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Life in a patchy world: species-habitat relationships link macroalgal communities to higher trophic levels in temperate rocky reefs


Benthic macroalgae occur on most temperate shallow rocky reefs and provide an important source of biogenic structure for associated fauna. The assemblage of low-lying foliose red (Rhodophyta) algae creates significant habitat for small, mobile invertebrates that fuel higher trophic levels. The changing composition and abundance of algal species can affect abundance and species composition of associated assemblages including prey items for fish predators. Availability and abundance of prey items for fish predators is therefore linked to availability of suitable biogenic habitat that harbors these prey items. Top-down effects by fish predators can also structure prey communities through selective predation, but predation itself can be affected by habitat complexity. In temperate rocky reefs, microcarnivorous fishes actively use benthic red algae to acquire prey. When availability of preferred prey items are not evenly distributed across a temperate reef, fish predators may choose to select for a specific habitat based on factors including resource availability, competition and predation levels, as well as physical features of the environment. For my research, I examined the links between algal habitat, associated invertebrate assemblages, and fish predators at shallow, rocky reefs of the Monterey Bay Peninsula.

This dissertation focuses on three components to understand how changes in algal habitat affect prey availability and habitat utilization of fish predators. In Chapter 1, I take advantage of a natural gradient of physical disturbance (exposure to ocean swell) to examine how changes in benthic macroalgal assemblages affect associated invertebrate assemblages and to determine if the two assemblages co-vary predictability along that gradient. I found that both algal and invertebrate assemblages had high spatial variability and that assemblages were more similar among spatially close sites than those more distant. In Chapter 2, I look at how top-down predation by Oxylebius pictus, a microcarnivorous fish, structures prey communities associated with different red algal species. I found that the abundance of different prey items of O. pictus are affected by host algal species and that predators disproportionately feed on prey items from specific algal hosts. In Chapter 3, I examined the ontogenetic shift in diet and habitat associations of O. pictus in relation to prey availability and changing feeding mechanics. I asked if different life stages associate with different algal species given prey preference and selectivity as well as availability of prey across different algal species. I found that the prey identity and size of prey of O. pictus shifted from juvenile to adult stages. Diet shifts coincided with changes in both vertical gape and mechanical advantage of the jaw. There was also a shift in algal habitat and benthic reef associations from juvenile to adult stages.

Together, results from this dissertation suggest that the red algal assemblage, which covers a considerable portion of shallow, temperate rocky reefs, is an important source of biogenic habitat, providing significant structure for prey items of microcarnivorous fishes. Results suggest that changes in species composition and identity of algal habitat have consequences to availability of prey items. In turn, changes in prey availability due to changes in algal species composition may help explain why fish exhibit algal-specific associations and prey selectivity and why these change with ontogeny.

Understanding how changes in plant assemblages influence faunal communities is central to our ability to both explain and predict overall (flora and fauna) spatial and temporal variation in community structure and functions. Additionally, understanding mechanisms that help explain how fish predators utilize habitat and if this changes with ontogeny is a key requirement to understanding important community interactions. The temperate reefs studied in this dissertation are examples of the natural complexity of an ecological community. Understanding how changes in biogenic habitat, caused by natural or anthropogenic sources, and how these changes will affect associated communities is an important goal of ecosystem monitoring and management.

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