Caching propensities and effectiveness of coexisting heteromyid rodent species as seed dispersers of Indian ricegrass (Achnatherum hymenoides)

Authors: Longland, William - Bill; VANDER WALL, STEPHEN - University Of Nevada

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/25/2019
Publication Date: 11/25/2019
Citation: Longland, W.S., Vander Wall, S.B. 2019. Caching propensities and effectiveness of coexisting heteromyid rodent species as seed dispersers of Indian ricegrass (Achnatherum hymenoides). Western North American Naturalist. 79:523-533. https://doi.org/10.3398/064.079.0406.
DOI: https://doi.org/10.3398/064.079.0406

Interpretive Summary: Indian ricegrass is an important forage grass for livestock, particularly on arid desert winter ranges. This native grass is propagated mainly from new seedlings that emerge from shallowly buried seed caches made by desert rodents. We compared the seed caching behavior of five rodent species to determine which species are likely to cache Indian ricegrass seeds under conditions that most favor the production of new seedlings. One kangaroo rat species was ruled out as an important seed disperser as they made caches almost exclusively inside of burrows rather than making shallow surface caches that can establish new seedlings. Individual animals of a kangaroo mouse species made the most surface caches and wasted fewer seeds because they placed fewer per caches. However, another kangaroo rat species, Merriam’s kangaroo rat, is much more abundant than the kangaroo mouse. Consequently, the kangaroo mouse species is probably most effective at dispersing Indian ricegrass seeds when considering individuals, but the kangaroo rat is the most effective seed disperser at the species level. All of the species that made surface caches placed them at depths that are conducive to emergence of Indian ricegrass seedlings.

Technical Abstract: Recruitment of Indian ricegrass (Achnatherum hymenoides) seedlings occurs primarily from surface seed caches made by scatter-hoarding desert rodents of the family Heteromyidae. We used radiolabeled Indian ricegrass seeds in field seed-caching experiments with 5 coexisting heteromyid species (Dipodomys deserti, D. merriami, D. microps, Microdipodops pallidus, and Perognathus longimembris) to compare their potential effectiveness as dispersers. Dipodomys microps individuals deposited seeds almost exclusively in larders rather than in scatter hoards, implying that this species is unlikely to be an important seed disperser. Among the other species, individual caches made by larger species had more seeds than those made by smaller species, but smaller species made more caches. At the level of individual animals, M. pallidus was the most effective disperser in a quantitative sense; they made more caches than other species tested and placed fewer excess seeds in caches relative to optimal cache sizes for Indian ricegrass seedling establishment. However, because D. merriami individuals were considerably more abundant at the study site than other species and were also avid scatter hoarders, D. merriami is likely to be the most quantitatively effective disperser of Indian ricegrass seeds at the species level. Ranking species according to qualitative effectiveness (e.g., by considering effects such as the caching microsite on seedling establishment) was more ambiguous. For example, P. longimembris made relatively shallow caches that most closely match optimal planting depths for Indian ricegrass seedling emergence, but such shallow caches are probably more likely than deeper caches to be discovered by foraging rodents before they can germinate. With the possible exception of D. microps, any of the species we tested may be effective dispersers of Indian ricegrass seeds.