Ant seed dispersal

Aphaenogaster woodland ants are important dispersers of understory plant seeds. What happens when an invader displaces them?

And now some bad news.

A new study by Mariano Rodriguez-Cabal in Biological Invasions appears to document an unfortunate effect of the ongoing Pachycondyla chinensis needle ant invasion: a decrease in wild evergreen ginger plants.

Abstract: By disrupting the structure of native ant assemblages, invasive ants can have effects across trophic levels. Most studies to date, however, have focused on the impacts just two species (Linepithema humile and Solenopsis invicta). The impacts of many other invasive ant species on ecological processes in their introduced range are unknown. In this study we tested the hypothesis that the invasive ant Pachycondyla chinensis disrupts ant-seed dispersal mutualisms by displacing native ant species, especially the keystone mutualist Aphaenogaster rudis, while failing to disperse seeds itself. In a paired design we measured the impact of P. chinensis on the native ant-plant seed dispersal mutualism. The number of A. rudis workers was 96% lower in invaded than in intact plots, and the number of seeds removed was 70% lower in these plots. Finally, in invaded plots the abundance of Hexastylis arifolia, a locally abundant myrmecochorous plant, was 50% lower than in plots where P. chinensis was absent. A parsimonious interpretation of our results is that P. chinensis causes precipitous declines in the abundance of A. rudis within invaded communities, thereby disrupting the ant-plant seed dispersal mutualisms and reducing abundances of ant-dispersed plants. In sum, the magnitude of the effects of P. chinensis on seed dispersal is quantitatively similar to that documented for the intensively studied invasive Argentine ant. We suggest that more studies on the impacts of less-studied invasive ant species on seed dispersal mutualisms may increase our knowledge of the effects of these invaders on ecosystem function.

In short- native ants help native plant seeds spread and sprout. Foreign ants displace native ants, and the native plants suffer.

asian needle ant

The culprit: Pachycondyla chinensis, the asian needle ant

One caveat. This study is correlative in nature. Invaded areas had lower ant-dispersed plant counts. It is possible, I suppose, that the causation runs the other direction. Dense ant-plant areas might be more resistant to invasion, leading to the same pattern but for different reasons. This strikes me as highly unlikely given what we know of the biology of these animals, though, but it wouldn’t hurt to experimentally confirm the chain of events.

source: Rodriguez-Cabal MA, Stuble KL, Guenard B, Dunn RR, Sanders NJ. 2011. Disruption of ant-seed dispersal mutualisms by the invasive Asian needle ant (Pachycondyla chinensis). Biological Invasions Online First DOI: 10.1007/s10530-011-0097-5