In many coastal and island settings, seabirds are the primary vehicles of nutrient transfer, substantially supplementing these environments with marine-derived nutrients. Changes in the density of seabirds can thus have important effects on the nutrient budgets and ecology of these ecosystems. For example, the introduction of novel avian predators has been shown to significantly disrupt seabird derived nutrient subsidies and affect fundamental ecosystems processes, in some cases triggering whole scale shifts in ecosystem structure and function. Yet top-down effects are not the only mechanism that can drive down bird densities and reduce avian nutrient depositions. Here we consider whether habitat modification by a widely disseminated plant, Cocos nucifera, can cause similar deleterious effects to seabird populations, nutrient subsidies, and ecosystem processes and structure. We conduct this work by comparing multiple ecological features of islets with different C. nucifera abundance on a tropical atoll (Palmyra Atoll, USA), where C. nucifera has proliferated in the last 200 years.
Results/Conclusions
We demonstrate that C. nucifera, the coconut palm, significantly interrupts the flow of allochthonous marine subsidies to terrestrial ecosystems This interruption is driven by a novel mechanism: C. nucifera provides suboptimal habitat for seabirds, thus lowering seabird abundance in forest ecosystems and reducing the critical nutrient inputs that they bring to these systems from the marine environment. These decreases in marine subsidies affect the ecology of these nutrient poor coastal forests by: reducing available soil nutrients, decreasing leaf nutrient quality, diminishing leaf palatability, reducing overall rates of herbivory, and changing abundance and trophic position of some consumer groups. This case offers an important example of how a plant can alter the movement of nutrients across ecosystem boundaries and affect the ecology of communities that depend on these subsidies. This observation sharply contrasts the patterns of nutrient enrichment that more typically follow plant range expansions/invasions. This work improves our understanding of the role that C. nucifera plays in coastal forests and more generally illustrates the need to better understand the role that plants play in mediating transfers of allochthonous nutrients.
