Patterns in the assembly of an island plant community

Aim To test for patterns in the assembly of an island plant community. Location Islands off the west coast of Vancouver Island, British Columbia, Canada. Methods Twenty-seven islands were visited by boat, and the abundance of six woody angiosperm species was quantified. Null models were then used to test whether: (1) some species co-occur less than expected by chance (i.e. co-occurrence assembly rule), (2) the incidence and abundance of some species are inversely related to the abundance of other species (i.e. incidence assembly rule), and (3) support for assembly rules precludes evidence for nestedness, which refers to a pattern in species composition in which the species present on depauperate islands form regular subsets of those occurring on progressively more diverse islands. Results Most species co-occurred with other species at frequencies expected by chance. However, one species (Sambucus racemosa) co-occurred with other species less frequently than randomized expectations. The observed incidence and abundance patterns of most species were also consistent with randomized patterns. However, the incidence and abundance of S. racemosa declined with the abundance of other plant species. Weak, variable support was found for nestedness of the total plant community. However, stronger, consistent support was found after removing S. racemosa from the matrix prior to analyses. Main conclusions Most species were assembled on islands in a manner consistent with randomized expectations. However, non-random distributional patterns were observed in one species whose distribution was consistent with the hypothesis that competition limits the assembly of island communities.