Right place, right height: Habitat connectivity and condition underpin fish community structure across coastal seascapes
Creators: name: Van Lier, Joshua
Publisher: name: The Australian National University
Year: 2020

description: Community ecology seeks to understand the variety of factors that influence biodiversity. Early work has recognised that local habitat quality is important (e.g., niche theory), but so is spatial context (e.g., Island Biogeography Theory). In my PhD, I build upon these concepts in marine biomes by combining underwater surveys with Geographic Information Systems to explore how habitat quality and spatial context shapes fish community structure and function in a variety of soft-structure coastal habitats. Local-scale aspects of habitat quality, like three-dimensional hard complexity, are recognised as key predictors of fish diversity. However, relatively little is known about the effects of soft-structure habitat complexity on fish diversity. Using a best-subsets modelling approach and data from three temperate mesohabitats, I assessed what aspects of soft structure best predicted differences in reef fish community composition. I found that high levels of canopy cover and height were key predictors in supporting a diverse fish community across mesohabitats. Thus, it is important that habitat quality in soft-structure environments account for variation across three dimensions. In addition to local habitat quality, the spatial context of an area within the wider seascape mosaic can influence patterns of diversity. Using a patch network of tropical macroalgal meadows that varied in size and distance to a coral reef, I assessed the relative importance of local patch quality and spatial context for predicting the diversity and structure of the local fish community. Multivariate analysis showed the relative importance of these variables differed depending on a taxonomic or functional focus of fish diversity. Most importantly, when species identity was explicitly examined, the placement of macroalgal patches within 500m of a coral reef was among the key predictors, along with hard complexity and soft canopy height. Many accounts of local habitat quality as a predictor of diversity are based on observational studies, such as the results I report on above. Therefore, I used an experimental unseasonal reduction of tropical macroalgal canopy height to explore the consequences of unseasonal canopy loss on resident fishes over short-to-long time-scales. Despite these meadows undergoing a similar, seasonal change in canopy structure, experimental sites, relative to nearby reference sites, had significant decreases in fish species richness and density within five days of canopy height reduction which was coupled with a short-term increase in the density of higher-order carnivores. Some of these effects were still evident two years later, despite an overall recovery in canopy structure. Habitat variation across multiple-scales has often shown to influence patterns of diversity, but their effects on ecological processes are less well known. Predators, through regulation of prey populations, may structure ecological communities via top-down effects. Therefore, I explored the behavioural ecology of a common mesopredatory fish (Thalassoma lunare) to understand how local habitat quality and spatial context may influence predation patterns. While T. lunare showed indications of habitat preference in early life history stages, adult fish were habitat generalists who varied their foraging behaviour according to spatial context. Specifically, individuals in isolated patches would forage across significantly smaller distances than those in well-connected patches, and may therefore contribute to differing local predation pressures in different spatial contexts. My work shows that a holistic, multi-scale approach is needed to understand ecological patterns and processes in marine biomes. I discuss how these insights add to our understanding of marine ecology, and can inform management and design of marine reserves by identifying well connected and structurally complex areas of the seascape to facilitate movement between these high quality areas.