Dynamics of competing herbivores: a test of theory using freshwater zooplankton

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The competitive exclusion principle states that no two species competing for shared, limiting resources can coexist at equilibrium. To explain the high species diversity found in natural systems, classical competition theory has focused on mechanisms such as resource or habitat partitioning that allow coexistence of competing species at equilibrium. However, few consumers or their resources maintain equilibrium densities, and the natural systems they are a part of can be viewed as 'nonequilibrium' systems. Recently, nonequilibrium existence theory has suggested that destabilizing factors in the environment, or from within the consumer-resource system itself in the absence of environmental variation, may perturb the consumer resource system and prevent it from reaching equilibrium. This destabilization may create shifts in the competitive abilities of the consumers as resource density fluctuates, and thus promote coexistence. The primary objective of this thesis is to test whether nonequilibrium dynamics in a nonvarying environment can promote coexistence among competing predators. The test system consisted of populations of two freshwater zooplankters (Daphnia pulex and rotifer species) which are known to compete for a shared food resource (phytoplankton). A quantitative synthesis of relevant biological parameters from literature studies revealed that Daphnia and rotifer biology satisfy all the theoretical criteria required for nonequilibrium coexistence in a nonvarying environment. To test for nonequilibrium coexistence, I performed two manipulation experiments using plankton communities maintained in a greenhouse environment. The first experiment tested whether Daphnia could 'invade' a rotifer-algal community at 'equilibrium', which is a key assumption of nonequilibrium theory. Results showed that Daphnia could successfully invade and become established in the community without any of the three populations being driven to extinction. The second manipulation experiment tested predictions made by nonequilibrium theory about the effect of Daphnia on: 1) the structure and diversity of rotifer communities, and 2) the dynamics of rotifer and phytoplankton populations. In general, the predictions were supported in systems with ambient nutrient levels. The presence of Daphnia changed the dynamics of rotifers and phytoplankton, and rotifer species diversity was significantly increased in systems with Daphnia populations present. However, the results from nutrient -enriched systems rejected predictions from nonequilibrium coexistence theory. The results from the manipulation experiments represent the first test of nonequilibrium coexistence in a nonvarying environment. A secondary objective of this thesis was to assess the relative importances of exploitative and interference competition in the suppression of rotifers by Daphnia. By analyzing the structure of rotifer communities in the presence and absence of Daphnia, I concluded that exploitative competition was the more important factor. Finally, I examined whether 'Paradox of Enrichment' behaviour occurs in the Daphnia-rotifer-algal and rotifer-algal systems. Theory predicts that nutrient enrichment should destabilize predator-prey dynamics, causing large amplitude fluctuations in density. By comparing dynamics of populations in nutrient enriched and unenriched systems, I found no evidence that enrichment destabilized the systems.
Bibliography: p. 146-153.
Mackey, R. L. (1992). Dynamics of competing herbivores: a test of theory using freshwater zooplankton (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/12413