Despite being recognized as a promoter of diversity and a condition for local coexistence decades ago, the importance of intraspecific variance has been neglected over time in community ecology. ...Recently, there has been a new emphasis on intraspecific variability. Indeed, recent developments in trait-based community ecology have underlined the need to integrate variation at both the intraspecific as well as interspecific level. We introduce new T-statistics (‘T’ for trait), based on the comparison of intraspecific and interspecific variances of functional traits across organizational levels, to operationally incorporate intraspecific variability into community ecology theory. We show that a focus on the distribution of traits at local and regional scales combined with original analytical tools can provide unique insights into the primary forces structuring communities.
Understanding the imprint of environmental filtering on community assembly along environmental gradients is a key objective of trait‐gradient analyses. Depending on local constraints, this filtering ...generally entails that species departing from an optimum trait value have lower abundances in the community. The community‐weighted mean (CWM) and variance (CWV) of trait values are then expected to depict the optimum and intensity of filtering, respectively. However, the trait distribution within the regional species pool and its limits can also affect local CWM and CWV values apart from the effect of environmental filtering. The regional trait range limits are more likely to be reached in communities at the extremes of environmental gradients. Analogous to the mid‐domain effect in biogeography, decreasing CWV values in extreme environments can then represent the influence of regional trait range limits rather than stronger filtering in the local environment. We name this effect the ‘trait‐gradient boundary effect’ (TGBE). First, we use a community assembly framework to build simulated communities along a gradient from a species pool and environmental filtering with either constant or varying intensity while accounting for immigration processes. We demonstrate the significant influence of TGBE, in parallel to environmental filtering, on CWM and CWV at the extremes of the environmental gradient. We provide a statistical tool based on Approximate Bayesian Computation to decipher the respective influence of local environmental filtering and regional trait range limits. Second, as a case study, we reanalyze the functional composition of alpine plant communities distributed along a gradient of snow cover duration. We show that leaf trait convergence found in communities at the extremes of the gradient reflect an influence of trait range limits rather than stronger environmental filtering. These findings challenge correlative trait–environment relationships and call for more explicitly identifying the mechanisms responsible of trait convergence/divergence along environmental gradients.
1. Understanding how environmental factors drive plant community assembly remains a major challenge in community ecology. The strength of different assembly processes along environmental gradients, ...such as environmental filtering and functional niche differentiation, can be quantified by analysing trait distributions in communities. While environmental filtering affects species occurrence among communities, functional divergence or convergence is strongly related to species abundances within communities, which few studies have taken into account. We examine the trait-mediated effect of these two processes along a stress-resource gradient. 2. We measured species abundances and the distributions of eight traits related to vegetative and regenerative phases in plant communities along a gradient of soil depth and resource availability in Mediterranean rangelands. We quantified environmental filtering, defined as a local restriction of trait range, and trait divergence, based on abundance-weighted trait variance, using a two-step approach with specifically designed null models. 3. Communities presented a clear functional response to the soil gradient, as evidenced by strong trends in community-weighted trait means. We detected environmental filtering of different traits at both ends of the gradient, suggesting that, contrary to widespread expectations, trait filtering may not necessarily be the result of abiotic filtering under harsh conditions but could likely also result from biotic interactions in productive habitats. 4. We found marked shifts in trait abundance distributions within communities along the gradient. Vegetative traits (e.g. leaf dry matter content) diverged on shallow soils, reflecting the coexistence of distinct water- and nutrient-use strategies in these constrained habitats and converged with increasing soil resource availability. By contrast, regenerative traits (e.g. seed mass) tended to diverge towards deeper soils, while plant reproductive heights diverged all along the gradient. 5. Synthesis: Our study highlights how the combination of abundance data with traits capturing different functional niches is critical to the detection of complex functional responses of plant communities to environmental gradients. We demonstrate that patterns of trait divergence and filtering are strongly contingent on both trait and environment such that there can be no expectation of a simple trend of increasing or decreasing functional divergence along a gradient of resource availability.
1. Climate change is expected to increase the magnitude and the frequency of extreme climatic events such as droughts. Better understanding how plant communities will respond to these droughts is a ...major challenge. We expect the response to be a shift in functional trait values resulting from both species turnover and intraspecific trait variability, but little research has addressed the relative contribution of both components. 2. We analysed the short-term functional response of subalpine grassland communities to a simulated drought by focusing on four leaf traits (LDMC: leaf dry matter content, SLA: specific leaf area, LNC: leaf nitrogen concentration and LCC: leaf carbon concentration). After evaluating species turnover and intraspecific variability separately, we determined their relative contribution in the community functional response to drought, reflected by changes in community-weighted mean traits. 3. We found significant species turnover and intraspecific variability, as well as significant changes in community-weighted mean for most of the traits. The relative contribution of intraspecific variability to the changes in community mean traits was more important (42–99%) than the relative contribution of species turnover (1–58%). Intraspecific variability either amplified (for LDMC, SLA and LCC) or dampened (for LNC) the community functional response mediated by species turnover. We demonstrated that the small contribution of species turnover to the changes in community mean LDMC and LCC was explained by a lack of covariation between species turnover and interspecific trait differences. 4. Synthesis. These results highlight the need for a better consideration of intraspecific variability to understand and predict the effect of climate change on plant communities. While both species turnover and intraspecific variability can be expected following an extreme drought, we report new evidence that intraspecific variability can be a more important driver of the short-term functional response of plant communities.
Successions are a central issue of ecological theory. They are governed by changes in community assembly processes that can be tracked by species' traits. While single-trait-based approaches have ...been mostly promoted to address community assembly, ecological strategies actually encompass tradeoffs between multiple traits that are relevant to succession theory. We analyzed plant ecological strategies along a 140-year-long succession primary succession of 52 vertical outcrop communities after roadwork. We performed a RLQ analysis to relate six functional traits, associated with resource acquisition, competition, colonization ability and phenology, to the age of the outcrops. We found the prominence of two main axes of specialization, one related to resource acquisition and the other to reproduction and regeneration. We further examined the community-level variation in ecological strategies to assess the abiotic and bio tic drivers of community assembly. Using trait-based statistics of functional richness, regularity and divergence, we found that different processes drove the variation in ecological strategies along the axes of specialization. In late succession, functional convergence was detected for the traits related to resource acquisition as a signature of habitat filtering, while the coexistence of contrasted strategies was found for the traits related to reproduction and regeneration as a result of spatial micro-heterogeneity. We observed a lack of niche differentiation along the succession, revealing a weak importance of biotic interactions for the regulation of community assembly in the outcrops. Overall, we highlight a prominent role of habitat filtering and spatial micro-heterogeneity in driving the primary succession governed by water and nutrient limitation.
Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV ...compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta‐analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole‐plant (e.g. plant height) vs. organ‐level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait‐based community and ecosystem studies.
A major objective in ecology is to determine how local species abundances relate to their functional trait values (i.e. trait–abundance relationship), under a combined influence of 1) environmental ...filters affecting local species performance conditionally to trait values, 2) neutral demographic and immigration dynamics affecting abundances independently from these trait values, and 3) varying availability and frequency of species at regional level. We examined the nature and strength of the trait–abundance relationship in 30 000 simulated communities covering a gradient of the relative importance of niche‐based environmental filtering and neutral stochastic processes, with heterogeneous regional species frequencies. We explored scenarios of directional, stabilizing and disruptive filtering differently affecting the success of species in communities, depending on their relative trait values. We evaluated how the four first moments of the trait distribution in a local community (i.e. abundance‐weighted mean, variance, skewness and kurtosis) were influenced by immigration, environmental filtering and neutral dynamics. Then we determined whether including constraints related to these moments in a Bayesian maximum entropy regression improved the prediction of the trait–abundance relationships. First, we found pervasive influence of regional frequencies on local species abundances, related to regular input of immigrants. Second, the first four moments of the local trait distribution were affected by environmental filtering, the shape of the response depending on the type of filtering. Third, with decreasing immigration rate, the imprint of local demographic stochasticity overrode the impact of environmental filtering on trait–abundance relationships. Lastly, accounting for the mean and variance of local trait distribution appeared sufficient to explain the trait–abundance relationships in our regression framework, although their contribution differed depending on the type of environmental filtering. Therefore, the mean and variance of trait values in communities, two pillars of trait‐gradient analyses in functional ecology, can capture the key influence of environmental filtering on local trait–abundance relationships.
Hutchinson's n‐dimensional hypervolume concept underlies many applications in contemporary ecology and evolutionary biology. Estimating hypervolumes from sampled data has been an ongoing challenge ...due to conceptual and computational issues.
We present new algorithms for delineating the boundaries and probability density within n‐dimensional hypervolumes. The methods produce smooth boundaries that can fit data either more loosely (Gaussian kernel density estimation) or more tightly (one‐classification via support vector machine). Further, the algorithms can accept abundance‐weighted data, and the resulting hypervolumes can be given a probabilistic interpretation and projected into geographic space.
We demonstrate the properties of these methods on a large dataset that characterises the functional traits and geographic distribution of thousands of plants. The methods are available in version ≥2.0.7 of the hypervolume r package.
These new algorithms provide: (i) a more robust approach for delineating the shape and density of n‐dimensional hypervolumes; (ii) more efficient performance on large and high‐dimensional datasets; and (iii) improved measures of functional diversity and environmental niche breadth.
Abstract
Identifying species that are both geographically restricted and functionally distinct, i.e. supporting rare traits and functions, is of prime importance given their risk of extinction and ...their potential contribution to ecosystem functioning. We use global species distributions and functional traits for birds and mammals to identify the ecologically rare species, understand their characteristics, and identify hotspots. We find that ecologically rare species are disproportionately represented in IUCN threatened categories, insufficiently covered by protected areas, and for some of them sensitive to current and future threats. While they are more abundant overall in countries with a low human development index, some countries with high human development index are also hotspots of ecological rarity, suggesting transboundary responsibility for their conservation. Altogether, these results state that more conservation emphasis should be given to ecological rarity given future environmental conditions and the need to sustain multiple ecosystem processes in the long-term.
Summary
Variation and tradeoffs within and among plant traits are increasingly being harnessed by empiricists and modelers to understand and predict ecosystem processes under changing environmental ...conditions. While fine roots play an important role in ecosystem functioning, fine‐root traits are underrepresented in global trait databases. This has hindered efforts to analyze fine‐root trait variation and link it with plant function and environmental conditions at a global scale. This Viewpoint addresses the need for a centralized fine‐root trait database, and introduces the Fine‐Root Ecology Database (FRED, http://roots.ornl.gov) which so far includes > 70 000 observations encompassing a broad range of root traits and also includes associated environmental data. FRED represents a critical step toward improving our understanding of below‐ground plant ecology. For example, FRED facilitates the quantification of variation in fine‐root traits across root orders, species, biomes, and environmental gradients while also providing a platform for assessments of covariation among root, leaf, and wood traits, the role of fine roots in ecosystem functioning, and the representation of fine roots in terrestrial biosphere models. Continued input of observations into FRED to fill gaps in trait coverage will improve our understanding of changes in fine‐root traits across space and time.
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