AimIn recent years evidence has accumulated that plant species are differentially sorted from regional assemblages into local assemblages along local-scale environmental gradients on the basis of their function and abiotic filtering. The favourability hypothesis in biogeography proposes that in climatically difficult regions abiotic filtering should produce a regional assemblage that is less functionally diverse than that expected given the species richness and the global pool of traits. Thus it seems likely that differential filtering of plant traits along local-scale gradients may scale up to explain the distribution, diversity and filtering of plant traits in regional-scale assemblages across continents. The present work aims to address this prediction.Location North and South America. MethodsWe combine a dataset comprising over 5.5 million georeferenced plant occurrence records with several large plant functional trait databases in order to: (1) quantify how several critical traits associated with plant performance and ecology vary across environmental gradients; and (2) provide the first test of whether the woody plants found within 1°and 5°map grid cells are more or less functionally diverse than expected, given their species richness, across broad gradients. ResultsThe results show that, for many of the traits studied, the overall distribution of functional traits in tropical regions often exceeds the expectations of random sampling given the species richness. Conversely, temperate regions often had narrower functional trait distributions than their smaller species pools would suggest. Main conclusionThe results show that the overall distribution of function does increase towards the equator, but the functional diversity within regional-scale tropical assemblages is higher than that expected given their species richness. These results are consistent with the hypothesis that abiotic filtering constrains the overall distribution of function in temperate assemblages, but tropical assemblages are not as tightly constrained.

Abstract. The study of biodiversity has tended to focus primarily on relatively information-poor measures of species diversity. Recently, many studies of local diversity (alpha diversity) have begun to use measures of functional and phylogenetic alpha diversity. Investigations into the phylogenetic and functional dissimilarity (beta diversity) of communities have been far less numerous, but these dissimilarity measures have the potential to infer the mechanisms underlying community assembly and dynamics. Here, we relate levels of phylogenetic and functional alpha diversity to levels of phylogenetic and functional beta diversity to infer the mechanism or mechanisms responsible for the assembly of tree communities in six forests located in tropical and temperate latitudes. The results show that abiotic filtering plays a role in structuring local assemblages and governing spatial turnover in community composition and that phylogenetic measures of alpha and beta diversity are not strong predictors of functional alpha and beta diversity in the forests studied.

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We present a theoretical framework to describe stochastic, sizestructured community assembly, and use this framework to make community-level ecological predictions. Our model can be thought of as adding biological realism to Neutral Biodiversity Theory by incorporating size variation and growth dynamics, and allowing demographic rates to depend on the sizes of individuals. We find that the species abundance distribution (SAD) is insensitive to the details of the size structure in our model, demonstrating that the SAD is a poor indicator of size-dependent processes. We also derive the species biomass distribution (SBD) and find that the form of the SBD depends on the underlying size structure. This leads to a prescription for testing multiple, intertwined ecological predictions of the model, and provides evidence that alternatives to the traditional SAD are more closely tied to certain ecological processes. Finally, we describe how our framework may be extended to make predictions for more general types of community structure. ignited an ideological debate in community ecology, challenging the viewpoint that deterministic forces play the dominant role in shaping patterns of biodiversity (2-9). NBT proposes that community-level patterns are primarily determined by the effects of demographic stochasticity, and that a detailed knowledge of the traits of and interactions between individuals comprising the community is irrelevant. It has been argued that the predictions of NBT are uninformative of process (10-12), and it has been demonstrated that the assumptions underpinning NBT are often manifestly violated: in the tropical forests where the theory has found striking success (1,13,14), there is a huge variation in demographic rates of individuals (15,16). Although there has been much debate (6,17,18) about the importance of neutral, stochastic processes, one common principle has emerged (17-20): the need for a unified, theoretical framework both to quantify the effects of demographic stochasticity relative to other forces, and to generate a broader range of predictions more closely tied to process.In this article, we integrate the effects of demographic stochasticity with ontogenetic variation in the size of individuals (21), and allow the demographic rates of individuals to explicitly depend on their size. We assume that this variation in demographic rates depends on size alone, and is not linked to species identity, an approach that is closely related to the philosophy of allometric scaling theory (8,(22)(23)(24): individuals of a given size play by the same rules, regardless of species identity. Given the strong evidence that demographic rates in nature are correlated with size (8, 16), this synthesis of size variation with demographic stochasticity may be thought of as adding a crucial extra layer of biological realism to NBT.From these ingredients we derive a functional differential equation to describe ecological communities, and use its analytical solution to answer three key questions. First, are our predict...

Summary1. Understanding ecological strategies of invasive species relative to the entire native community is important in understanding and managing both the mechanisms and the potential impacts of invasion, but few studies have taken this approach. 2. We utilize advances in plant ecology to compare functional traits of an invasive shrub species, autumn-olive Elaeagnus umbellata, to those of the understorey native woody plant community in a southeast Michigan forest. We estimate species trait distributions for six leaf functional traits (specific leaf area, leaf laminar area, leaf dry matter content, leaf nitrogen content, leaf carbon: nitrogen ratio and delta 15 N) using a kernel estimator. We then use pairwise trait distance and overlaps to describe the ecological strategy of the invasive shrub relative to that of the native understorey woody plant community. 3. The overlap of the invasive shrub's trait distribution with those of the native species was found to be smaller than the overlap of a native shrub's distribution with other native species, consistent with the empty niche hypothesis of invasion. However, the invasive species' mean leaf functional trait values typically fall outside of the range of most native species' mean trait values, suggesting the invasive shrub extends trait space for understorey individuals of the native woody plant community. 4. The invasive shrub's leaf trait values are characteristic of a sun-demanding species, contradicting its presence in the shaded forest understorey community and its persistence in an empty niche. We propose that this novel use of leaf functional traits by the invasive shrub could indicate that the invasion is facilitated by additional mechanisms such as release from natural enemies, association with nitrogen-fixing bacteria, use of an empty temporal niche (namely the light available to the understorey early in the growing season), bird dispersal or some synthesis of these factors. 5. Synthesis and applications. This study develops plant functional trait theory by studying an invasive species within the context of the entire native woody plant community. The approach can improve our understanding of the potential mechanisms of an invasion event and suggest ecological consequences and related management strategies for the maintenance of native forest communities. Specifically, this study, along with known data on the invasive species, suggests aggressive management in sunny gap and edge habitat provides the best strategy for long-term control.

Kraft et al. (Reports, 24 October 2008, p. 580) used a variety of metrics describing the distribution of functional traits within a tropical forest community to demonstrate simultaneous environmental filtering and niche differentiation. We discuss how these results could have arisen from sampling design and statistical assumptions, suggesting alternative approaches that could better resolve these questions.K raft et al.(1) used the distribution of functional traits in a diverse tropical forest community to test hypotheses of community assembly. The study focused on six traits that drive diverse ecological functions. By comparing metrics of observed distributions of each trait with those expected under different null models, the authors showed evidence for both classical niche differentiation and environmental filtering at the scale of 20-by 20-m quadrats. Relative to null expectations, environmental filtering was indicated by shifting trait means, decreased trait range, and variance within quadrats, whereas differentiation was indicated by overdispersion, measured by standard deviation (SD) of nearestneighbor distance in trait space, and kurtosis of the within-quadrat trait distribution. Combined, these tests were used to argue for the important role of stabilizing processes in the maintenance of biodiversity in a hyperdiverse tropical forest. This approach represents movement toward a more comprehensive use of functional traits in testing processes of community assembly. Nevertheless, there are some subtle problems in the Kraft et al. analysis that we expand upon here.First, Kraft et al.(1) inappropriately tested for both environmental filtering and niche differentiation using null models that select randomly from the entire species pool. If environmental filtering limits the range of traits of species that can occur in a given quadrat, an accurate test of niche differentiation within quadrats must sample only from species that fall between the minimum and maximum traits present in a quadrat. The larger the available trait space, the larger the possible range of nearest-neighbor distances in trait space, and hence the bigger the SD of nearest-neighbor distances will be under a random draw from that available trait space. Thus, if limitations on the range of trait values in each quadrat due to habitat filtering are not accounted for in the null trait distribution used in the niche differentiation test, one might artificially conclude that the observed SD of nearest-neighbor distances is smaller than expected by chance and that niche differentiation is occurring. Similar problems might arise in the comparison of kurtosis values.Second, Kraft et al. ignored intraspecific variation. Due to the obvious difficulty of collecting trait data for all individuals, the authors targeted outer canopy leaves of trees in the 1-to 5-cm diameter at breast height (dbh) size class growing under closed canopy that were readily accessed from the ground, leading to a collection of 2 to 5 leaves from 1 to 20 individuals for most of t...

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Local spatial distributions of populations are often studied in comparison to Complete Spatial Randomness (CSR) and are found to be ubiquitously aggregated, likely due to dispersal limitation. Here we theoretically examine the advantages of comparing observed distributions to simulated populations subject only to drift and Dispersal Limitation (DL). Compared to this DL null, local competition produces overdispersion out to surprisingly large scales, much larger than the scale of competitive interactions. Furthermore, strong overdispersion provides a hallmark that a key requirement of stable coexistence is met, as it can only be observed if intraspecific competition is substantially stronger than interspecific competition. Dispersion compared to CSR is insensitive to competition and as a result unreflective of its presence. Hence, we suggest DL as a complement to CSR since the former focuses on biologically relevant spatial scales and has the potential to detect biotic interactions and habitat specificity.