Burrow building in seabird colonies: a soil-forming process in island ecosystems

Soil modification via biopeciturbation by burrow-buitcling seabirds was examined in a Mediterranean, island ecosystem. Physical and chemical soil properties were compared between a colony of Wedge-tailed Shearwaters (Puffinus pacificus) and adjacent heath across a 14-month period. When compared to heath soil, the biopedturbated soil was 28% drier (6.04 +/- 5.40vol%), had increased bulk density (by 29% to 1.30 +/- 0.11 g cm(-3), 51% porosity), wetting capacity (by 83% to 0.55 +/- 0.83 motarity of ethanol droplet), hydraulic conductivity (by 266% to 398.91 +/- 252.04mmh(-1)), and a greater range in soil surface temperature (31.7 +/- 6.2 degrees C diurnally to 18.3 +/- 3.2 degrees C nocturnally). Soil penetration resistance was reduced by 26% at a depth of 0-100 mm (326.5 +/- 122.4 kPa) and by 55% at 500-600 mm (1116.8 +/- 465.0kPa). Colony soil also had increased Levels of nitrate (by 470%), phosphorous (118%), ammonium (102%), sulphur (69%), and potassium (34%), decreased levels of iron (by 50%) and organic carbon (61%), was more alkaline, and had a 78% greater conductivity. Shearwaters deposited guano at a rate of 234.4 kg ha(-1) yr(-1) (dry mass). Chemical analysis of guano equated this to 50.9, 5.7, 5.5, and 3.6 kg ha(-1)yr(-1) of nitrogen, potassium, sulphur, and phosphorous, respectivety. Experimentally constructed burrows demonstrated that digging alone can alter physical and chemical soil factors, but that changes in the nutrient profile of colony soils are predominantly guano-driven. We argue that the physical impact of seabirds on soil should not be overlooked as a soil-forming and ecosystem-shaping factor in island ecosystems, and that biopeclturbation can exert major bottom-up influences on insular plant and animat communities. (c) 2004 Etsevier GmbH. All rights reserved.