What are the individual effects of small mammals vs. lagomorphs on a semiarid ephemeral plant community?

Background/Question/Methods

In 1989 we initiated an LTER in Quebrada de las Vacas, Bosque Fray Jorge National Park, in north-central Chile: we focused on the following: 1) what is the role of biotic interactions such as vertebrate predation, interspecific competition, and herbivory on components of the semiarid community here?; 2) how do abiotic events such as aperiodic ENSOs alter the role of such interactions?; and 3) What are the individual effects of small mammals vs. lagomorphs on the plant community? To address the last question we have maintained five treatments (4 - 0.56 ha replicates ea.) since 2008.  These include control plots (C) with open access to all small mammals and predators, and four biotic treatments: i.e., predator exclusion plots with access to all small mammals (-P); exclusion of small mammals (-SM); exclusion of lagomorphs (rabbits and hares) (-L); and exclusion of small mammals + lagomorphs (–SM-L). We inventoried plant cover of ephemeral plants (annual and geophytes) in the experimental plots during growing season. Financed by several FONDECYT grants (most recent: N° 1110228), and the National Science Foundation (most recently: DEB 0948583 to DAK and 0978224 to PLM).

Results/Conclusions

Total ephemeral plant cover was strongly related to annual rainfall. Except for 2012, when both lagomorph and predator exclusion plots had lower plant cover, no other treatment effects were found. However, at the species level, several differences in cover were found: Bromus berterianus had higher cover in–SM, and –SM-L treatments, and Moscharia pinnatifida exhibited higher cover in the –SM plots but not in the –L or –SM-L plots. M. pinnatifida is restricted to sites under shrub canopies where small mammals forage. Finally, in the most recent years of analysis (2011 and 2012) the most abundant annual species in open microhabitats, Plantago hispidula, exhibited higher cover in–SM and –SM-L plots than in control plots. All three plant species had lower cover values in the –P plots. This may be due to higher numbers of small mammals in these plots, especially in high rainfall years.  Hence, the total cover of ephemeral plants generally tracked the rainfall regime, and overall, there were few differences among treatments. However, some plant species clearly responded to herbivore exclusions, most notably small mammals,  and (with a time lag)  lagomorphs.