Browsing down our natural heritage: Deer impacts on vegetation structure and songbird populations across an island archipelago

Declines in forest and woodland birds have largely been attributed to habitat loss and fragmentation. In the past decade, however, the potential for herbivores to influence bird species abundance and community composition via their direct impact on vegetation structure has also been recognised. We tested the hypothesis that deer influence vegetation structure and bird assemblages in a large island archipelago in western North America using surveys of 18 islands with deer densities ranging from 0 to over 1 deer/ha. Amongst these islands, reduced predation and hunting pressure has allowed deer populations to increase above those likely to have existed in pre-European times. Our results support a growing body of evidence that deer regulate both the cover and architecture of understory vegetation which in turn profoundly affects island bird assemblages. Deer-free islands supported the most abundant and diverse bird fauna. Iconic songbirds such as the rufous hummingbird, song and fox sparrow were abundant on islands with no deer but substantially reduced on islands with high deer densities. Only one bird species, the darkeyed junco, preferred moderate and high density deer islands. Our observations suggest that current cohorts of palatable shrubs on islands with high deer densities are relatively old and potentially represent an impending extinction debt, where the full effects of high deer density on island biotamay take decades to fully unfold. Our results suggest that deer densities below a threshold of 0.1 deer/ha should allow native vegetation to recover and a rich and diverse bird species assemblage to persist. We suggest that adaptive management be used to test the validity of this threshold, and that without active management of deer abundance, local extinctions of native flora and fauna appear likely to accelerate.

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Browsing down our natural heritage: Deer impacts on vegetation structure and songbird populations across an island archipelago (1.9 MB)


Abstract. The dominance of nonnative plants coupled with declines of native plants suggests that competitive displacement drives extinctions, yet empirical examples are rare. Herbivores, however, can alter vegetation structure and reduce diversity when abundant. Herbivores may act on mature, reproductive life stages whereas some of the strongest competitive effects might occur at early life stages that are difficult to observe. For example, competition by perennial nonnative grasses can interfere with the establishment of native seeds. We contrasted the effects of ungulate herbivory and competition by neighboring plants on the performance of native plant species at early and established life stages in invaded oak meadows. We recorded growth, survival, and flowering in two native species transplanted as established plants, six native species grown from seed, and five extant lily species as part of two 2 3 2 factorial experiments that manipulated herbivory and competition. Herbivory reduced the performance of nearly all focal native species at early and established life stages, whereas competition had few measurable effects. Our results suggest that herbivory has a greater local influence on native plant species than competition and that reducing herbivore impacts will be required to successfully restore endangered oak meadows where ungulates are now abundant.

Key words: black-tailed deer; British Columbia; competition; conservation; exotic; grass; invasive; oak savanna; Odocoileus hemionus; sheep.

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