The SLOSS dilemma of road ecology: Are several small wildlife crossing structures better than a single large?

Crossing structures for large wildlife are increasingly being constructed at major roads and railways in many countries, and current guidelines for wildlife mitigation at linear infrastructures tend to advocate for large crossing structures sited at major movement corridors for the target species. The concept of movement corridors has however been challenged, and pinching animal movements into bottlenecks entail risks. In this paper, I address the SLOSS dilemma of road ecology, i.e., the discussion whether a Single Large Or Several Small crossing structures along a linear barrier would produce the most benefit for wildlife. I point out risks, ecological as well as practical, with investing in one large crossing structure, and list a number of situations where it may be more beneficial to distribute the conservation efforts in the landscape by constructing several smaller crossing structures; for example when the ecological knowledge is insufficient, when animal interactions are expected to be significant, when the landscape changes over time, or when future human development cannot be controlled. I argue that such situations are often what infrastructure planning faces, and that the default strategy therefore should be to distribute rather than to concentrate passage opportunities along major transport infrastructures. I suggest that distributing passage opportunities over several smaller crossing structures would convey a risk diversification, and that this strategy could facilitate the planning of wildlife mitigation. What to choose would however depend on, i.a., landscape composition and ecology, and on relationships among target species. A single large should be selected where it is likely that it can serve a large proportion of target animals, and where the long-term functionality of the crossing structure can be guaranteed. I illustrate how species and regional differences may influence the choice, using the case of ungulates in Sweden. New research is needed to support trade-offs between size and number of crossing structures. Cost-effectiveness analyses of wildlife crossing structures are currently rare and need to be further explored. Camera trapping and video surveillance of crossing structures provide opportunities to analyze details concerning, for example, any individual biases according to sex, age, status and grouping, and any antagonism between species and individuals. Wildlife ecology research need to better address questions posed by road and railway planning regarding the importance of specific movement routes and movement distances.