Trophic ecology of large marine predators in the Southwest Atlantic

Globally marine ecosystems are changing dramatically under pressures from human activities, with potentially major disruption of the ecological roles of higher marine predators. As key players in the structure of marine ecosystems (either by direct or indirect effects on the food chain), loss of, or significant reductions in predators may result in top-down trophic cascades, which may then jeopardise the health of entire ecosystems. Understanding the trophic ecology of higher predators is a fundamental goal if we are to understand their responses to a changing environment. In this thesis, I assessed how different factors affect the trophic niches of higher marine predators. In Chapter Two, I used stable isotope analysis of tissues from two sympatric species with contrasting foraging strategies from the Falkland Islands (South Atlantic) the South American sea lion (Otaria byronia) and the South American fur seal (Arctocephalus australis), to understand how foraging strategies and competition affect individual specialisation. I found that foraging behaviour influenced the degree of individual specialisation. In the Falklands, the pelagically foraging fur seals feed in a dynamic environment with abundant but similar prey and are specialised populations of generalist individuals. By contrast, the benthically foraging sea lions fed in habitats with diverse but less abundant prey and had more generalist populations composed of highly specialised individuals. In Chapter Three, I tested the plasticity of population isotopic trophic niches in two widely distributed sympatric species (O. byronia and A. australis) from two contrasting environments, the Falkland Islands and the Río del Plata, Uruguay. There was an unexpected, complete inversion of the population trophic niche size for both species between the two sites. This striking result demonstrates considerable plasticity in the foraging niche and illustrates that predators may show divergent foraging strategies in different environments. Here it appears that when conditions dictate, populations of generalist individuals can result in broad population niches and that, if necessary, populations of specialist individuals can be confined to very narrow population niches. Finally, in Chapter Four, I explored how morphology (teeth shape and body length) influence trophic niche using a comparative analysis of two sympatric sharks of similar size but contrasting feeding modes. One, the grey nurse shark (Carcharias taurus), captures and swallow their prey whole with spear-shaped teeth, and the other, the sevengill shark (Notorynchus cepedianus), has multi-cuspid cutting teeth used to serrate larger prey. Using stable isotopes and stomach contents, I show that as body length increases, both species consumed species from higher trophic levels, but non-gape limited sevengill sharks consistently preyed over higher trophic levels and were less restricted by mouth size.

Overall, in this thesis, I hypothesised that differences in individual specialisation were related to prey availability and habitat differences. However, regardless of the degree of individual specialisation displayed by the individuals within a population, the population may be either specialist or generalist depending on the regional characteristics of the environment and associated prey available. Morphological characters play an important role in the predator-prey selection and may be the primary mechanism facilitating resource partitioning in large sympatric predators. Differences in trophic morphological traits, individual specialisation, and regional environmental characteristics on population niche plasticity are all key factors that should be considered to better understand how predators’ prey choices shape trophodynamics in marine ecosystems.