Biogeochemistry and forest composition shape nesting patterns of a dominant canopy ant
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Humans are increasing nutrient deposition across the globe, and we know little about how these changes influence consumer populations in tropical rainforests. We used a long-term fertilization experiment conducted in a Panamanian forest to explore how nutrient availability and tree traits affect abundance of a higher-level consumer. We added nitrogen, phosphorus and potassium in a factorial design for 18 years. Given that phosphorus often limits ecosystem processes in lowland tropical forests, and added nitrogen reduces insect abundance in our experiment, we first hypothesized that phosphorus addition would increase nest density and nest size of Azteca chartifex ants while nitrogen addition would have the opposite effects. We found 48% lower nest density in the canopy of nitrogen addition plots relative to plots that did not receive nitrogen. Phosphorus addition did not affect nest density or size. These nutrient effects were not diminished by the selectivity of host trees. In general, larger trees held more nests, despite their low frequencies across the forest, while some abundant species (e.g., palms) were rarely used. We further predicted higher nest frequency on trees with extrafloral nectaries, because this ant fuels its large colonies with extrafloral nectar. Despite the non-random distribution of A. chartifex nests, across tree species and nutrient treatments, trees with extrafloral nectaries did not host more nests. Our study suggests that areas of a tropical lowland forest which are not oversaturated with nitrogen, and contain large trees, have higher nest density. This could enable A. chartifex in similar areas to outcompete other ants due to high abundance.
KeywordsAzteca trigona Neotropics Nitrogen Nest density Nest size Phosphorus
Comments of Michael Patten, Ellen Welti, Lara Souza and Ana Ješovnik as well as suggestions of two anonymous reviewers and the handling editor greatly improved our manuscript. Oris Acevedo and Belkys Jimenez of the Smithsonian Tropical Research Institute provided logistical support in Panama. Arthur Escalas provided valuable R advice. This study was funded by the Biology Department (OU), the Graduate Student Senate (OU), and Frankopan Fund Fellowship awarded to JB, and National Science Foundation Macrosystems Grant EF-1065844 awarded to MK. The experiments in this study comply with the current laws of the Republic of Panama.
Author contribution statement
SJW designed and conducted the long-term fertilization experiment. JB, MK and SJW conceived and designed the study. JB conducted field work, analyzed the data and wrote the manuscript. MK and SJW provided editorial advice.
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