Effects of Macroalgal Habitats on the Community and Population Structure of Temperate Reef Fishes

摘要

Two families of brown macroalgae that occur in sympatry dominatetemperate subtidal rocky coasts: the Laminareales, and the Fucales. Both of thesefamilies are habitat-forming species for a wide variety of invertebrates and fishes.Variation in the presence, density, and composition of brown macroalgae canhave large influences on the evolution and ecology of associated organisms.Here, using a series of observational and experimental studies, I evaluated theeffects of heterogeneity in the composition of brown macroalgal stands at thepopulation and community levels for reef fishes.A central ecological challenge is the description of patterns that occur atlocal scales, and how these are manifested at larger ones. I conducted furthersampling across a set of sites nested within locations over three regions, JuanFernandez Islands (Chile), Northern New Zealand, and Tasmania (Australia), toevaluate patterns of variation in the diversity and composition of fishassemblages. Specifically, I explored spatial variation in fish assemblages as afunction of rocky reef habitats (dominated by brown-macroalgae) and othersources of variation (abiotic and biotic factors) that potentially mediate therelationship between fishes and reef habitats. Analyses suggest that spatialvariation in diversity (e.g., species and trophic) may be explained by spatialvariation in depth, temperature, and composition of macroalgal habitats. At eachlocation, only 2-3 families dominated the composition of fish assemblages, butspecies identities varied among locations. In a subsequent study, I assessed thefish-habitat associations from sites within the Juan Fernandez Islands, an isolatedeastern Pacific Island that lack large brown macroalgae. I found that, despite theclose proximity of these Islands to the South American continent, fish assemblages were mostly composed of endemic representatives from familiesthat dominate the fish assemblages in New Zealand and Australia. Spatialvariation in depth and temperature did not contribute to the observed variation infish abundance. Instead, I found that benthic habitat-forming species(particularly foliose brown macroalgae) appeared to limit the abundance of somereef fishes. These results suggest that a mixture of large-scale (e.g., stochasticrecruitment) and small-scale processes (i.e., relating to habitat heterogeneity)influence the diversity, composition and abundance of fish assemblages.Subsequently, I evaluated relationships between reef fishes andmacroalgae composition across multiple sites, surveyed repeatedly over fourseasons. I found that fishes were associated with different components ofheterogeneity in macroalgal habitats, potentially indicating interspecificpartitioning of resources that may arise from differential feeding habits and sizesusceptibilityto predation. Seasonal variation in the fish-habitat associations wasdetected, and site differences in macroalgal composition explained significantvariation in the local diversity of fishes.Using a series of small-scale lab and field-based manipulativeexperiments, I determined the demographic and behavioural responses of reefassociatedfishes to heterogeneity in the composition of brown macroalgalhabitats. I found that (i) different fish species distinguished betweenmonospecific macroalgae stands (macroalgal identity affected the abundance of7 of 15 reef fish species); (ii) there is within-species variation in the response offishes to macroalgal composition (suggesting ontogenetic habitat shifts); and (iii)the abundance of 5 of 7 reef fish species, and the overall structure of the localfish assemblage, varied with the composition of mixed-species macroalgal stands.Lastly, I evaluated the potential for fishes to provision demographicfeedbacks to macroalgae. Specifically, I conducted a mesocosm experiment toevaluate the effects of fishes on grazing amphipods, and therefore, the potentialindirect-effects of fishes on large-brown macroalgae. I found that only one of thetwo fish species studied reduced grazer abundance. Although the second fishspecies did not consume grazing amphipods, its presence altered amphipodbehaviour to significantly reduce grazing efficiency on the macroalgal-host.This study illustrates how density and trait-mediated indirect interactions canhave similar effects on primary producers.Overall, my observational and experimental components of this thesisemphasize the influence of heterogeneity in macroalgal structures on the breadthof habitat use for reef fishes at multiple locations. I found strong behaviourallymediated linkages between the abundance of reef fishes and composition ofmacroalgal stand. I also provide some evidence that mutualistic relationshipsmay exist between kelp and associated fishes.