PARENT SESSION
Poster Session #41: Plant Demography.
Thursday, August 9, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Effects of simulated grazing on the population demography of the exotic grass, Bothriochloa ischaemum.

GABBARD, BETHANY¹, FOWLER, NORMA¹, ¹

ABSTRACT- Because individual survival and fecundity are almost always highly correlated with plant size, herbivores can reduce the growth rate () of plant populations in at least three different ways: (1) by reducing average plant size; (2) by decreasing mean fecundity of plants of a given size; and (3) by decreasing the probability of individual survival in plants of a given size. In this study, the effects of simulated herbivory upon an exotic perennial grass, Bothriochloa ischaemum, were studied to determine which of these processes occurred. 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 three years to mimic cattle grazing. In each plot the size (as number of tillers) and seed set of each individual of B. ischaemum were repeatedly recorded. Clipping decreased the average biomass per plant; it increased the average number of tillers per plant but decreased the average size of a tiller even more. In unclipped plants fecundity was proportional to plant size; no clipped plants set seed. Clipping decreased the growth rate of the population (1) by altering the size distribution towards more small plants and fewer large ones and (2) by eliminating seed set (making fecundity zero), but (3) did not alter the rate of survival of plants of a given size.

KEY WORDS: Bothriochloa ischaemum, population, demography, herbivory