The influence of seabird-derived nutrients on island food-webs

There is an increasing understanding of the influence seabirds have on island food webs globally, which often arises from the trans-boundary input of nutrients. Seabird-derived nutrients, primarily in the form of guano, can have significant effects on island communities by increasing primary productivity and then indirectly influencing other species. However, there are few studies looking at how the influence of seabirds permeates island food webs to higher trophic levels, in particular within the United Kingdom, which holds globally significant populations of seabirds. To understand the extent to which seabirds influence islands, the size of the seabird population must be first reliably determined. With an increasing seabird population size and density the effects of seabirds on land increases concomitantly. The Atlantic puffin Fratercula arctica is a difficult species to reliably monitor, given its underground presence from the use of burrows and its notoriously erratic attendance at colonies. This study looks firstly at a novel method to monitor the Atlantic puffin using time-lapse photography. Time-lapse photography provides a way to derive an estimate of population size from counts of individuals, by repeated photographs across a period of time with relatively low cost and from areas normally considered inaccessible. The results showed there was a significant and positive relationship between the maximum numbers of Atlantic puffins observed and the size of the population; further work is required though to reduce the error associated with population size estimates. Data from high temporal resolution time-lapse photography shows how the attendance of Atlantic puffins at the colony varies over different temporal scales. Given the variability in sampling intensity the study stresses the need for standardised sampling intensity with the use of photography to monitor Atlantic puffins. Secondly, this study showed how the presence of two seabird species, the Atlantic puffin and the great skua Stercorarius skua, alters island food webs. These seabird species are likely to change plant community diversity, relative to areas without seabirds. The chemical concentration of grasses inside seabird colonies was also altered: grasses had significantly higher concentrations of nitrogen and also had higher values of δ15N, relative to areas without seabirds. These chemical alterations suggest that nutrients from seabirds are incorporated into local vegetation. Furthermore, samples of hair from rabbits and sheep found within puffin colonies also had significantly higher values of δ15N, suggesting that nutrients travel from seabirds into secondary consumers, via ornithogenic forage. An additional study on the transfer of nutrients within island food webs showed how ornithogenic nutrients deposited on an island in the Baltic Sea were incorporated into house martins, via aquatic insects. These studies, along with an understanding of seabird population size, suggest that the impact of seabirds on island food webs may be considerable and have large consequences for island conservation and management