One ant's trash is another ant's treasure: Army ant middens provide resources for diverse ant assemblages

The army ant Eciton burchellii boasts more animal associates than any other animal species yet described, but the relationship between army ants and other ant species has only been studied in the context of predation. The waste deposits (middens) of army ant colonies are nitrogen-rich, a potentially high-value nutrient source for leaflitter arthropods. We explored this bottom-up role of army ant middens in the context of tropical ant communities. Our three main questions were the followings: (1) Which ant species forage on army-ant middens? (2) How does the bi-phasic life cycle of army ant colonies (affecting midden size, persistence, and abundance) affect which and how many ant species a midden boasts? (3) How do the ants that forage on army ant middens differ across elevations? Across 39 bivouacs, we found 36 species of ants foraging on army ant middens. These included highly predatory ants, nitrogen-limited arboreal ants, and fungus-farming ants. Per-midden richness was significantly lower for the usually smaller middens deposited during the nomadic phase and was higher for the typically larger middens deposited during the statary phase. Per-midden richness was not significantly different across elevations, but there was far greater species turnover across elevations than across phases within the same elevational site. Our results suggest that army ant middens are an important resource for a wide variety of tropical ants, informing a better understanding of the complex network of associations revolving around this keystone species.

Abstract in Spanish is available with online material