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      Environmental harshness drives spatial heterogeneity in biotic resistance

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      NeoBiota

      Pensoft Publishers

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          Abstract

          Ecological communities often exhibit greater resistance to biological invasions when these communities consist of species that are not closely related. The effective size of this resistance, however, varies geographically. Here we investigate the drivers of this heterogeneity in the context of known contributions of native trees to the resistance of forests in the eastern United States of America to plant invasions. Using 42,626 spatially referenced forest community observations, we quantified spatial heterogeneity in relationships between evolutionary relatedness amongst native trees and both invasive plant species richness and cover. We then modelled the variability amongst the 91 ecological sections of our study area in the slopes of these relationships in response to three factors known to affect invasion and evolutionary relationships –environmental harshness (as estimated via tree height), relative tree density and environmental variability. Invasive species richness and cover declined in plots having less evolutionarily related native trees. The degree to which they did, however, varied considerably amongst ecological sections. This variability was explained by an ecological section’s mean maximum tree height and, to a lesser degree, SD in maximum tree height ( R 2 GLMM = 0.47 to 0.63). In general, less evolutionarily related native tree communities better resisted overall plant invasions in less harsh forests and in forests where the degree of harshness was more homogenous. These findings can guide future investigations aimed at identifying the mechanisms by which evolutionary relatedness of native species affects exotic species invasions and the environmental conditions under which these effects are most pronounced.

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          Consequences of dominance: a review of evenness effects on local and regional ecosystem processes.

          The composition of communities is strongly altered by anthropogenic manipulations of biogeochemical cycles, abiotic conditions, and trophic structure in all major ecosystems. Whereas the effects of species loss on ecosystem processes have received broad attention, the consequences of altered species dominance for emergent properties of communities and ecosystems are poorly investigated. Here we propose a framework guiding our understanding of how dominance affects species interactions within communities, processes within ecosystems, and dynamics on regional scales. Dominance (or the complementary term, evenness) reflects the distribution of traits in a community, which in turn affects the strength and sign of both intraspecifc and interspecific interactions. Consequently, dominance also mediates the effect of such interactions on species coexistence. We review the evidence for the fact that dominance directly affects ecosystem functions such as process rates via species identity (the dominant trait) and evenness (the frequency distribution of traits), and indirectly alters the relationship between process rates and species richness. Dominance also influences the temporal and spatial variability of aggregate community properties and compositional stability (invasibility). Finally, we propose that dominance affects regional species coexistence by altering metacommunity dynamics. Local dominance leads to high beta diversity, and rare species can persist because of source-sink dynamics, but anthropogenically induced environmental changes result in regional dominance and low beta diversity, reducing regional coexistence. Given the rapid anthropogenic alterations of dominance in many ecosystems and the strong implications of these changes, dominance should be considered explicitly in the analysis of consequences of altered biodiversity.
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            Phylogenetic patterns are not proxies of community assembly mechanisms (they are far better)

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              A guide to phylogenetic metrics for conservation, community ecology and macroecology

              ABSTRACT The use of phylogenies in ecology is increasingly common and has broadened our understanding of biological diversity. Ecological sub‐disciplines, particularly conservation, community ecology and macroecology, all recognize the value of evolutionary relationships but the resulting development of phylogenetic approaches has led to a proliferation of phylogenetic diversity metrics. The use of many metrics across the sub‐disciplines hampers potential meta‐analyses, syntheses, and generalizations of existing results. Further, there is no guide for selecting the appropriate metric for a given question, and different metrics are frequently used to address similar questions. To improve the choice, application, and interpretation of phylo‐diversity metrics, we organize existing metrics by expanding on a unifying framework for phylogenetic information. Generally, questions about phylogenetic relationships within or between assemblages tend to ask three types of question: how much; how different; or how regular? We show that these questions reflect three dimensions of a phylogenetic tree: richness, divergence, and regularity. We classify 70 existing phylo‐diversity metrics based on their mathematical form within these three dimensions and identify ‘anchor’ representatives: for α‐diversity metrics these are PD (Faith's phylogenetic diversity), MPD (mean pairwise distance), and VPD (variation of pairwise distances). By analysing mathematical formulae and using simulations, we use this framework to identify metrics that mix dimensions, and we provide a guide to choosing and using the most appropriate metrics. We show that metric choice requires connecting the research question with the correct dimension of the framework and that there are logical approaches to selecting and interpreting metrics. The guide outlined herein will help researchers navigate the current jungle of indices.
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                Author and article information

                Journal
                NeoBiota
                NB
                Pensoft Publishers
                1314-2488
                1619-0033
                December 04 2018
                December 04 2018
                : 40
                : 87-105
                Article
                10.3897/neobiota.40.28558
                © 2018

                https://creativecommons.org/share-your-work/public-domain/cc0/

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