Assembly Theory Research Articles

Vulnerable Bodies. On the Visibility of Political Action

This article aims to examine the category of carnality and the bodily risk of modern protests from the perspective of visual culture studies. The theoretical basis of the proposed approach is Judith Butler's performative theory of assembly and her concept of „precarious life”, transferred to the practice of producing images of protesting bodies. I examine the political power of selected visual representations of an active and vulnerable body and trace the visual figure of the „precarious body” in its various forms - as an "icon" and a "poor image" - and the circulation of protest images in social media. An important analytical frame of this article is the case study of the Pepsi advertising spot, in which the "precarious body" figure was used for commercial purposes, and which I treat as a kind of visual meta-commentary on different practices of producing and using visual representations of bodies during the act of political resistance.

Ecosystem Assembly: A Mission for Terrestrial Earth System Science

Organisms not only respond to their environment but also influence the availability of resources and change environmental conditions. Hence, the impacts of organisms on their environment shape the selective regimes that drive, on ecological time scales, the assembly of ecological communities and, on evolutionary time scales, diversification. Recent studies have drawn attention to the fact that feedbacks between organisms and the environment can prevent or induce catastrophic transitions in ecosystem states and argue that climate change increases the likelihood of such catastrophic regime shifts. Ecologists have very limited ability to predict the likelihood of such regime shifts or the properties of the ecosystems that assemble after such collapses. This is because ecology does not have a theory of ecosystem assembly, nor does it have an established way of translating such a theory into models capable of predicting future ecosystem states. Without knowing these potential endpoints, we cannot develop strategies for coercing ecosystems into desired states, severely constraining our capacity to mitigate climate change and climate change impacts. This paper outlines a roadmap for developing a theory of terrestrial ecosystem assembly. Recent progress in dynamic global vegetation modelling and community assembly provides a useful foundation for a theory of ecosystem assembly. Environmental filtering and limiting similarity are key principles, but to be useful, they need to be linked to resource consumption and environmental modulation, and be more strongly constrained by biophysics and the trade-offs defined by biophysical principles. Such a theory recognises that ecological and evolutionary history ensures that many different ecosystem assemblies are possible at any given point in space and time.

Contrasting Patterns of Species Richness and Functional Diversity in Bird Communities of East African Cloud Forest Fragments.

Rapid fragmentation and degradation of large undisturbed habitats constitute major threats to biodiversity. Several studies have shown that populations in small and highly isolated habitat patches are prone to strong environmental and demographic stochasticity and increased risk of extinction. Based on community assembly theory, we predict recent rapid forest fragmentation to cause a decline in species and functional guild richness of forest birds combined with a high species turnover among habitat patches, and well defined dominance structures, if competition is the major driver of community assembly. To test these predictions, we analysed species co-occurrence, nestedness, and competitive strength to infer effects of interspecific competition, habitat structure, and species′ traits on the assembly of bird species communities from 12 cloud forest fragments in southern Kenya. Our results do not point to a single ecological driver of variation in species composition. Interspecific competition does not appear to be a major driver of species segregation in small forest patches, while its relative importance appears to be higher in larger ones, which may be indicative for a generic shift from competition-dominated to colonisation-driven community structure with decreasing fragment size. Functional trait diversity was independent of fragment size after controlling for species richness. As fragmentation effects vary among feeding guilds and habitat generalists, in particular, tend to decline in low quality forest patches, we plead for taking species ecology fully into account when predicting tropical community responses to habitat change.

Ecospace: A unified framework for understanding variation in terrestrial biodiversity

Understanding patterns in biodiversity is a core ambition in ecological research. Existing ecological theories focusing on individual species, populations, communities, or niches aid in understanding the determinants of biodiversity patterns, yet very few general models for biodiversity have emerged from simplistic approaches. We propose that a systematic, low-dimensional representation of environmental space with building blocks adopted from gradient, niche, metapopulation and assembly theory may unite old and new aspects of biodiversity theory and improve our understanding of variation in terrestrial biodiversity.We propose the term ecospace to cover the local conditions and resources underlying diversity. Our definition of ecospace encompasses abiotic position, biotic expansion and spatiotemporal continuity, which all affect the biodiversity of a biotope (α-diversity). Position refers to placement along abiotic gradients such as temperature, soil pH and fertility, leading to environmental filtering known from classical community theory. Expansion represents the build-up and diversification of organic matter that are not strictly given by position. Continuity refers to the spatiotemporal extension of position and expansion.Biodiversity is scale dependent. The contribution of one biotope to large scale diversity must be estimated by considering its unique contribution to the species richness of the surrounding landscape or region or to the biodiversity of the entire planet. In addition to the relationship between ecospace and biotope richness (α-diversity), we also propose a relation between the uniqueness of the biotope ecospace and the unique contribution of species to the surrounding larger-scale richness.Whereas the impacts of ecospace position and continuity on biodiversity have been studied in isolation, studies comparing or combining them are rare. Furthermore, biotic expansion has never been fully developed as a determinant of biodiversity, ignoring the megadiverse carbon-depending groups of insects and fungi. Precursors of the ecospace concept have been presented over the last 70 years, but they were never fully developed conceptually for terrestrial biodiversity or applied to prediction of biodiversity.Ecospace unites classical and – at times – contradicting theories such as niche theory, island biogeography theory and a suite of community assembly theories into one framework for further development of a general theory of terrestrial biodiversity.

A Neural Assembly–Based View on Word Production: The Bilingual Test Case

I will propose a tentative framework of how words in two languages could be organized in the cerebral cortex based on neural assembly theory, according to which neurons that fire synchronously are bound into large‐scale distributed functional units (assemblies), which represent a mental event as a whole (“gestalt”). For language this means a word is engendered by widely distributed cell assemblies in which the different linguistic constituents are grouped together in action‐perception circuits and become activated in parallel. In this article I will discuss the advantages of assembly coding over the traditional hierarchical convergence scheme. Recent evidence from language comprehension and production supporting the notion of neural assemblies representing words are discussed and a spatiotemporal model for word production based on this concept is presented. Finally, I will show how this view may be generalized to bilingualism and explain mechanistically some key phenomena in the literature.

Macroinvertebrate community assembly in pools created during peatland restoration

Many degraded ecosystems are subject to restoration attempts, providing new opportunities to unravel the processes of ecological community assembly. Restoration of previously drained northern peatlands, primarily to promote peat and carbon accumulation, has created hundreds of thousands of new open water pools. We assessed the potential benefits of this wetland restoration for aquatic biodiversity, and how communities reassemble, by comparing pool ecosystems in regions of the UK Pennines on intact (never drained) versus restored (blocked drainage-ditches) peatland. We also evaluated the conceptual idea that comparing reference ecosystems in terms of their compositional similarity to null assemblages (and thus the relative importance of stochastic versus deterministic assembly) can guide evaluations of restoration success better than analyses of community composition or diversity. Community composition data highlighted some differences in the macroinvertebrate composition of restored pools compared to undisturbed peatland pools, which could be used to suggest that alternative end-points to restoration were influenced by stochastic processes. However, widely used diversity metrics indicated no differences between undisturbed and restored pools. Novel evaluations of restoration using null models confirmed the similarity of deterministic assembly processes from the national species pool across all pools. Stochastic elements were important drivers of between-pool differences at the regional-scale but the scale of these effects was also similar across most of the pools studied. The amalgamation of assembly theory into ecosystem restoration monitoring allows us to conclude with more certainty that restoration has been successful from an ecological perspective in these systems. Evaluation of these UK findings compared to those from peatlands across Europe and North America further suggests that restoring peatland pools delivers significant benefits for aquatic fauna by providing extensive new habitat that is largely equivalent to natural pools. More generally, we suggest that assembly theory could provide new benchmarks for planning and evaluating ecological restoration success.

Experimental insight into the process of parasite community assembly.

Community assembly is a fundamental process that has long been a central focus in ecology. Extending community assembly theory to communities of co-infecting parasites, we used a gastrointestinal nematode removal experiment in free-ranging African buffalo to examine the community assembly patterns and processes. We first asked whether reassembled communities differ from undisturbed communities by comparing anthelmintic-treated and control hosts. Next, we examined the temporal dynamics of assembly using a cross-section of communities that reassembled for different periods of time since last experimental removal. Next, we tested for evidence of assembly processes that might drive such reassembly patterns: environmental filtering based on host traits (i.e. habitat patches), interspecific interactions, priority effects and chance dispersal from the environmental pool of infective stages (i.e. the regional species pool). On average, reassembled parasite communities had lower abundance, but were more diverse and even, and these patterns varied tightly with reassembly time. Over time, the communities within treated hosts progressively resembled controls as diversity and evenness decreased, while total abundance increased. Notably, experimental removal allowed us to attribute observed differences in abundance, diversity and evenness to the process of community assembly. During early reassembly, parasite accumulation was biased towards a subordinate species and, by excluding stochastic assembly processes (i.e. chance dispersal and priority effects), we were able to determine that early assembly is deterministic. Later in the reassembly process, we established that host traits, as well as stochastic dispersal from the environmental pool of infective stages, can affect the community composition. Overall, our results suggest that there is a high degree of resiliency and environmental dependence to the worm communities of buffalo. More generally, our data show that both deterministic and stochastic processes may play a role in the assembly of parasite communities of wild hosts, but their relative importance may vary temporally. Consequently, the best strategy for managing reassembling parasite communities may also need to shift over time.

Faunal community consequence of interspecific bark trait dissimilarity in early‐stage decomposing logs

SummaryDead tree trunks have significant ecosystem functions related to biodiversity and biogeochemical cycles. When lying on the soil surface, they are colonized by an array of invertebrate fauna, but what determines their community composition is still unclear.We apply community assembly theory to colonization of tree logs by invertebrates. During early decomposition, the attached bark is critically important as an environment filter for community assembly through habitat provision. Specifically, we hypothesized that the more dissimilar bark traits were between tree species, the more their faunal community compositions would differ.We tested this hypothesis by investigating the effects of bark traits on the invertebrate communities in the early‐decomposing logs of 11 common, temperate tree species placed in the ‘common garden’ experimentLOGLIFE. Bark traits included bark looseness, fissure index, outer bark thickness, ratio of inner to outer bark thickness, punch resistance, water storage capacity and barkpH. The predominant faunal groups studied were Annelida, Isopoda, Chilopoda, Diplopoda, Diptera and Coleoptera.Our results showed (i) strong interspecific differences in bark traits, (ii) that bark traits related to environmental buffering had profound effects on the abundance of specific invertebrate groups, and (iii) the higher the overall bark trait dissimilarity between tree species, the more dissimilar these tree species were in faunal community composition, and the higher was the joint invertebrate family richness.A suite of bark traits together has fundamental afterlife effects on invertebrate community assembly, strongly filtering the colonizing invertebrates in early‐decomposing logs, driving variation in their community composition and diversity. Our findings indicate that bark trait dissimilarity among tree species in forest stands is likely a better indicator of early‐phase dead trunk fauna diversity than tree species diversityper se.Alay summaryis available for this article.

Same nurse but different time: temporal divergence in the facilitation of plant lineages with contrasted functional syndromes

Summary Plant facilitation assembles functionally contrasted communities in dry lands. Nurse plants are often early colonizers with xerophytic traits that were mostly selected during the dry Quaternary (Quaternary syndrome), while beneficiary plants tend to be late‐successional species with mesophytic traits that evolved mainly during the more humid Tertiary (Tertiary syndrome). Integrating plant facilitation within the community assembly theory requires a better understanding of the ontogenetic development of the nursing abilities that benefit functionally contrasted species. We assessed whether the same nurse plant facilitates species with Quaternary and Tertiary syndromes in an ecosystem under severe abiotic stress conditions imposed by aridity and gypsum soil toxicity. We hypothesized that both functional types find suitable microsites for seedling establishment underneath the same nurse but their optimal regeneration niches are temporally segregated along the ontogenetic development of the nurse. We carried out a sowing experiment along a 40‐year ontogenetic gradient of the nurse shrub Ononis tridentata. Seeds from five Tertiary and five Quaternary species were sown and seedling emergence monitored. While the nurse age did not affect the seedling emergence of Quaternary species, it significantly increased that of Tertiary species. These results were corroborated for elder ontogenetic stages in non‐manipulated plants in the field. Juveniles of Quaternary species were able to grow beneath nurse plants along their whole ontogenetic gradient excepting beneath Ononis seedlings, while Tertiary species were only facilitated by mature nurses. Synthesis. Our results show that plant nursing abilities evolve ontogenetically in a different way for beneficiary Quaternary and Tertiary plant lineages. The finding of a plant species that plays a role as key assembler of early‐ and late‐successional species in plant dynamics broadens the scope of facilitation in the community assembly theory. A lay summary is available for this article.

Notes toward a performative theory of assembly

Judith Butler elucidates dynamics of public assembly under prevailing economic and political conditions, analyzing what they signify and how. Understanding assemblies as plural forms of performative action, Butler extends her theory of performativity to argue that precarity-the destruction of conditions of livability-has been a galvanizing force and theme in today's highly visible protests. Butler broadens theory of performativity beyond speech acts to include concerted actions of body. Assemblies of physical bodies have an expressive dimension that cannot be reduced to speech, for very fact of gathering says something without always relying on speech. Drawing on Hannah Arendt's view of action, yet revising her claims about role of body in politics, Butler asserts that embodied ways of coming together, including forms of long-distance solidarity, imply a new understanding of public space of appearance essential to politics. Butler links assembly with precarity by pointing out that a body suffering under conditions of precarity still persists and resists, and that mobilization brings out this dual dimension of corporeal life. Just as assemblies make visible and audible bodies that require basic freedoms of movement and association, so do they expose coercive practices in prison, dismantling of social democracy, and continuing demand for establishing subjugated lives as mattering, as equally worthy of life. By enacting a form of radical solidarity in opposition to political and economic forces, a new sense of the people emerges, interdependent, grievable, precarious, and persistent.

Community assembly and coexistence in communities of arbuscular mycorrhizal fungi.

Arbuscular mycorrhizal fungi are asexual, obligately symbiotic fungi with unique morphology and genomic structure, which occupy a dual niche, that is, the soil and the host root. Consequently, the direct adoption of models for community assembly developed for other organism groups is not evident. In this paper we adapted modern coexistence and assembly theory to arbuscular mycorrhizal fungi. We review research on the elements of community assembly and coexistence of arbuscular mycorrhizal fungi, highlighting recent studies using molecular methods. By addressing several points from the individual to the community level where the application of modern community ecology terms runs into problems when arbuscular mycorrhizal fungi are concerned, we aim to account for these special circumstances from a mycocentric point of view. We suggest that hierarchical spatial structure of arbuscular mycorrhizal fungal communities should be explicitly taken into account in future studies. The conceptual framework we develop here for arbuscular mycorrhizal fungi is also adaptable for other host-associated microbial communities.

MONTAGEM DE COMUNIDADES: CONCEITOS, DOMÍNIO E ESTRUTURA TEÓRICA

COMMUNITY ASSEMBLY: CONCEPTS, DOMAIN AND THEORETICAL STRUCTURE Community assembly refers to any ecological process that operates to select species from a regional pool and put them together in local communities. Initially, Diamond (1975) proposed a community assembly model based on rules explaining the checkerboard distribution of co-occurring bird species in New Guinea archipelago. By that time, the assembly rules proposed interspecific competition as the only plausible explanation for the patterns observed. However, the lack of testable predictions, inappropriate statistical analyses and, mainly, the absence of alternative hypotheses have represented an obstacle for the theory development. With the introduction of phylogenetic/functional analyses in community studies, a meaningful increase of published papers on community assembly has been noticed, mainly due to the development of models providing more predictive power to assembly processes. Hence, we decided to elaborate a critical review about the conceptual progress in the community assembly theory. We point out that a) the community assembly theory has enhanced its predictive power when it became to be associated with solid statistics; b) the hypothesis test by null models has been reinforced as a consistent data analysis method applied in many studies; and c) the neutral theory has been used as a satisfactory alternative to the coexistence hypothesis by deterministic niche-based processes. However, the community assembly theory is not fully mature yet but has a high potential of near-future development, mainly through the consolidation of its conceptual base. We hope that our review can help in this development by systematizing the community assembly theory and clarifying some misunderstood and/or so-far poorly applied concepts.

Stochastic dilution effects weaken deterministic effects of niche-based processes in species rich forests.

Recent theory predicts that stochastic dilution effects may result in species-rich communities with statistically independent species spatial distributions, even if the underlying ecological processes structuring the community are driven by deterministic niche differences. Stochastic dilution is a consequence of the stochastic geometry of biodiversity where the identities of the nearest neighbors of individuals of a given species are largely unpredictable. Under such circumstances, the outcome of deterministic species interactions may vary greatly among individuals of a given species. Consequently, nonrandom patterns in the biotic neighborhoods of species, which might be expected from coexistence or community assembly theory (e.g., individuals of a given species are neighbored by phylogenetically similar species), are weakened or do not emerge, resulting in statistical independence of species spatial distributions. We used data on phylogenetic and functional similarity of tree species in five large forest dynamics plots located across a gradient of species richness to test predictions of the stochastic dilution hypothesis. To quantify the biotic neighborhood of a focal species we used the mean phylogenetic (or functional) dissimilarity of the individuals of the focal species to all species within a local neighborhood. We then compared the biotic neighborhood of species to predictions from stochastic null models to test if a focal species was surrounded by more or less similar species than expected by chance. The proportions of focal species that showed spatial independence with respect to their biotic neighborhoods increased with total species richness. Locally dominant, high-abundance species were more likely to be surrounded by species that were statistically more similar or more dissimilar than expected by chance. Our results suggest that stochasticity may play a stronger role in shaping the spatial structure of species rich tropical forest communities than it does in species poorer forests. These findings represent an important step towards understanding the factors that govern the spatial configuration of local biotic communities. The stochastic dilution effect is a simple geometric mechanism that can explain why species' spatial distributions in species-rich communities approximate independence from their biotic neighborhood, even if deterministic niche processes are in effect.

Environmental heterogeneity has a weak effect on diversity during community assembly in tallgrass prairie

Abstract Understanding what constrains the persistence of species in communities is at the heart of community assembly theory and its application to conserving and enhancing biodiversity. The “environmental heterogeneity hypothesis” predicts greater species coexistence in habitats with greater resource variability. In the context of community assembly, environmental heterogeneity may influence the variety and strength of abiotic conditions and competitive interactions (environmental filters) to affect the relative abundance of species and biodiversity. We manipulated key resources that influence plant diversity in tallgrass prairie (i.e., soil depth and nitrogen availability) to increase environmental heterogeneity prior to sowing native prairie species into a former agricultural field. We compared variability in nutrient availability, aboveground annual net primary productivity (ANPP), and the composition of species between replicate plots containing soil heterogeneity manipulations and plots with no resource manipulations (n = 4 per treatment) during the first 15 yr of community assembly as a test of the “environmental heterogeneity hypothesis.” The manipulations increased environmental heterogeneity, measured as the coefficient of variation in NO3‐N availability and ANPP. Plant diversity, however, was similar and decayed exponentially and indiscriminately over time between the heterogeneity treatments. Species richness declined linearly over time in both heterogeneity treatments, but richness was higher in the more heterogeneous soil 2 yr following a second propagule addition 8 yr after the initial sowing. As a result, there was a lower rate of species loss over time in the more heterogeneous soil (0.60 species yr−1) relative to the control soil (0.96 species yr−1). Communities in each treatment exhibited strong convergence over time resulting from a shift in dominant species across all treatments and a gradual increase in the clonal C4 grass, Andropogon gerardii. We attribute the weak effect of heterogeneity on diversity to increasing dominance of a clonal species, which decreased the scale of soil treatments relative to plant size, dispersal limitation, and absence of a key driver (grazing) known to increase plant diversity under a frequent fire regime. Thus, steering community assembly to attain high biodiversity may depend more on manipulating processes that reduce dominance and facilitate the arrival of new species than promoting environmental heterogeneity.

Preface to Special Topic on Protein Dynamics: Beyond Static Snapshots in Structural Biology.

As structural biology moves beyond the over dependence on the static snapshots to a deeper understanding of the role of dynamics in the function of the molecules, it has never been more apparent that experiment, theory, and simulation must be all considered together in our discourse. This special issue of Structural Dynamics is devoted to the subject of protein dynamics and comprises articles that span these areas nicely and show how the combined information coming from them has completely changed the field. The papers include descriptions of new experimental crystallographic evidence on the flexibility of loop dynamics in an enzyme involved in natural product biosynthesis (Han et al., 2016); the effects of quaternary structure on the dynamics of pilin as measured by HD exchange methods (Lento et al., 2016) and as predicted by simulations in hemoglobins (Gupta and Meuwly, 2016); efficient methods for constraining atomic level molecular dynamics trajectories (Chandrasekaran et al., 2016), theory and simulations of assembly of complicated fibrous structures in muscle contraction (Fischer et al., 2016); and theoretical methods for defining sections of proteins that form fairly rigid segments as a dimensionality reduction tool (Streinu, 2016).

Intersubjetividad y domesticación en el devenir de una región global: la territorialización del salmón en la Patagonia chilena

Este artículo examina transformaciones en la Patagonia chilena, región líder en la producción de salmón para mercados globalizados. Utilizando una aproximación etnográfica, se abordan las posibilidades de considerar las intersubjetividades en los procesos de conformación de regiones importantes para la producción global de alimentos, argumentando contra algunas perspectivas que enfatizan en la interobjetividad al explorar relaciones humanas y no humanas. En un mundo complejo y globalizado, la teoría del ensamblaje permite comprender cómo las transformaciones regionales son estimuladas por formas de biopoder que generan nuevas relaciones entre vida, agencia y naturaleza. Se rastrean experimentos, encuentros y afectos, descentrando así miradas convencionales sobre domesticación de especies, desarrollo regional y producción de alimentos, y abriendo un debate sobre las potencialidades y límites de interpretaciones no lineales de la realidad.

EXPLORING THE DYNAMICS DURING COMMUNITY ASSEMBLY THROUGH COMMUNITY PHYLOGENETICS

Species diversify through speciation and accumulate in ecological community, a process known as community assembly. Relying on both evolutionary mechanisms acting at regional scale and ecological mechanisms acting at local scale, the process of community assembly results from intricate interactions among mechanisms at play across varying spatial and temporal scales. During the last decade, community assembly theory has been reconsidered in the light of evolutionary dynamics of species diversification and ecological dynamics have been formalized in an explicit spatial framework (i.e. metacommunity theory). The aims of the present review are: (1) to present the community assembly theory and the main paradigms that have been proposed, (2) to discuss how the metacommunity theory as defined an explicit spatial framework for community ecology, (3)to discuss the potential mechanisms at play during community assembly and their associated predictions, (4) to present new approaches to study community assembly based on phylogenetics approaches and discuss how they have been integrated in empirical studies.

Increased taxonomic and functional similarity does not increase the trophic similarity of communities

AbstractAimDespite a long‐standing research interest in the association between the biodiversity (i.e. taxonomic and functional composition) and trophic structure of communities, our understanding of the relationship remains limited. Community assembly theory predicts that niche partitioning will result in communities with a diverse array of functional traits, which in turn may facilitate a correspondingly diverse array of trophic interactions that define the trophic niche of those communities. The aim of our study is to test this prediction.LocationNorth America.MethodsWe built a database composed of functional traits and stable isotope values (δ13C and δ15N) for 63 freshwater fish communities containing 109 species in 34 lentic and 29 lotic ecosystems. First, using linear mixed models (i.e. an alpha‐diversity approach), we tested whether the taxonomic diversity of communities was positively associated with their functional diversity and if their functional diversity was positively associated with their trophic diversity. Second, we assessed the taxonomic, functional and trophic similarity of communities using multiple regression on distance matrices (MRM) and their respective ‘turnover’ and ‘nestedness‐resultant’ components to test if the taxonomic similarity of communities was positively correlated with their functional similarity and if their functional similarity was positively associated with their trophic similarity (i.e. a beta‐diversity approach).ResultsWe found that the functional diversity of communities increased as their taxonomic diversity increased. Similarly, the trophic diversity of communities increased as their functional diversity increased. The pairwise taxonomic and functional similarity of communities were also positively associated, but there was a weak relationship between the functional and trophic similarities of communities.Main conclusionsOur study demonstrates that communities with similar functional characteristics can have disparate food web structures, suggesting that additional site‐specific factors influence community variation in trophic niche geometry. Determining the relative importance of functional characteristics and site‐specific factors in shaping trophic interactions is crucial for a better understanding of how future species loss and species introductions will affect food web structure and ecosystem functioning.

Diversity Increases Indirect Interactions, Attenuates the Intensity of Competition, and Promotes Coexistence.

A fundamental assumption of coexistence theory is that competition inevitably decreases species diversity. Consequently, in the quest to understand the ecological regulators of diversity, there has been a great deal of focus on processes with the potential to reduce competitive exclusion. However, the notion that competition must decrease diversity is largely based on the outcome of two-species interaction experiments and models, despite the fact that species rarely interact only in pairs in natural systems. In a field experiment, we found that competition among native perennial plants in multispecies assemblages was far weaker than competition between those same species in pairwise arrangements and that indirect interactions appeared to weaken direct competitive effects. These results suggest that community assembly theory based on pairwise approaches may overestimate the strength of competition and likelihood of competitive exclusion in species-rich communities. We also found that Centaurea stoebe, a North American invader, retained strong competitive effects when competing against North American natives in both pairwise and multispecies assemblages. Our experimental results support an emerging body of theory suggesting that complex networks of competing species may generate strong indirect interactions that can maintain diversity and that ecological differentiation may not be necessary to attenuate competition.

Intense ranchland management tips the balance of regional and local factors affecting wetland community structure

Greater understanding of land management effects on species richness and composition is needed to manage biodiversity-related ecosystem services in agricultural ecosystems. Here we studied responses of vascular plants, macroinvertebrates, and ectothermic vertebrates to local and regional factors in two ranchland management intensities (semi-natural and highly-managed). Samples were collected in 40 separate wetlands embedded in the two management intensities. Based on community assembly theory, we expected a shift from regional to local-based factors as predictors of communities where local conditions become more limiting. We also expected highly managed ranchlands to most strongly “filter” community membership to become more homogeneous. Both predictions were supported though results sometimes differed among taxa. Wetland communities embedded in highly managed pastures were more homogeneous and significantly different from those in semi-natural pastures. Overall, regional factors related to propagule dispersal were most important in semi-natural pastures, and local factors (especially nutrients) were most important in highly managed pastures. However, some community metrics were consistently affected most by local or regional factors regardless of land use intensity. For instance, vegetation richness and vertebrate composition were consistently influenced by local factors, whereas macroinvertebrate composition was consistently influenced by regional factors. Based on our results, biodiversity conservation will be most effective if management efforts focus on improving local habitat conditions in highly managed areas, and on preserving regional heterogeneity in more natural areas. Taxon-specific conservation strategies should account for taxon-specific sensitivities to local and regional processes.