Apparent competition among the seeds of desert plants: Causes and consequences

Rodents in the family Heteromyidae are highly granivorous and as such, they can induce apparent competition among the seeds of different plant species. In general terms, apparent competition between two prey species is the decreased survival (due to predation) of one or both prey species when in the presence of the other prey species as compared to survival when the other prey species is absent. Apparent competition between seeds is the decreased survival of seeds in patches containing two or more seed species compared to patches containing only one seed species. By measuring rates of seed removal from trays, I documented apparent competition between seeds of Indian ricegrass (Oryzopsis hymenoides) and seeds of other less common desert plants. Behavioral studies of captive rodents revealed that both seed preference and density-dependent foraging caused apparent competition. I developed a patch-occupancy model to examine the consequences of apparent competition for the coexistence of adult plants in small individual-plant-sized patches. The model revealed that apparent competition as documented in my study was strong enough to decrease the frequency of coexistence if competition among seedlings was limited to conspecifics. Competition among heterospecific seedlings generally overwhelmed the effect produced by apparent competition among seeds. Under some conditions, apparent competition among seeds does affect the spatial structure of plant communities. The effects of apparent competition between other species of prey in other types of ecosystems should be examined further and the behaviors of the predator species should be studied so as to understand the causes of apparent competition.