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Published online 23 February 2009 | Nature | doi:10.1038/news.2009.114
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Uncertainty over animal crossings
Are bridges over busy roads helping wildlife to breed?
There is little evidence that the bridges built to allow wildlife to cross roads are helping them to thrive, a review suggests. The study by researchers in Austria points to a lack of genetic studies investigating whether groups of animals on either side of a bridge are breeding with each other.
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This article fails to distinguish among the possible objectives of crossing structures, which in turn determines how effectiveness may be defined. Since many structures have been installed without a clear identification of the management objectives, and without defining associated performance measures, it's easy to see how structures can be determined to be failures. It is key to defining the effectiveness of wildlife crossing structures and associated fencing to identify the objectives. Three possible objectives relate to the article's discussion of how effective crossing structures are, or how well they 'work'. Certainly there are more objectives possible, but this article mixes these three. 1. Genetic interchange. In this case, a few individuals of the target species crossing every generation meets the objective. Those that cross do need to breed to meet this objective, but it wouldn't take many. This is the easiest performance measure to meet because the fewest number of individuals need to make it across, and depending on the area and species, may be adequate to meet management goals. It's also an objective that may be best met with another type of management approach, say, transplanting individuals. 2. Dispersal and habitat recolonization. More individuals would need to successfully use the structures and thus contribute to keeping the local and/or larger scale populations intact, and suitable habitat occupied at a level that keeps the larger population healthy and stable. This level of effectiveness is harder to obtain, particularly with species that are wide-ranging and for which we lack movement data. I think this level of objective will become more important as we try to plan for animal/plant movements in response to climate change. 3. Daily or seasonal movements. This includes the concern of population-level effects from vehicle mortality. Effectiveness at maintaining seasonal or daily movements contributes to population health if not viability because these can be key to accessing needed life history elements. Good examples are migratory mule deer herds or amphibians accessing isolated breeding ponds. This can be the most difficult level of effectiveness to achieve if we lack the understanding of how many individuals use the structures we create, as well as how many individuals need to cross in order to breed at a level consistent with a healthy population. It is very possible that it is not the lack of effectiveness of crossing structures that is the problem, but rather the insufficient quantity of them. Daily or seasonal movements are also usually where safety objectives come into play, but the article discussed animal movement needs, not safety issues. Although most projects to date have not wrestled with a good quantifiable performance measures, having some understanding of what the desired outcome is (ie, how many individuals cross, how many go on to breed, and other meaningful metrics) would enable project planners to better explain to the public what went well or what needs to be improved.
What about smaller animals for which a road is a nearly complete barrier? Butterflies, turtles, etc. Though I guess the question with them is whether they can even find the crossing.