Functional trait patterns in grassland communities, and the importance of scale
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For a long time, ecologists have worked on gaining insight into the processes that govern the assembly of natural plant communities. Plant trait-based approaches have great potential to improve our understanding of community assembly and assessment of ecosystem services. In many trait-based studies, trait-for-species substitutions are used by assigning mean trait values to each species in a community, which means that only between-species trait variability (i.e. interspecific trait variability) is considered. As there is growing evidence of the importance of within-species trait variability (i.e. intraspecific trait variability), this work is dedicated to studying how the applicability of these trait-based approaches is constrained by intraspecific variability and scale dependence. It could be shown that both interspecific trait variability and intraspecific trait variability indeed determine the trait patterns among habitats, communities and species. As a consequence, neglecting either type of trait variability by relying on species potential trait values derived from a much larger scale than the processes studied can lead to misleading conclusions, also on the community level. The benefits of using species mean trait values derived from large databases for a trait-based study will strongly depend on the level and scale of the question. In summary, there is a need to clearly differentiate between realized and potential traits on all levels in trait-based studies.Keywords:
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Community
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Abstract Turnover in species composition and community-wide functional traits across environmental gradients is a ubiquitous pattern in ecology, and is generally assumed to reflect shifts in trait optima across these gradients. However, the demographic processes that give rise to these trait turnover patterns at the community level remain unclear. We asked whether shifts in the community-weighted means of three key functional traits across an environmental gradient in a southern California grassland reflect variation in the trait-performance relationship across the landscape. We planted seeds of 17 annual plant species in cleared patches with no competitors, and quantified the lifetime seed production of 1360 individuals. We then asked whether models that included trait-environment interactions help explain interspecific variation in demographic responses to the environment. This allowed us to evaluate whether observed shifts in community-weighted mean traits matched the direction of any trait-environment interactions detected in the plant performance experiment. Our results indicate that commonly-measured plant functional traits help explain variation in species responses to the environment – for example, high-SLA species had a demographic advantage in soils with high soil Ca:Mg levels, while low-SLA species had an advantage in low Ca:Mg soils. We also found that shifts in community-weighted mean traits often reflect the direction of these trait-environment interactions, though not all trait-environment relationships at the community level reflect interactive effects of traits and environment on species performance. Our results support the value of plant functional traits for predicting species responses to environmental variation, and highlight a need for more detailed evaluation of how trait-performance relationships change across environments to improve such predictions.
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Community
Environmental change
Environmental gradient
Variation (astronomy)
Functional ecology
Specific leaf area
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There is growing recognition of the need to incorporate within-species trait variability into trait-based studies to improve understanding of community assembly and how plant communities drive ecosystem processes. Given that many plant species can occupy a wide range of environmental conditions, studies that have traditionally focused solely on between-species trait variability and neglected within-species trait variability could lead to an incomplete picture of how plant traits influence community- and ecosystem-level properties. In this thesis, within-species trait variability of all component species across a well-studied system of 30 forested lake islands in the boreal zone of northern Sweden were characterized to understand how differences among individual species traits contribute to community level properties and community assembly. Collectively, the islands represent a long-term chronosequence across which there are large changes in plant community composition, diversity and above- and belowground resource availability and heterogeneity. Significant within-species trait variability was found among all dominant species that were widespread across the chronosequence. In addition, within-species trait variability was highly responsive to differences in environmental conditions among ecosystems, in a manner mostly consistent with patterns observed at the across- species level. Across contrasting environments, trait variability within species sometimes explained a greater amount of variation in overall community-level responses than did among-species variation. There was also significant within-species variation in biomass allocation patterns of co-occurring dominant dwarf shrub species across the chronosequence. This, together with directional shifts in within- and between-species functional trait diversity of both dominant and subordinate species across the gradient, provides insights on how changes in resource availability drive community trait composition, species coexistence and consequently community responses. These findings overall highlight the importance of within-species variability for understanding the responses of whole plant communities to environmental changes, and potentially to ongoing global changes. Further, given the importance of plant traits in governing ecosystem processes such as net primary productivity, carbon sequestration, biogeochemical cycling and decomposition, knowledge of the extent and magnitude of within-species trait variability is imperative for better understanding these processes and their drivers, especially in ecosystems with low species diversity and turnover such as boreal forests.
Chronosequence
Trait
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Abstract Issue Approaches to predicting species assemblages through stacking individual niche‐based species distribution models (S‐SDMs) need to account for community processes other than abiotic filtering. Such constraints have been introduced by implementing ecological assembly rules (EARs) into S‐SDMs, and can be based on patterns of functional traits in communities. Despite being logically valid, this approach has led to a limited improvement in prediction, possibly because of mismatches between the scales of measurement of niche and trait data. Evidence S‐SDM studies have often related single values of a species’ traits to environmental niches that are captured by abiotic conditions measured at a much finer spatial scale, without accounting for intraspecific trait variation along environmental gradients. Many pieces of evidence show that omitting intraspecific trait variation can hinder the proper inference of EARs from trait patterns, and we further argue that it can therefore also affect our capacity to spatially predict functional properties of communities. In addition, estimates of environmental niches and trait envelopes may vary depending on the scale at which environmental and trait measurements are made. Conclusion We suggest that to overcome these limitations, surveys sampling both niche and trait measurements should be conducted at fine scales along wide environmental gradients, and integrated at the same scale to test and improve a new generation of spatial community models and their functional properties.
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Niche differentiation
Community
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Abstract It has been widely recognized that species show extensive variation in form and function. Based on species’ attributes, they can be positioned along major axes of variation, which are often defined by life‐history traits, such as number of offspring, age at maturity or generation time. Less emphasis has been given in this respect to tolerance traits, especially to resistance to abiotic stress conditions, which often determine community (dis)assembly and distribution. Soil fauna species distribution is governed to a large extent by environmental conditions that filter communities according to functional traits, such as abiotic stress tolerance, morphology and body size. Trait‐based approaches have been successfully used to predict soil biota responses to abiotic stress. It remains unclear, though, how these traits relate to life‐history traits that determine individual performance, that is, reproduction and survival. Here, we analyse patterns in multidimensional trait distribution of dominant groups of soil fauna, that is, Isopoda, Gastropoda and Collembola, known to be important to the functioning of ecosystems. We compiled trait information from existing literature, trait databases and supplementary measurements. We looked for common patterns in major axes of trait variation and tested if vertical distribution of species in the soil explained trait variation based on three components of trait diversity (trait richness, evenness and divergence). Our results showed that two to three axes of variation structured the trait space of life‐history and tolerance traits in each of the taxonomic groups and that vertical distribution in soil explained the main axis of trait variation. We also found evidence of environmental filtering on soil fauna along the vertical soil distribution, with lower trait richness and trait divergence in soil‐dwelling than in surface‐living species. Our study was partially limited by the lack of detailed trait measurements for the selected taxonomic groups. In this regard, there is an urgent need for standardized trait databases across invertebrate groups to improve trait‐based diversity analysis and fill gaps in the mechanistic understanding behind trait distribution, trait filtering and the link with species fitness and performance.
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Life History Theory
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Ecological communities and their response to environmental gradients are increasingly being described by measures of trait composition at the community level – the trait‐based approach. Whether ecological or non‐ecological processes influence trait composition between communities has been debated. Understanding the processes that influence trait composition is important for reconstructing paleoenvironmental conditions from fossil deposits and for understanding changes in community functionality through time. Here, we assess the influence of ecological and non‐ecological processes on the distribution of traits within North American mammals. We found that non‐ecological processes including historical contingency, spatial autocorrelation, and evolutionary history do not influence trait composition; however, the variance in trait composition is highly explained by climate gradients. Our results suggest that habitat breadth, terrestriality, diet breadth, and reproductive traits are strong candidates as proxies for measuring functional aspects of environments in the past and present.
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Community
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Summary A central aim of forest ecologists is to quantify the relative importance of different community assembly mechanisms in tropical and subtropical tree communities. Recent work in this field has focused on the importance of functional trait similarity and abiotic filtering. While important, none of this work has simultaneously: linked these trait dispersion patterns to the underlying abiotic environment, considered dispersal limitation and quantified the degree to which patterns of trait dispersion may be explained simply by shared ancestry. Here we use data from a subtropical Chinese forest to accomplish this goal. We first examine the trait dispersion (leaf area, specific leaf area, seed mass, wood density, maximum height and five traits together) on local scales by comparing the observed trait dispersion pattern to that expected from a null model. Then we use a variance partitioning approach to examine the degree to which spatial proximity, environmental similarity or the phylogenetic dispersion of the species determine the observed trait dispersion. The results show that, on local scales, trait dispersion is often non‐randomly filtered. Further the widespread trait clustering observed is largely explained by the environment and space, while the phylogenetic dispersion of species in a sample explains relatively little. This result further underscores that inferring an assembly mechanism from a pattern of phylogenetic dispersion is tenuous. The work is important in that it is the first to partition the variation in tree trait diversity into its spatial, environmental and phylogenetic components and that it demonstrates that functional trait data often lack enough phylogenetic signal on local scales to confidently link patterns of trait and phylogenetic dispersion. Ultimately, the findings suggest a strong role for abiotic filtering and dispersal limitation during community assembly on local spatial scales and that shared evolutionary history plays a relatively small role.
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Null model
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Trait variation underlies our understanding of the patterns and importance of biodiversity, yet we have a poor understanding of how variation at different levels of biological organization structures communities and ecosystems. Here, we use a mesocosm experiment to test for the effects of a larval dragonfly functional trait on community and ecosystem dynamics by creating artificial populations to mirror within- and between-population trait variation observed in our study area. Specifically, we manipulate variation in activity rate, a key functional trait shaping food webs, across three levels of biological organization: within-populations (differences in trait variation in a population), among-populations (differences in population mean trait values), and among-species (species-level differences of co-occurring dragonflies). We show that differences in activity rate alter prey communities, trophic cascades, and multiple ecosystem processes. However, trait variation among populations had much larger effects than differences between co-occurring species or even the presence of a predator, whereas within-population variation had a relatively minor impact. Interestingly, combined with earlier work in the same system, our study suggests that the relative importance of species vs. individual level differences for ecosystem functioning will depend on the spatial scale considered. Ecological processes, including biodiversity-ecosystem-functioning relationships, cannot be understood without accounting for trait variation across biological scales of organization, including at fine scales.
Trait
Community
Functional ecology
Variation (astronomy)
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Trait
Community
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Abstract Aim To evaluate how environment and evolutionary history interact to influence global patterns of mammal trait diversity (a combination of 14 morphological and life‐history traits). Location The global terrestrial environment. Taxon Terrestrial mammals. Methods We calculated patterns of spatial turnover for mammalian traits and phylogenetic lineages using the mean nearest taxon distance. We then used a variance partitioning approach to establish the relative contribution of trait conservatism, ecological adaptation and clade specific ecological preferences on global trait turnover. Results We provide a global scale analysis of trait turnover across mammalian terrestrial assemblages, which demonstrates that phylogenetic turnover by itself does not predict trait turnover better than random expectations. Conversely, trait turnover is consistently more strongly associated with environmental variation than predicted by our null models. The influence of clade‐specific ecological preferences, reflected by the shared component of phylogenetic turnover and environmental variation, was considerably higher than expectations. Although global patterns of trait turnover are dependent on the trait under consideration, there is a consistent association between trait turnover and environmental predictive variables, regardless of the trait considered. Main conclusions Our results suggest that changes in phylogenetic composition are not always coupled with changes in trait composition on a global scale and that environmental conditions are strongly associated with patterns of trait composition across species assemblages, both within and across phylogenetic clades.
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Null model
Macroecology
Mammal
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Abstract Questions Community structure is the outcome of individual‐level interactions. Recent work has shown that disaggregating trait information from the species to the individual level can elucidate ecological processes. We aim to integrate trait dispersion analyses across different aggregation levels including a broad range of traits that allow assessment of patterns of variation among co‐occurring and non‐co‐occurring individuals. We ask the following questions: (1) what is the role of intra‐ and inter‐specific dissimilarity within neighbourhoods vs. across neighbourhoods in promoting trait dispersion; (2) how is trait variation partitioned across all individuals in each study system; and (3) are the results consistent across traits and forests? Location Puerto Rico and China. Methods We measured allocation and organ‐level (e.g. specific leaf area) traits on every individual in two seedling censuses in two tropical rain forests. Then, we partitioned trait variation within and across species, considering its impact on patterns of trait dispersion, and quantifying how these outcomes vary depending on whether allocation‐related or organ‐level traits are considered. Results We found an increase in trait dispersion when individual‐level traits are considered, reflecting conspecific differentiation for allocation of traits. Organ‐level traits, however, do not necessarily promote strong phenotypic displacement within conspecifics. Consistent with this, we found that the majority of variation in allocation of traits was between conspecifics, while most of the variation in organ‐level traits was found between species. Conclusions Overall, trait displacement occurs within and across neighbourhoods, reflecting differentiation at inter‐ and intra‐specific levels. Also, we identify two major phenotypic groups of variation, allocation and organ‐level traits, that constitute two contrasting strategies for response to biotic and abiotic contexts: one highlights ecological differences among individuals, while the other highlights differences among species.
Trait
Variation (astronomy)
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