Relationships between the demography and distribution of two bird-dispersed plants in an island archipelago

Aim I evaluated relationships between the demography and distribution of two bird-dispersed shrubs in an island archipelago to better understand the life history precursors of different island distributional patterns. Location Barkley Sound, Vancouver Island, British Columbia, Canada (48degrees80' N, 125degrees20' W). Methods The abundance and occurrence of Lonicera involucrata Rich. (Caprifoliaceae) and Ribes divaricatum Dougl. (Grossulariaceae) were measured on 69 islands measuring between 25 and 800 m(2). Light and soil conditions were quantified to broadly characterize changes in environmental conditions with island area and isolation. Several experiments and field observations were conducted to evaluate the life history response of each species to changes in island environments. Results Lonicera showed a typical island distribution; it increased in abundance and occurrence with island area. Ribes had a more unusual distributional pattern; it showed no differences in occurrence or abundance with island area. The distribution of both species was unrelated to island isolation. Small islands had less soil and more light than large islands, while environmental conditions were unrelated to island isolation. Lonicera had higher seed germination rates and adult survivorship in large island environments, but similar rates of seed dispersal, seedling survivorship and sapling survivorship in large and small island environments. Ribes had similar rates of seed dispersal, germination and adult survivorship in large and small island environments. However, slightly higher seedling survivorship in large island environments was offset by decreased sapling survivorship. Conclusions Demographic advantages accrued at particular life history stages were either enhanced (Lonicera) or erased (Ribes) at other life history stages. Nevertheless, overall life history trends were consistent with distributional patterns, and suggest different suites of life history characteristics promote different island distributions.