RABBIT HERBIVORY AND ITS EFFECT ON CLADODE, FLOWER AND FRUIT PRODUCTION OF OPUNTIA-VIOLACEA VAR MACROCENTRA (CACTACEAE) IN THE NORTHERN CHIHUAHUAN DESERT, NEW-MEXICO

We Studied the timing and effect of black-tailed jackrabbit and desert cottontail herbivory on a platyopuntia species, Opuntia violacea var macrocentra at a site on the Jornada del Muerto, 35 km north of Las Cr uces, New Mexico in the northern Chihuahuan Desert. Total monthly feca l pellet dry mass collected around the base of individual opuntias ove r 15 months indicates that both rabbit species exhibit a seasonal pref erence for this opuntia with the greatest grazing pressure evident dur ing dry seasons or when little annual or new growth of other perennial plants is present. Neither rabbit species appears to graze small opun tias possessing less than three cladodes. They also appear to avoid gr azing new cladodes, since more than 80% of the cladodes produced in Ma y 1990 survived for six months. The proportion of individual opuntias grazed increases with increasing cladode number size class as does the mean fecal-pellet dry mass collected around the base of individual op untias. Spinescence did not affect grazing intensity over the 15-month study period. Instead, plant size and prating history appear to be th e most important determinants of grazing intensity. Although statistic ally not significant, there is a trend in our data which suggests that above-average rabbit herbivory may negatively affect cladode, flower and fruit production in intermediate opuntia size-classes only. For in dividuals with either very few or very many cladodes these parameters appear unaffected by rabbit herbivory. However, the strongly-skewed si ze-class frequency distribution indicates that recruitment of juvenile opuntias into the population is little affected by rabbit herbivory.