Marine Birds on Land: A Review of Plant Biomass, Species Richness, and Community Composition in Seabird Colonies

Abstract

Seabirds are chemical and physical engineers that are capable of transforming terrestrial vegetation by altering edaphic conditions, generating physical disturbance, and affecting seed dispersal. Substantial changes in seabird populations are occurring worldwide and are likely to have important consequences for plant community composition on islands and coastal areas. This review focuses on the impact of seabirds on plant biomass, species richness and community composition. A total of 57 publications (42 studies) were selected for review. Of the 42 studies represented in the publications, 55% were descriptive. Most studies took place in Australia, New Zealand, the British Isles, Japan, North America, and sub-Antarctic islands. A few studies showed that aboveground plant biomass in seabird colonies increased with sufficient rainfall and moderate temperatures. The majority of studies on plant species richness showed a decrease in seabird colonies compared to areas unaffected by birds. However, species richness was higher in areas of intermediate seabird disturbance, compared to undisturbed areas. Moreover, the effects of seabirds on species richness varied with respect to island size. Most studies of plant community composition indicated that annuals, ȁ8ruderals”, and cosmopolitan species increased in abundance in seabird colonies. Changes in plant communities in seabird colonies appear to result mainly from altered soil nutrient concentrations and pH, increased physical disturbance, and seed dispersal by seabirds and humans. However, few studies have rigorously studied the relative importance of these alterations. Both the direction and magnitude of seabird effects are modified by: (1) density of birds, (2) temperature and precipitation, and (3) proximity to human habitation. A reduction in seabird populations is likely to have negative consequences for native plant species that rely on seabird disturbance for their persistence. However, seed dispersal by nesting seabirds, especially gulls, frequently leads to invasion by cosmopolitan plant species and declines of native species. Further studies that incorporate both quantitative sampling and manipulative experiments would go a long way in improving our understanding of how seabirds affect plant communities.