Burrowing seabirds drive decreased diversity and structural complexity, and increased productivity in insular-vegetation communities

Burrow-nesting seabirds, such as the wedge-tailed shearwater (Puffinus pacificus (Gmelin)) physically and chemically engineer the soil of their colonies in a manner that is likely to affect plant growth and ecology. We examined this functional interaction by measuring the diversity, vertical structure and productivity of vegetation in shearwater colonies on Rottnest Island, Western Australia, and by comparing these with those in the adjacent, non-colonised heath. The colony supported a distinct, less diverse vegetation community and was dominated by short-lived, succulent exotics. An ecotone was present between the two communities. Across all species, vegetation was shorter and denser in the colony and individual species that co-occurred in both locations were stunted in the colony. The percentage of bare soil in the colony was double that of the heath. The productivity of a phytometer (Rhagodia baccata) was significantly higher in colony soil than in heath soil. In a glasshouse experiment, cuttings grown in colony soil had 337% of the root mass and 537% of the foliage mass of plants grown in heath soil. Field measurements demonstrated increased leaf set and foliage extension in colony plants. Seed germination from the colony soil (2674 seedlings m(-2)) greatly exceeded that of the heath (59 seedlings m(-2)). Dense, productive and species-poor colony vegetation supports the assemblage-level thinning hypothesis as the mechanism for vegetation change, but we argue that prominent colony species are simply better adapted to high nutrient loads and frequent disturbance. A model of vegetation succession is also proposed.