A regular theme on this blog (here, here, here, etc…) is how fascinated all of our contributors are by the factors that promote and maintain biodiversity. Personally, I am really into some of these such as positive interactions between species, such as mutualisms (like the legume-rhizobium mutualism I study) and facilitative effects between species. In some systems, negative interactions between species can result in indirect facilitation of other species by inhibiting a shared competitor. These competitive facilitative interaction combinations could result in increased coexistence between species in a community. A recent paper by Palmer et al. explores how the competitive hierarchy between 4 ant species known to be mutualists on Acacia drepanolobium trees can lead to coexistence between these species through indirect facilitation.
Ant-plant mutualisms are pretty amazing and well-studied, but pushing it one step further are ant-ant interactions on plants! The ant participants in this account are (in order of dominance, from highest to lowest, established in some of the author’s previous studies): Crematogaster sjostedti, C. mimosae, C. nigriceps, and Tetraponera penzigi.
These ants compete strongly for trees and only one species occupies a tree at a given time, but a single ant colony can occupy multiple trees, creating a territory. After noticing some of the least competitive species were living near the most competitive species (big woot for natural history observations!), Palmer et al sought to explore whether the niche differences between species might lead to indirect facilitation of T. penzigi by C. sjostedti. They then employed a thorough series of observations and experiments to test their hypotheses.
It turns out that the ant species occupy different sized trees, following the hierarchy with the more dominant species residing in larger trees. Moreover, the dominant C. sjostedti were more tolerant of T. penzigi, with lots of T. penzigi queens and colonies being found in the dominant’s territory, especially on smaller trees, but not in the territories of the other two species. Manipulations also revealed that the T. penzigi was more likely to colonize trees in the dominant’s territory than was the 2nd weakest species, C. nigriceps. However, both intermediates were found to displace T. penzigi more than the dominant, indicating the potential for succession in ant establishment on Acacia trees. This is pretty exciting – the dominant ant species, C. sjostedti, likely does not go after the subordinate, T. penzigi, on smaller trees because it is not as capable of using smaller trees (different niches). This is partially because C. sjostedti requires a beetle to excavate the stem first, which the beetle can not do on trees that are too small. So, as the trees grow, the two intermediates appear and dispace T. penzigi, then C. sjostedti comes in (getting rid of the intermediates), thus allowing T. penzigi to colonize nearby smaller trees, starting the whole process over again. The authors even go on to discuss a nearby site with mainly smaller trees in which they find none of the main dominant ant species discussed here as well as none of the subordinate, further bolstering their results.
This study an exciting summary of the ecological interactions in a complex competitive hierarchy on Acacia trees in Kenya. Not only do the authors discuss and utilize their tremendous knowledge of the system to ask really interesting questions very relevant to demonstrating how facilitation between species can help maintain diversity, but they also manage to convey the excitement of this system to the reader. I, for one, am anticipating reading further about their future studies into these indirect effects!
Palmer, T.M., M.L. Stanton, T.P. Young, J.S. Lemboi, J.R. Goheen, R.M. Pringle. 2013. A role for indirect facilitation in maintaining diversity in a guild of African acacia ants. Ecology 94: 1531-1539.