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Tropical rainforests are hotspots of species diversity for many taxonomic groups. But why should there be so many species? How are they all able to coexist? These are the questions I have been seeking to answer during the course of my PhD. The number of species and the complexity of the interactions between them makes investigating this through the whole of the rainforest ecosystem an impractical task. I have taken a microcosm approach to this question and have focussed on a smaller part of the system in the hope that patterns observed at this scale will have relevance to larger scales. The microcosm I have used is the ant community living inside bird's nest ferns (Asplenium spp.). Bird's nest ferns are litter basket epiphytes, meaning that they intercept falling leaf litter from higher in the canopy, and use the nutrients resulting from the breakdown of this leaf litter to power their own growth. The resulting spongy mass of decomposing leaf litter retains water and provides a vital refuge for canopy animals from arthropods to bats away from the hot dry conditions prevalent in the rainforest canopy. Ants are the most commonly found animal group in the ferns and perform important ecological functions, such as soil aeration, seed dispersal and predation. Multiple colonies can be found in a single fern, making them ideal for studying the interactions that occur between species. Understanding such interactions is a vital first step towards building a model of how species diversity is maintained.

Initially I needed to document the distribution of the ferns themselves and understand the factors affecting this distribution. I did this at the site where all of my work was to be carried out in Danum Valley, Sabah, Malaysia. I surveyed the densities of the ferns in primary (unlogged) forest and measured a number of canopy architecture variables to understand how the density of the ferns varied with these. In this group of ferns it has always been difficult to distinguish between species in the field. I assigned putative species in the field, and took frond samples from a number of individuals in order to identify those using molecular techniques. I found that the majority of my field identifications had been correct. Two species of fern coexisted in the forest at Danum: Asplenium phyllitidis and Asplenium nidus. A. phyllitidis was found at lower levels in the canopy below 30m, whereas A. nidus was found at all levels in the canopy, up to 60m, and was often associated with emergent trees and more open areas. It seemed likely that microclimate was affecting the distribution of the two species, with A. nidus being able to withstand hot dry conditions found higher up in the canopy, while A. phllyitidis thrived in the cool damp conditions in the understory.

Having documented the distribution of the ferns I was then able to study the ant communities themselves. Initially I made collections of the ants from 87 ferns in order to take a snapshot of the communities present at a single time. I found that the bird's nest ferns supported an extremely diverse ant community consisting of 71 species across 27 genera. This level of ant diversity has never before been documented in a single type of epiphyte. Both the height of the fern in the canopy and the size of the fern had a small but statistically significant effect on which species of ants were able to live in it. More importantly, colonies of the same species never coexisted, and colonies of the same genus did so less often than would be expected by chance. This suggests that strong competition within species and within genera is responsible for maintaining the high diversity of ants found in the ferns. No single species or genus can become very dominant, as at high abundances they compete very strongly with other colonies in that species or genus.

I then wanted to investigate experimentally whether or not competition within species and genera really was important. To do this I cultured ant colonies in the laboratory and introduced them into ferns. I used a single species of ant as a model invader. A colony of this species was introduced into a fern containing either a colony of the same species, a colony of the same genus (but of a different species), or a colony from a completely different genus. The results of these experiments reinforced the findings from my surveys: colonies of the same species and the same genus aggressively defended ferns making it impossible for the invading colony to successfully take residence. Colonies from a different genus did not react to the invaders at all. Competition between colonies of the same species and colonies of the same genus, mediated through aggressive defence of territories, is the driving force behind maintenance of species diversity in this microcosm.

But how do communities change over time? I wanted to understand how ant communities assembled themselves in the ferns. To this end I identified the ants from collections made during a previous experiment in which bird's nest ferns had been cleared of their ant faunas and then replaced either high in the rainforest canopy or low in the understory. The ferns were then resampled for ants after one month and then again after nine months. I found that the ant communities before clearing were the same in the high canopy and the understory, but that after one month and nine months there were differences in which sets of species were present low down and high up. It is possible that it takes a long time for the communities to reach equilibrium, much longer than the nine months that the experiment lasted. Such slow dynamics may also contribute to maintaining species diversity, as non-equilibrium communities tend to be higher in diversity.

One major factor affecting species diversity is deforestation by humans. I investigated the impacts of deforestation and conversion to oil palm plantation, a majorly understudied tropical land use, on the ant communities in the bird's nest ferns and in the rest of the canopy. My hypothesis was that the bird's nest ferns, which are often left to grow in oil palm plantations, would be a reservoir for ant species away from the hot dry conditions found in the plantations. I found that the number of species present in the canopy, as sampled by insecticide fogging, decreased dramatically upon conversion to oil palm. This was not the case for the ant communities in the ferns, which harboured the same number of species in oil palm plantation and unlogged forest. However, the species that were found in the oil palm ferns were not the same species as those found in the ferns from unlogged forest. Instead many were widespread species often found in human-modified habitats. So while the ferns do increase diversity in plantations, they are no substitute for having areas of unlogged forest, as it is only in these that a subset of ant species are able to survive.

Article courtesy Tom Fayle.

Tags: Keeping Ants | Evolution | Behaviour

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