Ecology of the Opuntiae in the Big Bend region of Texas

Anthony, Margery

Opuntia populations were studied in Jeff Davis, Presidio, and Brewster counties, with the latter receiving the most detailed consideration. Great variety of topography and resultant localization of soil and moisture conditions have produced a corresponding diversity of habits. These are occupied by at least thirty-six plant communities of which four feature cacti as follows: cresote-tasajillo; Coryphantha-Opuntia; tobosa-platysopuntia; and short grass-cholla associations. Thirty-one species, varieties, and hybrids of Opuntia are found in the area. Elevation, through its concurrent effect on climate and drainage constitute the most evident factors in local distribution of Opuntiae. Type of country rock, texture of soil, degree and exposure of slope, and amount of grazing are also determining factors. Therefore, local distribution of Opuntiae can be closely correlated with the plant associations since they are an accurate index to topographic and climatic conditions. The richest cactus flora is developed on high gravelly desert plains in Big Bend National Park. Vicissitudes of change, such as transportation by torrential drainage or by animals, account for occasional isolates not in the usual habitats. Desert eminences have produced well-defined races and disjunct distributions of Opuntiae through isolation effect. Prevalence of vegetative reproduction among Opuntinae is a very significant factor explaining their: 1) rapid invasion and establishment in newly available habitats, 2) general ubiquity, and 3) variety of forms. Animals browsing and eating fruits, overgrazing, and disturbance around habitations stimulate increase in local populations of Opuntiae. Proximity of four adjacent biotic provinces has made the Big Bend region a meeting ground for woodland, high plains, arid grassland and desert forms of Opuntiae. Most of the Opuntiae with restricted distribution in the Big Bend region also have a relatively small area of general distribution, indicating low degrees of ecological variability or relatively new species. Joints of Opuntiae which look very different in the field may, when grown in a greenhouse, produce new joints lacking such differences. This shows that those dissimilar forms all belong to one taxonomic entity since their new growth is identical. Three of the species are considered as polymorphous complexes, having intergrading variants within each group.

Ecology of the Opuntiae in the Big Bend region of Texas

Series