Investigations of Altered Aquatic Ecosystems: Biomonitoring, Disease, and Conservation
Changes to hydrology, water chemistry, physical habitat, and the landscape surrounding aquatic ecosystems can have profound effects on their biological communities, potentially reducing important ecosystem services and functions provided to wildlife and humans. In this dissertation, I examine how environmental variables, human activities, and management practices have affected stream and wetland biota throughout Northern California.
The parasitic trematode Ribeiroia ondatrae has been shown to cause limb malformations in amphibian species through laboratory studies, but these small-scale experiments lacked important ecological co-factors that could alter parasite transmission rates and host susceptibility. Therefore, I conducted a multi-year, nested experiment at Hog Lake in Mendocino County to test host responses to Ribeiroia infection. The addition of Ribeiroia-infected snails to half of Hog Lake caused an increase in severe limb malformations in Pacific chorus frogs (Pseudacris regilla) and monitoring at different spatial scales revealed unique dose-response relationships because of ecological interactions and environmental co-factors. To determine if the parasite might contribute to population declines, I monitored juvenile and adult P. regilla at Hog Lake and a second wetland for four years. The consistently low (< 5%) malformation prevalence in adult frogs despite a generally high malformation prevalence (30-50%) in juvenile frogs confirmed that malformed amphibians do not survive to adulthood, indirect evidence that Ribeiroia may cause population declines. Multi-year surveys at 17 Northern California wetlands showed a strong dose-response relationship among the parasite and P. regilla malformation levels, providing evidence that Ribeiroia is an important cause of limb malformations in this region.
Effective conservation of wetland species depends on understanding how anthropogenic stress, natural environmental variables, and management activities affect biological communities. To identify which of these explanatory variables were most influential and determine the conservation value of created wetlands, I sampled amphibians and insects from a total of 49 stormwater ponds, stockponds, and natural ponds in Northern California. Overall, landscape variables (i.e., percent urban and natural, number of ponds within 1 km, and elevation) were associated with differences in macroinvertebrate community structure, but specific conductance and percent littoral vegetation present were important co-factors. Compared to natural ponds, stockponds supported the most similar invertebrate assemblages and greater amphibian species richness. Light cattle-grazing of areas surrounding natural ponds and stockponds was associated with increased amphibian richness, whereas the addition of exotic fishes was associated with mild differences in invertebrate populations. This research showed that stockponds are conservation resources for amphibians and invertebrates but that proper management is necessary to maximize this potential.
To objectively quantify the ambient condition of wetlands, ecologists use the plants and animals found in aquatic habitats as biological indicators. Because no biological indicator has been developed for wetlands in California, I designed an index of biotic integrity (IBI) to determine the ecological condition of the wetland based on macroinvertebrate community structure. Eight metrics that showed significant responses to urbanization, had adequate range, and lacked redundancy with each other were incorporated into the IBI. The IBI was successfully validated and showed no bias along natural gradients (e.g., ecoregion, elevation, and precipitation), except that non-perennial and perennial ponds did support slightly different communities, indicating that the IBI is applicable across this region.
Although biomonitoring data have been successfully used to quantity the ecological condition of perennial streams in many regions of California, no analysis tool (e.g., IBI) has been developed for the San Francisco Bay Area, a region which contains a large proportion of non-perennial streams. To identify least-disturbed reference sites in a highly urban region and examine how inter-site and temporal variability affect bioassessment data, I analyzed a large (> 400 sites) stream bioassessment dataset. I found that using geographic information system (GIS) watershed analysis and local physical habitat data were both necessary to identify least-disturbed reference sites. Among reference sites, the macroinvertebrate assemblages of perennial streams were different from non-perennial streams, a result which demonstrates the need for separate IBIs for these two habitat types. Interannual variability was moderate across 2000 to 2007, with index scores ranging from 10-15 out of 100 points, but some sites showed extreme variation of more than 50 points.
In summary, this dissertation shows that effective management of wetlands and streams can be accomplished with the use of biomonitoring data to infer ecological condition and that stockponds are valuable conservation resources when properly managed.