Abstract
BackgroundThe prevalence of different biotic processes (limiting similarity, weaker competitor exclusion) and historical contingency due to priority effects are in the focus of ongoing discussions about community assembly and non-random functional trait distributions.Methodology/Principal FindingsWe experimentally manipulated assembly history in a grassland biodiversity experiment (Jena Experiment) by applying two factorially crossed split-plot treatments to all communities: (i) duration of weeding (never weeded since sowing or cessation of weeding after 3 or 6 years); (ii) seed addition (control vs. seed addition 4 years after sowing). Spontaneous colonization of new species in the control treatment without seed addition increased realized species richness and functional richness (FRic), indicating continuously denser packing of niches. Seed addition resulted in forced colonization and increased realized species richness, FRic, functional evenness (FEve) and functional divergence (FDiv), i.e. higher abundances of species with extreme trait values. Furthermore, the colonization of new species led to a decline in FEve through time, suggesting that weaker competitors were reduced in abundance or excluded. Communities with higher initial species richness or with longer time since cessation of weeding were more restricted in the entry of new species and showed smaller increases in FRic after seed addition than other communities. The two assembly-history treatments caused a divergence of species compositions within communities originally established with the same species. Communities originally established with different species converged in species richness and functional trait composition over time, but remained more distinct in species composition.Conclusions/SignificanceContrasting biotic processes (limiting similarity, weaker competitor exclusion) increase functional convergence between communities initially established with different species. Historical contingency with regard to realized species compositions could not be eradicated by cessation of weeding or forced colonization and was still detectable 5 years after application of these treatments, providing evidence for the role of priority effects in community assembly.
Highlights
A deeper insight into the mechanisms which control community assembly is central to understand ecosystem functioning and the maintenance of biodiversity
We used the opportunity to study biotic processes of community assembly under relatively homogeneous environmental conditions in split-plots of a large grassland biodiversity experiment opened for colonization by new species and removed dispersal limitation through a seed addition treatment [21]
Species richness and functional trait diversity Species gain and increase in functional richness (FRic) after the opening of experimental communities for colonization by new species was reduced by a higher number of initially established species and a longer colonization period
Summary
A deeper insight into the mechanisms which control community assembly is central to understand ecosystem functioning and the maintenance of biodiversity. It has been discussed that competition may increase similarity among species ( = trait convergence), when species with similar traits compete relatively while excluding weaker competitors with different traits (i.e. equalizing fitness processes [8,9]) This niche-based view of community assembly has been challenged by the ‘‘neutral theory’’ [10] assuming that functional differences do not play a role in community assembly and that the fitness of all species in a community is equivalent. The prevalence of different biotic processes (limiting similarity, weaker competitor exclusion) and historical contingency due to priority effects are in the focus of ongoing discussions about community assembly and non-random functional trait distributions
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