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Research Paper - Size selective predation by goldfish and...

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Size selective predation by goldfish and behavioral effects of predation on Daphnia manga Melissa Harintho Heather Eisler BIOS 10162 Section 3 March 19, 2008
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Abstract Predation is one of the most important ecological interactions affecting natural populations and communities; it can determine whether an entire species can survive in a habitat, and heavily influences patterns of population cycles. The study of predation however, is limited by the strong effect that the characteristics of a system has in the degree of importance predation plays in a particular system. Fish predation on zooplankton communities is the most clearly and repeatedly demonstrated example of the powerful impact of predation. In this experiment, 30 large (≥ 2.5 mm) and 30 small (≤ 1 mm) sized plankton were distinguished using Greatest Axial Linear Dimension measurement (GALD) and placed into control and experimental (fish- containing) tanks. As expected, the results reveal a statistically significant difference (t= 2.18, p= 0.037, using a t-test) between the amounts of large (1.58± 0.17 mm, using a 95% confidence interval) and small (1.30± 0.24 mm) Daphnia magna present after the goldfish had been allowed to forage for prey. The results suggest that goldfish prefer large daphnids to small daphnids in food predation, and imply that a process of selection based on maximizing rate of energy intake plays a significant role in predation. Introduction Predation is a key component of any biological system. Past studies have shown that the presence of fish has a direct impact on the population of plankton. A study of Crystal Lake between 1942 and 1964 examined the patterns of plankton body size in the absence and presence of Alosa fish. While larger plankton were found in abundance in the lakes before the presence of Alosa fish, smaller plankton dominated the lakes when the Alosa fish population grew. Moreover, a study delineating the size- efficiency hypothesis describes the factor that the level of
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predation intensity has on plankton populations. The size- efficiency hypothesis explains that when there is intense predation, a greater amount of large plankton (relative to the amount of small plankton) will be eliminated since they are easier to visualize. Alternately, predators will be more likely to eliminate smaller sized plankton when there is not as intense predation, because smaller sized plankton can be collected more efficiently (Brooks et al. 1965). An additional study also examined the effects that the type of predator had on size distribution of plankton. In this study, scientists observed that larger plankton were directly impacted by the predation of vertebrate preys, whereas the small plankton population was influenced by the invertebrate preys. Thus, while the population densities of large daphnids preferred by the fish is controlled directly by fish predation, the population densities of smaller and less preferred zooplankton is controlled indirectly through the food-web cascade (Chang et al. 2004).
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