Effects of road fencing on population persistence: Under what conditions will fences be beneficial?
Written By Jochen A. G. Jaeger
Ottawa-Carleton Institute of Biology, Carleton University
25th November 2021
Roads act as barriers to the movement of wildlife (reduced landscape connectivity), increase mortality due to collisions with vehicles, and reduce the amount of habitat. Fences can be used to reduce roadkill, but their use is sometimes controversial because fences also create a barrier to animal movement. They may separate a wildlife population into smaller subpopulations, each of which will have a higher extinction risk. Recolonization of habitats to compensate for local extinctions will then not be possible, unless wildlife passages are installed as well. We address the following question: Under what conditions do fences along roads reduce or increase population persistence?
The trade-off between traffic mortality and barrier effect (reduced connectivity)
Installing fences removes the problem of road mortality for many species but in so doing it necessarily increases the barrier effect. The relative importance of the effects of increased traffic mortality and reduced connectivity on wildlife populations has important consequences for the question of how to effectively mitigate the impacts of roads. What factors determine whether fences will reduce or enhance population persistence? An obvious factor is the magnitude of traffic mortality. If there is no traffic mortality then fences are useless and may be detrimental. A second factor is the degree to which an animal that encounters a road does not attempt to cross it, which we call “road avoidance” (Figure 1). If the animals avoid the road entirely, then no fences are needed.
Figure 1. Illustration of road avoidance, R, and the proportion of animals killed on the road when trying to cross, K. The two variables are specified independently of each other, ranging from 0 to 100%. Barrier strength, B, comprises both effects, B = 1 – (1 – R)⋅(1 – K).
We studied this trade-off using a simulation model of population dynamics to investigate the conditions under which fences reduce the impact of roads. We identified the ranges of traffic mortality (K) and road avoidance (R) within which the effect of an unfenced road is more detrimental to population persistence than the effect of a fence. Both variables ranged from 0 to 100%. Barrier strength (i.e., the combination of these effects), B, also ranged from 0 to 100%: B = 1 − (1 − R)⋅(1 − K). For our model, we assumed that installing fences corresponds to 100% road avoidance (R = 100%).
The fence threshold line
Our results showed that a fence may or may not reduce the effect of the road on population persistence, depending on the degree of road avoidance by the species and the probability that an animal that enters the road is killed by a vehicle (Fig. 2). Traffic mortality alone had a much stronger effect on persistence probability than did road avoidance alone. For K between 10% and 70%, there was an optimal level of road avoidance for population persistence, which depended on K: The higher K, the higher the predicted optimal level of road avoidance. Average time to extinction (of the model runs in which the population went extinct) was also strongly affected by the levels of traffic mortality and road avoidance. Higher road mortality decreased the length of time for which the population persisted, whereas stronger road avoidance led to longer persistence times in most cases. A high level of road avoidance could compensate for high road mortality.
Figure 2. Probability of population persistence as a function of traffic mortality (K) and road avoidance (R). R = 1 corresponds to the effect of fencing.
Our results indicate that, in general, fences increase the likelihood a population will persist when the species has a high probability of traffic mortality, i.e., a high proportion of those individuals that try to cross get killed (Figure 3). In contrast, fences reduce population persistence for species with lower probability of traffic mortality. Between these two extremes, the benefit of fences depends on the combined effect of traffic mortality and road avoidance. Fences are more likely to be beneficial for species that are more inclined to make attempts to cross the road and the higher the probability of an animal trying to cross the road being killed.
Figure 3. Fence threshold line separating the ranges of traffic mortality (K) and road avoidance (R) within which fencing a road would increase or decrease population persistence.
Other factors that should also be considered
Several factors would likely shift the fence threshold line downward or upward by affecting the relative susceptibility of the population to additional mortality and population subdivision. For example, if the animals need access to resources on both sides of the road (e.g., breeding habitat is on one side, foraging habitat on the other), crossing the road is mandatory for survival and a fence will never be beneficial, unless combined with wildlife passages. Our model predicts that fences combined with crossing structures that are sufficiently used by the species of interest will always result in a higher probability of population persistence, corresponding to a road where animals attempt to cross the road (R < 100%) and there is no mortality (K = 0).
Recommendations regarding the use of wildlife fences along roads
Based on our results, we recommend the use of fences in 5 cases:
(1) When populations of the species of interest are declining and animal mortality on roads is a relevant contributor to the decline;
(2) when traffic volume is so high that animals rarely succeed in their attempts to cross the road;
(3) in road sections that are roadkill hotspots;
(4) when populations have already been reduced due to roadkill (to allow for the recovery of these populations); or
(5) when animals use wildlife passages or other existing crossing structures (e.g., water culverts) to cross the road.
In contrast, if population size is stable or increasing, adding fences could be harmful, unless wildlife passages are included. When the animals need access to resources on both sides of the road, fences should be used in combination with wildlife crossing structures. In many cases, fences will be beneficial as an interim measure until more permanent measures are implemented (Figure 4). For new roads that are in the planning stages, if the species of interest do not show any road- or traffic-avoidance behavior, mortality due to traffic collisions is expected to be high. In this case, fencing should be included in the road construction plan. The lower the degree of road avoidance and the higher the anticipated amount of traffic on the road, the more likely it will be that fences will be beneficial. When none of the above-mentioned information is available for the species of interest, fences should be used in combination with wildlife crossing structures.
Figure 4. A fence for large and medium-sized mammals along Highway 175 in Quebec, Canada. Fencing was combined with several wildlife passages for large mammals and many wildlife passages for medium-sized and small mammals along the road. The fences guide animals to the wildlife passages. Photo: Jochen Jaeger
Author information:
Jochen A. G. Jaeger
Ottawa-Carleton Institute of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada.
Department of Geography, Planning and Environment, Concordia University Montreal, 1455 de Maisonneuve Blvd. West, Suite H1255, Montréal, Québec, H3G 1M8, Canada.
For requesting a PDF of the article, please send an email to J. Jaeger at jochen.jaeger@concordia.ca
Source:
Jaeger, J.A.G., Fahrig, L. (2004): Effects of road fencing on population persistence. Conservation Biology 18(6): 1651-1657. https://doi.org/10.1111/j.1523-1739.2004.00304.x or https://conbio.onlinelibrary.wiley.com/doi/abs/10.1111/j.1523-1739.2004.00304.x
Editor:
Michelle Gadd and Rodney van der Ree
Cite this summary:
Jaeger, J.A.G. (2021): Effects of road fencing on population persistence: Under what conditions will fences be beneficial?. Edited by Gadd, M & van der Ree, R. TransportEcology.info, Accessed at: https://transportecology.info/research/road-fencing-population-persistence[Date accessed].
