crossing Corridors

1. Avoid building roads, canals, or railroad tracks in linkages and avoid having these infrastructures bisect linkages

whenever possible.

2. When roads are unavoidable, use speed abatements to reduce trafﬁc speed within and adjacent to corridors.

3. Use seasonal road closures during critical wildlife life history periods.

4. Reduce or eliminate artiﬁcial lighting.

Provide a variety of safe crossing structures over or under roads, railways or canals, and whenever possible maintain natural

vegetation in the structure.

6. Ensure the protective fencing prevents wildlife from accessing the road/railway or canal interior while simultaneously

funneling wildlife to a safe crossing structure.

7. Maintain high-quality natural areas on either end of crossing structures.

8. Aside from protective fencing along roads, rails and canals, ensure that all other fencing within the ecological corridor is

wildlife friendly and clearly marked with safe deterring markers.

Provide safe drinking water sites near canals/aqueducts to avoid wildlife seeking to access water in unsafe canals/aqueducts.

Section 4. Recommendations for Streams and Riparian Zones

1. Maintain dams and impoundments to ensure that they are functioning properly.

Manage the release of water from the impoundment to mimic the natural water cycle of the stream, and to prevent scouring

ﬂoods.

3. Ensure that water levels below the impoundment are maintained at a level that supports natural vegetation growth.

4. Provide riparian zone buffers.

5. Actively remove invasive species using chemical or mechanical means as necessary.

Section 5. Recommendations for Urban and Suburban Development in Corridors

1. Whenever possible, avoid urban development within the linkage design.

2. Minimize the road infrastructure associated with urban development within or adjacent to the linkage.

3. Strive to maintain residential parcel sizes >20 ha.

4. Use signage and education to explain the value of the linkage and encourage good stewardship by visitors

5. Minimize artiﬁcial lighting.

6. Discourage wildlife feeding on trash or other practices that attract animals to unsafe areas or disrupt natural communities.

7. Encourage a leave-no-trace ethic associated with recreational use of the linkage.

8. Reduce use of fertilizers and pesticides on urban lawns.

9. Encourage good pet ownership to reduce domestic animal damages to wildlife within the linkage.

Section 6. Recommendations for Agricultural Development Within Corridors

1. Encourage limited use of herbicides, pesticides, and rodenticides.

2. Use controlled burns or mechanical methods to remove debris from the corridor.

3. Use conservation easements or other incentives to promote appropriate practices on land adjacent to the linkage.

4. Use available government programs to help restore and manage natural vegetation within the linkage.

5. Encourage good ranching husbandry practices to reduce livestock depredations, and consider payment programs to

compensate producers for livestock losses from wildlife.

6. Reduce or eliminate grazing within or adjacent to the linkage.

Land 2021, 10, 140 4 of 25

Table 1. Cont.

Section 7. Knowledge Gaps, Summary, and Conclusions

1. Do synurbanized wildlife alter human x wildlife x domestic animal x landscape interactions and to negatively affect

linkage functionality?

2. How wide do corridors need to be for a given length to ensure functionality?

3. What human activities within or adjacent to the linkage are compatible with linkage functionality?

4. What, if any, time lags occur between human activities and the effects of those activities on linkage function?

5. What state and national level polices best promote successful linkage designs?

2. Methods

We searched for relevant literature (books, peer-reviewed articles, white papers, link-

age designs, and theses) using Cambridge Scientiﬁc Abstracts, ISI Web of Science, and

Google Scholar, searching various combinations of keywords related to roads, canals,

railroads, fences, border security, rivers, streams, riparian areas, urbanization, artiﬁcial

night lighting, livestock grazing, and agriculture in combination with keywords related to

biodiversity, conservation, linkages, and corridors. Citations in these documents yielded

additional literature. We last searched for literature on 17 April 2020.

We used this literature to recommend management practices for human-created

linear barriers, streams and riparian areas, and farms in and near corridors, to conserve

connectivity and ecosystem processes. The most effective recommendation would be to

“remove all roads, canals, and human land uses, and retain and restore all streams and

riparian areas in the corridor.” Because such a recommendation is impractical, we took a

precautionary “no regrets” approach by making recommendations that least restrict human

land use and are likely to conserve connectivity, while assuming relatively high estimates

of the impacts of human land alterations on wildlife movement. We avoided the alternative

strategy of making recommendations that would succeed only under the lowest estimates

of such impacts.

3. Recommendations for Human-Created Linear Barriers Crossing Corridors

Linear structures such as highways [

], canals [

], and utility lines can impede

corridor utility if they cross a corridor and are not mitigated. These structures cause frag-

mentation primarily through behavioral avoidance and mortality. Linear barriers can break

large habitat areas into small, isolated habit patches which support few individuals; these

small populations lose genetic diversity and are at greater risk of local extinction

[22,23]

. Al-

though fragmentation effects are most severe for wide roads with heavy high-speed trafﬁc,

some mammals avoid crossing two-lane roads with <100 slow-moving vehicles/day [

and some birds may avoid crossing electric transmission lines [

]. These linear fea-

tures impede movement of animals, and movement of plants that depend on animals

to transport their propagules. Here we consider roads (with special measures regarding

highways), canals, fences, border security barriers, and transmission lines. We assume that

our recommendations on highways also apply to railroads.

3.1. Inﬂuence of Roads

High vehicle trafﬁc in potential linkage areas increases the mortality and repellent

effect of the road system [

–

], resulting in roads causing considerable loss and frag-

mentation of habitat. For example, the U.S. has approximately 6.5 million km of roads,

covering approximately 1% of the nation’s land surface [

]. However, the spatial extent of

road impacts is much larger due to mortality on highways, barrier effects that fragment

habitat, accumulation of road salts and other pollutants, and animal avoidance of road

noise and vibrations.

Road mortality (road kill) occurs when animals die in collisions with vehicles. These

collisions kill between 89 million and 340 million birds annually on US roads [

], and

may contribute to global declines of insect populations [

]. In one desert ecosystem, at

Land 2021, 10, 140 5 of 25

least 22.5 snakes were killed by vehicles per road kilometer per year [

], but no reliable,

large-scale roadkill estimates exist for amphibians, reptiles, and small mammals. The best

information regarding impacts of roads comes from studies of large mammals. Roadkill

can have severe impacts on wide-ranging animals that occur in low population densities,

such as the cougar in southern California [

–

], the Florida panther [

], the ocelot in

south Texas [27], and the Iberian lynx [34].

Roads can also signiﬁcantly affect animal communication. Road noise and vibration

interfere with the ability of reptiles, birds, and mammals to communicate, detect prey,

or avoid predators [

]. The intensity of these impacts is related to trafﬁc volume and

speed [

]. For example, some reptiles (which “hear” ground-transmitted vibrations

through their jaw [

]) are repelled even from low-speed 2-lane roads, resulting in

reduced species richness [40].

In addition, wildlife fencing along roads (meant to reduce wildlife road collisions),

increase the barrier effects of roads and result in greater fragmentation impacts associated

with roads. Road surfaces act as heat traps, which may attract certain reptiles species

seeking basking sites [

]. Roads often increase the spread of exotic plants, promote

erosion, create barriers to ﬁsh, and pollute water sources with roadway chemicals [

Vehicles can deposit 300 to 800 exotic seeds per square meter per year to roadside areas,

often from several kilometers away [

]. Highway lighting causes some animals to avoid

areas near roads [

]. Together, these road effects cause animals to avoid crossing roads

(thus increasing fragmentation), increase animal mortality (indirect roadkill), and reduce

access to otherwise usable habitat.

3.2. Inﬂuence of Canals on Connectivity and Ecosystems

Canals are typically a nearly-complete barrier to wildlife movement because they are

often bordered by tall chain-link fences, have nearly vertical concrete sides, and are ﬁlled

with swiftly-moving water that is often >2 m deep and >10 m wide. Such a conﬁguration

is almost certainly a complete barrier for all or almost all reptiles, non-ﬂying mammals,

and non-ﬂying invertebrates (Figure 2).

Land 2021, 10, x FOR PEER REVIEW 6 of 26

Figure 2. Canal in the Mojave showing wildlife-proof protective fencing along the edges. Photo taken from an improved

wildlife crossing structure facilitating movement across the canal.

Canals can cause linear discontinuities in natural land cover, and also kill some ani-

mals, such as small animals that can pass through chain-link fences to drown in the canal.

Where fences are absent or breached, large mammals such as desert mule deer, bighorn

sheep, and pronghorn drown in canals [45].

3.3. Influence of Border Security Structures on Connectivity and Ecosystems

Security measures at national borders include roads, fences, walls, bright outdoor

lighting, vegetation clearing, and increased human activity. In some cases, the eventual

abetment of these measures may improve wildlife habitat and connectivity over the long

term, such as the EU Greenbelt that now exists along the former Iron Curtain [46], and the

demilitarized zone between North and South Korea. These promote connectivity because

semi-natural conditions flourished in the 1 to 25 km wide “no man’s lands” between border

fences [46]. In other instances, such as along parts of the U.S.–Mexico border and India–

Pakistan border, fences impede movement for some species resulting in population sub-

division [47–49].

3.4. Influence of Other Transmission Lines and Livestock Fences on Connectivity

Collision and electrocution with electrical transmission lines and entanglement in

livestock fences are significant sources of mortality for some species [50,51], even for short

distribution lines [32]. By providing perches for raptors, powerlines and fences likely

cause local decreases in some raptor prey, although rigorous studies of population im-

pacts are lacking. Moreover, powerlines in rural areas typically maintain a clear right of

way or improved access of 30–60 m wide, with width increasing with voltage, to reduce

risk of fire. This can enhance the perceived or actual barrier effect of the powerlines. Wind

turbines and the associated infrastructure with wind parks may also have an impact on

wildlife corridor efficacy due to avoidance behavior by some wildlife species [52].

Fences are barriers that can enhance the fragmentation impacts of the road, or cause

direct mortality when wildlife are ensnared trying to cross the fence [19]. Although strips

of natural vegetation along fencerows can enhance connectivity for some species [53], live-

stock and property boundary fences impede movement of many species [54].

Figure 2.

Canal in the Mojave Desert of California showing wildlife-proof protective fencing along

the edges. Photo taken from a road across the canal.

Canals can cause linear discontinuities in natural land cover, and also kill some

animals, such as small animals that can pass through chain-link fences to drown in the

Land 2021, 10, 140 6 of 25

canal. Where fences are absent or breached, large mammals such as desert mule deer,

bighorn sheep, and pronghorn drown in canals [45].

3.3. Inﬂuence of Border Security Structures on Connectivity and Ecosystems

Security measures at national borders include roads, fences, walls, bright outdoor

lighting, vegetation clearing, and increased human activity. In some cases, the eventual

abandonment of these measures may improve wildlife habitat and connectivity over the

long term, such as the EU Greenbelt that now exists along the former Iron Curtain [

and the demilitarized zone between North and South Korea. These promote connectivity

because semi-natural conditions ﬂourished in the 1 to 25 km wide “no man’s lands” between

border fences [

]. In other instances, such as along parts of the U.S.–Mexico border and

India–Pakistan border, fences impede movement for some species resulting in population

subdivision [47–49].

3.4. Inﬂuence of Other Transmission Lines and Livestock Fences on Connectivity

Collision and electrocution with electrical transmission lines and entanglement in

livestock fences are signiﬁcant sources of mortality for some species [

], even for short

distribution lines [

]. By providing perches for raptors, powerlines and fences likely cause

local decreases in some raptor prey, although rigorous studies of population impacts are

lacking. Moreover, powerlines in rural areas typically maintain a clear right of way 30–60

m wide, with width increasing with voltage, to reduce risk of ﬁre. This can enhance the

perceived or actual barrier effect of the powerlines. Wind turbines and the associated

infrastructure with wind parks may also have an impact on wildlife corridor efﬁcacy due

to avoidance behavior by some wildlife species [52].

Fences are barriers that can enhance the fragmentation impacts of the road, or cause

direct mortality when wildlife are ensnared trying to cross the fence [

]. Although strips

of natural vegetation along fencerows can enhance connectivity for some species [

livestock and property boundary fences impede movement of many species [54].

3.5. Avoid Building Roads in Corridors

Clearly, the best way to avoid the barrier effect of roads is to avoid building roads in

wildlife corridors. An unmitigated major road crossing a corridor might render it unusable

for some species [

]. Small roads that do not create signiﬁcant barriers, can degrade

habitat as road density increases. For example, Mladenoff et al. [

] suggested that road

densities >0.6 km/km

render landscapes inhospitable to wolves, and suggested that road

avoidance behavior has removed >10.9 million ha of potential elk habitat in the U.S. [57].

3.6. Minimize Artiﬁcial Lighting on Roads That Pass through the Linkage Design

It is a best practice to avoid artiﬁcial lighting, because artiﬁcial lighting has been shown

to disrupt natural daily, seasonal, and lunar light cycles as experienced by a diversity of

organisms, leading to altered cues for the timings of many biological activities [

]. If light-

ing is deemed necessary in an area, the following guidelines will limit impact on wildlife:

(i) dim lighting (reducing the intensity of artiﬁcial lighting to optimize the balance between

what is required for human activities and deterioration of the natural nighttime environ-

ment); (ii) part-night lighting (switching off lighting when use is low/motion activated

lighting); (iii) change the spectra of the lighting (avoid blue light emissions); (iv) minimize

light trespass (improving the design and use of light sources so as to direct artiﬁcial light

where it is actually required and to prevent it from being directed elsewhere [59]).

3.7. Reduce Vehicle Speeds on Roads in Corridors

As vehicle speed decreases, roads pose less of a barrier to wildlife movement and areas

near roads are less avoided by wildlife [

], in part due to less noise and vibration [

For many animals, speed has a greater effect on mortality and avoidance than trafﬁc

volume [

]. Vehicle speed and collisions with wildlife can be reduced by making roads

Land 2021, 10, 140 7 of 25

narrower, using trafﬁc-calming devices (curves, bumps), and installing more road signs

(speed limit signs, wildlife crossing signs) per unit of road length [

]. If periods of

high vehicle trafﬁc coincide with peak periods of animal activity (e.g., large mammal

migrations), lowering speed limits during peak activity, installing temporary road signs

that warn drivers of animal activity, or installing speed bumps in key sites will alert drivers

to be more cautious and careful driving through these areas [

]. To the extent that it

is socially or politically practical, the size and types of vehicles allowed to utilize roads

within a corridor should be limited, and seasonal road closures during critical breeding

and dispersal periods should also be considered [3,7,19].

3.8. Raise the Highway Bed above the Surrounding Terrain to Minimize Road Kill and Direct

Animals toward Crossing Structures

Clevenger et al. [

] found that vertebrates were 93% less susceptible to road kills

on sections of both 2-lane and 4-lane highways raised on embankments, compared to

segments at the natural grade of the surrounding terrain. Raised sections of road can

funnel animals toward crossing structures. Raised road beds require less maintenance than

wildlife fencing.

3.9. Design Highway and Canal Crossing Structures Speciﬁcally to Provide for Animal Movement

There are two general classes of wildlife crossing structures: overpasses (sometimes

also referred to as green bridges or faunal passes) and underpasses. Wildlife overpasses

(sometimes also referred to as green bridges or faunal passes), bridges, culverts, and pipes.

While many of these structures were not originally constructed with ecological connectivity

in mind, many species beneﬁt from them [

]. Clevenger and Huijser [

] provide

detailed recommendations on planning, placement, and design of crossing structures. In

Banff National Park, Alberta, grizzly bears, wolves, and ungulates (bighorn sheep, deer,

elk, and moose) prefer overpasses to underpasses, while species such as mountain lions

and black bear prefer underpasses [

]. Here we describe important considerations for

managers developing both types of structures.

Wildlife overpasses are typically 30 to 50 m wide, but can be as wide as 200 m. Wildlife

underpasses include viaducts, bridges, culverts, and pipes, and are often designed to allow

water to ﬂow beneath highways. A bridge is a road supported on piers or abutments above

a watercourse or ravine, whereas a culvert is a round or rectangular tube under a road.

The most important difference is that the streambed under a bridge is mostly native rock

and soil, instead of concrete or corrugated metal as in culverts, and the area under the

bridge is large enough that a semblance of a natural stream channel and riparian vegetation

typically returns a few years after construction, even when rip-rap or other scour protection

is installed to protect bridge piers or abutments. In contrast, vegetation does not grow

inside a culvert, and hydrology and stream morphology are permanently altered, not only

within the culvert, but for some distance upstream and downstream from it.

Most ungulates will not use culverts, but readily pass under tall bridges with long

spans [

]. Because most small mammals, amphibians, reptiles, and insects need vegeta-

tive cover for security, bridged under-crossings should extend to uplands beyond the scour

zone of the stream, and should allow enough light for vegetation to grow underneath. In

the Netherlands, rows of stumps or branches under crossing structures have increased

connectivity for smaller species crossing under bridges on ﬂoodplains [

]. Because trafﬁc

noise within an under-crossing discourages use by ungulates [

], new designs should

minimize vehicle noise in underpasses. Ungulates prefer under-crossings with a high

openness ratio (height x width divided by length) and sloped earthen sides (instead of

vertical concrete sides) [67].

Despite their disadvantages, well-designed and located culverts can mitigate the

effects of busy roads for small- and medium-sized mammals [

], including mice, shrews,

foxes, rabbits, armadillos, river otters, opossums, raccoons, ground squirrels, skunks,

coyotes, bobcats, mountain lions, black bear, long-tailed weasel, amphibians, lizards,

snakes, and frogs [

]. In locations where the ﬂoor of a culvert is persistently

Land 2021, 10, 140 8 of 25

covered with water, a concrete ledge above water level can provide terrestrial species with

a dry path through the structure [

]. It is important for the lower end of the culvert to

be ﬂush with the surrounding terrain. Many culverts are built with a concrete pour off of

8–12 inches

, and others develop a pour-off lip due to scouring action of water. A sheer pour

off of several inches makes it unlikely that many small mammals, snakes, and amphibians

will ﬁnd or use the culvert. Culverts intended to promote wildlife passage should have

both upstream and downstream openings ﬂush with the surrounding terrain and native

land cover.

The best crossing structure for a canal is to bury segments of the canal below ground.

For narrow canals, such as those irrigating ﬁelds, it may be cheaper to cover the canal with

metal or concrete slabs, and cover these plates with soil and vegetation. To prevent damage

from natural ﬂoods, many canals often include a buried siphon to convey water under a

natural watercourse; these siphon gaps create opportunities for wildlife to cross the canal.

Siphons intended for wildlife use should create a passageway at least 40–50 m wide that

has natural vegetation and follows the natural grade of the surrounding landscape.

3.10. Build Multiple Types of Crossing Structures Spanning Highways and Canals to Provide

Connectivity for All Species, Preferably with Appropriate Structures No More Than One Home

Range Width Apart

Viaducts (bridges that span an entire valley), long railroad or highway tunnels, and

long canal siphons can support movement by all wildlife species in an area. For sections

of linear barriers where such structures cannot be built, it may be necessary to build

different types of structures to accommodate all species [

], including wildlife

overpasses for ungulates, underpasses such as large box culverts for bears and felids [74],

and pipe culverts from 0.3 to 1 m in diameter for small mammals [61,68].

Because most reptiles, small mammals, and amphibians have small home ranges,

Clevenger et al. [

] recommend culverts at intervals of 150–300 m. For ungulates and

other large mammals, Mata et al. [

] and Clevenger and Wierzchowski [

] suggest that

bridges or wildlife overpasses should be located no more than 1.5 km apart. Although

these spacing guidelines are ideal, we believe they can be relaxed in areas where the more

widely-spaced structures connect to large intact habitats on both sides of the linear barrier

in such a way that genetic and demographic connectivity can be maintained.

3.11. Maintain Suitable Habitat on Both Sides of Highway and Canal Crossing Structures

Suitable habitat should occur on both sides of the crossing structure [

]. This

applies to both local and landscape scales. At the local scale, vegetative cover should be

present near entrances to give animals security, and to reduce negative effects of lighting

and noise [

]. At the landscape scale, for key focal species, it may be important to

manage vegetation, land use, and human behavior to ensure that individuals from nearby

population centers can reach the structure [63].

Whenever possible, suitable habitat should occur within the crossing structure. This

can best be achieved by having a bridge high enough to allow enough light for vegetation to

grow under the bridge, and by making sure that the bridge spans upland habitat that is not

regularly scoured by ﬂoods. Where this is not possible, rows of stumps or branches under

large-span bridges can provide cover for smaller animals such as reptiles, amphibians,

rodents, and invertebrates; regular visits are needed to replace artiﬁcial cover removed by

ﬂoods. Within culverts, mammals and reptiles prefer earthen to concrete or metal ﬂoors.

In the southwestern US, most box culverts smaller than 2.4

2.4 m have large ac-

cumulations of branches, Russian thistle, sand, or garbage (Beier, personal observation).

Structures should be monitored for, and cleared of, obstructions such as detritus or silt that

impede movement of small mammals, carnivores, and reptiles [67,69].

Clevenger and Waltho [

] suggest that human use of crossing structures should be

restricted and foot trails relocated away from structures intended for wildlife movement.

However, a large crossing structure (viaduct or long, high bridge) may be able to accom-

modate both recreational and wildlife use. Furthermore, if recreational users are educated

Land 2021, 10, 140 9 of 25

to maintain utility of the structure for wildlife, they can be allies in conserving wildlife

corridors. At a minimum, nighttime human use of crossing structures should be restricted.

3.12. Use Fencing along Highways and Canals to Keep Animals off the Roadway and Funnel Them

toward Crossing Structures

Fencing should never block entrances to crossing structures, and instead should direct

animals toward crossing structures [

]. In one study, such funnel fencing reduced

roadkill by 93.5% and increased the number of species using the culvert from 28 to 42 [

Fences, guard rails, and embankments at least 2 m high discourage animals from getting on

roads [

]. For animals that cross such fencing, one-way escape ramps can prevent

animals from being trapped on a road [25].

3.13. Provide Alternative Water Sources near Canals and Escape Structures along Unfenced Canals

To discourage wildlife from attempting to drink water from a canal, some water

should be diverted to catchments where wildlife can drink without risk of drowning [

Cable-and-ﬂoat directors in conjunction with stairs or ramps can allow ungulates and

other species to escape. Rautenstrauch and Krausman [

] found that desert mule deer

could swim an average distance of 947 m before escaping via escape structures, and

recommended escape structures

≤

2 km apart, with at least one structure upstream from

hazards such as a siphon entrance.

3.14. Use Wildlife-Friendly Fencing

Highway fences should prevent wildlife from entering highways, as noted above.

However, for fences used to mark grazing allotments, mark property lines, or keep livestock

off rural roads, planners should: use wildlife-friendly fencing, raise the lower wire to allow

wildlife to pass under the fence, and use ﬂashing or other wildlife-friendly markings on the

top wire to alert wildlife to the fences presence and dimensions [

]. Do not use ﬂashing,

ﬂadry, or other wildlife markings on the lower wire of the fence, as this may be perceived

by some species, in particular carnivores, as a barrier [79].

4. Recommendations for Streams and Riparian Zones in Corridors

Streams form natural corridors connecting most protected wildlands, provide travel

paths required by many aquatic species, and are strongly preferred by most terrestrial

wildlife species [

]. As such, perennial and ephemeral streams have strong positive

inﬂuences on connectivity. The only potential negative impact is that a major river cutting

across the main axis of the corridor could be a linear barrier for animals that cannot ﬂy or

swim across it.

In this section, we focus solely on streams that run along the long axis of a wildlife

corridor and we give less attention to aspects of stream ecology, restoration, and conser-

vation unrelated to connectivity. For example, we oppose actions that would dewater a

stream, making it a less attractive wildlife travel route, but we do not advise against actions

such as discharge of treated wastewater that extend the annual ﬂow period of ephemeral

streams, as long as it does not harm corridor function by; for example, creating perennial

ﬂow that allows invasive frogs to displace native amphibians.

4.1. Support Native Riparian Ecosystems along Streams

Minimize risk of scouring ﬂoods, reduce erosion, and support native riparian plants

over invasive plant species. To preserve the ability of streams to promote animal movement

along a corridor, we recommend ﬁve actions. First, maintain settling basins within the ﬂood

plains of dams to ensure they are present and functioning properly, and these should be

required in upstream urban areas along tributaries to the riparian corridor so as to minimize

risk of unnatural ﬂoods that would otherwise scour riparian vegetation in the corridor [

Flooding impacts stream morphometry in ways that make stream beds and adjacent land

inhospitable to many wildlife species [

]. Second, use native vegetation to stabilize banks

Land 2021, 10, 140 10 of 25

of riparian corridors to reduce erosion, and reduce siltation from adjacent agricultural or

urban areas [

]. Third, in an intermittent reach with native amphibians, releases of treated

wastewater should not create perennial ﬂows that might disrupt the ability of the stream

to support native wildlife. For example, in many streams of the western US, modiﬁed

ﬂow regimes have resulted in bullfrogs displacing native amphibians [

]. Fourth,

manage ﬂow regimes to favor native over invasive species. For example, in the western

US, most spring soils favor native riparian trees over invasive tamarisk [

]. Fifth, manage

groundwater pumping near the riparian corridor to maintain native riparian vegetation

and natural duration of stream ﬂow [86].

4.2. Create and Protect Buffer Regions of Natural Habitat Beyond the Riparian Zone

Buffer strips can protect and improve water quality, provide habitat and connectiv-

ity for many species, improve quality of life for human neighbors, and increase nearby

property values [

–

]. Recommended buffer widths to sustain riparian plant and animal

communities vary from 30 to 500 m [

–

]. At a minimum, buffers should contain

the stream channel and the terrestrial landscape affected by ﬂooding and elevated water

tables. Wider buffers are needed to protect edge sensitive species; for example, Ficetola

et al. [

] suggest that buffers for amphibians should protect 400 m wide swaths of suitable

vegetation, that roads be minimized, and stream connectivity maximized within 1 km of

the main channel. Thus, to support dispersal, gene ﬂow, and meta-population stability

of riparian zone wildlife and plants, we recommend delineating a buffer that extends a

minimum of 200 m beyond the annual high-water mark on each side of the channel.

4.3. Maintain Biotic/Abiotic Interactions

Riparian zones contain a diverse array of species and environmental processes as a

result of variable ﬂood regimes, geographically unique channel processes, altitudinal and

climate shifts, and drainage inﬂuences on the ﬂuvial corridor [

]. As a result, riparian

zones each support a unique community adapted to a particular riparian zone [

Many regulated rivers and streams are characterized by an inundated upstream region and

a downstream reach with an unnaturally regulated ﬂow. Such systems modify the riparian

zone, increase salinity, and are ultimately more prone to invasion by exotic species [

For example, in Kansas, channelization reduced water ﬂow along the Cimarron River and

has led to the collapse of local plant biodiversity as a result of Tamarix species invasion [

Within the arid southwestern United States, riparian systems evolved with grazing and

browsing pressure from deer and pronghorn antelope, which are highly mobile grazers

and browsers. High-intensity livestock grazing by cattle poses a very different grazing

pressure than native grazers and is a major stressor for riparian systems in hot southwest

deserts [

]. Thus, livestock should be excluded from stressed or degraded riparian

areas, and hydrologic management should strive to mimic natural ﬂow regimes as best

as possible.

4.4. Eradicate or Control Invasive Riparian Plants

Some invasive species in riparian corridors create signiﬁcant ecological problems,

such as tamarisk in the western US, which depletes soil water, displaces native species,

increases sedimentation, and increases ﬂood damage [

]. Additional work is needed to

determine the extent to which such disruption reduces habitat connectivity for species

that depend on displaced native riparian species. Removing stressors and reestablishing

natural ﬂow regimes can help restore riparian communities, but physical control of some

persistent exotics is necessary ([83,98], but see [100]).

4.5. Enforce Existing Regulations

Finally, we recommend aggressive enforcement of existing regulations. Within the

United States, the Clean Water Act of 1972, as amended in 2009, restricts dumping of soil,

agricultural waste, and trash in streams and restricts the intensity of farming, mining, and

Land 2021, 10, 140 11 of 25

building along streams and on ﬂoodplains. Adequate enforcement of these existing policies

would go a long way toward improving riparian zone and stream habitat quality along

corridors. In areas of the world where such policies do not exist, the enactment of policies

that restrict the dumping of soil, human waste, and trash in streams, as well as policies

which restrict the intensity of farming, mining, and building allowed along streams and on

ﬂoodplains should be sought [101].

5. Recommendations for Urban and Suburban Development in Corridors

Today >80% of the world’s population lives in urban and suburban settings [

102

103

Urbanization includes high-density and low-density residential areas, as well as factories,

gravel mines, and shopping centers. In addition, many urban corridors are being designed

and implemented after build out has occurred using least cost modeling to identify and

link existing green infrastructure throughout the urban setting [

104

]. For example, the city

of Shenzhen in Guandong, China used this approach to identify and protect a 9.53 km

ecological network embedded in a city of 10.37 million people [

105

]. While such approaches

are beneﬁcial, care needs to be taken when assessing the efﬁcacy of such approaches, as

centuries of urbanization can result in the urban green spaces being populated with urban

adapted species and lead to a false sense of corridor success. In such instances, the goal

of the corridor and selection of a suite of focal species should be carefully considered and

articulated prior to corridor development.

Urbanization within a corridor stimulates development of a network of local roads.

Rural subdivisions require more road length per dwelling unit than more compact res-

idential areas. Increased vehicle trafﬁc in potential linkage areas increase the mortality

and repellent effect of the local road system [

]. For example, even low-speed

2-lane roads can have a severe repellent effect on reptiles and amphibians that “hear”

ground-transmitted vibrations through their jaw [38,39].

Another way urbanization impacts corridors is through the removal and fragmenta-

tion of natural vegetation. CBI (2005) evaluated four measures of habitat fragmentation

in rural San Diego County, namely, percent natural habitat, mean patch size of natural

vegetation, percent core areas (natural vegetation > 30 m from non-natural land cover),

and mean core area/habitat patch at seven housing density levels (Figure 3) [

106

107

Fragmentation effects were negligible in areas with

≤

1 dwelling/80 acres, and severe in

areas with

≥

1 dwelling/40 acres [

108

]. Similarly, “ranchette” development (40 acre lots) in

Colorado harbored eight non-native plants not found on nearby ranchlands or parklands

of the same elevation and soil type [

104

]. The negative effects of urbanization were evident

at housing densities as low as one dwelling unit per 40–50 acres. Wagner [

109

] observed

similar effects of urban sprawl on wildlife in Texas. In Arizona, some species of birds [

110

]

and lizards [

111

] were absent as housing density increased. Similar patterns were observed

for birds and butterﬂies in California [

112

–

114

], birds in Washington state [

114

], mam-

mals and forest birds in Colorado [

115

], and migratory birds in Ontario [

116

]. In general,

housing densities below the 1 unit/>70 acres threshold had little impact on birds and

small mammals.

Urban and rural development leads to increased numbers of dogs, cats, and other pets

that act as subsidized predators, killing millions of wild animals each year [

117

118

]. Do-

mestic dogs were detected at 65% and 35% of sampling locations respectively on ranchette

developments in Colorado compared to <3% of sampling locations on working ranches

and parks [

119

]. Subsidized suburban/urban native predators such as raccoons, foxes,

coyotes, and crows, may exploit garbage and other human artifacts to reach unnaturally

high density with negative impacts on native prey [118–120] and disease spread [104].

Land 2021, 10, 140 12 of 25

Land 2021, 10, x FOR PEER REVIEW 12 of 26

ecological network embedded in a city of 10.37 million people [105]. While such ap-

proaches are beneficial, care needs to be taken when assessing the efficacy of such ap-

proaches, as centuries of urbanization can result in the urban green spaces being popu-

lated with urban adapted species and lead to a false sense of corridor success. In such

instances, the goal of the corridor and suite of focal species sought to protect should be

carefully considered and articulated prior to corridor development.

Urbanization within a corridor stimulates development of a network of local roads.

Rural subdivisions require more road length per dwelling unit than more compact resi-

dential areas. Increased vehicle traffic in potential linkage areas increase the mortality and

repellent effect of the local road system [27,30,36]. For example, even low-speed 2-lane

roads can have a severe repellent effect on reptiles and amphibians that “hear” ground-

transmitted vibrations through their jaw [38,39].

Another way urbanization impacts corridors is through the removal and fragmenta-

tion of natural vegetation. CBI (2005) evaluated four measures of habitat fragmentation in

rural San Diego County, namely, percent natural habitat, mean patch size of natural veg-

etation, percent core areas (natural vegetation > 30 m from non-natural land cover), and

mean core area/habitat patch at seven housing density levels (Figure 3) [106,107]. Frag-

mentation effects were negligible in areas with ≤ 1 dwelling/80 acres, and severe in areas

with > 1 dwelling/40 acres [108]. Similarly, “ranchette” development (40 acre lots) in Col-

orado harbored eight non-native plants not found on nearby ranchlands or parklands of

the same elevation and soil type [104]. The negative effects of urbanization were evident

at housing densities as low as one dwelling unit per 40–50 acres. Wagner [109] observed

similar effects of urban sprawl on wildlife in Texas. In Arizona, some species of birds [110]

and lizards [111] were absent as housing density increased. Similar patterns were ob-

served for birds and butterflies in California [112–114], birds in Washington state [114],

mammals and forest birds in Colorado [115], and migratory birds in Ontario [116]. In gen-

eral, housing densities below the 1 unit/>70 acres threshold had little impact on birds and

small mammals.

Urban and rural development leads to increased numbers of dogs, cats, and other

pets that act as subsidized predators, killing millions of wild animals each year [117,118].

Domestic dogs were detected at 65% and 35% of sampling locations respectively on ran-

chette developments in Colorado compared to <3% of sampling locations on working

ranches and parks [119]. Subsidized suburban/urban native predators such as raccoons,

foxes, coyotes, and crows, may exploit garbage and other human artifacts to reach unnat-

urally high density with negative impacts on native prey [118,119, 120] and disease spread

[104].

Figure 3.

Percent natural land cover in relation to residential housing density in rural San Diego

County, California, USA. The study excluded cells with orchards and farms to focus solely on the

impact of single-family residences [121].

Urbanization can lead to increased numbers of wild predators removed for killing

pets or hobby animals. Although exurban development may bring little increase in the

number of depredation incidents per unit area, each incident is more likely to lead to death

of predators because humans are emotionally attached to pets and prompt to notice loss

or injury [

122

123

]. Orlando [

123

] found that pumas strongly avoided parcels less than

8 ha, preferred parcels greater than 16 ha, and experienced high mortality and conﬂict with

hobby livestock owners on parcels of 16–32 ha.

Urban development along streams or other traditional wildlife movement corridors

may divert wildlife from traditional paths, leading to increased human wildlife con-

ﬂicts [

102

]. For example, black bears in Bozeman, Montana frequently follow stream

drainages into town each spring, where they apparently become lost and confused, feed on

residential trash, and are subject to lethal control actions [

124

125

]. Humans living in or

adjacent to corridors may use rodenticides, which kill not only target species (e.g., domestic

rats) but also secondary scavengers and predators (e.g., raccoons and coyotes that feed on

poisoned rats) and tertiary carnivores such as mountain lions which feed on raccoons and

coyotes [126].

Urbanization is associated with artiﬁcial night lighting, which can impair the ability of

nocturnal animals to navigate through a corridor [

127

] and has been implicated in decline

of reptile populations [

128

]. Finally, human-produced noise associated with development

may disturb or repel some animals and present a barrier to movement [129–131].

Synurbanization is the process by which wildlife become acclimated to human pres-

ence and activity on the landscape, and can result in wildlife populations that inhabit

urbanized landscapes having distinct behaviors, life histories, and physiology from con-

speciﬁcs inhabiting agricultural or natural landscapes [

102

]. For example, anthropogenic

noise can change the frequency and amplitude of bird songs [

], the timing of social

courtship displays [

132

], diet [

133

–

135

], number of offspring or number of breeding at-

tempts per year/season [

133

], density and social structure [

102

], and dispersal distance and

movement propensity [

136

]. Consequently, synurbanized wildlife may behave differently

than conspeciﬁcs living in wild or agricultural landscapes [102,137].

5.1. Avoid and Minimize Urbanization in or Adjacent to Corridors

Unlike road barriers (which can be modiﬁed with fencing and crossing structures),

urbanization creates barriers to movement which cannot easily be removed or restored.

Land 2021, 10, 140 13 of 25

If possible, planners should keep residential lot sizes in corridors above 20 ha per resi-

dence, ban artiﬁcial night lighting, keep vehicle speeds low by limiting lane width and

incorporating curves, and specify what plant species are allowed and prohibited. Local

governments or homeowners’ associations are typically reluctant to require a homeowner

to retain natural ﬁre-prone vegetation, remove artiﬁcial night lighting, or tolerate wild

predators that kill companion animals [

138

]. Therefore, precluding urbanization is the

best way to manage urban impacts in a wildlife linkage. However, most corridors are

designed after urbanization and suburban sprawl has begun, such that humans reside in

the corridor before it is designated. Further, virtually all corridors will be bordered by

urban and residential areas as human footprint expands. Therefore, local jurisdictions

striving to conserve corridors will have to engage local residents as cooperative stewards.

5.2. Encourage People Residing in or Adjacent to the Linkage to Be Proud Stewards

Speciﬁcally, residents should be encouraged to landscape with natural vegetation,

minimize water runoff into streams, manage ﬁre risk with minimal alteration of natural

vegetation, keep pets indoors or in enclosures (especially at night), accept depredation on

domestic animals as part of the price of a rural lifestyle, avoid all use of rodenticides, use

herbicides and insecticides carefully, and direct outdoor lighting toward houses and walk-

ways (and away from the linkage area). These activities can be implemented by builders of

planned communities, homeowners’ associations, local jurisdictions, conservation NGOs,

and owners of conserved lands in the corridor. When the local land permitting agency

considers a proposal for new urban development in the linkage area, the agency should

stipulate appropriate conditions. Even if some clauses are not rigorously enforced, such

stipulations promote awareness of how to live in harmony with wildlife movement.

In most situations, it is impossible and politically undesirable to prohibit nearby

residents from enjoying the natural areas in the corridor. Instead, the number of access

points should be limited to trailheads where signage can engage and educate people

entering the corridor. Signage should provide a link to a website where people can learn

more about the value of wildlife corridors and how they can engage in good corridor

stewardship practices. The signage and website should be part of a public education

campaign that allows people living in and visiting the linkage to educate each other about

living with wildlife and the importance of maintaining ecological connectivity [102].

5.3. Encourage Small Building Footprints on Large Parcels with a Minimal Road Network

Where development is permitted within the linkage design, minimize impacts by

keeping the footprint of the building small and limit roadways, with fewer than 1 residence

per 20 ha and requirements to maintain native vegetation on most of each residential lot.

The Santa Lucia Preserve, an 8300 ha residential development in central California USA,

provides an example of such a development (https://slconservancy.org/). Most of the

preserve consists of wildlands permanently protected and managed for biodiversity, and

funded by homeowners. The 350 privately-owned parcels for residences average 12 ha

in size, of which 10 ha is natural open land and 2 ha is designated as homeland, which is

the only place that fences, outdoor lighting, off-leash pets, and non-native vegetation can

occur. The homelands have a well-crafted strict plant palette (no invasive exotic plants),

strict lighting codes, and other rules that create a setting in which wildlife including large

carnivores and rare species thrive with even less human disturbance than on neighboring

publicly-owned wildlands.

5.4. Regulate Access by Recreational Users

Beier et al. [

] recommend that each strand of the linkage design be wider than 1 km so

that it can accommodate a well-designed trail system without compromising the usefulness

of the linkage for wildlife. The trail network should be laid out so that most of the corridor

is relatively undisturbed by humans. People should be encouraged to stay on trails, keep

Land 2021, 10, 140 14 of 25

dogs on leashes, and discouraged from collecting reptiles and harassing wildlife. Off-road

motorized vehicles should be prohibited within the linkage.

5.5. Discourage Residents and Visitors from Feeding Wildlife

Feeding wildlife can encourage undesirable human–wildlife interaction and a cycle of

dependency [

139

]. For example, bears that supplement their diets with, or depend upon,

human garbage generally are not as healthy as their wild counterparts, and small mammal

densities are artiﬁcially inﬂated when their resources are supplemented by trash [

140

Feeding is not only deliberately offering food to animals, but also the accidental provision

of food via discarded food and wildlife accessible trash containers. Trash receptacles at

trailheads should be wildlife-proof, and trailhead signs should encourage leave-no-trace

practices and inform recreational users that they are responsible for bringing out whatever

they bring in.

5.6. Take Special Steps to Protect Species That Might Be Perceived as Undesirable

Venomous snakes, poisonous plants, alligators, and other predators may be perceived

as dangerous, but these species may need the linkage, and may be important to maintaining

ecological function in the linkage. Managers should use signage and other methods to

educate visitors not to harm these species. In some cases, managers may need to protect

critical habitat areas at some distance from trails in order to provide core refuges for them.

6. Recommendations for Agricultural Development in Corridors

The two main types of agricultural development are intensive row-crop farming (in-

cluding orchards) and livestock grazing. Both types of agriculture impact connectivity

due to fencing (covered in Section 3). Both activities alter ﬁre regimes by increasing the

number of wildﬁre ignitions, especially those outside the natural burning season [

141

], sup-

pressing what might otherwise be beneﬁcial ﬁres, and requiring ﬁrebreaks and vegetation

manipulation, sometimes at considerable distance from human-occupied sites [

142

143

Fire suppression in rangeland ecosystems has led to increased woody encroachment. Con-

versely, over-burning has resulted in decreased biodiversity, decreased habitat structure,

and increased invasibility [

144

145

]. Many forested ecosystems are ﬁre adapted and have

suffered from ﬁre suppression, resulting in increased coarse woody debris accumulation

leading to increased fuel loads and increased risk of type-converting ﬁres [146].

Intensive cultivation degrades connectivity by causing loss of native woodlands and

grasslands [

143

] and declines of many wildlife populations [

147

148

]. Only approximately

50% of agricultural fertilizers are used by crops; the rest runs off into drainage ditches and

ground-water [

149

]. Agricultural herbicides, pesticides, and fertilizers have negatively

impacted biodiversity [

150

], causing reduced hatch rates among birds, increased suscepti-

bility to disease in small mammals, extirpation of lizards and amphibians, hemorrhagic

fever in ﬁsh, and stream eutrophication [

150

151

]. Buffer strips of natural vegetation

between agricultural ﬁelds and wildlife-sensitive areas can prevent or reduce agricultural

herbicides and fertilizers from reaching sensitive wildlife [

152

–

154

]. Such buffers harbor

a higher density of predators of agricultural pests (reducing the need for pesticides) and

pollinators (reducing the need for fertilizers) [

154

–

157

]. No-till farming practices may

reduce or eliminate farmers’ need to apply fertilizers and pesticides, and the cost savings

can compensate for lower yields [

150

]. No-till farming practices also increase soil carbon

sequestration, reducing atmospheric carbon accumulation [

158

]. When fertilizers and

pesticides are required or no-till farming practices are not practical, producers should be

encouraged to follow the “4R’s” of fertilizer use: right source of nutrients/fertilizers; right

rate or density of nutrient application; right timing of fertilizer application to best coincide

with crop growth; right place for nutrient/fertilizer application [159].

Wildlife depredation of both livestock and crops may be a major concern for farms

and ranches abutting wildlife corridors [

160

]; in the US, damage to crops may exceed

$4.5 billion annually [

161

]. The economic impact of direct livestock depredation by wildlife

Land 2021, 10, 140 15 of 25

is poorly known, but most loss is felt by a few landowners whose herds are repeatedly

depredated [

158

160

161

]. Such losses can erode local support for the corridor. Although

ﬁnancial compensation reduces the economic impact, these payments do little to reduce

the animosity felt by farmers and ranchers [

162

]. Farmers and ranchers prefer to have the

knowledge and tools to proactively reduce economic damage from wildlife [163–167].

Horizontal transmission of disease between infected wildlife and domestic livestock

is an emerging concern for corridors in agricultural regions for diseases such as bovine

tuberculosis, West Nile Virus, and malaria [

168

169

]. In agricultural landscapes, it is there-

fore important that the corridor does not become the major vector of disease transmission,

which could erode public support for the corridor [170].

6.1. Encourage Farmers to be Good Stewards of the Corridor

Encourage farmers in or adjacent to the linkage to be proud stewards. In some

ways, this recommendation is simpler for farmlands than for urbanization (Section 5)

because farmers and ranchers are stewards of the land, and correctly view themselves

as such. Consequently, engagement can focus on providing farmers and ranchers with

information to allow them to become better stewards and minimizing impacts to their

livelihoods. Such engagement should be led by familiar partners such as agricultural

extension agencies, government forestry, and agricultural entities accustomed to working

with farmers and ranchers, and local agricultural community organizations to disseminate

information to local landowners about the beneﬁts of good husbandry practices. Outside

NGOs should take the time to develop good relationships by, for example, attending annual

agricultural fairs and sponsoring competitions with cash prizes and a physical trophy that

can be displayed by the farmer or rancher with the most corridor-friendly farming and

ranching practices.

6.2. Encourage Farmers to Reduce the Amount of Herbicide, Pesticide, and Fertilizer They Use and

Provide Information about No-Till Farming Practices

In particular, farmers should be encouraged to establish semi-natural transition zones

between agricultural ﬁelds and the corridor, emphasizing the beneﬁts that such buffers

may have for their crops [152–154].

6.3. Encourage Controlled Burns or Mechanical Removal of Debris along Corridor and Adjacent

Lands That Most Closely Mimic Natural Fire Regimes

In rangeland systems, recommend a patch-burn patch-graze rotational grazing sys-

tems for ranchers with lands adjacent to corridors. Along with the patch-burn patch-graze

ﬁre regime, season-long cattle stocking at low to moderate stocking densities should be

encouraged. The interaction of this level of burning and stocking intensity will most

likely be able to maintain adequate grassland habitat heterogeneity necessary to support

wildlife [

171

], without adversely affecting ranchers’ proﬁtability [

172

]. In forested corridors,

encourage mechanical removal of coarse woody debris and mechanical stand adjustments

to lessen fuel loads, increase stand diversity and otherwise mimic the effects of ﬁre without

the risk of catastrophic ﬁre that might disrupt connectivity [173].

6.4. Encourage Conservation Easements or Acquisition of Conservation Land

Recognizing that there may never be enough money to buy easements or land for

an entire linkage, encourage innovative cooperative agreements with landowners that

may be less expensive [

174

175

]. Use available government programs (such as the US

Farm Bill, and IUCN Ag. Lands Programs) to establish natural land buffers between

farmland and the corridor and to expand the size of the corridor. Encourage the use of

wildlife-friendly fencing on property and pasture boundaries, and wildlife-proof fencing

around gardens and other potential wildlife attractants. Encourage landowners to remove

unnecessary fences.

Land 2021, 10, 140 16 of 25

6.5. Use National and Local Agricultural Programs

The United States’ Farm Bill [

176

] includes the Conservation Reserve Program, the

Wetlands Reserve Program, the Wildlife Habitat Incentives Program, the Environmental

Quality Incentives Program, and the Conservation Securities Program, which provide

managers with the ability to enhance connectivity across agricultural gaps in the corridor,

establish natural land buffers between cropland and corridor lands along corridor routes

to reduce edge effects, and increase the amount of suitable habitat within or adjacent to the

corridor to enhance meta-population dynamics [

177

]. Similar programs are included in the

European Union’s Natura 2000 initiative and the EU’s Common Agricultural Policy

[7,178]

These programs provide cost-sharing opportunities, technical assistance, and other ﬁnan-

cial incentives to restore or enhance habitats, and protect habitats through long-term or

permanent conservation easements [

176

179

]. For example, the US Farm Bill’s Wildlife

Habitat Incentives Program provides up to 75% cost share for wildlife habitat improvement

under contracts lasting ﬁve to 15 years (Public law 104–127). Similar CAP policy programs

have had similar beneﬁcial effects for wildlife in Europe [

]. In African nations, laws

expanding and protecting traditional grazing and nomadic lifestyles of tribal peoples,

such as the Masai, have been used to enhance connectivity [

180

181

]. Additionally, the

government of Australia makes an effort to preserve the Stock Route Network, historic

cattle-droving routes, that connects a network of protected areas in New South Wales

and Queensland [

179

]. The creation of other similar programs in other nations should be

encouraged and supported [182].

6.6. Help Ranchers Reduce Depredation Losses

Provide farmers and ranchers with information on the use of livestock or crop guard-

ing dogs [

164

183

], scarecrows, the use of ﬂadry or ﬂashing along fence lines [

], and

good husbandry practices. Cleaning up livestock carcasses may be the single best prac-

tice to reduce wildlife depredation on livestock and domestic companion animals [

184

Managers can provide information on non-lethal predator control [

160

] and on relevant

laws and regulations (Information available for all U.S. regions is available through the United

States Department of Agriculture, Animal and Plant Health Inspection Service. Website:

http://www.aphis.usda.gov/wildlife_damage).

6.7. Concentrate Supplemental Feeding and Water Sites Away from Natural Areas

Many wildlife species will make use of livestock feed and mineral blocks, especially

during winter months when forage may be scarce. Wildlife use of livestock feed areas is

intensiﬁed when such food plots are far from human-use areas and near wildlife utilized

resources [

185

]. In many instances, horizontal disease transmission between wildlife and

livestock occurs at shared food plots [

186

187

]. Therefore, managers should encourage

farmers with land within or adjacent to corridors to locate livestock watering facilities,

supplemental feeding stations, and salt licks close to farm buildings and away from

the corridor.

6.8. Prohibit Farmed Wildlife within or Adjacent to Corridors

The presence of farmed wildlife species such as deer, elk, buffalo, gazelle, kudu, bison,

or others increases the likelihood of horizontal disease transfer among livestock and wildlife

utilizing the corridor [

163

]. Further, landowners should be encouraged to vaccinate all

domestic companion animals and livestock for distemper and other transmissible diseases.

6.9. Limit the Number of Landowners Whose Property Is Either within or Adjacent to the Corridor

This will limit the number of parties with vested interest in the corridor and make

implementing the above management practices easier.

Land 2021, 10, 140 17 of 25

7. Knowledge Gaps, Summary, and Conclusions

In general, the best management practices for corridors can be summed up in the

adage less is more. The less human activity within or adjacent to a linkage, the better and

more successful it will be. Thus, for areas in, or adjacent to, wildlife linkages, our recom-

mendations strive to: (1) minimize the number and intensity of human activities within,

(2) maintain or re-create natural processes in linkages, (3) create buffers between linkage

lands and human-use areas, and (4) help wildlife cross linear barriers, and (5) encourage

recreationists and other people using the corridor to behave as well-informed stewards.

Project proposals that would impact designated wildlife corridors should include

detailed environmental analyses of the impacts and beneﬁts of several alternative strategies,

should consider an array of mitigation strategies, and should provide opportunities for

public review and comment. Following the precautionary principle, proponents of a project

that would degrade connectivity should bear the burden of proving that such impacts will

be minimal.

We offer this paper as a ﬁrst attempt to outline good practices for corridors, and we

look forward to improvements. We encourage others to improve on our recommendations

by conducting research on poorly-understood relationships. One promising approach

would be to replace some of our reasonable inferences from observed ecological patterns

with more targeted observations or experiments. For example, in Section 5.1, we note that

synurbanization can cause wildlife behavior to differ between populations living close

to humans and more remote populations. Additional research is needed to determine if

these behavioral differences may impede movement and gene ﬂow between populations

in corridors and the natural landscape blocks the corridors are intended to connect. Future

improvement should strive to include more evidence from areas outside North America

and Europe, including pastoral regimes that may differ from the farming and livestock

practices on these two continents, and a broader array of legal and cultural settings.

Below, we call attention to several knowledge gaps that, if resolved, would produce

better management practices for wildlife corridors.

7.1. What Are Critical Dimensions of Corridors?

Is there an interaction between how wide corridors must be with corridor length or

type of human use within or adjacent to corridors? What impact do edge effects have on

corridor functionality? Is the edge to area ratio a meaningful metric of corridor function?

7.2. Is There a Critical Threshold in Intensity of Land Use?

Wildlife seem to tolerate certain low-impact land use practices (Burchett and Burchett

2011). For example, converting up to half of a landscape to grain cultivation can beneﬁt

some wildlife populations, with steep declines as larger areas are converted [

188

]. Do these

observations apply to most wildlife species, and to use of land for movement as strongly as

for occupancy? Is there a non-linear relationship between the intensity of adjacent human

land use and wildlife use of a corridor? Thresholds are common in wildlife ecology [

189

but to date little to no work has been completed with regards to ecological thresholds

and corridors.

7.3. How do Landscape Traits of Corridors Affect Different Species?

Individual species and types of species almost certainly vary in their response to cer-

tain corridor traits. For example, forest rodents have limited tolerance to open landscapes,

whereas grassland and rangeland rodents readily enter forests [

190

191

]. For some wide-

ranging and edge-tolerant species, roads, grazing, and even limited human development

of the corridor will have only minimal impacts on corridor functionality [

192

]. For other

species, even light recreational use creating a narrow hiking path will be sufﬁcient to create

a signiﬁcant and measurable barrier [

193

]. To date, little work has evaluated the relative

effect of species versus landscape attributes on corridor use.

Land 2021, 10, 140 18 of 25

7.4. How Far Away from the Linkage Edge Do We Need to Manage Human Uses?

Our recommendations are vague about human activities “adjacent to” a linkage. What

inﬂuence do second- or third-order spatial lag effects have on corridor use? The increasing

spatial extent of population dynamics synchronization under climate change suggests that

managing successful linkages might also include managing areas outside of the linkage as

intensively as the linkage itself, but the intensity, types, and spatial extent of these activities

is currently unrecognized and unstudied [191].

7.5. How Do Time Lags Affect Corridor Use?

Past land use practices incur extinction debts [

194

]. Do they impede wildlife use of

a newly conserved corridor? The environmental history of a corridor could be critical to

understanding how the corridor functions today, whether and how fast a landscape returns

to equilibrium. Adaptive landscape models and neutral theory models could be useful in

this context [195].

7.6. Which Governance and Management Activities Are Most Acceptable to People Living in or

near Corridors and Which Policies Are Beneﬁcial for Corridors?

Corridor establishment and management is an issue of coupled natural–human sys-

tems [

196

]. There are socio-political aspects to developing and managing corridors and

human access to natural areas. For example, in the Czech Republic, federal laws prohibit

any management legislation that would limit citizen access to forests [

197

]. Similarly, in

New South Wales Australia, human desire to preserve the droving history of the land-

scape resulted in the preservation of the Stock Route Network, and mandated that those

areas be maintained to allow livestock grazing [

182

]. Lastly, sometimes well-intentioned

conservation practices can have unintended interactions. For example, in both China

and Kenya, fortress conservation practices of forests created a high-value commodity

resource that created market-economy incentives for people to violate conservation laws

and utilize the resources to a greater/more intense degree than had been previously ob-

served [196,198–201].

Public perception of the quality and legitimacy of the science supporting corridor

development and management is also an important consideration [

202

]. In some instances,

public and political interest will spur the creation of a corridor, but that support will wane

later if the goals and reasoning for the corridor and regulations protecting the corridor

are ambiguous [

203

204

]. This requires the balancing of social, ecological, and political

factors during the planning stages of the corridor and integration of all major stakeholder

perspectives into the corridor management plan. An example of how this was performed

well can be seen in the Green River Corridor in the Netherlands [205].

Author Contributions:

Conceptualization, P.B., E.G., and A.G.; methodology, A.G., E.S., and P.B.;

investigation, P.B., A.G., E.G., and E.S.; writing—original draft preparation, P.B. and E.G.; writing—

review and editing, A.G., E.S., P.B., and E.G.; supervision, A.G.; project administration, A.G. and E.S.

All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not Applicable.

Informed Consent Statement: Not Applicable.

Data Availability Statement:

Not Applicable. This is a literature review and all data were already in

the public domain.

Acknowledgments:

We thank colleagues whose linkage designs, scientiﬁc papers, and oral pre-

sentations provided many of the sources we used to develop our recommendations, including

Andrew Bennett (Deakin Univ., AU), Annika Keeley (Delta Stewardship Council, Sacramento, CA

US), Burkhard Vogel (BUND; Germany), Dan Majka (The Nature Conservancy, US), Reed Noss (U

Central Florida, US), Kristeen Penrod (Science & Collaboration for Wildlands, US), Uwe Riecken (BfN

Germany), and Wayne Spencer (Conservation Biology Institute, US). We also thank Joan Berning

Land 2021, 10, 140 19 of 25

(Eden to Ado, South Africa), Jonathan Caruthers-Jones (Leeds University, UK), Thomas Mölich

(Bund, Germany), Janella Hodel (Toledo Metro Parks, US), Gary Tabor (Center for Large Landscape

Conservation, US), and Eric Nelson (MN Department of Natural Resources, US) for technical review

of early drafts of this manuscript.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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