Mitigating road effects on wildlife populations:
Why we know less than we think we know
Many studies have shown that roads can have large, negative effects on animal populations (Fahrig and Rytwinski 2009). How should these effects be mitigated? I argue that appropriate mitigation depends on the mechanism through which roads affect animal populations. The three most frequently hypothesized mechanisms are (i) mortality effects, (ii) roads as movement barriers (limiting access to habitat and resources), and (iii) roads as sources of disturbance such as noise. Appropriate mitigation of road effects depends on which of these mechanisms is most important. In the case of mortality, mitigation measures should aim to keep animals off roads. If the important mechanism is the movement barrier effect then mitigation measures should focus on connections that allow animals to move across roads (ecopassages). If traffic noise is the main mechanism then mitigation should involve structures to block noise, and/or the use of materials (e.g., tires and pavements) that reduce noise levels.
Three types of population-level studies have been taken as evidence of these three mechanisms, respectively: (i) depressed populations next to high-traffic roads, (ii) absence or rarity of a species in areas with high road densities, and (ii) increasing animal numbers with increasing distance from roads. However, inferences from these patterns to the road effect mechanisms are flawed. Mortality, movement barrier effects, and/or noise could cause depressed populations next to high-traffic roads. Likewise, all three mechanisms could cause the absence or rarity of a species in areas with high road densities. Finally, either road mortality or traffic noise (or another form of road disturbance) could cause patterns of increasing animal numbers with increasing distance from roads. Since the current state of research does not allow a general conclusion about the relative importance of the different mechanisms, we are currently unable to make rationale choices about the most appropriate type of mitigation for population-level road effects.
I outline some initial attempts from research in my lab to compare the relative importance of the three mechanisms in causing population-level effects of roads. Preliminary results suggest that road mortality may be generally a more important effect than the effects of either movement barriers or noise. A simulation study (Jaeger and Fahrig 2004) predicted that mortality is more important than the movement barrier effect in about 82% of the parameter space examined. This conclusion is supported by a comparison of two species, the white-footed mouse and the leopard frog.
Translocation studies showed that the white-footed mouse avoids roads, so roads act as movement barriers to this species (McGregor et al. 2008). In contrast, leopard frogs do not avoid roads, so roads are a significant source of mortality for this species (Bouchard et al. 2009). Population-level studies showed a positive effect of roads on the mouse (Rytwinski and Fahrig 2007) but a negative effect of roads on the frog (Carr and Fahrig 2001, Eigenbrod et al. 2008). Therefore, the negative effect of roads at the population level was much stronger for the species exposed to road mortality than for the species for which roads act as a movement barrier.
A study of bird abundance vs. noise levels across 10 high-traffic sites showed that bird abundance is not related to noise level, after controlling for distance from the road, and that birds with lower song frequencies (KHz) (so more likely to be masked by traffic noise) are not more strongly affected by roads than those with higher song frequencies (Summers and Fahrig submitted). Similarly, male frogs calling in the presence of traffic noise adjust their calls, and these adjustments completely compensate for any effect of traffic noise on mate attraction (Cunnington and Fahrig in press; submitted). In all cases the most likely mechanism for the negative effects of roads appears to be mortality. Therefore, I conclude that, unless in a particular situation there is evidence to the contrary, the first priority for road mitigation should generally be reducing road kill. Increasing connections and reducing noise disturbance are secondary.
What is actually known about effectiveness of mitigation measures for reducing road mortality? An experimental study on effectiveness of culverts for reducing mortality in amphibians suggests that culverts are not effective at reducing mortality unless they are combined with fencing to keep the animals off the roads (Cunnington et al. in prep). I propose that an effective mitigation for reducing mortality would be to replace culverts at road-stream intersections with expanded crossings that allow for the natural stream and stream banks to flow beneath the roads. This proposal is based on the notion that road-stream intersections are road-kill hotspots. Finally, I suggest that there is an urgent need for long-term, experimental (before-after-control-impact), population-level tests of road mitigation effectiveness (Roedenbeck et al. 2007).
- Carr, L.W. and L. Fahrig. 2001. Impact of road traffic on two amphibian species of differing vagility. Conservation Biology 15: 1071-1078.
- Cunnington, G.M. and L. Fahrig. In press. Plasticity in the vocalizations of anurans in response to traffic noise. Acta Oecologica.
- Cunnington, G.M. and L. Fahrig. In review. Alteration of male anuran vocalizations compensates for negative effects of traffic noise on mate attraction.
- Eigenbrod, F., S.J. Hecnar and L. Fahrig. 2008. The relative effects of road traffic and forest cover on anuran populations. Biological Conservation 141: 35-46. Fahrig, L. and T. Rytwinski. 2009. Effects of roads on animal abundance: An empirical review and synthesis. Ecology and Society 14: 21.
- Jaeger, J.A.G. and L. Fahrig. 2004. Under what conditions do fences reduce the effects of roads on population persistence? Conservation Biology 18: 1651-1657.
- McGregor, R., D.J. Bender and L. Fahrig. 2008. Do small mammals avoid roads because of the traffic? Journal of Applied Ecology 45: 117-123.
- Roedenbeck, I.A., L. Fahrig, C.S. Findlay, J.E. Houlahan, J.A.G. Jaeger, N. Klar, S. Kramer-Schadt and E.A. van der Grift. 2007. The Rauischholzhausen-agenda for road ecology. Ecology and Society 12 (1): 11.
- Summers, P.D. and L. Fahrig. In review. Are negative effects of roads on breeding birds caused by traffic noise?
- Rytwinski, T.D.M. and L. Fahrig. 2007. Effect of road density on abundance of white-footed mice. Landscape Ecology 22: 1501-1512.