Virtually all empirical ecological interaction networks to some extent suffer from undersampling. However, how limitations imposed by sampling incompleteness affect our understanding of ecological networks is still poorly explored, which may hinder further advances in the field. Here, we use a plant-hummingbird network with unprecedented sampling effort (2716 h of focal observations) from the Atlantic Rainforest in Brazil, to investigate how sampling effort affects the description of network structure (i.e. widely used network metrics) and the relative importance of distinct processes (i.e. species abundances vs. traits) in determining the frequency of pairwise interactions. By dividing the network into time slices representing a gradient of sampling effort, we show that quantitative metrics, such as interaction evenness, specialization (H2 '), weighted nestedness (wNODF) and modularity (Q; QuanBiMo algorithm) were less biased by sampling incompleteness than binary metrics. Furthermore, the significance of some network metrics changed along the sampling effort gradient. Nevertheless, the higher importance of traits in structuring the network was apparent even with small sampling effort. Our results (i) warn against using very poorly sampled networks as this may bias our understanding of networks, both their patterns and structuring processes, (ii) encourage the use of quantitative metrics little influenced by sampling when performing spatio-temporal comparisons and (iii) indicate that in networks strongly constrained by species traits, such as plant-hummingbird networks, even small sampling is sufficient to detect their relative importance for the frequencies of interactions. Finally, we argue that similar effects of sampling are expected for other highly specialized subnetworks.
1. Metacommunity phylogenetics aims at evaluating environmental and/or historical factors driving clade distribution. Phylogenetic fuzzy weighting (PFW) describes clade distribution across metacommunities based on fuzzy sets defined by phylogenetic relatedness among species. The method enables analysing environmental and/or biogeographic determinants of clade distribution. PFW also offers an exploratory tool for visualizing clade distribution via Principal Coordinates of Phylogenetic Structure (PCPS). In this article, we describe the theoretical properties and biological backgrounds of PFW and evaluate its statistical performance (type I error and statistical power) in assessing environmental and phylogenetic determinants of species distribution in comparison with other phylobetadiversity methods (COMDIST, COMDISTNT, Rao's H and UniFrac). 2. The statistical performance of PFW and the other phylobetadiversity metrics was tested by (i) simulating metacommunities under different species assembly scenarios (species distribution influenced or not by environment and/or phylogeny), niche breadth tolerance and species pool sizes; (ii) submitting community matrices to PFW and deriving pairwise phylogenetic dissimilarities between communities (D P ) and PCPS; (iii) submitting these metrics and the other phylobetadiversity methods to different analytical approaches (Mantel test, regression on dissimilarity matrices -ADONIS, and GLM) to evaluate the influence of environment and phylogeny on metacommunity phylogenetic structure; and (iv) estimating type I error and power estimates via alternative permutation procedures. 3. Results demonstrated that PFW provides robust assessment of environmental and phylogenetic drivers of species distribution across metacommunities. Although all methods had acceptable type I error for both Mantel test and ADONIS, only PFW showed acceptable power for both tests. Rao's H had acceptable power only for Mantel test, while COMDIST had acceptable power only for ADONIS. COMDISTNT and UniFrac showed poor statistical performance for both tests. Conversely, GLM had acceptable power only for the first PCPS. 4. Performing ADONIS on D P provides a robust overall assessment of environmental and phylogenetic drivers of species distribution. On the other hand, performing PCPS analysis after rejecting the null hypotheses via ADONIS allows identifying the phylogenetic nodes mostly associated with environmental gradients. PFW enables synthesizing and analysing phylogenetic patterns in metacommunities, allowing attaining a more complete portrait of ecological and evolutionary drivers of species distribution.
Functional traits are commonly used in predictive models that link environmental drivers and community structure to ecosystem functioning. A prerequisite is to identify robust sets of continuous axes of trait variation, and to understand the ecological and evolutionary constraints that result in the functional trait space occupied by interacting species. Despite their diversity and role in ecosystem functioning, little is known of the constraints on the functional trait space of invertebrate biotas of entire biogeographic regions. We examined the ecological strategies and constraints underlying the realized trait space of aquatic invertebrates, using data on 12 functional traits of 852 taxa collected in tank bromeliads from Mexico to Argentina. Principal Component Analysis was used to reduce trait dimensionality to significant axes of trait variation, and the proportion of potential trait space that is actually occupied by all taxa was compared to null model expectations. Permutational Analyses of Variance were used to test whether trait combinations were clade‐dependent. The major axes of trait variation represented life‐history strategies optimizing resource use and antipredator adaptations. There was evidence for trophic, habitat, defence and life‐history niche axes. Bromeliad invertebrates only occupied 16%–23% of the potential space within these dimensions, due to greater concentrations than predicted under uniform or normal distributions. Thus, despite high taxonomic diversity, invertebrates only utilized a small number of successful ecological strategies. Empty areas in trait space represented gaps between major phyla that arose from biological innovations, and trait combinations that are unviable in the bromeliad ecosystem. Only a few phylogenetically distant genera were neighbouring in trait space. Trait combinations aggregated taxa by family and then by order, suggesting that niche conservatism was a widespread mechanism in the diversification of ecological strategies. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13141/suppinfo is available for this article.
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Questions What is the magnitude of between‐species trait variability (BSV) and within‐species trait variability (WSV) of specific leaf area (SLA) in a sapling meta‐community? To what extent do species turnover and WSV influence community‐level mean trait responses to an environmental gradient and trait spread patterns across this gradient? What is the role of WSV for mean plant responses to environmental variation and niche partitioning in structuring sapling communities? Location Forest patches within a native grassland matrix in southern Brazil. Methods We recorded saplings in community plots across a canopy openness gradient in forest patches and described each of the 1129 individuals using SLA. First, we partitioned trait variation into BSV and WSV irrespective of co‐occurrence in plots. Then, using the community data, we partitioned the total variation of community‐weighted trait means (CWM) and Rao's functional diversity (FD) into components explained by canopy openness, species turnover and WSV. We also partitioned the effects of WSV between and within plots on FD. Finally, we explored the responses of CWM and FD to the gradient using the whole trait variability, only BSV or only WSV. Results Specific leaf area presented a substantial proportion of variation within species (37%), although it varied more between species (63%). Species turnover and WSV explained 48% and 19% of the variation in CWM across the gradient, respectively. Species turnover and WSV explained 51% and 45% of the variation in FD across the gradient, respectively. SLA varied within species more along the gradient than within communities. Within‐species variability enhanced shifts in CWM and FD across the gradient. Canopy openness significantly predicted CWM at all levels, and FD at all but the within‐species level. Conclusions Plastic responses of species mirrored the average response of communities to the environmental gradient. Within‐species trait variability enhanced the mean plant responses to environmental variation as well as niche partitioning, and was especially important in enabling species to establish in a wider portion of the environmental gradient. Our study provides new evidence that population‐level phenomena matter for community assembly.
Functional traits mediate ecological responses of organisms to the environment, determining community structure. Community-weighted trait means (CWM) are often used to characterize communities by combining information on species traits and distribution. Relating CWM variation to environmental gradients allows for evaluating species sorting across the metacommunity, either based on correlation tests or ordinary least squares (OLS) models. Yet, it is not clear if phylogenetic signal in both traits and species distribution affect those analyses. On one hand, phylogenetic signal might indicate niche conservatism along clade evolution, reinforcing the environmental signal in trait assembly patterns. On the other hand, it might introduce phylogenetic autocorrelation to mean trait variation among communities. Under this latter scenario, phylogenetic signal might inflate type I error in analysis relating CWM variation to environmental gradients. We explore multiple ways phylogenetic history may influence analysis relating CWM to environmental gradients. We propose the concept of neutral trait diffusion, which predicts that for a functional trait x, CWM variation among local communities does not deviate from the expectation that x evolved according to a neutral evolutionary process. Based on this framework we introduce a graphical tool called neutral trait diffusion representation (NTDR) that allows for the evaluation of whether it is necessary to carry out phylogenetic correction in the trait prior to analyzing the association between CWM and environmental gradients. We illustrate the NTDR approach using simulated traits, phylogenies and metacommunities. We show that even under moderate phylogenetic signal in both the trait used to define CWM and species distribution across communities, OLS models relating CWM variation to environmental gradients lead to inflated type I error when testing the null hypothesis of no association between CWM and environmental gradient. To overcome this issue, we propose a phylogenetic correction for OLS models and evaluate its statistical performance (type I error and power). Phylogeny-corrected OLS models successfully control for type I error in analysis relating CWM variation to environmental gradients but may show decreased power. Combining the exploratory tool of NTDR and phylogenetic correction in traits, when necessary, guarantees more precise inferences about the environmental forces driving trait-mediated species sorting across metacommunities.
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