Reevaluation of a classic phylogeographic barrier: new techniques reveal the influence of microgeographic climate variation on population divergence

We evaluated the mtDNA divergence and relationships within Geomys pinetis to assess the status of formerly recognized Geomys taxa. Additionally, we integrated new hypothesis‐based tests in ecological niche models (ENM) to provide greater insight into causes for divergence and potential barriers to gene flow in Southeastern United States (Alabama, Florida, and Georgia). Our DNA sequence dataset confirmed and strongly supported two distinct lineages within G. pinetis occurring east and west of the ARD. Divergence date estimates showed that eastern and western lineages diverged about 1.37 Ma (1.9 Ma–830 ka). Predicted distributions from ENMs were consistent with molecular data and defined each population east and west of the ARD with little overlap. Niche identity and background similarity tests were statistically significant suggesting that ENMs from eastern and western lineages are not identical or more similar than expected based on random localities drawn from the environmental background. ENMs also support the hypothesis that the ARD represents a ribbon of unsuitable climate between more suitable areas where these populations are distributed. The estimated age of divergence between eastern and western lineages of G. pinetis suggests that the divergence was driven by climatic conditions during Pleistocene glacial–interglacial cycles. The ARD at the contact zone of eastern and western lineages of G. pinetis forms a significant barrier promoting microgeographic isolation that helps maintain ecological and genetic divergence.     

Patterns of size change in late Neogene pocket gophers from the Meade Basin of Kansas and Oklahoma

A dense late Neogene history of pocket gophers from the Meade Basin of south-western Kansas and north-western Oklahoma preserves a detailed record of size change, as indicated by mean length of the lower fourth premolar. A ‘time for space substitution’ interpretation results in the tentative recognition of a pattern analogous to character displacement, in which the small to medium-sized Pliogeomys buisi, P. louderbachi, Geomys minor and G. floralindae are considered ecological analogues and treated as a single ecological entity, Avatar A. The larger G. jacobi and G. quinni are combined as Avatar B. Statistical analyses confirm that populations of Avatar A are of equal size when allochronic from Avatar B, and significantly smaller when synchronic (and sympatric) with Avatar B. However, this pattern is complicated by the observation that Avatar A began dwarfing before the appearance of Avatar B. This historical perspective suggests that static size patterns among modern taxa may have a complex history. Occasional diminutive geomyid transients (G. adamsi, Thomomys cf talpoides) are always paired with significantly larger, medium-sized Geomys. Although a random walk explanation for the pattern of size changes is unlikely, we are currently unable to distinguish between environmental change and competition as the likely forcing mechanisms.     

Response of pocket gophers (Geomys) to late Quaternary environmental change on the Edwards Plateau of central Texas

Fossils of pocket gophers (Geomys) recovered from Hall's Cave, Texas, comprise a large sample of specimens spanning the late Pleistocene through Holocene. Statistically adequate sample sizes, a well-constrained chronology and independent palaeoenvironmental data allowed for the testing of both taxonomic change and predictions of ecophenotypic change in the dentary of Geomys. A time-transgressive reduction of diastema length (DL) in Geomys was predicted based on known, regional palaeoenvironmental changes and known ecophenotypic response in the closely related genus, Thomomys. Statistical evaluation suggests that although DL in Geomys is correlated to tooth row length (TRL), changes in DL may reflect an independent ecophenotypic response. Testing was hampered by the inadequacy of TRL as a diagnostic feature of most Geomys from Texas. A statistical test using coefficient of variation revealed the presence of multiple species of Geomys in some stratigraphic levels. Reduced local richness of Geomys to a single species coincides with the loss of other burrowing taxa in the vicinity of Hall's Cave. Based on the results of an analysis of variance (ANOVA) test, no significant changes in DL occurred in populations of Geomys from Hall's Cave.     

Survivorship of penstemon grandiflorus in an oak woodland: combined effects of fire and pocket gophers

Summary The effects of fire and pocket gophers, Geomys bursarius, on the survivorship of Penstemon grandiflorus growing in an oak woodland in Minnesota were studied from 1986 to 1990. Plants growing in sparse vegetation experienced mortality rates twice that of plants growing in dense vegetation. This difference was due partly to pocket gophers whose earth moving activities reduce the density of vegetation and bury and kill individual Penstemon plants. Laboratory feeding trials showed that gophers readily eat Penstemon, particularly the fleshy roots. An experiment involving the removal of 25–75% of the root tissue in 90 plants showed that root loss significantly reduced survivorship, suggesting that gopher herbivory might also kill plants. When gophers were experimentally excluded, plants growing in sparse vegetation exhibited significantly lower mortality rates than those growing in dense vegetation. Plants in the smallest size class exhibited reduced survivorship following a late spring burn; however, overall patterns of survivorship of plants in burned areas did not differ markedly from those in the unburned areas. A longitudenal analysis of plants with different reproductive histories revealed no survivorship cost to reproduction. Mortality rates decreased with increasing plant size. Small plants were more likely to be killed by fire and by being buried under gopher mounds. Differences in underground energy reserves of small and large plants can account for most of the survivorship patterns observed in this study. The study shows that within openings of the oak woodland, fire and gophers reduce the survival of individual Penstemon plants. Nevertheless, since both gophers and fire also serve to perpetuate suitable habitat in the woodland, Penstemon is ultimately dependent on both for its long term persistence in the landscape.     

Area-restricted search by the plains pocket gopher (Geomys bursarius) in tallgrass prairie habitat

Because pocket gophers have the high energetic cost of excavatingburrows and an inability to detect distant food items throughthe soil, I hypothesized that individuals within establishedburrow systems would use area-restricted search as a foragingstrategy. To examine this hypothesis I compared gopher foragingeffort over a 10-month period between areas in which overallplant densities were experimentally varied. Gophers expendedapproximately 50% of their foraging effort in areas with thehighest plant density, even though these made up only 33% ofthe available area in experimental plots. In large, griddedareas sampled for an entire season as well as in small areasin which gophers foraged for less than 1 week, gopher foragingeffort was related to the density of a single leguminous plantspecies, Psoralea argophylla. In small plots where this plantspecies was at high density, gophers created more tunnel branches,thereby intensifying their search effort. Thus, area-restrictedsearch appears to increase the rate of encounter with the patchilydistributed Psoralea plants.     

Morphology, genetics, and ecology of pocket gophers (genus Geomys) in a narrow hybrid zone

A population of hybrid pocket gophers ( Geomys bursarius × G. lutescens ) exists 1–2 km west of Oakdale, Antelope County, Nebraska, U.S.A. The hybrids occur in soil that has characteristics intermediate between that occupied by G. lutescens (sand) and by G. bursarius (silt loam); the vegetation associations on the different soils are Sandhills Prairie and Tall-grass Prairie, respectively, with mixed prairie on the intermediate soils. Hybrids are identifiable on the basis of both qualitative and quantitative morphological characteristics, allozymes and karyology. Concordance between morphological, allozymic, karyotypic and ecological data sets is very high. Hybrids appear to reproduce normally and survive well; i.e. they suffer no obvious loss of fitness. Backcrossing to either parental type is apparently rare. The parental species each support obligate parasitic lice ( Geomydoecus : Trichodectidae) of different species; these species are not sister species. We suggest that hybrid zones resulting from primary and secondary contact may be distinguished by (1) concordance of clines in different character sets, (2) fossil and biogeographic data, and (3) parasite data. We conclude that this zone resulted from secondary contact, and that the zone is maintained either by selection against hybrids (less likely) or by hybrid superiority (more likely).