Abstract
Although we understand many initial effects of invasive deer on songbirds, we do not yet understand how their longer-term effects unfold. We looked for such potential long-term effects on songbirds in the context of an archipelago where deer were introduced over a century ago. Initial data consist of vegetation plots and songbird point-counts on 57 islands in Haida Gwaii (British Columbia, Canada) taken in 1989 when the impacts of introduced deer lacking predators had already developed for [50 years. Twenty years later, we surveyed these islands using the same methods. To isolate the effects of deer, we compare results to nearby islands never colonized by deer and assess how canopy birds have fared relative to understory birds. We also compare responses between islands of ‘‘moderate deer impact’’ where the understory vegetation was only moderately depleted by deer in 1989 and those with ‘‘severe deer impact’’ where understory vegetation was strongly depleted even in 1989. In 1989 all islands with deer were impoverished in songbirds that depend on understory vegetation, but the moderate impact islandsstill had richer and more abundant understory forestbird communities than found on severe impact islands.Islands with the fewest deer impacts in 1989 were small and isolated from sources of deer colonization.By 2009, severe deer impacts extended to islands that were initially less affected by deer. The severity ofimpacts also increased even on islands that had been dramatically affected by 1989. Declines in bird abundance occurred before declines in bird diversity. These results support the need for actions by wildlife managers to curtail deer impacts as soon as these become evident, especially in reserves and protected areas that lack hunters and other deer predators.