I developed a spatially explicit individual-based model (IBM) to capture the dynamic behavioral interaction between a fierce predator (killer whale, Orcinus orca) and a clever prey (dusky dolphin, Lagenorhynchus obscurus), and to answer the ultimate question of costs vs. benefits for dusky dolphins when making anti-predator decisions. Specifically, I was interested in calculating time/distance budgets for dusky dolphins in the presence/absence of killer whales and the presence/absence of movement and behavioral rules, which presumably evolved in response to spatial and temporal variations in predation risk. Results reveal that dusky dolphins rest less, travel more and have reduced foraging time when killer whales are present. These effects are more pronounced with increased presence of killer whales. The model suggests that a strong reason favoring the adoption of short and long-term anti-predator mechanisms is increased survival resulting from decreased encounters with killer whales. Further, a mother with calf rests less and travels more when killer whales are present relative to a dolphin without calf. However, a mother with calf on average, flee shorter distances and have fewer encounters with killer whales than a dolphin without calf. Thus, despite ecological costs, it makes evolutionary sense for dusky dolphins to adopt anti-predator rules. Bioenergetic consequences for dusky dolphins with and without calf were estimated as total energetic costs and foraging calories lost due to low/high presence of killer whales. I calculated total energy costs as: Foraging costs (FC) Locomotor costs (LC) (Travel) or LC (Travel) LC (Flee) based on the absence, as well as low/high presence of killer whales. Foraging costs contributed significantly to total energetic costs estimated. Travel costs are minimal owing to proximity to deep waters. The total energy costs were not significantly higher from low or high presence of killer whales for mother with calf, but increases by about 90 kcal/day for a dusky without calf. However, I estimate foraging calories lost due to increased killer whale presence is almost 5 times more for mother with calf. Therefore, it might be important to consider indirect predation risk effects by social type in future studies on animal bioenergetics.

The hierarchical levels at which resource selection occurs can have important consequences for individual and population energy budgets and structure the impacts of a forager on its ecosystem. Assessing factors affecting resource selection of large marine herbivores across scales is important because of their potentially large impacts on seagrass community dynamics and historical and current changes in their population sizes and those of their potential predators. I explored the factors (predation risk, resource abundance, quality and identity) affecting resource use of large marine herbivores (green turtles, Chelonia mydas) from the scale of habitat patches to forage species within patches. I used a combination of in-water surveys, aerial drone video transects, baited camera surveys, and seagrass community and nutrient content analyses to provide insights into resource use by turtles in multiple ecological contexts. In Abaco, The Bahamas I found relatively intact shark populations, including apex predators, relative to other parts of the Caribbean. In the context of healthy predator populations in Abaco, I tested a priori predictions rooted in Ideal Free Distribution (IFD) theory. Green turtles off Abaco deviated from predictions of an IFD determined by the standing stocks of seagrass. Instead, distributions are consistent with predictions of the foraging arena hypothesis with turtles largely restricted to safe habitat patches and selecting locations within these where seagrass N content is relatively high. Marine invasive species can have detrimental effects on coastal ecosystems and economies. Therefore, understanding the effects of, and factors influencing the rate of spread of the invasive seagrass Halophila stipulacea in the Caribbean is important. In the French West Indies (Guadeloupe, Martinique and St. Martin), I investigated foraging preferences for native versus invasive seagrass species and whether green turtles might facilitate or attenuate the invasion through their choice of habitats and feeding patterns. Green turtle distributions were correlated with native seagrass distributions. Also, despite similar nutrient contents, turtles preferred feeding on native seagrasses irrespective of their relative abundance within a patch. These results suggest that, as predicted by the Enemy Release Hypothesis, green turtles likely facilitate the invasion and spread of the invasive seagrass that may reduce energy flow into turtle populations.

Elasmobranchs are among the oldest and most successful predators in the ocean, yet one of the most vulnerable to the direct and indirect effects of fishing. Many populations are rapidly declining around the world, and an increasing number is listed as threatened or endangered. The broader ecosystem consequences of these declines, and whether other marine predators can replace sharks, are open questions. In this thesis, I used a diverse set of data and modeling techniques to analyze long-term changes in elasmobranch populations in the Mediterranean Sea, and the consequences of shark declines on marine ecosystems. Because of its long history of fishing, the Mediterranean offers a unique perspective on the response of marine communities to exploitation over long time scales. Here, I reconstructed the history of elasmobranch exploitation over the past 200 years in pelagic, coastal and demersal communities. Results were combined meta-analytically to derive a general pattern of change for the entire region. Overall, I detected multiple cases of regional species extirpations, a strong correlation between historical intensity of exploitation and the stage of community degradation, and some cases of compensatory species increases. My results suggest that compared to other marine ecosystems worldwide, the Mediterranean Sea might be in an advanced stage of overexploitation. To gain more general conclusions about the patterns and consequences of shark declines in the ocean, I reviewed and reanalyzed documented changes in exploited elasmobranch communities around the world, and synthesized the effects of sharks on their prey and wider communities. This work revealed that sharks are abundant and diverse in little exploited or unexploited marine ecosystems but vulnerable to even light levels of fishing. The decline in large sharks has reduced natural mortality in a range of their prey, contributing to changes in abundance, distribution, and behaviour of marine megafauna that have few other predators. In some cases, this has resulted in cascading changes in prey populations and food-web structure. Overall, my thesis greatly enhanced our knowledge about the critical state of elasmobranchs in the Mediterranean Sea and the consequences of the declines of these important marine predators on marine ecosystems.

As climate change continues, climactic extremes are predicted to become more frequent and intense, in some cases resulting in dramatic changes to ecosystems. The effects of climate change on ecosystems will be mediated, in part, by biotic interactions in those ecosystems. However, there is still considerable uncertainty about where and how such biotic interactions will be important in the context of ecosystem disturbance and climactic extremes. Here, I review the role of consumers in seagrass ecosystems and investigate the ecological impacts of an extreme climactic event (marine heat wave) and subsequent widespread seagrass die-off in Shark Bay, Western Australia. Specifically, I compare seagrass cover, shark catch rates, and encounter rates of air breathing fauna in multiple habitat types before and after the seagrass die-off to describe post-disturbance dynamics of the seagrass community, shifts in consumer abundances, and changes in risk-sensitive habitat use patterns by a variety of mesoconsumers at risk of predation from tiger sharks (Galeocerdo cuvier). Finally, I conducted a 16 month field experiment to assess whether xi loss of top predators, and predicted shifts in dugong foraging, could destabilize remaining seagrass. I found that the previously dominant temperate seagrass Amphibolis antarctica is stable, but not increasing. Conversely, an early-successional tropical seagrass, Halodule uninervis, is expanding. Following the die-off, the densities of several consumer species (cormorants, green turtles, sea snakes, and dugongs) declined, while others (Indo-Pacific bottlenose dolphins, loggerhead sea turtles, tiger sharks) remained stable. Stable tiger shark abundances following the seagrass die-off suggest that the seascape of fear remains intact in this system. However, several consumers (dolphins, cormorants) began to use dangerous but profitable seagrass banks more often following seagrass decline, suggesting a relaxation of anti-predator behavior. Experimental results suggest that a loss of tiger sharks would result in a behaviorally mediated trophic cascade (BMTC) in degraded seagrass beds, further destabilizing them and potentially resulting in a phase shift. My work shows that climactic extremes can have strong but variable impacts on ecosystems mediated in part by species identity, and that maintenance of top predator populations may by important to ecological resilience in the face of climate change.