Ecosystem engineering by invasive beavers reduces in-stream diversity and enhances ecosystem function via habitat and trophic pathways in Cape Horn, Chile

Predicting the impacts of invasion is difficult. Examining the effects of invasive ecosystem engineers on species diversity and ecosystem function provides an opportunity to explicitly test mechanistic effects of these invaders.  Ecosystem engineers are predicted to primarily affect ecosystems via altering nutrient, trophic or habitat resources to other organisms. Here, we quantified the impacts of habitat and resource modifications caused by introduced beavers on aquatic macroinvertebrate community structure and function in Cape Horn, Chile. We compared responses to beavers in three habitats: 1) forested (unimpacted) streams, 2) beaver ponds and 3) reaches downstream of beaver dams in four streams. We found that beaver engineering in ponds created taxonomically simplified, but more productive, macroinvertebrate assemblages. Macroinvertebrate richness, diversity and number of functional feeding groups were reduced by half, while abundance, biomass and secondary production increased 3-5x in beaver ponds compared to forested sites. Reaches downstream of beaver ponds were similar to natural forested sections. Reductions in species richness as well as increased secondary production appeared to be driven by the same engineering effect: increased retention of fine particulate organic.  Ecosystem engineering appeared to affect species richness via a habitat modification pathway and affected secondary production via a carbon availability (trophic) pathway.  Engineering-induced modification of resource nutrient content was not related to macroinvertebrate response. Quantitative analysis of the effects of other ecosystem engineers via trophic, nutrient or habitat-modification pathways will potentially inform predictions concerning their effects as invasive species.