Thomomys bottae

Abstract: Proportional to their diversity, fossorial rodents have received more attention regarding their patterns of genetic variation than any other mammal exclusive of man. This interest has undoubtedly stemmed from the peculiarities of their general biology dictated by the subterranean habitus, features which have been developed convergently in at least five totally unrelated lineages. Among the attributes associated with the fossorial life-mode .are morphological adaptations for burrowing, low individual vagility, often disjunct distribution patterns resulting from patchy soil conditions, parapatric distributions of species, and a closed, individually territorial social system (see Pearson, 1959, for review). Consequently, a high degree of both intra and interpopulation divergence in genetic systems of fossorial rodents has been predicted and generally found where studied. Genetic data, both chromosomal and genic (=allozyme), are available for two groups of fossorial rodents, the North American pocket gophers (family Geomyidae) and Middle Eastern mole rats (genus Spalax], Chromosomal variability is indeed extreme in all cases studied, with extensive inter and intrapopulation variation involving both diploid number and arm number changes known. Often this diversity is associated with reproductively isolated units and the chromosomal reorganizations involved were probably instrumental in the development of that incompatability (in the \"talpoides\" complex of Thomomys [Thaeler, 1968, 1974a, b] and the \"ehrenbergi\" complex of Spalax [Wahrman et al., 1968]) . In other cases the variation recorded is apparently independent of reproductive isolation (in the bottae group of Thomomys (Patton, 1972) and the bursarius group of Geomys (Davis et al., 1971; Baker et al., 1973). Fewer studies are available on allozyme variation and those display two general modes which are also concordant with exhibited chromosomal patterns. Groups of species with chromosomally determined reproductive isolation tend to exhibit quite low levels of intrapopulation genic variability (Nevo and Shaw, 1972; Nevo et al., 1974) whereas the other group has moderate to high levels (Patton et al., 1972; Selander et al., 1974). In both cases genic differentiation between populations does not appear to have been of strong influence in the development of reproductive isolation. The present effort documents the geo-

Abstract: Species definitions are commonly based on documentation either of genetic or ecologic cohesion (biological species models) or of phylogenetic relationship (phylogenetic spe? cies concept). However, these philosophical positions are often incompatible with data on ge? nealogical relationship of and genetic/reproductive interactions among member populations. The difficulty in defining species in nature based on either viewpoint is exacerbated when times between divergence events are short, when differentiation among populations prior to speciation is extensive, and when reticulation events persist subsequent to speciation. We illustrate this set of problems with data on mitochondrial DNA sequences and both nuclear sequences and allozyme electromorphs for sets of populations of two currently recognized species of pocket gophers, Thomomys bottae and T. townsendii in the western United States. These molecular per? spectives give somewhat conflicting views of polyphyly, paraphyly, and monophyly at the population and species level due, in part, to probable differences in times to monophyly, dif? ferential lineage sorting, retention of ancestral polymorphisms, and/or episodes of asymmetrical introgressive hybridization. As a consequence, strict adherence to any species concept in the objective recognition of evolutionary units within this complex is difficult at best. (Species concepts; paraphyly; polyphyly; allozymes; mtDNA sequence; 0-globin sequence; pocket go? phers; Thomomys.)

Abstract: We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.