Island biogeography theory predicts that species richness will reflect a balance of colonization and extinction probabilities that relate to island size and isolation. Less thought has been given to island productivity and habitat diversity. In the fall of 2016, we conducted quantitative surveys of the flora and fauna on all islets of Palmyra Atoll, a low-lying coral atoll, 1000 miles south of Hawaii. A National Wildlife Refuge, Palmyra has never supported permanent human habitation and geckos are the only terrestrial vertebrates. In this arthropod-dominated system, ants occupy a unique position in the food web functioning as the sole generalist omnivores. In addition to their unique trophic position, ants on Palmyra are one of the most important consumers in terms of both density and biomass. Here, we examine the distribution, diversity, density, and biomass of 11 species of ants across 11 of the 23 islets that comprise Palmyra Atoll. We examined how these species distribute across individual trees, canopies and finally habitat types: native vs. non-native forests. These islets vary independently in size and productivity but are similar in isolation. Thus Palmyra provides a unique opportunity to understand how these abiotic forces determine the diversity and composition of ant establishment.
Results/Conclusions
All ants are non-native to Palmyra, and our comprehensive survey identified 11 species spanning 9 genera, including two recent invasions (post-2005) and a possible local extinction. At the Atoll level, the community is dominated by the globally invasive big-headed ant (Pheidole megacephala), which is present on all islets. Less dominant ant species were present on only a few islets and vegetation types. To better understand this, we are currently investigating how these 11 ant species partition resources at finer scales and interact with other arthropod species on the island. Our preliminary results indicate that productivity, and not ecosystem size is the primary driver of ant diversity, densities, and biomass on Palmyra Atoll. We found that the relative abundance of ants on islands was correlated to accumulation rates of phosphorus (grams per day) (r=0.721, df=10, p-value = 0.008), and ant species richness was correlated to accumulation rates of nitrogen (grams per day) (r=0.604, df=10, p-value= 0.037). Overall, our results suggest that the drivers of community structure and abundance are highly context dependent, depending on fine-scale habitat features. Given the global threats of invasive ants, understanding the drivers of invasive ant community composition and abundance is of critical conservation concern.