Facilitation Cascades

Foundation species, such as kelps and trees, are defined as organisms that ameliorate physical (i.e., desiccation, heat) and biological (i.e., competition, predation) stress to enhance species diversity and productivity. To date, experimental studies have revealed direct, obligate interactions between foundation species and associated organisms in a variety of ecosystems, suggesting the presence of a single, dominant facilitator can give rise to an entire community.

Many ecosystems, however, are structured by multiple foundation species (i.e., corals-sponges, macroalgae-clams), whose interactions can produce emergent effects on community structure and diversity. My collaborative study in New England cobble beaches (Altieri, Silliman, Bertness 2007, American Naturalist), for example, found that facilitation cascades arise when an independent foundation species, cordgrass, harbors a second, dependent foundation species, mussels, and combined they generate conditions amenable to other plants and animals. Although this was the first-ever experimental demonstration of a facilitation cascade, these positive interactions among foundation species, or facilitation cascades, may be pervasive phenomena that organize communities, and explain biodiversity patterns on regional scales. Over the past three years, experimental work in my lab (Angelini and Silliman 2015, Ecology) in coastal oak tree hammocks along the Southeastern Atlantic coast provides evidence for this possibility, and reveals that the rich diversity of micro-animals (insects, spiders, reptiles) associated with oak tree canopies are in large part maintained via a facilitation cascade, where live oak trees (Quercus virginiana) facilitate Spanish moss (by ameliorating heat stress), which in turn facilitate local insect communities by protecting them from both predation and desiccation stress. Over the past 5 years, we have tested for occurrence of facilitation cascades and their impacts on food web structure in mangrove-oyster and bivalve-seagrass systems (Tjisse, H.,et al, 2016, Proceedings of the Royal Society), shown that facilitation cascades regulate marsh ecosystem function and these impacts are density dependent (Angelini et al, 2015, Proceedings of the Royal Society) and tested for global meta-analysis (Thomsen et al, in revision, Nature Ecology and Evolution). Combined, these results are supporting the idea show that positive interactions hierarchically organize associated animal communities, an idea not currently incorporated into ecological theory.