Community assembly by limiting similarity vs. competitive hierarchies: testing the consequences of dispersion of individual traits

Abstract

Summary1. The degree of dispersion of trait values among species in a community has frequently been used to infer processes of community assembly. However, multiple assembly processes can lead to the same pattern of trait dispersion or the same process can lead to different patterns of dispersion. In particular, competitive processes can lead both to trait overdispersion (if the trait controls niche differentiation and only substantial differences allow coexistence) or to trait underdispersion (if the trait controls position in a competitive hierarchy and only similar values enhance coexistence). 2. Because different traits are likely to contribute to stabilizing (niche differentiation) and equalizing (competitive hierarchies) mechanisms of coexistence, we compare the role of dispersion of a number of individual traits for species diversity by conducting in silico experiments using field-parameterized spatially explicit models of communities of clonally growing plants. We manipulate both dispersion and means of different traits and examine consequent changes of species diversity in the whole community. We hypothesize that growth traits, which are directly linked to resource acquisition, are likely related to position in competitive hierarchies and thus diversity and trait dispersion will be negatively associated. In contrast, we hypothesize that architectural traits, which control spatial deployment of new plants and are thus less directly linked to resource acquisition, are more likely to be linked to niche differentiation and thus diversity and trait dispersion will be positively associated. 3. Individual traits differed considerably in effects of trait dispersion on community diversity. Specifically, increasing dispersion in growth traits often decreased diversity, presumably reflecting widening competitive differences and exclusion of weaker competitors, which is consistent with the action of equalizing mechanisms of species coexistence. In contrast, increasing dispersion in architectural traits either increased diversity or had no effect, which possibly indicates niche-based mechanisms of diversity maintenance. Changes of community-wide trait means can have large effects on diversity for a given degree of dispersion, although not usually changing the sign of the relationship. 4. Synthesis. These results suggest inference of community assembly processes from patterns of trait dispersion without understanding how particular traits function in community assembly may often be misleading. Effects of dispersion of traits that are likely associated with position in a competitive hierarchy are very different from those of traits associated with niche differences.

Different sets of belowground traits predict the ability of plant species to suppress and tolerate their competitors

“…Root tissue density was negatively related to root system size, suggesting a common link with growth rate (Ryser 1996;Kramer-Walter et al 2016). This finding is in agreement with other studies showing that large plant size and acquisitive traits in the leaf economics spectrum translate into high competitive ability (Goldberg and Landa 1991;Keddy et al 2002;Herben and Goldberg 2014). However, we found that size-related root traits were not related to the ability to tolerate competition (i.e.…”

“…strongly related to competitive hierarchies than to niches (Kunstler et al 2012;Herben and Goldberg 2014;Kraft et al 2015). This apparent contradiction highlights the current lack of a mechanistic understanding of the way in which easily measured traits relate to plant function and the way in which different traits contribute to competitive hierarchies versus niche differentiation between species (Kraft et al 2015;Shipley et al 2016).…”

“…Plant size has been repeatedly identified as a trait that is central to competitive success (Goldberg 1996;Keddy et al 2002). Several aboveground traits such as greater maximum height, larger leaf area, higher leaf nitrogen content and lower leaf dry matter content have also been shown to provide an advantage in interspecific competition (Goldberg and Landa 1991;Herben and Goldberg 2014;Bennett et al 2016). Competitively superior species are also frequently characterised by high specific leaf area (Kunstler et al 2012;Fort et al 2014;Kraft et al 2015).…”

Competitive network determines the direction of the diversity–function relationship

“…Intransitive competition and competitive hierarchies are important drivers of coexistence and biodiversity in nonfungal communities [e.g., grassland plant communities (16,38)]. Nevertheless, an important outstanding question is whether or not these results extend to other systems.…”

Successional colonization of temporary streams: An experimental approach using aquatic insects

“…Historically, colonization has been reported as the main determinant of community succession, with species showing a tradeoff between traits that favor dispersal and persistence (Herben and Goldberg, 2014;Ricklefs, 1987). Species with high dispersal ability can colonize recently created or remote habitats, causing species occupancy to become a dynamic process in biological communities (Hanski and Ovaskainen, 2000).…”