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Factors influencing the coexistence of bromeliad-dwelling chironomids on Ilha do Cardoso, Brazil Letaw, Alathea Diana


Species interactions can influence the spatial distribution of organisms and the composition of local communities. To investigate how interactions influence the coexistence of invertebrates living in bromeliad phytotelmata, I combined methodological development and empirical exploration with the aim of understanding: 1) Which species in a community show signs of strong interactions, 2) Whether predators influence the outcome of competitive interactions and 3) Whether equalizing or stabilizing interactions between species change depending on context. To detect interactions between species given observational field data, I designed a method of finding negative co-occurrence patterns (using checkerboard units) between species based on their abundances in nature. Using this method, I found that three chironomid (Diptera: Chironomidae) species showed very strong negative co-occurrence patterns, suggesting that they experience net negative interactions (e.g. competition) or habitat filtering. Next, I performed a predator-addition experiment to assess the importance of predators in mediating the coexistence of the three chironomid species. Three predator species were added to bromeliads containing the three chironomid species. Although field observations suggested that at least one chironomid species should improve performance in the absence of predators, there was only a slight differential response to predators. Furthermore, one species of chironomid was competitively superior to the others in both the presence and absence of predators. We suspect that differing habitat preferences and the presence of other prey may be more important to coexistence than the presence or absence of predators alone. Finally, I performed an experiment to assess how habitat and ontogeny affect the outcome of competition between the two most common chironomid species. When reared at the same body size, the two chironomids exhibited a stable relationship that we term here asymmetric equivalence: in one species experiences the world neutrally but the other does not. However, when species differed in their ontogenetic stage, the asymmetric equivalence disappeared. Taking all three studies together, I found that competition, but not predation, is an important factor in chironomid coexistence, but that differences in context lead to different coexistence outcomes.

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