Streptanthus bracteatus(Brassicaceae), an endangered annual plant, occurs only on the eastern and southeastern edge of the Edwards Plateau of central Texas. We collected data on vegetation, soil depth, and canopy cover in occupied and unoccupied plots in twelve of the fourteen known sites that had extant populations at the time of the study. We also recorded plant number and the size, fecundity, and herbivory and disease status of individual plants. We combined these data with information from soil, geologic, and topographic data bases for analysis.
We identified an unusual, perhaps unique, geological association: S. bracteatus only occurs in close proximity to the geological boundary between two apparently similar Cretaceous limestone formations. The cause of this association is still unknown; dolomitization is suspected but experiments with Mg levels have so far been inconclusive and no relationships with soil variables were found. No indicator or unusual associated plant species were identified. We confirmed earlier findings that rates of deer herbivory are high. The relationships of individual size and fecundity with canopy cover indicated that many S. bracteatus plants were growing in sub-optimal light levels; this may reflect reduced fire frequency and/or some protection from deer afforded by Juniperus ashei. While two populations were stable or increasing, we documented the loss of two populations and most of a third one to development of private land. As is true for many endangered species, conservation of this species is complex because there are so many different threats. For example, reducing canopy cover is precluded in most sites by the conservation of an endangered warbler species. In addition to the challenges presented by deer, lack of fire, development of private land, and the higher priority given to bird conservation on public land, S. bracteatus also suffers from a fungal disease that may or may not be native, lack of legal protection (it is stalled in Candidate status), and public land recreation, especially mountain bikes. Effective conservation will require taking all of these factors, in addition to its habitat requirements, into account.