SEEDLING RECRUITMENT INORYZOPSIS HYMENOIDES: ARE DESERT GRANIVORES MUTUALISTS OR PREDATORS?

Abstract

Granivorous animals that cache as well as consume seeds may actually serve as mutualists to their plant resources. Seeds of Indian ricegrass (Oryzopsis hymenoides), a perennial bunchgrass in North American deserts, are consumed by various desert granivores and dispersed by seed-caching heteromyid rodents. We used a three-way factorial design at a western Nevada site to selectively exclude or allow access to experimental plots by granivorous rodents and seed harvester ants, and to subsequently follow the fate of radiolabeled Indian ricegrass seeds introduced to the plots. In addition to the presence or absence of rodents or ants, the third experimental treatment factor was to allow “initial caching” of the radiolabeled seeds by single Merriam's kangaroo rats (Dipodomys merriami), which were confined to certain plots for one night. Both rodents and ants larder-hoarded seeds in their burrows, but seedlings rarely established from larders. Only rodents also placed seeds in scatterhoards: shallowly buried surface caches distributed about an animal's home range. Following initial caching by a kangaroo rat, the number of seedlings established from scatterhoards was significantly greater (usually by more than an order of magnitude) than those from seeds unharvested by either type of granivore. With no initial caching, rodents reduced seedling recruitment from unharvested seeds but facilitated compensatory seedling recruitment from scatterhoards. Seeds harvested by ants seldom established seedlings. We used a seed fate model to estimate that, on average, rodents and ants harvested 96% and 7%, respectively, of seeds to which they had exclusive access, and that the probability of seedling establishment for a seed harvested by a rodent was an order of magnitude greater than from a seed harvested by an ant and slightly greater than for an unharvested seed. The predicted rank order of seedling recruitment among nine experimental treatments based on expected effects of rodent seed caching closely matched the observed ranking pattern, indicating that rodents determined seedling recruitment patterns of Indian ricegrass. Because rodents harvested such a large majority of seeds and their caches enhanced seedling establishment, they played a central role in the population dynamics of Indian ricegrass.