Local Extinction in Perennial Grasslands: A Life-History Approach

Abstract

A common element of many perennial grasslands, defined as an extinction-prone perennial grass, is a palatable obligate seed reproducer, producing low numbers of larger diaspores that are poorly dispersed. Such a species can become locally extinct on a regional or landscape scale in grasslands that experience both a variable rainfall, in which drought is characteristic, and sustained heavy grazing. The changes in the temporal and spatial patterns of grazing in human-influenced systems has created the possibility of local extinction. Catastrophic mortality of established populations occurs as a consequence of droughts in combination with sustained grazing, which eventually leads to the elimination of the population. Elimination of the established population is more likely to occur in semiarid than moist areas because of the greater frequency and duration of droughts. The maintenance or reestablishment of the population depends on successful recruitment, but seedling mortality is high because of intraseasonal drought and trampling. The seed bank of an extinction-prone species is rapidly eliminated if the input of seed is curtailed. These species do not form persistent seed banks because the seeds have a limited (2-3 yr) intrinsic capacity for survival when in a state of induced or enforced dormancy. Innate dormancy lasts for only a few months. Therefore, the size of the seed bank is determined primarily by the annual input of seed. But even under favorable rainfall and light grazing, 100% of seed produced can be lost to predation. Seed production can fail from drought and from the direct consumption of inflorescences. An extinction-prone species is self-dispersed and therefore poorly dispersed. The rate and scale of incidental animal dispersal are unlikely to assist in the maintenance or reestablishment of a population because of the low probability of seedling survival. The decline of an extinction-prone species under heavy grazing is compounded by the concomitant competitive influences of invasive species and unpalatable species, which both benefit from heavy grazing. Irreversible changes in the physicochemical environment may also result in local extinction, but these processes are not mutually exclusive. The autogenic model of secondary succession is inadequate for these grasslands if the present concept of local extinction is upheld.