Desert Rodent Activity: Response to Seed Production by Two Perennial Plant Species

Abstract

Activity of heteromyid rodents in the Sonoran Desert of Arizona [USA] was measured around 2 different sources of seed production to test the hypothesis that the spatial distribution of rodent activity was contingent on current seed production. In early July, 1979, fruit fall of saguaro cactus (Carnegiea gigantea) had peaked and by Aug. it had terminated. Fruit fall of prickly pear cactus (Opuntia engelmannii) began in late July and was extensive in Aug. During these 2 mo., rodent activity, as revealed by smoked paper tracking, did not change around 7 saguaro, but increased significantly around 6 monitored patches of Opuntia. In Aug. and Sept., there was a large increase in the proportion (60% and 75%, respectively) of cheek pouches containing Opuntia seeds. Collection of saguaro seeds in July increased (20%) from April and May levels, but not to the extent shown by prickly pear later in the summer. The Opuntia seed resource was extensively collected during seed production and rodent activity increased around Opuntia patches. The diversity of seeds in the cheek pouches of rodents was compared to the cumulative total diversity of seeds collected by rodents during the spring and summer. This comparison tests the hypothesis that rodents collect a restricted variety of seed species during a limited period of time. The results showed that mean seed species diversity in cheek pouches was significantly less than cumulative seed species diversity. This pattern held for 4 spp. of heteromyid rodent [Dipodomys merriami, Perognathus amplus, P. baileyi, P. penicillatus] and is consistent with the opportunistic exploitation of current seed production.