Abstract 1 | Evaluating the success of wildlife crossing structures using genetic approaches and an experimental design: Lessons from a gliding mammal

Millions of dollars are spent on wildlife crossing structures intended to reduce the barrier effects of roads on wildlife. However, we know little about the degree to which these structures facilitate dispersal and gene flow.

Our study incorporates two elements that are rarely used in the evaluation of wildlife crossing structures: an experimental design including a before and after comparison, and the use of genetic techniques to demonstrate effects on gene flow at both population and individual levels.

We evaluated the effect of wildlife crossing structures (canopy bridges and glider poles) on a gliding mammal, the squirrel glider (Petaurus norfolcensis). We genotyped 399 individuals at eight microsatellite markers to analyse population structure, first-generation migrants and parentage relationships.

We found that the freeway was not a complete genetic barrier, with a strong effect evident at only one site. We hypothesise that the presence of corridors alongside the freeway and throughout the surrounding landscape facilitated circuitous detours for squirrel gliders. Installing a crossing structure at the location with a strong barrier effect restored gene flow within just 5 years of mitigation.

Our study highlights the importance of using genetic techniques not just to evaluate the success of road-crossing structures for wildlife, but also to guide their placement within the landscape.

Managers wishing to reduce the effects of linear infrastructure on squirrel gliders and other arboreal mammals should aim to preserve and enhance vegetation along roadsides and within centre medians, as well as mitigate large gaps by implementing wildlife crossing structures.

Abstract 2| How effectively can we mitigate the barrier impacts of roads on wildlife movement? A global assessment and meta-analysis

Millions of dollars are spent around the world in an attempt to reduce the impacts of roads and linear infrastructure on wildlife movement. Evaluating the effectiveness of these measures is critical if we are to ensure that conservation goals are met, and financial investments have been worthwhile.

Hundreds of research projects have focused on evaluating road mitigation, however there has been no quantitative synthesis of their findings. What have we learned so far? To what extent do mitigation measures reduce barrier effects for wildlife?

We assessed the global evidence for the effectiveness of mitigation intended to reduce the barrier effect of roads on wildlife using meta-analysis. We searched the literature for research that quantified the effect on wildlife movement, and considered any action that was intended to lessen a potential barrier effect as mitigation (including crossing structures, modified drainage, roadside management or crosswalks).

For a study to demonstrate ‘effectiveness’, it had to compare mitigation to an unmitigated situation (i.e. compare taking action to taking no action). More than 400 studies evaluated the use or effectiveness of barrier mitigation.

Wildlife crossing structures were the most common measure evaluated. However, fewer than 50 studies evaluated effectiveness. Instead, most research efforts focused on:

1. Documenting the use of crossing structures by wildlife;
2. Evaluating the willingness of wildlife to use mitigation (e.g. ‘acceptance’); or,
3. Identifying factors affecting wildlife use of crossing structures.

The lack of comparison with ‘unmitigated’ data is a key factor limiting our ability to evaluate the effectiveness of wildlife mitigation, leaving us unable to answer some of the most pressing questions that road planners and agencies ask.

We will discuss what this means for our ability to guide effective on-ground actions, and suggest how future research can address this critical gap.

Bio | Kylie Soanes

Kylie is an ecologist focusing on the impacts of roads and urban development of nature. She received her PhD in 2015 for her research project evaluating the effectiveness of road-crossing structures for arboreal mammals in Australia. Kylie now works for the National Environmental Science Programme in the Clean Air and Urban Landscapes Hub and Threatened Species Recovery Hub, investigating ways to promote biodiversity and conservation of native species in cities and towns. She is based in the School of Ecosystem and Forest Sciences at the University of Melbourne.