An Evaluation of Prior Residency and Habitat Effects on the Persistence of Settling Reef Fishes

Abstract

Both habitat complexity and competitive interactions can shape patterns of distribution and abundance of species. I evaluated the separate and joint effects of competitive interactions and habitat complexity on the survival of young fishes (Family Labridae) on coral reefs. First, I developed (in Chapter 2) a quantitative approach to evaluate potential resource (i.e., niche) overlap among groups of co-occurring species. Using appropriate transformations and probability models, I show that different types of data (e.g., categorical, continuous, count or binary data, as well as electivity scores) give rise to a standard measure of niche overlap, with the overlap statistic between two species defined as the overlapping area between the distributions for each species. Measurements derived from different types of data can be combined into a single multivariate analysis of niche overlap by averaging over multiple axes. I then describe null model permutation tests that differentiate between species occupying similar and different niches within my unified indices. I then implemented this approach (in Chapter 3) to evaluate potential habitat overlap among eight species of wrasse (Gomphosus varius, Halichoeres hortulanus, H. trimaculatus, Pseudocheilinus hexataenia, Scarus sordidus, Stethojulis bandanensis, Thalassoma hardwicke and T. quinquevittatum), and used these results to inform my subsequent field experiments. In a field assay, I identified the presence of T. quinquevittatum as having the greatest negative effect on survival of transplanted T. hardwicke from a suite of three candidate species which were most similar in habitat use to T. hardwicke (the other two candidate species were G. varius and P. hexataenia). In a subsequent field experiment, I tested how competition with T. quinquevittatum and structural refuge interact to influence the postsettlement survival of T. hardwicke. Competition with T. quinquevittatum and structural refuge both altered the survival of T. hardwicke, although their effects were not interactive, indicating that structural complexity did not mitigate the negative effects of competition. Survival of T. hardwicke was 2.3 times greater in treatments without T. quinquevittatum relative to those with T. quinquevittatum, and 2.8 times greater in treatments with structural refuge relative to treatments without structural refuge. Thalassoma hardwicke and T. quinquevittatum often enter reef communities asynchronously, resulting in competitive pressures faced by earlyarriving individuals that potentially differ from those experienced by late-arriving individuals. In a series of field experiments, I investigated whether the strength of intra-cohort competitive interactions between recent T. hardwicke and T. quinquevittatum settlers were dependent upon the sequence and temporal separation of their arrival into communities. Survival rates for both species were greatest in the absence of competitors, but when competitors were present, survival rates were maximized when competitors arrived simultaneously. Survival rates declined as each species entered the community progressively later than its competitor. Further, reversals in the sequence of arrival reversed competitive outcomes. Results provide empirical evidence for competitive lotteries in the maintenance of species diversity in demographically open marine systems, while also highlighting the importance of temporal variation in the direction and magnitude of interaction strengths. To further our understanding of how timing of arrival influences interaction strengths, I tested whether increasing the availability of complex habitat attenuates or enhances timing-of-arrival effects. Results from this field experiment indicated that aggression by early-arriving individuals towards late-arriving individuals increased as arrival times diverged. When aggression was weak, subordinate individuals were not displaced from complex habitat. Experimental increases in the availability of complex habitat resulted in increased survival of subordinates, presumably by disrupting predation pressure. However, when aggression was intense, competitive subordinates were displaced from complex habitat (regardless of the amount of complex habitat available), and this likely increased their exposure to predators. Overall, the experimental and observational components of this thesis emphasise heterogeneity in competitive environments experienced by recently settled reef fishes. These results highlight the important role that priority effects and habitat complexity play in determining the persistence of reef fish settlers, and illustrate how ecological contexts can add considerable variation to realised interaction strengths.