Genesis, evolution, and trends of a large-scale field manipulation in semiarid north-central Chile

Background/Question/Methods

In the 1980’s, much attention was focused on the role of biotic interactions as regulatory factors in communities.  Preliminary data on small mammals at a semiarid site in north-central Chile suggested the importance of top-down biotic interactions including predation and interspecific competition.  In 1989, we initiated a large-scale ecological experiment in a thorn scrub community using a reductionist approach. Fenced exclosures were installed to selectively exclude vertebrate predators (raptors and foxes), and putative competitors/herbivores (a medium-sized rodent, the Chilean degu) from replicated 0.56 ha grids.  Small mammals have been inventoried monthly using mark-recapture techniques.  Additional metrics collected include predator activity and diets, perennial shrub and ephemeral cover, and soil seed densities.  Over time, we expanded our work with food addition experiments in 1996-1999, and monitoring of other organismal groups including lizards, insects, birds, and lagomorphs [rabbits and hares]). Manipulations were later directed to address the putative role of lagomorphs as well as potential interactions among herbivores in the community.  On this 25^thanniversary, we reexamine the major results as well as the future directions in this ongoing project.  Other presentations in this session will address the role of biotic as well as environmental factors. 

Results/Conclusions

Although transitory effects of predation, as well as indirect facilitation by degus were detected, bottom-up environmental factors play a major, overriding role.  These are tied to varying rainfall which in turn is strongly affected by El Niño Southern Oscillations (ENSO).  Rainfall has dramatic effects on the ephemeral plant community, and thus, resources in the community; food addition experiments verified the importance of resources for herbivores during droughts.  High rainfall events trigger percolating-upward increases in their predators.  Since 2000 there has been a pattern of higher, less variable precipitation here, which has led to a major shift in the small mammal assemblage.  Degus now comprise a consistently higher proportion of the small mammal biomass, and there has been less year-to-year variation in assemblage composition and diversity.  As degu foraging activities indirectly facilitate invasion of exotic ephemerals, this has the potential to greatly alter local community structure.  Such changes may constitute a critical transition (sensu Scheffer), and in turn be a product of ongoing climate change. Work continues to elucidate these consequences, and their implications for the ecology and sustainability of the north-central semiarid zone in Chile.