Islands have provided insights into some of the most fundamental patterns of diversity. For example, the influences of area and isolation on diversity have been studied extensively in the context of the equilibrium theory of island biogeography (IBT). However, on small islands area may be a weaker predictor of diversity than other factors. The transfer of nutrient subsidies from marine ecosystems to terrestrial ecosystems is a process that can influence the diversity and composition of insular plant communities. Characteristics of islands, such as their perimeter-to-area ratio and their permeability to vectors of subsidies, could determine the influence of subsidies on plant communities—but little is known about how those characteristics interact with subsidies and other drivers of plant communities across scales. Our objective is to determine how island-scale variables mediate the effect of marine nutrient subsidies on insular plant communities. To address this objective, we surveyed plant communities on 74 small islands on the Central Coast of British Columbia. We collected plot-scale abiotic and biotic variables, and used remote sensing to derive island-scale variables. We used hierarchical, mixed effects modelling and model selection to select among competing hypotheses of drivers of plant community composition and diversity.
Results/Conclusions
Between islands, island-scale species density increased weakly with island area, while mean soil nitrogen and soil δ15N at shoreline and 40m inland plots decreased with island area. Within islands, the major gradient in community composition was characterised by a shift from nitrogen-poor indicators to nitrogen rich indicators, and strongly correlated with a nitrogen-indicator index. Distance to shore and soil moisture were the strongest predictors of plot-scale community composition, while island-scale IBT variables were not important predictors. Community composition was dominated by nitrogen rich indicators at shoreline plots, and species density was highest at shoreline plots. Shoreline plots had higher total soil nitrogen and higher soil and foliar δ15N values. Our preliminary results indicate that the influence of subsidies on soils is greater on small islands. The impact of subsidies on within-island plant communities is greatest at the shoreline, closest to the source of the subsidies. We will show the interaction of other island characteristics with subsidies, and their cross-scale and cross-boundary effects on plant community composition and diversity. Our work will extend our understanding of the drivers of patterns of diversity, using island plant communities as a model.