Boisterous bathing birds bias β–diversity: Frequent dispersal by gulls homogenizes invertebrate taxa in a rock-pool metacommunity

Background/Question/Methods

For aquatic invertebrates, freshwater habitats represent hospitable islands embedded in a hostile terrestrial landscape. Although some species are capable of actively dispersing among habitats by flying or walking, many aquatic invertebrates must rely on passive dispersal via external mechanisms (e.g., wind, animals) to colonize new habitats. Despite the potential importance of passive dispersal for the distribution and diversity of aquatic invertebrate species, its effects remain poorly understood. Since Darwin’s seminal experiment suggesting that ducks could transport snails among wetlands, waterbirds have been implicated as an important vector for passively dispersing invertebrates among aquatic habitats. Indeed, many studies since Darwin have demonstrated that birds can transport invertebrates, either externally (on feathers, feet, etc.) or internally (in the gut), between habitats.  However, most of these reports lack data regarding the spatial and temporal context of bird-mediated dispersal. Further, the effects of bird-mediated dispersal on populations and communities of aquatic invertebrates remain unknown. Here, we use a system of freshwater rock pools as a model field metacommunity to quantify passive dispersal of invertebrates via gulls (Larus spp.) and determine if, and how, it influences local and regional species diversity.

Results/Conclusions

We use a combination of field observations and experiments to show that gulls frequently visit rock pools and regularly transport invertebrates between pools. A second set of field observations demonstrated that this gull-mediated dispersal had a significant homogenizing effect on the invertebrate communities: pools in areas of higher gull densities had significantly lower among-pool variation in species composition (i.e., lower β-diversity) compared with pools in areas with lower gull densities. However, neither local (α) nor regional (γ) species richness was significantly affected by the density of gulls. Our results demonstrate that waterbirds, as suggested by Darwin, are an important vector for dispersing aquatic invertebrates and can strongly influence the species composition of aquatic habitats, in this case for the spatial scale at which juvenile gulls move.