The Role of Dispersal During the Recovery of Acid-Damaged Zooplankton Communities
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Ecologists studying acid-damaged zooplankton communities have often documented a time lag in recovery following pH increases. While previous work has provided a solid understanding of the local factors that may delay recovery (e.g. competition), less is known about the role of dispersal. The work in this thesis was conducted to test the hypothesis that dispersal limitation contributes to delays in the recovery of acid-damaged zooplankton communities. To assess the role of dispersal during recovery I pursued three objectives: 1) To measure dispersal in the field and determine the relative importance of various dispersal vectors for contributing acid-sensitive colonists to lakes; 2) to determine if spatial structure in recovering zooplankton communities exists across the landscape independent of environmental gradients; and 3) to determine if an interaction between dispersal and local environmental variables could influence recovery. Data collected for Objective 1 demonstrated that overland dispersal rates for acid-sensitive species in Killarney Park were relatively low, but some species were found emerging from the diapausing egg bank or dispersing through streams to recovering lakes. Spatial modeling and variation partitioning analyses for Objective 2 revealed spatial patterns indicative of dispersal limitation in recovering Killarney Park zooplankton communities. Enclosure experiments conducted for Objective 3 suggested that the colonization of the acid-sensitive copepod Epischura lacustris may be influenced by an interaction between dispersal levels and pH, such that higher dispersal levels may be required for establishment in lakes that are early in the process of pH recovery. Enclosure experiments also indicated that community resistance and low dispersal levels might hinder the reestablishment of the acid-sensitive copepod Skistodiaptomus oregonensis. Taken together, my results strongly suggest that dispersal limitation could contribute to delays in zooplankton community recovery. The recovery of acid-sensitive copepod species may be particularly difficult, as their reestablishment in recovering lakes appears to be influenced by Allee effects, community resistance, and an interaction between pH and dispersal levels. While dispersal rates could be artificially increased by human intervention, this would carry the risk of introducing invasive species. As a result, patience and continued monitoring of recovering lakes may be the best management approaches at this time.