This dissertation consists of three chapters, each of which addresses a topic in one of three related categories of research as required by the Ph.D. program in ecology as directed through the Department of Zoology at Miami University.
Chapter 1, Phylogeographic analysis reveals multiple cryptic species of amphipods (Crustacea: Amphipoda) in Chihuahuan Desert springs, investigates how biodiversity conservation and the identification of conservation units among invertebrates are complicated by low levels of morphological difference, particularly among aquatic taxa. Accordingly, biodiversity is often underestimated in communities of aquatic invertebrates, as revealed by high genetic divergence between cryptic species. I analyzed PCR-amplified portions of the mitochondrial cytochrome c oxidase I (COI) gene and 16S rRNA gene for amphipods in the Gammarus pecos species complex endemic to springs in the Chihuahuan Desert of southeast New Mexico and west Texas. My analyses uncover the presence of seven separate species in this complex, of which only three nominal taxa are formally described. The distribution of these species is highly correlated with geography, with many present only in one spring or one spatially-restricted cluster of springs, indicating that each species likely merits protection under the U.S. Endangered Species Act. I present evidence suggesting that habitat fragmentation, long-distance colonization, and isolation-by-distance have occurred at different temporal and spatial scales within this system to produce the lineages that I report.
Chapter 2, Detecting conservation units using morphological versus molecular criteria: evaluating the Gammarus pecos species complex as a test case, compares the results of morphological versus molecular biodiversity assessments within the G. pecos species complex. I compared results from an earlier morphology-based study to my results from screening 166 COI gene sequences according to Moritz’ Evolutionarily Significant Unit (ESU) concept and a DNA barcode-based Species Screening Threshold (SST) concept. I found strong concordance between the two molecular screening methods, but these two molecular methods separated populations as distinct from one another whereas the morphological method alone failed to separate the same populations. Overall, I found that morphological and molecular techniques for biodiversity estimation should be combined, when possible, to produce a powerful tool for addressing taxonomic and conservation issues.
Chapter 3, Salinity tolerance as a potential driver of ecological speciation in amphipods (Gammarus spp.) from the northern Chihuahuan Desert, examines population level responses to salinity. My results suggest barriers to gene flow between populations as a result of ecologically-based divergent selection, and that tolerance to habitat salinity has structured biodiversity across springs in the northern Chihauhuan Desert. Furthermore, salinity tolerance is correlated with environmental salinity. This work shows a role for both putatively neutral processes (e.g. isolation and genetic drift) and natural selection (acting on population-level physiological responses to habitat salinities).
Taken together, these results provide clues important for future biodiversity investigations in geographically isolated aquatic habitats, and shed light on the understudied and underestimated levels of biodiversity present in desert spring systems.