How collectively integrated are ecological communities?

Abstract

Are the population dynamics of species mainly determined by direct interactions with predators, preys and conspecifics? Or, instead, are those dynamics more dependent on indirect feedbacks that ripple across the whole interaction network? Here we show that, from a basic spectral feature of the interaction network, we can predict the length of indirect interaction pathways that contribute to community-level dynamical patterns, such as the depth of a perturbation's reach, or the contribution of biotic processes to realized species niches. In doing so, we propose a measure of collectivity that integrates existing approaches to community complexity, collective integration and indirect interactions. By revisiting classic concepts of theoretical community ecology, our work proposes an original perspective on the question of to what degree communities are more than loose collections of species or simple interaction motifs. This perspective can help clarify when reductionist approaches, focusing on particular species and small interaction motifs, ought to suffice or fail when applied to ecosystems.

Getting More by Asking for Less: Linking Species Interactions to Species Co-Distributions in Metacommunities

“…Yet, the ‘net effects’ between species, defined to include all indirect impacts arising over time and through intermediates (e.g. indirectly helping a species by directly hindering its competitors), are found to be highly context-dependent as soon as interaction strength is not very small (Zelnik et al ., 2022). On the other hand, our choice of looking at long-term abundance patterns, letting species reach an equilibrium, is giving maximal opportunity for such indirect effects to play out – even a rare or slow-growing species has time to exert or mediate significant impacts on others, hence the fact that the matrix of net effects can change drastically when we remove a species, no matter how rare.…”

“…While it may seem counter-intuitive that, say, net effects between two dominant species might change drastically even from adding a rare third species, it is important to point out that these effects are fully realized only in the long term: the inverse matrix appears naturally when computing equilibrium abundances (4) so that even a rare or slow-growing species has time to exert or mediate significant impacts on others. Observing abundances out-of-equilibrium could lessen the importance of very indirect paths, and limit context dependence (Zelnik et al ., 2024).…”

“…The former have been termed ‘direct effects’, and the latter ‘net effects’ (Lawlor, 1979). Even if the direct effects of one species on another’s dynamics were context-independent, its net effects on abundances in the long term can be mediated many other species, along chains of indirect impacts playing out over time, and can thus depend on the whole community’s composition (Schaffer, 1981; Zelnik et al ., 2024). We must therefore carefully define direct and net effects and specify which we are trying to infer.…”

Chemical Mixtures and Multiple Stressors: Same but Different?

“…Trophic (Eklöf et al, 2013) and other (Kéfi et al, 2016) interactions generate a complex network of species interactions. These networks are typically well connected, such that direct effects of a stressor on a species propagate to other species (Zelnik et al, 2022). Importantly, indirect effects can overweight or reverse direct effects (Fleeger, 2020; Spaak et al, 2017).…”

Plant interaction networks reveal the limits of our understanding of diversity maintenance

“…Consequently, various limiting assumptions which may not hold in empirical communities still underpin and co‐construct current mathematical frameworks for predicting diversity patterns and must be challenged together. These assumptions include: that interactions cannot be facilitative, in the sense that species have important positive effects on their or another species' growth rate (Bruno et al., 2003); that interactions are constant or vary linearly across environments and modelling them as such reliably captures the relevant processes driving diversity (Bimler et al., 2018); and that diversity can be treated as an extension of coexistence between sets of species pairs (Barabás et al., 2016; Levine et al., 2017), rather than as an emergent process that is not reflective of additive pairwise outcomes alone (Maynard et al., 2017; Zelnik et al., 2022).…”