The foraging behavior of granivorous rodents and short-term apparent competition among seeds

The foraging behavior of a predator species is thought to be the cause of s hort-term apparent competition among those prey species that share the pred ator. Short-term apparent competition is the negative indirect effect that one prey species has on another prey species via its effects on predator fo raging behavior. In theory, the density-dependent foraging behavior of gran ivorous rodents and their preference for certain seeds are capable of induc ing short-term apparent competition among seed species. In this study, I ex amined the foraging behavior of two heteromyid rodent species (family; Hete romyidae). Merriam's kangaroo rats (Dipodomys merriami) and little pocket m ice (Perognathus longimembris). In one experiment I tested the preferences of both rodent species for the seeds of eight plant species. Both rodent sp ecies exhibited distinct hut variable preferences for some seeds and avoida nce of others. However, the differences in preference appeared to have only an occasional effect on the strength of the short-term apparent competitio n detected in a field experiment. In another experiment, I found that capti ve individuals of both rodent species had approximately equal foraging effo rt (i.e., time spent foraging) in patches that contained a highly preferred seed type (Oryzopsis hymenoides) regardless of seed density and the presen ce of a less preferred seed type (Astragalus cicer) in the patches. The rod ents also harvested a large proportion of O. hymenoides seeds regardless of initial seed density; this precluded a negative indirect effect of A. cice r on O. hymenoides. But there was a negative indirect effect of O hymenoide s on A. cicer caused hy rodents having a lower foraging effort in patches t hat only contained A. cicer seeds than in patches that contained A. cicer a nd O. hymenoides seeds. The indirect interaction between O. hymenoides and A. cicer thus represented a case of short-term apparent competition that wa s non-reciprocal. h lost importantly, it was caused by the foraging behavio r of the rodents.