Persisting in the pelagic: environmental, behavioral, and morphological controls on predator-prey interactions
Hansen, Adam Garner
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Persisting in the pelagic is not easy. The physical environment of pelagic ecosystems is highly dynamic. Unlike terrestrial systems where habitat complexity is driven by physical structure (e.g., vegetation and terrain), habitat complexity in the pelagic is driven by vertical gradients in light, turbidity, temperature, and oxygen. All of these factors change over time, and can mediate predator-prey interactions given ontogenetic or asymmetric responses of predators and prey to diel and seasonal changes in these factors. Additionally, pelagic predators and prey rely primarily on vision for feeding. Therefore, changes in photic conditions (light and turbidity) in particular can have a strong impact on the structure of predator-prey interactions. Yet, it remains unclear how habitat heterogeneity over different dimensions of time and space interacts with perception, behavior, and physiological tolerance to mediate the foraging success of predators and predation risk for prey in pelagic ecosystems. Pelagic environments are not static. They will change given continued human-induced alterations to the landscape, shifts in climate, and unanticipated introductions of nonnative predators and prey. Knowing how the pelagic foraging-risk environment changes in response to shifts in physical habitat over many different temporal-spatial scales should improve predictions regarding how aquatic food webs will respond to different perturbations. For my dissertation I addressed the following series of questions: 1) how do light and turbidity effect the visual prey detection responses of pelagic planktivores and piscivores, and do the consumer groups differ?, 2) how does natural variation in photic conditions (diel and seasonal light regimes at different latitudes and turbidity) shape the foraging-risk environment for visually-feeding planktivores and piscivores in pelagic ecosystems?, 3) in addition to photic conditions, how do seasonal shifts in the thermal environment shape the foraging-risk environment for pelagic planktivores and piscivores?, and 4) do fluctuations in the abundance, distribution, visual detectability, and vulnerability to predation of different prey groups alter the diet selection of piscivores and relative predation risk for planktivores in diverse pelagic communities? To address the first question, I conducted a series of controlled laboratory experiments and measured light- and turbidity-dependent reaction distances by piscivores. To address the remaining questions, I linked individual-based, mechanistic models (visual foraging and bioenergetics models) that capture important fine-scale behavioral and physiological processes with empirical data on physical habitat, predator diet, movement, and distribution (from netting, ultrasonic telemetry, and hydroacoustics) to estimate changes in feeding rates for piscivores and planktivores and predation risk for planktivores over time and space. First, reaction distance responded asymptotically with increasing light, but declined quickly with increasing turbidity for both planktivores and piscivores. The maximum reaction distance for piscivores was 5-6 fold greater than for planktivores, but planktivores achieved their maximum reaction distance at a much lower light level, and the decline in reaction distance with turbidity was much steeper for piscivores. Second, based on these asymmetric visual prey detection responses, the foraging-risk environment for pelagic planktivores and piscivores changed considerably in systematic ways with changes in diel patterns of illuminance along a broad latitudinal gradient and to increases in turbidity. These changes have different implications for the structure of pelagic predator-prey interactions over a broad latitudinal gradient. Third, like shifts in photic conditions, seasonal shifts in the thermal environment also mediated the foraging success of piscivores and predation risk for planktivores. Here, periods of environmental stress (i.e., high temperature and low dissolved oxygen) greatly reduced both the foraging success of piscivores and predation risk for planktivores by creating thermal refugia for the planktivores. Lastly, the nature of the feeding selectivity (random or opportunistic versus non-random or targeted) of visually-oriented piscivores was highly dependent on fluctuations in the abundance and susceptibility of key prey to visual detection and capture. Results suggested that pelagic piscivores are flexible predators, and can adapt their feeding behavior to take advantage of large influxes of highly catchable prey. Overall, by observing through the eyes of pelagic predators and prey, my results show that the foraging-risk environment for piscivores and planktivores can look very different as physical habitat changes over many different dimensions of time and space.
- Fisheries