Effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla).

Abstract

The effectiveness of six species of rodents as dispersers of singleleaf piñon pine (Pinus monophylla) was investigated in field enclosures. The rodent species were Panamint kangaroo rat (Dipodomys panamintinus), Great Basin pocket mouse (Perognathus parvus), pinyon mouse (Peromyscus truei), deer mouse (Peromyscus maniculatus), least chipmunk (Tamias minimus), and white-tailed antelope ground squirrel (Ammospermophilus leucurus). Five measures of seed harvesting and caching behavior were assessed and used to calculate the effect of species on the probability of effective dispersal (i.e., movement of seeds to potential establishment sites) of piñon pine seeds. These measures were proportion of seeds harvested, proportion of harvested seeds that were scatter hoarded, relative dispersion of scatter-hoarded seeds, seedling emergence and seedling establishment. To evaluate the effects of the last two variables, a 2-year study of seed germination and seedling establishment was conducted in the field. Seedling survival was 72-90% for seedlings emerging from 1-4 cm deep but was markedly lower from surface caches and for depths >4 cm. All seedlings in the open microsite perished before the end of the first year, whereas 38-97% of seedlings under shrubs survived. Cache size did not affect seedling survival over the 2 years, but large caches reduced the total number of potential establishment sites. All six species harvested over 90% of the piñon seeds presented and scatter hoarded from 31% (pocket mouse) to 66% (chipmunk) of the seeds they harvested. Piñon mice and deer mice distributed seeds to more cache sites (i.e., put fewer seeds in caches) than the other species and, consequently, created more potential establishment sites. All species scatter hoarded seeds at depths suitable for seedling emergence, but pocket mice and piñon mice cached under shrubs more than the other species, the microhabitat where seedlings had the greatest probability of establishing. Factors that had the greatest effect on the probability of effective dispersal were microhabitat selection and cache size.