Mexican buckeyes, with their dangling empty seed pods, and chubby-twigged Ohio buckeyes reminded me that the ravine is a meeting-place for west and east with some tropical and transcontinental species in attendance. Two-thirds of our biota is eastern or southeastern in geographic affiliation -- Ohio buckeye and green anole, for instance. A quarter is western or southwestern-Mexican, such as Mexican buckeye and ringtail. Eastern ruby-throated and western black-chinned hummingbirds nest among eastern red cedars and western Ashe junipers. Transcontinental species include fragrant sumac and common garter snake, while Turk's cap and Inca dove represent the tropical element.

Eastern deciduous forest meets western evergreen woodland along the north-south trending and rapidly urbanizing Balcones Escarpment. These large-scale, structurally distinct communities (biomes), are mostly separated by a forty-mile-wide tongue of tallgrass prairie, now mostly cropland. Before cultivation, prairie life from the Great Plains ranged southward to the Gulf of Mexico. Most forest and woodland creatures can't cross prairie or cropland, although they traversed the land in a trans-regional forest fifteen thousand years ago. Today, such species as Shumard oak and tufted titmouse travel in wooded riverside corridors through the central Texas meltdown.