Picking on someone your own size: ant competition in epiphytic ferns is based on body size

Competition occurs when two species of animals or plants use the same resource, for example food or nesting space. Competition is surprisingly difficult to detect. If you see that two species are not often found together, this is not sufficient evidence to show that competition is occurring: the two species may just prefer different environments. This means that experiments are needed, in which pairs of species are introduced into the same environment. Furthermore, even if experiments show that pairs of species compete, this might not make any difference to species distributions in the wild, relative to other factors. We tested whether or not competition was occurring between ants living in epiphytic ferns (ferns living in the forest canopy, supported by trees). We assumed that ant species that were similar in size are likely to use the same resources. First we collected ants from ferns up to 50 m high the rain forest canopy in Malaysian Borneo. Similar-sized ant species were not often found in the same fern. We then tested whether this result was due to competition between similar-sized species by introducing pairs of ant colonies into ferns in the laboratory. We tried this for a range of different-sized ants. We found that when the ants were similar in size there was strong competition, with one colony ejecting the other from the fern. Ants of different body sizes co-existed peacefully. Then we used computer simulations to see what would happen if we considered ants colonising all of the ferns in an area of forest. When we included the result from the experiments (similar sized species competing), our simulations gave results very similar to the patterns seen in the wild, but when the experimental results were not included, the simulations did not give realistic results. Our results show that the fact that similar-sized ant species do not co-occur is due to competition, and that only when this relationship is included in simulations do they give realistic results.

Fayle T.M., Eggleton P., Manica A., Yusah K.M., Foster W.A. (2015) Experimentally testing and assessing the predictive power of species assembly rules for tropical canopy ants. Ecology Letters: Published online: 27 Jan. 2015.

DOI: 10.1111/ele.12403