Competitive coexistence, it's all about individuals.

Understanding how species coexist has been the raison d'etre for many ecologists over the past 100 years. The quest to understand and explain why so many species coexist together has really been a journey of shifting narratives. The major road stops on this journey have included searching for niche differences among species -from single resources to multidimensional niches, elevating the role for non-equilibrial dynamics -namely disturbances, and assessing the possibility that species actually differ little and diversity patterns follow neutral process. Along this entire journey, researchers (especially theoreticians) have reminded the larger community that that coexistence is a product of the balance between interactions among species (interspecific) and interactions among individuals within species (intraspecific). Despite this occasional reminder, ecologists have largely searched for mechanisms dictating the strength of interspecific interactions.

Image used under Flickr creative commons license, taken by Tinken

In order for two species to coexist, intraspecific competition must be stronger than interspecific -so sayeth classic models of competition. While people have consistently looked for niche differences that reduce interspecific competition, no one has really assessed the strength of intraspecific competition. Until now that is. In a recent paper in Science, Jim Clark examines intra- vs interspecific interactions from data following individual tree performances, across multiple species, for up to 18 years. This data set included annual growth and reproduction, resulting in 226,000 observations across 22,000 trees in 33 species!

His question was actually quite simple -what is the strength of intraspecific interactions relative to interspecific ones? There are two alternatives. First, that intraspecific competition is higher, meaning that among species differences only need to be small for coexistence to occur; or secondly, that intraspecific competition is lower, requiring greater species niche differences for coexistence. To answer this he looked at correlations in growth and fecundity between individuals either belonging to the same or different species, living in proximity to one another. He took a strong positive correlation as evidence for strong competition and a negative or weak correlation as evidence for resource or temporal niche partitioning. What he found was that individuals within species were much more likely to show correlated responses to fluctuating environments, than individuals among species.

This paper represents persuasive evidence that within-species competition is generally extremely high, meaning that to satisfy the inequality leading to coexistence: intra > inter, subtle niche differences can be sufficient. These findings should spur a new era of theoretical predictions and empirical tests as our collective journey to understanding coexistence continues.

Clark, J. (2010). Individuals and the Variation Needed for High Species Diversity in Forest Trees Science, 327 (5969), 1129-1132 DOI: 10.1126/science.1183506