Diversity and Distribution of California Dragonflies and Other Aquatic Taxa Over the Past Century

Climate and land-use change have altered and continue to affect the diversity, composition, and distribution of freshwater organisms throughout the world. This is particularly true in arid and semi-arid regions, where aquatic organisms may experience more pronounced reductions in available habitat with declines in precipitation, increases in water demand, and habitat degradation through human land-use. However, documentation of changes in taxonomic assemblages over long-time periods has been rare because of the difficulty in obtaining historical occurrence data. This dissertation used data from previously published literature, a resurvey study, museum specimens, and enthusiast sightings to document changes in the occurrence rates and distribution of freshwater organisms throughout California over the past century./nSummary information regarding freshwater taxa known to occur in California did not previously exist in a central publication. I therefore conducted a review of several primary groups of stream organisms found in the Mediterranean region of California and statewide. For this work, I gathered data from a variety of literature sources and museum specimens to summarize species composition and endemism in the region, and to identify data gaps and conservation priorities for the examined groups. The remainder of this dissertation focuses largely on changes in Odonata species diversity, composition, and occurrence rates over time in California. This charismatic group was ideal for study of change over time because of their relatively low diversity, well-known taxonomy, and the existence of sufficient historical and current specimen records and more recent enthusiast sightings of odonates./nI conducted a resurvey of sites originally sampled for Odonata by Clarence H. Kennedy 1914-1915. This work involved surveys of odonates at 81 sites throughout central California and northwestern Nevada, 45 of which were directly comparable to Kennedy's original sites. I found that while site-level species richness has not changed significantly, assemblages have become more homogeneous across sites. Habitat generalists have generally expanded in the extent of their distribution while habitat specialists have declined. In examining current local and regional factors influencing the occurrence of Odonata species in this region, I found that species occurrence was higher during site visits with higher degree-days, especially for highly mobile groups, including dragonflies and migratory species. The probability of presence across species was lower in highly urban sites, particularly for habitat specialists. Overall, both regional and local factors influenced the occurrence of odonates in the study with implications for conservation./nA large component of this dissertation included development and analysis of a database of over 33,000 Odonata occurrence records throughout California over the past century. This database included specimen records from museums in California and large odonate collections elsewhere, as well as statewide enthusiast sightings from recent years. I noted that these unstandardized data contain biases with regards to uneven sampling effort, which must be addressed in analysis. Subsequent analyses of occurrence records before and after 1975 indicated that Odonata distribution may have generally shifted northwards with temperature warming and to lower minimum elevations in response to increased summer water-availability in low-elevation agricultural regions. Similar to results from the resurvey study, the museum specimen data indicated that highly mobile migratory species have increased while habitat specialists have declined. I concluded that a combination of sampling biases, species traits, and climate that have influenced the probability of detection of Odonata species over the last century.

Advisor: Resh, Vincent H.