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Mathematical models describing the components of predator-prey interactions are reviewed and evaluated, and new equations representing selected aspects of the relationship are proposed. A model of prey selection that distinguishes between predator performance and prey vulnerability is devised and shown to lead to conclusions that may be qualitatively different from those produced using previous methods. The feeding habits of staghorn sculpins (Leptocottus armatus), the extent to which they utilize estuarine habitats and their predatory response to chum salmon fry (Oncorhynchus keta) are examined for the purposes of (i) ascertaining the factors shaping sculpin foraging behaviour and (ii) assessing their potential for limiting survival of juvenile salmon. During periods of fry migration, sculpin populations in the estuaries of Big Qualicum River, Salmon Creek and Rosewall Creek (on Vancouver Island, B. C.) were composed predominantly of small juveniles less than 80 mm in length. Tolerance to waters of low salinity, which decreased with sculpin size, was found to be the major variable governing residence in these areas. There was little evidence that the migration of fry was important in attracting sculpins to estuaries. Sculpins preyed on a wide diversity of fauna concentrating on benthic crustaceans, particularly the amphipod Eogammarus confervicolus. Juveniles were active throughout the day, but feeding became progressively more restricted to periods of low light intensity as they grew. The smallest that captured fry were 40-45 mm in length. When chum fry were offered to starved sculpins in field enclosures, the response of those less than 80 mm in length was type 2 (Holling 1965) whereas that of 80-99 mm sculpins was type 3 (sigmoid). Predation on fry was inversely related to light intensity from dawn to dusk, and positively correlated with' light levels during the night. When benthic invertebrates were added, sculpins exhibited an overall preference for fry, which were 4-5 times more profitable in terms of net energy intake. However, preference for fry declined markedly as their abundance relative to other prey increased, indicating a divergence from the usual predictions of optimal foraging theory. Capture rates by sculpins initially naive to salmon fry increased up to three-fold over 3-5 two hour trials. It is suggested that the foraging strategy of sculpins given a choice between salmonid fry and benthic invertebrates represents a balance between the requirement of minimizing risk of starvation and the need to evade their own predators (particularly birds). The schooling behaviour of fry requires that sculpins, even when experienced, must devote considerable attention to the attack process and in so doing, run the risk of being eaten themselves. The combined effects of the schooling response, which reduces the incentive to attack fry, and a profusion of alternative prey, which decreases average hunger levels, were thought to be responsible for low fry consumption in natural situations. In Big Qualicum River, an estimated 240,000 and 40,500 chum were captured by sculpins in 1979 and 1980, respectively. This represents corresponding percentages of only 0.51% and 0.06% of the fry populations, and was calculated to be less than one-tenth of the potential that could have been realized. Predation rates on coho fry (0. kisutch) were considerably greater, despite a smaller population size. Estimated consumption was 817,700 (42.97%) and 144,000 (9.09%) in 1979 and 1980. Systems where sculpins could consume higher proportions of chum fry populations were identified as small, shallow, warm estuaries of intermediate to high salinity with relatively few suitable benthic invertebrates and small numbers of fry. Recommendations for reducing sculpin predation in such cases are proposed. Birds, particularly Bonaparte's gulls (Larus Philadelphia), were found to be even more avid predators than sculpins on juvenile salmon in Big Qualicum River. In contrast to sculpins, they exhibited pronounced numerical responses to the appearance of fry in the estuary. An estimated 10-25% of the hatchery-reared chinook salmon (O. tshawytscha) and 2-4% of the coho were removed by birds in the years 1979-81. / Science, Faculty of / Zoology, Department of / Graduate

Predation by large fissipeds is examined using the specific example of the African lion. It is found that the lion is able to adjust its strategy and tactics at each phase of prey capture (search, stalk, attack, and subdue) so as to achieve a consistently high overall capture success with different prey and conditions. / Science, Faculty of / Zoology, Department of / Graduate

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Although predation has been identified as a potentially important driver in terrestrial and freshwater ecosystems, estuarine planktonic research has focused largely on the so-called "bottom-up" drivers of community assemblages. As such, this thesis focuses on the direct and indirect effects of zooplanktivorous predators on the planktonic community in an estuarine environment. By using a suite of in situ mesocosm experiments, a number of hypotheses, pertaining to the major research themes associated with predator-prey interactions, are tested. These themes included trophic cascading, risk effects associated with predation events and the importance of predator diversity in maintaining prey communities. The first experiment assessed the significance of apex predation pressure for the planktonic community through trophic cascades. Various treatments using in situ mesocosms were established in a closed oligotrophic estuary to highlight the importance of predation in stabilising estuarine plankton abundances. Through either the removal (filtration) or addition of certain planktonic groups, varied trophic scenarios were established. The experimental treatment containing apex zooplanktivores had consequences for multiple trophic levels, exerting a stabilising pressure throughout the food web (Chapter 3). Furthermore, pyrosequencing of filtered water samples revealed that when compared to the remaining treatments, the treatment containing stable apex predatory pressure experienced limited temporal deviation-from-initial in bacterial community structure (Chapter 4). These findings are consistent with trophic cascade theory whereby predators mediate interactions at multiple lower trophic levels with consequent repercussions for diversity. To assess the non-consumptive effects of predators on prey, two experiments were conducted. Firstly, using egg numbers per clutch as a measure of potential reproductive output, the non-lethal effects of predatory pressure on reproductive success in a key planktonic copepod was investigated. In this study, the average clutch size of fecund female copepods was found to be consistently lower in the presence of predators when compared to females not exposed to predation threat (Chapter 5). The second study assessed the effects of conspecific chemical alarm cues associated with predation, on population dynamics of a copepod species. This study revealed that the copepods appear to detect the presence of chemical alarm cues associated with predation events, with repercussions for population demographics over time. Furthermore, it showed that in the absence of actual predation, copepod prey responses to alarm cues were adjusted over time, consistent with the threat sensitive predator avoidance hypothesis (Chapter 6). The final data chapter dealt with predator diversity and its implications for zooplankton community structure. By experimentally monitoring the effects of two alternate model predators on the metazoan community over time, dissimilarities in community level control emerged. Alternate key prey populations were regulated by the different model predators, highlighting the importance of predator and prey behaviour in mediating predator-prey interactions (Chapter 7). These results highlight the potential importance of predators in maintaining community dynamics in estuarine planktonic communities under certain conditions. This study represents some of the first work to address these various aspects of predator-prey dynamics within the context of planktonic estuarine ecology.

Biotic interactions are known to shape natural community assemblages and biodiversity. Positive interactions such as facilitation have recently received attention in ecological food webs. Mechanistic models have improved our understanding of these complex food web interactions. Here, focus is given to a three-species food web system with a beach dune natural community in mind. In the last decade, there has been a series of studies investigating intraguild predation between two major loggerhead sea turtle nest predators, North American raccoons and Atlantic ghost crabs. Studies have also highlighted that ghost crab predation assists raccoons in finding nests (i.e., facilitated predation). However, the combined effects of these two intraguild interactions and their consequences on nests have not been examined explicitly. The aims of this study were to (i) develop a three-species, ordinary differential equation model (ii) implement a sensitivity analysis to understand the influence of facilitation and other factors in driving species richness and abundance and (iii) characterize the dynamic interactions between intraguild predators and their effects on a shared resource. Interactions between ghost crabs and sea turtle eggs and facilitation can yield a wide variety of species abundance responses and were influential factors in the model. I found that high secondary sea turtle egg depredation and low facilitated predation by raccoons led to three species co-existence regions in the model. Controlling for nest predators at higher abundance levels showed that ghost crabs had a larger negative effect on sea turtle egg abundance responses when compared to raccoons. This suggests that interactions between sea turtle eggs and ghost crabs appear to be important and potential sea turtle nest management implications are discussed such as the use of ghost crab exclusion devices.

The white perch (Morone americana) is a highly successful invader of freshwater systems, with negative consequences to some resident fishes. White perch are ovivores and may prey upon as well as compete with juvenile sportfishes. Since 1988, an introduced population of white perch has proliferated in the 19,790-ha Kerr Reservoir, NC-VA, potentially threatening popular sport fisheries for largemouth bass (Micropterus salmoides), crappie (Pomoxis spp.), and self-sustaining striped bass (Morone saxatilis). Trophic relationships between white perch and resident sportfish were examined in the Kerr Reservoir system to determine the white perch's impact on them. Striped bass eggs were found in the stomach contents of white perch collected from the Roanoke River during their concurrent spawning runs in May, and at times egg predation was intense (frequency of egg occurrence in white perch individuals up to 100 %). However, modeling simulations indicated that observed densities of white perch in the Roanoke River during the peak spawning period of striped bass (May) were too low to have a substantial effect on striped bass recruitment. Crappie eggs were found in the diets of white perch collected from Kerr Reservoir during April, but the significance of this predation was not determined. Trophic overlap (Schoener's Index) was high (> 0.5) between age-0 white perch and age-0 crappie, largemouth bass, and striped bass in June, but only remained high between white perch and crappie in the remainder of the growing season (July – September). After June, largemouth and striped bass were primarily piscivorous, whereas white perch and crappie remained primarily invertivorous. Trophic overlap was high between adult white perch and adult crappie (> 0.6), but not between white perch and any other species of adult sportfish. The utilization of white perch by adult piscivores (Ictalurids, Pomoxis spp., Moronids, and Percids) as a food source was low (< 2% of diets by weight). Piscivorous sportfish primarily ate clupeids, which are highly abundant in Kerr Reservoir. Analysis of sportfish performance before (1974 – 1987) versus after (1996 – 2005) white perch establishment indicated no change in abundance and growth of striped bass and largemouth bass. Performance data for crappie prior to white perch introductions were unavailable, but observed egg predation by and trophic overlap with white perch suggests that crappie have the highest potential to suffer deleterious interactions with the white perch. Abundance of white bass has significantly declined in years since white perch introductions. Future research in Kerr Reservoir should concentrate on interactions between white perch and these two species. / Master of Science

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