The abiotic environment and predator-prey interactions: direct and indirect effects within aquatic environments with a specific look at temperature

Species have specific tolerances to a variety of environmental variables including temperature, dissolved oxygen (DO) and turbidity. Changes in either of these variables can therefore be expected to affect predator-prey interactions in shallow water ecosystems. Temperature drives the metabolic rates of poikilotherms, including fish. Hypoxic conditions generally affect larger fishes to a greater degree than smaller fishes, though the presence of physostomous swim bladders in certain species can alter that relationship. Finally there are species of fish that rely on vision for food acquisition while other species rely on other senses such as chemical cues. Changes in turbidity levels could therefore affect foraging efficiency of visual foragers. This thesis examines the role that each of these environmental variables (temperature, DO and turbidity) can have on community composition and therefore predator prey interactions, with a specific focus on the role of temperature in structuring predator-prey interactions. Laboratory, field and theoretical studies suggest that as temperature increases, encounter rates between predators and prey will increase. Prey are more active, spend more time foraging, and increase their use of risky habitats in warmer environments in laboratory experiments. In the field, prey and predator activity and/or abundance is positively related to temperature. These laboratory and field studies suggest that temperature increases should result in increased predation rtes of prey. Finally, the results of a dynamic state dependent optimization model also suggest that periods of warming will result in a lowering of the probability of survival of the fathead minnow, Pimephales promelas, a prey species, over the-ice free season. A reduction in DO levels in aquatic ecosystems results in a reduction in the number of and/or activity of predators present. This should result in a reduction in predation risk to prey. However, when endothermic predators are factored in to this equation, this reduction in risk may not occur. The presence of avian predators of small forage fish are directly related to the level of DO in the water, regardless of the abundance of prey fish present. This relationship is likely a result of behavioural decisions of prey that occurs in hypoxic conditions. In periods of low DO, prey fishes may exploit areas of higher DO that are closer to the surface of the waters. While their piscine predators may not be able to tolerate the low DO levels regardless of the position of prey in the water column, avian predators appear to be able to cue in to this increase in availability of potential prey, reducing any benefits that might occur by occupying surface areas where DO levels might be slightly higher than lower in the water column. As compared to temperature and DO, turbidity does not appear to affect the potential risk of predation to forage fish. The catch per unit effort (CPUE) of foragers who rely on vision and those that rely on chemical cues to forages, were not related to turbidity levels. Turbidity levels were also not related to the abundance of avian predators. This suggests that in this generally turbid, shallow water ecosystem, changes in turbidity do not affect the overall species composition of the system. Predator-prey interactions in the system are also not likely to be affected by turbidity. In contrast to this, temperature and DO are likely to influence the interactions between predators and their prey in a shallow water ecosystem. Both increases in temperature and decreases in DO may result in increases in predation pressure on prey. While temperature increases will likely result in increased predation on prey by piscine predators, a reduction in DO, which often occurs as temperature increases, will likely result in increased predation on prey by avian predators, even as predation pressure by piscine predators decrease.

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