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Tiny Ants, Big Impacts

How weaver ants might be shaping distribution of insect-eating birds

Weaver ants
Weaver ants could be eating up bird food sources, thereby shaping their distribution. (Photograph by Tuancao1/Wikimedia Commons)

Are low-land ants in South Asia pushing insect-eating birds up to higher elevations by consuming all their food source? That is what E&E professor Trevor Price and his co-authors set out to explore in a recent study published in Ecology Letters. The results suggest that competition between weaver ants (Oecophylla) and invertivorous birds could have implications for bird elevational diversity. Featured in a Science story, this hypothesis—that ants might shape the distribution of insect-eating birds—still needs to be further tested, but already offers intriguing insights into the interplays between distantly related taxa and their potential outcomes.

 

“The mass of ants in the world is more than that of all wild birds and mammals combined, so in retrospect it is not surprising that they should have a large effect on where birds and mammals are found.”
Ecology and Evolution Professor Trevor Price

Paper Abstract

Using data on bird species elevational distributions from the world's mountain ranges, bird diets, and the distribution of the ant genus Oecophylla, we report that global patterns in bird elevational diversity show signals of competition with ants. Oecophylla is an abundant and effective predator of invertebrates, preying on the same species that invertivorous birds feed on. In mountain ranges with Oecophylla present in the foothills, the maximum species richness of invertivorous birds (but not other trophic guilds) occurs, on average, at 960 m, ca. 450 m higher than in mountain ranges without Oecophylla, resulting in a mid-elevation peak in bird species richness. Where Oecophylla is absent, bird species richness for all guilds generally show monotonic declines with increasing elevation. We argue that Oecophylla reduces prey density for invertivorous birds and that low prey abundance reduces invertivorous bird density, which in turn is correlated with lower bird species richness. These findings suggest that competition between distantly related taxa can set range limits, leading to emergent diversity patterns over large scales.