Experimental removal of introduced hedgehogs improves wader nest success in the Western Isles, Scotland.
Abstract
Introduced predators are a major threat to island avifaunas world-wide. In the Western Isles of Scotland, recently introduced hedgehogs Erinaceus europaeus have become a serious predator of bird eggs and are an important cause of declines of some waders (Charadrii). Experiments at 2 sites in 1998 measured the effect on wader nest success resulting from hedgehog removal from fenced exclosures. The nest success of waders inside the plots (where hedgehog densities were zero or low) was approximately 2.4 times that of birds nesting in the adjacent control areas (where hedgehog densities were high). There was no evidence of a compensatory increase in egg loss to native avian predators. The experiment was an integral part of a research programme to support wader conservation efforts. On the basis of the experiment it can be predicted that the removal of hedgehogs on a larger scale would result in a large increase in nest success. The study also tested the practicalities of using relatively cheap fences against hedgehogs. Fences were generally effective, but on dry sandy ground rabbits Oryctolagus cuniculus burrowed under fences, enabling some hedgehogs to re-enter plots. Well-designed fences could be used as a conservation tool, both as a barrier to protect key sites, and to aid the trapping and removal of hedgehogs. However, fences cannot be seen as a long-term solution to the problem. Radio-tagged hedgehogs removed from the plots and released nearby all attempted to re-enter the plots. Two tracking methods revealed that displaced hedgehogs followed fences for distances up to 500 m looking for an entry point. There was no evidence that hedgehogs were able, or even attempted, to climb over or dig under fences. The establishment of hedgehogs in the Western Isles provides an example of a threat to biodiversity following human-mediated redistribution of a species native to the UK to parts of the UK (Scottish islands) outside the species' natural range, an activity not currently prohibited by law. Policy action to deter or control species introduction should consider ecological range even within national boundaries.