Size‐dependent Herbivore Attack and the Demography of an Arid Grassland Shrub

I examined the demographic impact of two specialist insect herbivores on an arid grassland population of the shrub Gutierrezia microcephala in central New Mexico. The grasshopper Hesperotettix viridis may defoliate plants of all sizes, while the root—boring cerambycid beetle Crossidius pulchellus exploits only large, mature plants. During a year of high H. viridis population density (1980), no G. microcephala escaped severe defoliation regardless of size or distance from conspecifics. Over 80% of the plants failed to flower, and mean plant fecundity was over 400 times lower than that of plants experimentally protected from H. viridis attack. Juvenile plants were frequently killed by H. viridis, but only 7% of mature plants of intermediate size (20—30 cm canopy diameter) died following defoliation in 1980. Larger plants had higher mortality rates; this increased risk of death was correlated with a higher incidence of beetle attack in larger plants. Although defoliation virtually eliminated seed production in 1980, seedling abundance in the following spring was not measurably depressed. In addition, G. microcephala seeds germinated abundantly in small areas from which the established vegetation had been experimentally removed. Carry—over of seeds produced in previous years may this provide a source of recruits, despite the occasionally severe impact of grasshoppers on plant fecundity. The local age structure of G. microcephala, obtained by counts of annual growth rings in root crowns, highlights two phenomena that make G. microephala populations vulnerable to local extinction: (1) frequent failures of seedling recruitment (due to drought, grasshopper attack, and competing plants) and (2) the limited longevity of large mature plants, indicated by the rarity of plants older than one decade. Two critical life history stages buffer G. microcephala from these threats to population persistence: the seed pool, which permits opportunistic seedling recruitment during favorable periods; and mature plants of intermediate size, which are resistant to attack by both major herbivores.