Population Age Structure and Its Relationship to the Maintenance of a Semidesert Grassland Undergoing Invasion by Mesquite

Abstract

Data from long-term chart quadrats recorded from the Jornada Experimental Range in south-central New Mexico were used to develop demographic measurements for black grama and mesa dropseed. These measurements included mean and maximum lifespans, survival proba- bilities, and rates of establishment and were used to derive the stable age structure for black grama. They were combined with factors such as climate and grazing in a model designed to sim- ulate the long-term dynamics of a stable perennial grassland. Using the stable age structure as a starting point, the model was perturbed by differentially lowering the age-specific survival rates in an effort to mimic the effect of mesquite competition. A stable age structure for black grama was achieved after a period of 40 years. A similar structure for mesa dropseed could not be derived. Output of the model run over a 47-year period in the absence of mesquite shows that stability of numbers in each class can be achieved over the entire time period. Grazing does not appear to disrupt this stability, but only decreases the number of plants established each year. Simulation of increased mesquite competition showed that in the first 12 years there may be a concomitant decline in the number of new individuals. By the 25th year, the survival rates for all but the oldest individuals have been adversely affected and there are few individuals alive.