Higher Order Interactions

Symbiont cooperation protects corals under stress

Community ecology has long sought to understand structurally diverse ecosystems by decomposing communities into sets of pairwise interactions. However, summing the effects of these pairwise interactions to predict a whole community response has remained challenging. The leading hypothesis for the challenges associated with this approach is the existence of Higher-Order Interactions (HOI), whereby one species affects the strength of interactions between two other species. The goal of studying these interactions is to quantify the prevalence HOIs, their underlying mechanisms, and to understand their consequences on community structure and dynamics. To date, nearly all studies describing the existence of HOIs have so identified them by isolating small groups of species from the greater community (e.g., two predators and one prey), and the use of modules has been key to gaining greater insight into trait and density mediated mechanisms driving HOIs. Modules are thought to be recurrent and empirically manageable structures within the tangled web of interactions found in diverse ecosystems; however, our understanding of whether or how the strength and direction of interactions measured at the module level operate similarly in diverse natural systems remains largely untested.

My research in this area has been heavily focused on developing and testing null models for HOIs involving predation and mutualism. Our capacity to detect HOIs at either the module or whole community level is limited by the null models used to compare the expected effects of two guild members acting independently to the observed effects when HOIs, e.g., intraguild dynamics, are allowed to occur. Higher Order Interactions among predators are driven by direct predator-predator interactions (e.g. antagonism from interference or intraguild predation, or synergy from cooperative hunting), as well as indirectly through common or conflicting behavioral responses of prey to predators. While dozens of experiments have tested the prevalence of HOIs among predators, the classic null models used to evaluate these studies are restricted to a limited set of assumptions. During my PhD, I developed and tested a new null model to assess the strength of intraspecific predator-predator interactions and quantify how predator foraging behavior (i.e., attack and maximum-intake rates) changes with interactions between predators. We recently used simulations to demonstrate the bias in previous null models of HOIs between predator species and proposed an alternative that provides appropriate null models for these interactions. I have also developed null models for HOIs and tested them to show the benefits corals receive from a diverse community of crustacean mutualists that remove sediment and defend corals from predatory seastars.


McCoy M.W., Stier A.C., and C.W. Osenberg. 2012. Predicting emergent effects of multiple predators on prey survival: The importance of depletion and the functional response.  Ecology Letters. 15: 1449-1456. PDF

Stier A.C.= and Leray M.=. Multiple predator effects drive community organization of reef invertebrates. Coral Reefs. 10.1007/s00338-013-1077-2. PDF

Stier A.C.Δ, M.A. Gil Δ, C.S. McKeon Δ, S. Lemer‡, M. Leray‡, S.C. Mills‡, and C.W. Osenberg. Housekeeping mutualisms: do more symbionts facilitate host performance. PLoS ONE. PDF in press

Stier A.C., McKeon C.S.G, Osenberg C.W. and J.S. Shima. 2010. Guard Crabs Alleviate Deleterious Effects of Vermetid Snails on a Branching Coral. Coral Reefs. 29 (4) 1019 – 1022. PDF

McKeon C.S.G, Stier A.C., and Boyer S.E.* Multiple Defender Effects: Synergistic coral defense by exosymbiotic crustaceans. Oecologia. PDF in press