Ant community structure in Madagascar

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Title: Ant community structure in Madagascar
Author: Budaviciute, Silvija
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences, Department of Biosciences
Publisher: Helsingin yliopisto
Date: 2015-11-03
Thesis level: Licentiate thesis
Abstract: Insects are important components of ecosystems they are diverse, sustain multiple ecosystem services and play an important role in shaping the overall community, including other taxa. What then structures insect communities? Are there certain rules that govern their assembly, or are such communities just a random collection of species? Does habitat disturbance affect community structure and if so, how? And does habitat disturbance change the rate of important ecological functions sustained by insects? To address these questions, I used ants as a model system. More specifically, I used the morphological traits of ants and measures of ecological functions sustained by ants in different habitats. The thesis is based on two chapters, each a manuscript formatted for a scientific journal: I) Budaviciute, S., Cabeza, M., Blanchet, F. G. and Roslin, T. Disentangling the impact of environmental filtering from competition in structuring Malagasy ant assemblages and II) Budaviciute, S., Cabeza, M. and Roslin, T. Diversity and functioning of Malagasy ant assemblages along a disturbance gradient. To understand whether environmental filtering or competition shapes ant communities, I compared the distribution of trait values within and among habitats to distributions generated by null models. As indicators of environmental filtering, I searched for the aggregation of particular trait values in specific environments; as a sign of competition, I searched for the overdispersion of trait values compared to the random null model. To understand how habitat disturbance affects the community composition and ecological functions performed by Malagasy ants, I carried out field experiments in three types of habitats varying in their level of disturbance (primary forest, secondary forest and banana plantations). In each of them, I measured the functions of predation, seed removal and mutualism between ants and honeydew-producing Hemiptera. The morphological analyses suggested that ant communities are assembled at random among and within habitats, with a few exceptions. More precisely, when partitioning ant communities into groups of differentially-sized species, I found signs of environmental filtering among assemblages of large ants in a dry habitat, and among assemblages of medium-sized ants in a dry and an open habitat. Signs of the competition were evident among small ants in a disturbed habitat. Habitat disturbance left no detectable imprint on the species richness, abundance or composition of ant communities. Likewise, measures of key functions sustained by ants (i.e predation, seed removal and mutualism between ants and honeydew-producing Hemiptera) did not detectably differ among habitats varying in their degree of disturbance. The factor best explaining the lack of significant structuring of ant traits among habitats might be the scale at which I was trying to detect the processes. Random patterns within habitats could be a result of individuals simply modifying their foraging behaviour or activity without accompanying changes in morphology. For some groups of ants, like large and medium ants in dry habitats, local conditions may still pose a strong environmental filter. As a result, particular trait values may be selected under such conditions. In the case of small ants, I found signs of competition only in disturbed habitats. One potential reason is that small ants, due to the small grain size at which they experience their environment, are forced to compete for similar food and nesting resources. Perhaps such resources are particularly uniform in disturbed habitats, thus accentuating competition. Factors contributing to the lack of detectable differences in species composition between the habitats and in the rates of ecological functions examined may be three-fold: first, the disturbance was low to start with, with the most disturbed habitat still featuring ample vegetation. Thus, communities may not have been that strongly affected even by the initial disturbance. Second, communities may have been recovering quickly with the regrowth of secondary vegetation. Third, all disturbed habitats remained in spatial proximity to undisturbed habitats, which may have contributed to sustaining diversity in the disturbed areas. Overall, this work contributes to our understanding of forces governing community structure, and of the effects of habitat disturbance on insect communities. Importantly, the fact that habitat modification left no detectable imprints on ant community composition, ant abundance or on ecological functions sustained by ants may be seen as positive news for nature conservation. Perhaps ant communities may be both structurally and functionally resilient to habitat disturbance if the level of disturbance is only not too drastic?
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