Evaluating the role of biotic interactions in structuring communities using a gradient analysis of multiple interacting guilds

Abstract

The extent to which ecological communities are coherent entities as opposed to mere intersections of individual species distributions has long been one of the fundamental questions of ecology. Gradient analysis is one commonly used tool for addressing this question; however, all such studies have used organisms from a single taxon or guild. This risks missing important connections due to non‐competitive interactions, which should be more likely to occur between members of different guilds. Such organisms are unlikely to compete for resources and can have complementary niches that promote non‐competitive interactions. We examined the abundances of taxa in four interacting guilds along an elevation gradient in a forest in the southern Appalachian mountains. A causal discovery algorithm was used to investigate the relative frequencies of interguild and intraguild interactions. These were approximately equally common once taxonomic richness was taken into account. We used elements of metacommunity structure analysis to study the extent to which species distributions are non‐independent and tested the hypothesis that combinations of two or more interacting guilds exhibit more coherence than single guilds. In this analysis, all guilds other than collembola were classified as Clementsian or quasi‐Clementsian. (Collembola were classified as random.) When sets of multiple guilds were examined, Clementsian and quasi‐Clementsian structures predominated. We also compared boundary conjunction, measured as Morisita’s index (MI) for these sets of guilds to the weighted average of the guilds’ MI values. Only sets of directly interacting guilds had higher‐than‐baseline boundary conjunction values, and such boundary conjunction values are found in all but one set of directly interacting guilds. Our results highlight the importance of inter‐guild interactions in structuring patterns of cooccurrence. Trophic interactions and plant–fungus symbioses (mutualistic and/or pathogen–host) appear particularly important.