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Spatial Variation in Army Ant Swarm Raiding and its Potential Effect on Biodiversity



Invertebrate communities of the tropical rain forest floor are highly diverse, characterized by patchy species distribution patterns and high variation in species density. Spatial variation in the foraging activity of swarm raiding army ants, prime invertebrate predators in tropical rain forests, is discussed as a mechanism contributing to these patterns, but highly resolved long-term data on army ant raiding on the local and landscape scale are hitherto lacking. In this study, 196 positions in 11 study sites in a tropical rain forest in western Kenya were continuously monitored over ~4 mo for the occurrence of swarm raids of army ants. Using population simulation analyses, the consequences of army ant raiding for prey communities were assessed. We found an unexpectedly high variation in raid rates at the study site and landscape scale. The weekly chance of communities to become raided by army ants was on average 0.11, but ranged from 0 to 0.50 among the 196 positions. Simulating population developments of two Lotka–Volterra species—showing slight trade-offs between competitive strength and resistance to army ant raids—in the real raiding landscapes showed that the observed spatial variation in raid rates may produce high prey diversity at larger spatial scales (due to high β-diversity) and strong variation in species density. Our results indicate that high spatial variation in army ant swarm raiding is a mechanism capable of generating patchy species distribution patterns and maintaining the high biodiversity of invertebrate communities of the tropical rain forest floor.