Editor’s note

A recently published paper in Journal of Ecology, entitled Size asymmetry of resource competition and the structure of plant communities by DeMalach et al. 2016, garnered a lot of excitement during the review process. Associate Editor James F Cahill kindly agreed to share and comment the novel findings of this paper below.

Pierre Mariotte
Blog Editor, Journal of Ecology

New Insights into Size Asymmetry of Resource Competition

The foundations of modern community ecology lie in the study of competition, coexistence, and exclusion. Many plant ecological research programs have been based upon understand the mechanisms that cause observed relationships between species richness and resource availability (e.g. Fraser et al., 2015), with competitive exclusion due to light limitation a commonly ascribed culprit (Grime 1973). The centrality of competition in classical theory developed by plant ecologists (Grime 1979; Tilman 1982) is not surprising given that the sessile nature of plants make social interactions a chronic aspect of plant-life.

Challenges to the idea that plant-plant interactions are necessarily competitive has come from many fronts, most notably in the context of positive interactions (Bertness & Callaway 1994). However, there has also been an empirically-driven attack against the idea that competition, even when present, necessarily causes species loss and structures communities (Lamb & Cahill 2008; Hautier et al. 2009; Bennett et al. 2013). At the heart of the idea that competition can be both strong, while also not necessarily being a major driver of community assembly, is the issue of competitive size-asymmetry, the degree to which larger individuals gain disproportionate advantages in competitive encounters (Schwinning & Weiner 1998). The general result emerging from experiments is that when competition is belowground, community structure is relatively unaffected, while competition aboveground for light can cause dramatic changes (Lamb et al. 2009; Hautier et al. 2009). This is typically interpreted as root competition being size-symmetric under most conditions, and light competition size-asymmetric (Schwinning & Weiner 1998). Lacking, however, has been a general theory integrating the idea that competitive size-asymmetry may shift depending upon the identity of the limiting resources.

It is in filling this conceptual gap that the paper authored by Niv DeMalach, Eli Zaady, Jacob Weiner and Ronen Kadmon titled Size asymmetry of resource competition and the structure of plant communities (DeMalach et al. 2016) makes a significant contribution to plant community ecology. By extending Tilman’s resource competition model, DeMalach and colleagues are able to test whether size-asymmetric interactions associated with competition for light can generate two fundamental patterns associated with soil resource gradients: decreased species richness and species compositional shifts towards larger and faster growing species with increases in soil resources. Importantly, their model framework includes both growth rate and species diversity components, allowing for a mechanistic understanding of how resource supply can influence patterns in plant communities.

Through a series of permutations to the model framework, DeMalach and colleagues are able to show that size-asymmetric competition for light can generate both patterns.  Critically, this work reinforces the increasing empirical support suggesting competitive size-asymmetry, rather than simply the intensity of competition, can governs its influence in impacting community assembly. Additionally, their model indicates that the plant traits favored under competitive scenarios will vary with the size-asymmetry of competition.  Thus, their work suggests there are no single traits that are advantageous under all competitive regimes, and instead the relative value of a given trait will be contingent upon the size-asymmetry of competition that is occurring.

The work by DeMalach and colleagues opens the doors to greater understanding of plant competition, moving well beyond simple intensity. At a minimum, their work raises significant questions about the biological justification for ignoring size-asymmetry in plant interactions, with implications for both a general understanding of community dynamics as well as more applied studies focusing on biodiversity loss associated with species invasion.  With this novel contribution, we will hopefully see resurgences in field-based tests of the role of plant-plant interactions in structuring natural communities and a better understanding of the traits causing increased fitness under different competitive regimes.

James F Cahill
Associate Editor, Journal of Ecology