Abstract

Niche partitioning, or differential resource use, is a possible explanation for the coexistence of multiple species that would otherwise be in competition. In this study, we examined two sympatric desert snakes, the eastern black-tailed rattlesnake (Crotalus ornatus) and western diamondback rattlesnake (Crotalus atrox) on Indio Mountain Research Station (IMRS) in Hudspeth County, Texas, for evidence of partitioning along the spatial dimension of the niche. We tracked the movements of the two species using radiotelemetry and analyzed macrohabitat and microhabitat data using a combination of multinomial logit models and chi-square tests. Evidence of niche partitioning along the spatial dimension of the niche exists between the two species, with C. ornatus showing a higher affinity for southwest facing, rocky slopes and C. atrox showing a relatively diverse use of all habitats with a slight preference for northeastern slopes. Our results largely compare to previous studies at both IMRS and throughout the range of each species.

Abstract

Groundwater-obligate organisms are nearly always small range endemic species with a high-degree of imperilment because of a combination of anthropogenic threats and inherent ecological traits such as low reproductive potential. Accurate assessments of conservation status are foundational for development of species-specific research and conservation priorities. Fourteen species of endemic ostracods are restricted to groundwater habitats in Texas. We used NatureServe methodology to evaluate the conservation status of all 14 species. All evaluated taxa were identified as warranting G1S1 (critically imperiled) or G2S2 (imperiled) designation, primarily due to small-range endemism (particularly small area of occupancy and few known occurrences). Most species are recorded from five or fewer locations and occupy 4-km² grid cells. Threat levels varied by site type, with all springs and one surface water site facing the most severe threats and highest number of threats. Deeper, phreatic groundwater sites were typically more secure. Most ostracod sites harbor only a single groundwater species, but two sites, Comal Springs and the San Marcos Artesian Well, harbor six and seven species, respectively. Based on this status assessment, we recommend that all 14 ostracod species be designated as species of greatest conservation need (SGCN). Additionally, we propose that the top priorities for these taxa are 1) identification of existing lots in museum holdings, 2) molecular analyses of populations to better delimit species boundaries, 3) additional field work to fully delineate range sizes and area of occupancy, and 4) long-term sampling at diverse sites to track population trends.

Supplemental material is available for this article online.