Graduate Thesis Or Dissertation
 

Effects of perturbation on community structure and organization in aquatic microcosms

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  • Aquatic communities consisting of guppies, amphipods, copepods, snails, nematodes, rotifers, protozoans, euglenoid flagellates, and other microorganisms were established in sixteen laboratory microcosms. Guppies were exploited at four different levels (0 percent, 10 percent, 20 percent, and 40 percent of the population biomass) with four communities at each level of exploitation. An alfalfa ration (0.4 g/day) was introduced as a nutrient source for the communities. The general objective of this study was to advance the understanding of community structure and organization by conceptualizing communities as hierarchical arrangements of subsystems influenced by the external environment and by explaining how both the interactive performances of these subsystems and the external environmental conditions may influence the dynamics of community structure. Environmental perturbations imposed on the sixteen communities included introduction of a toxicant, dieldrin, an increase in nutrient input and habitat availability, and exploitation of the guppy populations at four levels. The level of guppy exploitation had a direct effect on community structure. The biomasses of the guppy populations that were maintained in the systems decreased as the level of exploitation increased. The guppies apparently preyed on the amphipod and nematode populations as well as competed with the amphipods for the alfalfa nutrient source. As the biomass of the guppy populations decreased with increased exploitation, the populations of amphipods and nematodes increased. Dieldrin was introduced into four systems (one at each level of exploitation) after these systems had exhibited near steady-state behavior on a guppy-amphipod phase plane for approximately one year. The most direct impact of the toxicant was on the guppy population. Guppy biomass was decreased substantially at 0 percent exploitation, 10 percent exploitation, and 20 percent exploitation. A subsequent increase in amphipods occurred as a result of the decreased intensity of predation and competition. New near steady-state biomasses were established for these populations. The system at 40 percent guppy exploitation showed no apparent immediate response to the dieldrin. Microinvertebrate populations also showed no apparent response to the toxicant. In eight of the remaining systems, nutrient input was increased and additional rock habitat was introduced to provide increased cover fo7 the invertebrates. This environmental perturbation greatly altered community structure. Immediate and dramatic increases in the guppy and amphipod populations occurred. The increased rock cover caused an oxygen depletion in the sediments and thus decreased the habitat available for the microinvertebrates. Substantial decreases occurred in the populations of nematodes, protozoans, and other previously abundant organisms. A system of intersecting isoclines is used to explain the general locations of the domains of behavior of these systems in phase space before and after the introduction of dieldrin and before and after the increase in nutrient and habitat availability. The increase in nutrient and habitat availability altered the domain of behavior for the communities more dramatically than did dieldrin.
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