The population dynamics and distribution of the exotic grass, Bothriochloa ischaemum

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Gabbard, Bethany Lynn

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I investigate the population dynamics and distribution of the exotic grass, Bothriochloa ischaemum. I conducted a set of surveys to which habitat types are occupied by B. ischaemum in central Texas, to determine the extent to which disturbances are necessary to its spread, and to measure its effects on plant diversity. The only habitat in which B. ischaemum was never found was under the canopies of woody plants. It was more common in plots near roads, probably because roads facilitate seed dispersal. Grazing and fire history did not affect the distribution of this species. B. ischaemum-dominated plots had lower species diversity and species richness than plots without B. ischaemum. The effects of simulated grazing on B. ischaemum were studied to determine how grazing affects plant size, morphology, survival, fecundity, and ‘population’ growth rate. In a natural population of this species, half of the study plots were unclipped and all plants in the remaining plots were clipped to ‘bite height’ every two weeks for two years to mimic cattle grazing. Clipping decreased the average above-ground dry biomass per plant. After two years of clipping, clipped plants had lower survival rates. Clipped plants never set seed. Neighboring unclipped plants provided a source of seeds in clipped plots. Despite the lower seed input, recruitment tended to be higher in clipped plots than in unclipped plots perhaps due to increased seedling survival. Clipping did not affect ‘population’ growth rate. Increased recruitment levels seem to have compensated for the negative effect of clipping on survival. Size-based population projection matrices were constructed and used to determine the potential population growth rate, potential stable population size distribution, and expected individual lifespan of clipped and unclipped plants. Elasticities and the proportional change in each matrix element caused by the clipping treatment were also calculated. After two years of clipping treatments, clipped plots had a lower potential population growth rate, proportionally fewer small individuals, and a shorter expected lifespan than unclipped plots. The ability of adults to survive and maintain their size was more important for the maintenance of λeig than was fecundity.




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