DOES SEABIRD COLONY SIZE DETERMINE THE PHYSIOCHEMICAL PROPERTIES OF ISLAND SOILS?

摘要

Many species of gulls have expanded their range worldwide and massively occupied coastal islands. These colonisations have not only affected the biotic interactions among seabird colonies, but also altered the soil chemistry through excrement accumulation. To test whether the seabird colony size determines nutrient levels of island soil, we carried out a field study during a breeding season on six Mediterranean islands in central North Algeria which harbour different population sizes of the yellow-legged gull (Larus michahellis). We sampled the soil and measured a suite of physical (area, floral richness, clay, fine silt, large silt, fine sand and large sand content) and chemical (pH, electric conductivity, limestone, organic matter, N, P, Ca, Mg, K and Na) properties and the colony size and density of the yellow-legged gull. Using principal component analysis, we found that the six islands showed some physicochemical similarities, but island area, colony size and soil N and P levels structured the islands into different groups. Although there was evidence of a positive relationship between colony size and N and P soil levels, this relationship was not linear. Our results suggest that historical rather than punctual data on the colony size of seabirds might give more robust predictions of soil physicochemistry of islands.