Spatial subsidies affect plant diversity and distribution patterns within and among islands in the San Juan Archipelago, Washington, USA

Background/Question/Methods

Cross ecosystem flux of resources subsidizes recipient systems with well-demonstrated patterns of enhanced or modified abundance of populations and predictable changes in trophic interactions. However, the influence of spatial subsidies on diversity patterns and spatial distributions of communities across space is less well established.  Subsidized Island Biogeography Theory predicts that the presence of spatial subsidies could explain departures from species-area curves for small islands, either increasing or decreasing island species richness, depending on inherent productivity of the region. Field surveys on islands in the San Juan Archipelago, Washington, USA explored if spatial subsidies in the form of nutrient inputs from decomposing shore wrack, nesting and roosting Canada geese, and denning river otters created differences within and among islands in the soil resource base that could influence physiological and competitive mechanisms that underlie diversity and distribution patterns of island plants.  Using soil nutrient status and ¹⁵N signatures of soils and plants as a indicators of degree of subsidization, islands that are more heavily subsidized by one or more of these three inputs have fewer plant species island-wide, and lower Simpson’s Indices and fewer species encountered per 0.25 m² basis than islands that have fewer subsidies. 

Results/Conclusions

Less subsidized islands tend to fall above the Species-Area Z-curve, and more subsidized islands tend to fall below the Z-curve.  Within islands, 0.25 m² sample quadrats that are closer to the shore and more heavily influenced by one or more of these subsidy sources have lower richness and Simpson’s than plots further inland, although plots at distances of 30-40 m or more from the shore tend to decline in richness and Simpson’s again as community composition shifts to more inland species that have different substrate requirements.  Richness and Simpson’s are highest in 10m and 20m plots where subsidies are less evident, and where coastal and inland species overlap.  While spatial subsidies tend to decrease local diversity within coastal zones on islands and across whole islands that are more subsidized, subsidies also tend to facilitate different species composition.  Thus spatial subsidies enhance regional diversity by creating and maintaining areas where certain pioneer species might otherwise be displaced, and allow coexistence of species adapted to high nutrient, high disturbance conditions with species less adapted to those conditions across islands and archipelagos.