Exploring the internal and external wildlife gradients created by conservation fences

Spillover effects are an expansion of conservation benefits beyond protected areas through dispersal of species that reside within. They have been well documented in marine but not terrestrial systems. To understand the effects on wildlife created by conservation fences, we explored the internal and external gradients of activity in mammal, reptile, and bird species at a conservation reserve in arid Australia that is fenced to exclude invasive rabbits (Oryctolagus cuniculus), cats (Felis catus), and foxes (Vulpes vulpes). Two methods were used: counts of animal tracks along transects on sand dunes and captures at pitfall-trapping sites. In both cases, sites were spaced at different distances from the reserve fenceline inside and outside the reserve. We recorded a range of spillover, source-sink, step, and barrier effects that combined to create a zone within and around the reserve with fence-induced species-specific wildlife gradients. Two endemic rodents but none of the 4 mammal species reintroduced to the reserve showed positive spillover effects. Barrier effects, where activity was highest close to the fence, were recorded for the feral cat and native bettong (Bettongia lesueur), species that could not breach the fence. In comparison, some reptiles and native mammal species that could permeate the fence displayed source-sink effects; that is, their activity levels were reduced close to the fence likely due to constant emigration to the side with lower density. Activity of some reptiles was lowest at sites inside the reserve and gradually increased at outside sites with distance from the fence, a gradient likely related to trophic cascades triggered by predator exclusion. Our result shows that fenced reserves can create overlapping layers of species-specific gradients related to each species' ability to permeate the fence and its varying susceptibility to threats. Managers should be aware that these gradients may extend for several kilometers either side of the fence and that not all contained species will increase in abundance. Creating wider conservation benefits may require increased fence permeability and threat reduction outside the fence.