Small mammals in semi-arid Chile do not support energetic compensation and zero-sum ecological dynamics

Background/Question/Methods .

Understanding how species coexist remains central to ecology. Research in North American deserts provides evidence of energetic compensation and zero-sum dynamics, suggesting that species in natural assemblages may be replaced with relatively little (albeit some) impact on ecosystem function.  We tested the generality of this at our long-term site in north-central Chile by excluding a dominant small mammal (degu, Octodon degus) from replicate plots in semi-arid coastal thorn scrub habitat.  We also pursued a macroecological assessment of the likelihood of energetically equivalent species occurring in the geographic species pool for 401 sites in aridlands of North America, the Gobi Desert, and South America.  Potential energetic equivalent species were defined as trophically similar species within 10% of the weight of focal species in each assemblage, and we tallied the number of such species across all sites.

Results/Conclusions .  

Experimental removal of a rodent species at our long-term site in north-central Chile revealed no evidence for energetic or functional compensation; energetic consumption remained significantly lower on degu exclusions relative to control plots.  A review of literature from other drylands in South America and the Gobi Desert suggest that North American aridlands may be atypical in supporting large numbers of ecologically similar species.  A macroecological assessment suggests that typical sites in North American aridlands have nearly twice as many potential energetic equivalent species as South American sites, and five times as many as sites in the Gobi Desert.  This difference is due primarily to granivorous species; South American sites tend to have more potential replacements in omnivorous and folivorous guilds.  These results reflect other studies that have highlighted the novelty and importance of the radiation of heteromyid rodents in North America and the importance of historical contingency.  Moreover, this suggests that zero-sum ecological dynamics may be regional in nature and scope, necessitating clarification of the underlying causality.  This also adds to a large number of studies underscoring the importance of long-term field studies for understanding ecological dynamics as well as potential responses to climate change and other extrinsic stressors.