Effects of seed-caching desert rodents on seedling survival of Indian ricegrass(Achnatherum hymenoides)

Background/Question/Methods

In addition to consuming seeds, many granivorous small mammals also cache seeds in shallowly buried scatterhoards, and seeds of many plant species are known to germinate and establish aggregated clusters of seedlings from these caches.  Scatterhoards made by desert heteromyid rodents provide the primary source of seedling recruitment for Indian ricegrass (Achnatherum hymenoides), a perennial bunchgrass species occurring widely across the southwestern deserts of North America, but effects of the resulting clumping of seedlings on subsequent survival have not been quantified under field conditions.  I compared survival rate curves of clumped versus single Indian ricegrass seedlings over their first year at two western Nevada study sites (Flanigan and Hot Springs Mountains) to test the null hypothesis that clumping of seedlings has no effect on the survival component of fitness.  At Flanigan, I counted single seedlings and seedlings within clumps (i.e., scatterhoards) on 4 fenced plots weekly from April (shortly after their emergence) to November and again the following March.  At the Hot Springs Mountains site, I followed and compared the survival status of 75 clumps and 75 paired single Indian ricegrass seedlings over their first year from June 2005 to May 2006.

Results/Conclusions

Individual seedlings within clumps at Flanigan had significantly higher survival rates than seedlings growing singly in 2 of 4 plots, but single seedlings had higher survival than seedlings in clumps on another plot.  There was an overall inverse effect of clump size on seedling survival.  However, one or more seedlings survived its first year in nearly all clumps, and individual seedlings within intermediately-sized clumps of 41-60 seedlings had significantly greater survival (56%) than those in smaller or larger clusters (all < 45%), suggesting that clumping of seedlings may benefit fitness.  Although individual seedlings within caches were not counted at the Hot Springs Mountains study site, whole caches had significantly higher survival curve functions than single seedlings.  Overall, results suggest that benefits accruing to Indian ricegrass due to seed-caching desert rodents often extend past the seedling establishment phase and into the longer-term survival of the plant.