Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co-occur randomly but are restricted in their co-occurrence by interspecific competition. This concept can be redefined in a more general framework where the co-occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta-analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co-occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta-analyses suggest that non-random co-occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non-random aspect of assembly in plant communities.

Mycorrhizal fungi provide plants with a range of benefits, including mineral nutrients and protection from stress and pathogens. Here we synthesize current information about how the presence and type of mycorrhizal association affect plant communities. We argue that mycorrhizal fungi regulate seedling establishment and species coexistence through stabilizing and equalizing mechanisms such as soil nutrient partitioning, feedback to soil antagonists, differential mycorrhizal benefits, and nutrient trade. Mycorrhizal fungi have strong effects on plant population and community biology, with mycorrhizal type–specific effects on seed dispersal, seedling establishment, and soil niche differentiation, as well as interspecific and intraspecific competition and hence plant diversity.

The species pool and its relation to species richness: evidence from Estonian plant communities.-Oikos 75: 111-117. Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (I) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HOI postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO z postulates that any value of species richness per unit area (I m-) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P =0.041 for HOI and P = 0.002 for HO z). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per I m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Functional trait differences among species are increasingly used to infer the effects of biotic and abiotic processes on species coexistence. Commonly, the trait diversity observed within communities is compared to patterns simulated in randomly generated communities based on sampling within a region. The resulting patterns of trait convergence and divergence are assumed to reveal abiotic and biotic processes, respectively. However, biotic processes such as competition can produce both trait divergence and convergence, through either excluding similar species (niche differences, divergence) or excluding dissimilar species (weaker competitor exclusion, convergence). Hence, separating biotic and abiotic processes that can produce identical patterns of trait diversity, or even patterns that neutralize each other, is not feasible with previous methods. We propose an operational framework in which the functional trait dissimilarity within communities (FDcomm) is compared to the corresponding trait dissimilarity expected from the species pool (i.e., functional species pool diversity, FDpool). FDpool includes the set of potential species for a site delimited by the operating environmental and dispersal limitation filters. By applying these filters, the resulting pattern of trait diversity is consistent with biotic processes, i.e., trait divergence (FDcomm > FDpool) indicates niche differentiation, while trait convergence (FDcomm < FDpool) indicates weaker competitor exclusion. To illustrate this framework, with its potential application and constraints, we analyzed both simulated and field data. The functional species pool framework more consistently detected the simulated trait diversity patterns than previous approaches. In the field, using data from plant communities of typical Northern European habitats in Estonia, we found that both niche-based and weaker competitor exclusion influenced community assembly, depending on the traits and community considered. In both simulated and field data, we demonstrated that only by estimating the species pool of a site is it possible to differentiate the patterns of trait dissimilarity produced by operating biotic processes. The framework, which can be applied with both functional and phylogenetic diversity, enables a reinterpretation of community assembly processes. Solving the challenge of defining an appropriate reference species pool for a site can provide a better understanding of community assembly.