Competition and habitat selection in coral-dwelling fishes

Ecologists have long been interested in competition because of its potential role in population and community regulation. A large body of competition theory has accumulated, much of which remains to be empirically tested. To date, the occurrence of competition in natural communities and the mechanisms by which competing species coexist are not fully understood. The close relationship between coral-dwelling fishes and a limiting resource (coral colonies) makes them ideal models for testing hypotheses and questions about competition in natural communities. In this thesis I examine the mechanisms of competition and their influence on resource use in two ecologically similar coral-dwelling gobies (Gobiodon histrio and Gobiodon erythrospilus) that are known to compete for access to preferred coral habitat. The chapters in this thesis address four fundamental questions: 1) the mechanisms of competitive coexistence between the two species, 2) the role of resource availability in shaping the outcome of competitive interactions, 3) the fitness-associated traits of coral colonies that drive habitat preferences and competitive interactions, and 4) the influence of benthic substratum around preferred coral colonies on habitat use and competitive interactions. Niche and lottery mechanisms of competitive coexistence have traditionally been viewed as mutually exclusive alternatives. However, recent theory suggests that a mix of these processes can facilitate coexistence between competing species. In Chapter 2 I tested the hypothesis that the mechanism of competition between G. histrio and G. erythrospilus changes with ontogeny, from a lottery for space at settlement to niche partitioning in adults. Field observations and experiments showed that juveniles of the two species settled at the same size, had similar patterns of habitat use, and similar competitive abilities, supporting the lottery mechanism at settlement. In contrast, habitat use differed in adults suggesting that resource partitioning occurs in larger individuals. In laboratory experiments, adults of each species preferred colonies of Acropora nasuta, however G. histrio was a superior competitor and prevented G. erythrospilus for using A. nasuta in more than 70% of the trials. In a fieldbased transplant experiment, G. erythrospilus (inferior competitor) suffered less of a fitness loss when occupying the non-preferred coral (A. spathulata) compared with G. histrio, which could explain its ability to persist when displaced by the superior competitor. These results suggest that the competitive mechanism operating between the two Gobiodon species shifts from a lottery for space to niche-partitioning through ontogeny and that these two mechanisms of competitive coexistence are not mutually exclusive. Competition arises from the shared use of limited resources. Consequently, spatial and temporal variation in resource availability could influence competition among coral reef fishes. In Chapter 3 I investigated how variation in the abundance of A. nasuta coral colonies, the preferred habitat of G. histrio and G. erythrospilus, influences the outcome of competitive interactions between these two species. First, the relative abundance and patterns of habitat use of the two goby species was compared among sites that varied in the absolute and relative abundance of their preferred habitat. Then a recolonization experiment was used to test the prediction that the effects of competition are greatest where preferred habitat is relatively less abundant. The proportional occupancy of A. nasuta by the superior competitor, G. histrio, increased as the relative abundance of A. nasuta declined. In the recolonization experiment the effects of preferred coral availability differed between juveniles and adults. For juveniles, where a competitive lottery operates, the proportional use of preferred coral was the same for the two species, regardless of the relative abundance of A. nasuta. In contrast, for adults niche-partitioning was greater at locations that had a lower relative abundance of A. nasuta. These results show that changes in the relative abundance of preferred resources can influence competitive interactions between reef fishes, but the effects differ depending on the mechanism of competitive coexistence. Competition for space affects patterns of habitat use and individual performance of coral-dwelling fishes; however, the physical attributes of corals that influence habitat preferences are uncertain. Chapter 4 investigated the influence of coral colony size and branching structure on habitat use and growth rate of the two coral gobies, G. histrio and G. erythrospilus. The preferred coral species, A. nasuta had smaller interbranch width compared with an alternative coral habitat, A. spathulata. A binary-choice laboratory experiment demonstrated that both gobies preferred coral colonies with smaller interbranch width, except when they had the opportunity to occupy A. nasuta over A. spathulata. A field transplant experiment showed that both goby species grew faster on larger coral colonies and in colonies with smaller interbranch width. G. erythrospilus grew faster than G. histrio on A. spathulata, indicating that it suffers less of a fitness loss occupying this alternative habitat. The results of this chapter show that coral physical attributes are important factors driving habitat preference of coral-dwelling gobies; however, there must also be additional factors related to coral species identity that influence their habitat preferences. The distribution and abundance of habitat specialists is often associated with the availability of preferred habitat; however, other environmental features can also influence habitat selection. Coral-dwelling gobies depend on the availability of a few key species of coral for their survival and also possess skin toxins that reduce predation risk. In Chapter 5 I analysed the influence of benthic substratum around preferred coral colonies on patterns of habitat use and toxicity of G. erythrospilus and G. histrio. Field surveys demonstrated that juveniles, single adults and breeding pairs of G. erythrospilus mostly inhabited A. nasuta colonies surrounded by branching corals. In contrast, G. histrio juveniles and single adults associated with A. nasuta coral colonies with adjacent epilithic algal matrix and G. histrio breeding pairs inhabited colonies surrounded by sand/rubble. Habitat-choice experiments showed that both gobies species prefer A. nasuta coral colonies with benthic substratum mainly composed by epilithic algal matrix and sand. Lastly, the substratum around preferred coral colonies also influenced the toxicity levels of the associated fishes. Gobies inhabiting A. nasuta coral colonies with more epilithic algal matrix and sand were more toxic than fishes collected from colonies surrounded by branching corals. Given the potential for toxicity level to reduce the risk of predation, this could explain why gobies compete for access to preferred coral species surrounded by epilithic algal matrix and sand. This thesis empirically demonstrated that different mechanisms of competitive coexistence (such as a lotteries and niche partitioning) are not mutually exclusive and may operate at different stages in an organism’s life history. Furthermore, it has shown that changes in the relative abundance of preferred resources can influence competitive interactions, but the population level effects depend on the mechanisms of competitive coexistence that operate. Additionally, this research highlights that both coral species identity and colony structural features influence the growth of coral-dwelling fishes and thus play a key role in shaping habitat preferences and competition for space in coral-dwelling fishes. Finally, the benthic composition around preferred coral species influences the toxicity of coral-dwelling gobies, and this further influences their habitat preferences and competitive interactions. This thesis answered some fundamental questions about the mechanisms of competition in animal communities with broader implications for predicting the effects of climate change and anthropogenic impacts on reef fish communities. Coral cover, benthic community composition and reef structural complexity are declining due to the combined effects of storms, crown of thorns starfish outbreaks, coral bleaching and diseases. This degradation will affect habitat use and fitness of coral associated fishes and ultimately influences the outcome of ecological process such as recruitment and competition within reef fish communities.