Contemporary patterns of vegetation seen in the region (e.g., Davis and Spicer, 1965) result from a variety of factors including climatic history, recent climatic trends, and grazing and ranching practices that include barbed wire fences to modern herbicides for shrub control. Unfortunately, we have little scientific understanding of the basic biological and geochemical (e.g., Jones and Woodmanse, 1979) processes which maintain the productivity and diversity of natural communities in this sub-tropical "steppe" ecosystem. Even its geographical position raises important questions which cannot be adequately answered by climatology (Norwine, 1978).
This semi-arid, shrub-grassland on the Rio Grande Plain is also distinct from similar ecosystems in northwest Africa and with respect to the increased involvement of energy intensive technology in managing the vegetation for grazing. The region is being transformed so rapidly that the opportunity to study and understand this part of the Tamaulipan biotic province at the community and ecosystem levels is quickly fading. Long established, natural ecological links and networks are being disrupted, uncoupled, and destroyed. Since the region possesses no large habitat reserves in Texas or Mexico, conservation of natural systems falls to private landowners, many of whom see the native woody vegetation (mesquite in particular) as totally detrimental to ranching. In a worldwide survey (IUCN, UNEP, WWF, 1980), the Tamaulipan province was identified as a high priority area for the establishment of protected areas. Meanwhile, maintenance of natural habitat for the hunting of white-tailed deer and other game accounts for much of the remaining natural habitat north of the Rio Grande.
Unfortunately most research applied to range management in this region suffers not only from the general lack of basic biological information on the region, but also from the acceptance of firmly held popular views about the history and place of woody plants like mesquite in the region. To quote a 1959 Saturday Evening Post article entitled "Texas grass is coming back," about brush control in Dimmit County:
...the thickets of South Texas are no modest collection of shrubbery, but rather a vicious invading jungle that has engulfed millions of acres of once-rich grasslands, described in early days as "the best wild pastures in the world," bringing calamity that could be equaled only if the state's oil wells went dry. For more than half a century hard-pressed cattlemen have tried to beat back this creeping blight of spiny trees and cactus, only to see it spread, driving out the prairie grass, leaving the soil as bare and unproductive as an old stove lid.
This quote is, of course, a journalist's impression of the prevailing folk wisdom about shrubs and grasslands in the Rio Grande Plain, but over the years, this folk wisdom has shaped the kinds of questions asked by range scientists researching in the region, and has been reflected in the rhetoric used in connection with mesquite and other woody species. Such rhetoric tends to suppress very basic, yet unanswered, questions such as what part mesquite plays in the nitrogen economy of the system.
Likewise, range scientists, at least in practice, seem to regard mesquite as an invader of shrub-free grasslands even in regions, such as the Rio Grande Plain, where all available historical evidence implicates mesquite as a dominant native species which has naturally fluctuated in density through time and from place to place (Inglis, 1962; Lehmann, 1969). Should mesquite and other woody species be managed differently where they are clearly elements of climax vegetation? This is another question which is perhaps slighted by the location of brush control research centers in true prairie regions where mesquite clearly has invaded (but probably locally, from stream bottoms). Basically once mesquite has been tagged a "noxious" species, an assumption has been made about its biology which directs research, however careful and rigorous, away from many key questions critical to both biological conservation and economic uses of the ecosystem.
Since the fight against mesquite is practically a religious war, it is almost heresy, if not lunacy, to suggest "the enemy" may have redeeming qualities from a ranching perspective. Yet my own experience in South Texas, an ever increasing international literature on the ecology of arid environments, and data from California and Arizona on the nitrogen fixing ability of mesquite (Felker and Clark, 1980) indicate that the prevailing attitudes and approaches are, at best, simplistic and inefficient, and at worst may be sacrificing the ecosystem and its long term economic potential for short term ends.