The Big Ifs in The Outcomes of Species Interactions: Review and Insights from the Interaction Function (IF) Model

Abstract

The conceptual framework for species interactions calls for a new perspective: conditional outcomes are not rare; but, rather, closer to the norm. Interactions may shift or switch between beneficial and detrimental following variations in the (context-dependent) balance of costs and benefits involved for each species. A classic (simple) model such as a Lotka-Volterra can be adjusted to represent these (complex) dynamics by introducing the appropriate nonlinearities in the interspecific terms. The Interaction Function (IF) model introduces a density-dependent function that takes values along the negative to positive continuum; a change in sign denotes a shift in the effect of one species on the other. In this paper I discuss the foundations and development of the IF model, along with other models and conceptual bodies that have emerged on the topic. A brief advance on the central results: (i) the systems present multiple stable equilibria where species may coexist at any combination of interaction outcomes (+ , – , 0), or one be excluded, (ii) transitions between outcomes occur either gradually (environmental conditions vary) or abruptly (drastic change in population abundances leads to another domain of attraction); catastrophic jumps are also possibilities, (iii) fragmentation of space and dispersal trigger source-sink dynamics that cause further variation in the outcomes at local and regional levels, and, remarkably, dynamic variations in the sink or source roles of localities and populations also occur. The fundamental conclusion is that the beneficial or detrimental role of a species in an association is a spatiotemporal dynamic quality, determined by the balance of many concurrent density-dependent effects, some reinforcing, some counteracting, one another.