Apparent competition in the Great Basin Desert: A test of small mammal facilitation of invasion by cheatgrass

Background/Question/Methods

Biological invasions are detrimental to biodiversity, ecosystem quality, and economic output.  Several hypotheses have been posited to explain mechanisms leading to invasion, including the Apparent Competition Hypothesis (ACH).  The ACH suggests that indirect effects of enemies shared by native and potentially invasive species may facilitate the process of invasion.  More specifically, the shared enemy must benefit by the presence of the invasive species, thereby increasing the relative negative effect on the native species.  Potential benefits to the enemy can be food-mediated, refuge-mediated, or behavior-mediated.  Our objective was to determine if small mammals facilitate invasion of cheatgrass in the Great Basin via food-mediated, refuge-mediated, or behavioral-mediated apparent competition.  To test for food-mediated competition, we employed a BACI design where we provided cheatgrass seed, ad libitum, to experimental plots and compared small mammal communities in experimental plots to communities in control plots. To test for refuge-mediated competition, we sampled small mammal communities at 30 sites that varied in cheatgrass coverage from very low to very high.  To test for behavioral-mediated competition, we performed modified Giving-Up Density (GUD) experiments at 30 randomly selected sites that varied in cheatgrass coverage from very low to very high. 

Results/Conclusions

Small mammal communities and abundances were not different between food-supplemented and control sites.   In addition, both small mammal abundance and diversity declined with degree of invasion by cheatgrass.  In contrast, Giving-Up Densities of native seeds increased with level of invasion by cheatgrass and there were no interactive effects of small mammal abundance and level of invasion.  Predictions based on food-mediated, refuge-mediated, and behavioral-mediated apparent competition were not supported by our data.  Therefore, we found no evidence that apparent competition is facilitating the invasion of cheatgrass in the Great Basin.