Seed-caching by heteromyid rodents enhances seedling survival of a desert grass, Indian ricegrass (Achnatherum hymenoides)

Background/Question/Methods

Seeds of many plant species germinate and establish aggregated clusters of seedlings from shallowly buried seed caches (i.e., scatterhoards) made by granivorous animals. Scatterhoarding by desert heteromyid rodents facilitates the vast majority of seedling recruitment in Indian ricegrass (Achnatherum hymenoides), but any benefits resulting from rodent seed dispersal depend on subsequent survival of clustered seedlings. In previous studies, we monitored survival of Indian ricegrass seedlings that emerged from scatterhoards and seedlings growing singly at two western Nevada study sites, Flanigan and Hot Springs Mountains. We counted clumped seedlings from rodent scatterhoards and single seedlings in fenced plots at Flanigan for a year beginning shortly after seedling emergence in 1993, and monitored survival of 75 seedling clumps from rodent scatterhoards and 75 paired single Indian ricegrass seedlings at Hot Springs Mountains from June 2005 to May 2006. We also followed up these observational studies with a 2009-10 field experiment at both sites in which we planted Indian ricegrass seedlings inside fenced plots either singly or in clumps of 25 or 35 seedlings and compared survival through their first winter. Here, we use survival analysis to compare post-establishment survival of clumped versus single Indian ricegrass seedlings in each of these studies.

Results/Conclusions

Survival curves differed significantly between single and clumped seedlings in all three studies, and clumping enhanced survival in most cases. At Flanigan, individual seedlings within clumps generally had higher survival rates from April 1993 to March 1994 than seedlings growing singly. Although there was an overall inverse effect of clump size on seedling survival, one or more seedlings survived its first year in nearly all clumps. Individual seedlings within intermediately-sized clumps of 41-60 seedlings had significantly higher survival (mean = 56%) than seedlings in smaller or larger clumps (< 45%), implying a fitness benefit resulting from clumping. Seedlings were not counted within caches in the 2005-06 Hot Springs Mountains study, but whole caches had significantly higher survival than single seedlings. Likewise, in the experimental study, clumped seedlings transplanted to both sites had higher survival than single seedlings, and clumps of 25 seedlings generally had the highest survival. Taken together, these results provide further evidence of a mutualism involving heteromyid rodents and Indian ricegrass. Indian ricegrass seeds provide a highly preferred food resource to heteromyids, and benefits to Indian ricegrass can extend beyond seed dispersal and seedling establishment and into the longer-term survival of the plant.