HERBIVORY, SEED DISPERSAL, AND THE DISTRIBUTION OF A RUDERAL PLANT LIVING IN A NATURAL HABITAT

Geranium purpureum is a winter-annual, self-pollinating herb common in anthropogenic habitats of southern Spain. It is rare, however, in natural habitats such as the open woodland-scrub vegetation of Donana, where populations are fragmented into patches associated with the dominant tree in the area. Factors limiting the fecundity and growth of individuals and, hence, population spread and persistence, were studied by combining seedling transplants, herbivore exclosures, and data on seed dispersal distances. Results indicate that the association is not due to properties of the soil under trees. Rather, the observed distribution results from the combined effects of intense herbivory in inter-tree areas, and a relatively inefficient (ballistic) mechanism of seed dispersal. The spatially fragmented structure of the population is discussed in relation to the ballistic system of dispersal, and the occasional ingestion of seeds by vertebrates.