Global trends of seabird impacts on terrestrial and near shore food webs

Background/Question/Methods

Seabirds are keystone species linking marine and terrestrial systems via their role as nutrient vehicles. Their role in each of these systems differs substantially; their primary role in marine environments is as top predators, while in terrestrial and near shore systems it is predominantly as providers of allochthonous resources. Seabird derived resource subsidies lead to both direct and indirect impacts throughout food webs. These impacts can be complex, and have not been widely studied, and so their universalities are unknown. Notably, the nature of seabird direct and impact impacts may depend on community assemblage at a given site, which is tightly tied to regional location (‘island identity’). Furthermore, the introduction of non-native organisms, particularly mammalian predators, often leads to seabird decline and extinction, which further alters food web dynamics. We review studies documenting the impacts of seabirds on terrestrial and near shore food webs, focusing particularly on work from islands in the Gulf of California and islands in the Hauraki Gulf of New Zealand.   

Results/Conclusions

Our analysis confirms that seabird bottom-up subsidies play a universally major role in terrestrial and near shore food webs, and that the introduction of non-native predators will drastically alter the nature of the role of seabirds in these systems. In particular we find that seabirds generally have positive effects on higher trophic levels, but that this trend can be confounded by variations in input rate and timing and in productivity of the recipient system, as well as by the presence of higher-level interactions in food webs, particularly when higher order interactions include changes in predator abundance which can switch the system to top-down regulation. At a broad level seabird species identity can plays some role in moderating ecosystem function, but the identity of other components in the food web consistently plays a major role. Thus community composition, reflecting archipelago identity, frequently alters the flow of energy and hence indirect interactions in any given system. From a restoration perspective, this suggests that even following eradication of introduced predators, it will prove difficult to fully rehabilitate systems, including reinstating indirect linkages, particularly since they are rarely well understood.