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

Abstract

Abstract. 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.

Stable isotopic (δ15N, δ13C) analysis of wood in trees growing in past and present colonies of burrow‐nesting seabirds in New Zealand. I. δ15N in two species of conifer (Podocarpaceae) from a mainland colony of Westland petrels (Procellaria westlandica), Punakaiki, South Island

“…In other studies, the presence of seabird breeding increased plant productivity (Bancroft et al 2005;Wait et al 2005) and leaf nutrient status (Anderson & Polis 1999;García et al 2002), although high burrow densities can reduce growth rates and seedling establishment (Mulder & Keall 2001). If the higher nutrient status and herbivory found in red mangroves in Florida, fertilised by the addition of bird guano to the surface of the plants, soil, and water is also present in podocarp conifers in the Westland petrel colony, the flow-on effects to higher trophic levels could be substantial.…”

A population and harvest intensity estimate for Sooty Shearwater, Puflinus griseus, on Taukihepa (Big South Cape), New Zealand

“…Although the true extent of their role and influence is only now starting to be understood (Harding et al 2004, Hawke & Holdaway 2005, Hawke & Newman 2005, Bancroft et aL 2005, Fukami et al 2006, McKechnie 2006, clearly the contributions seabirds can make to island ecosystems are considerable. Our estimate of over five million Sooty Shearwaters associated with an island of only 930 ha in size is astounding and provides some insight into the scale of the island-seabird interactions and processes operating there.…”

Characteristics of Coral Cay Soils at Coringa-Herald Coral Sea Islands, Australia

“…Activities of the burrowing wedge-tailed shearwaters (Puffinis pacificus) within the Pisonia forests have an important influence on soil development on coral cays (Bancroft et al 2005). According to Jones et al (1994), these birds are considered ''ecosystem engineers'' because they displace at least 11.5 t ha À1 (8.5 m 3 ha À1 ) of soil annually ( Jones et al 1994).…”

“…According to Jones et al (1994), these birds are considered ''ecosystem engineers'' because they displace at least 11.5 t ha À1 (8.5 m 3 ha À1 ) of soil annually ( Jones et al 1994). Bancroft et al (2005) ). This bioturbation is an important regolith process, comparable with soil formation, and enriches the A horizon with nutrients captured from aerosols, marine debris, guano, and plant litter from the marine ecosystem (Field and Little 2008).…”