Angiosperm growth habit,dispersal and diversification reconsidered

Summary Previous studies have sought to elucidate the relationship between dispersal mode (biotic versus abiotic) and the taxonomic diversification of angiosperm families, but with ambiguous results. In this study, we propose the hypothesis that the combination of (1) the large seed size required of plants germinating in closed, light-poor environments and (2) the necessity to move disseminules away from the maternal plant in order to avoid intraspecific competition, predation and pathogens should favour biotically-dispersed relative to abiotically-dispersed woody arborescent angiosperms, resulting in higher diversification of the former. In this paper, we seek patterns of diversification that support this hypothesis. We examine the association between dispersal mode, growth habit and taxonomic richness of monocotyledon and dicotyledon families using (1) contingency table analyses to detect the effect of dispersal mode on the relative abundances and diversification of woody versus herbaceous taxa and (2) non-parametric analyses of variance to detect the statistical effect of dispersal mode on taxonomic diversification (mean number of species per genus, genera per family and species per family) in monocot and dicot families dominated by biotic or abiotic dispersal. We found a clear statistical effect of dispersal mode on diversification. Among families of woody dicots, dispersal by vertebrates is associated with significantly higher levels of species per genus, genera per family and species per family than is abiotic dispersal. The same pattern is observed among woody monocots, but is not significant at the 0.05 level. Among families of herbaceous monocots and dicots, the situation is reversed, with abiotically-dispersed families exhibiting higher levels of diversification than vertebrate-dispersed families. When woody and herbaceous families are pooled, there is no association between dispersal mode and diversification. These data coincide with evidence from the fossil record to suggest vertebrate dispersal has positively contributed to the diversification of woody angiosperms. We suggest that vertebrate dispersal may have promoted the diversity of extant taxa by reducing the probability of extinction over evolutionary time, rather than by elevating speciation rates. Our results suggest vertebrate dispersal has contributed to, but does not explainin toto, the diversity of living angiosperms.     

Species and individual replacements contribute more than nestedness to shape vertebrate scavenger metacommunities

Understanding the mechanisms that organize biodiversity is central in ecology and conservation. Beta diversity links local (alfa) and regional (gamma) diversity, giving insight into how communities organize spatially. Metacommunity ecology provides the framework to interpret regional and local processes interacting to shape communities. However, the lack of metacommunity studies for large vertebrates may limit the understanding and compromise the preservation of ecosystem functions and services. We aim to understand the mechanisms underlying differences in species composition among vertebrate scavenger communities ? which provide key ecosystem functions, e.g. carrion consumption ? within a metacommunity context. We obtained species richness and abundances at scavenger communities consuming ungulate carcasses monitored through motion‐triggered remote cameras in seven terrestrial ecosystems in Spain. We partitioned beta diversity to decompose incidence‐based (species presence/absence) and abundance‐based dissimilarities into their components (turnover/balanced variation and nestedness/abundance gradient, respectively). We identified the environmental factors explaining the observed patterns. The vertebrate scavenger metacommunity consisted of 3101 individuals from 30 species. Changes in composition among ecosystems were mostly (> 84%) due to species or individual replacement (i.e. turnover or balanced variation). Species or individual loss/gain (i.e. nestedness or abundance gradient) accounted for 13–16% of these changes. Mean carcass weight, elevation and habitat diversity were the main factors explaining species/individual replacement. Our findings suggest that local processes such as species‐sorting through habitat heterogeneity would dominate scavenger metacommunity dynamics together with stochastic forces (i.e. related to carrion unpredictability and scavenging being a widespread strategy among vertebrates). The presence of structured patterns (i.e. nestedness) in beta diversity could reflect a role of deterministic processes: mass‐effects through dispersal and defaunation. Vultures are long‐distance foragers and functionally dominant species, which would connect local assemblages within the metacommunity, supporting scavenger diversity and functions across space. These results highlight the importance of managing vertebrate scavenger assemblages within a metacommunity context.     

Does size matter for dispersal distance?

Aim The aim of this study is to answer the questions: (1) do small organisms disperse farther than large, or vice versa; and (2) does the observed pattern differ for passive and active dispersers? These questions are central to several themes in biogeography (including microbial biogeography), macroecology, metacommunity ecology and conservation biology. Location The meta‐analysis was conducted using published data collected worldwide. Methods We collected and analysed 795 data values in the peer‐reviewed literature for direct observations of both maximal dispersal distance and mass of the dispersing organisms (e.g. seeds, not trees). Analysed taxa ranged in size from bacteria to whales. We applied macroecology analyses based on null models (using Monte Carlo randomizations) to test patterns relative to specific hypotheses. Results Collected dispersal distance and mass data spanned 9 and 21 orders of magnitude, respectively. Active dispersers dispersed significantly farther (P < 0.001) and were significantly greater in mass (P < 0.001) than passive dispersers. Overall, size matters: larger active dispersers attained greater maximum observed dispersal distances than smaller active dispersers. In contrast, passive‐disperser distances were random with respect to propagule mass, but not uniformly random, in part due to sparse data available for tiny propagules. Conclusions Size is important to maximal dispersal distance for active dispersers, but not for passive dispersers. Claims that microbes disperse widely cannot be tested by current data based on direct observations of dispersal: indirect approaches will need to be applied. Distance–mass relationships should contribute to a resolution of neutral and niche‐based metacommunity theories by helping scale expectations for dispersal limitation. Also, distance–mass relationships should inform analyses of latitudinal species richness and conservation biology topics such as fragmentation, umbrella species and taxonomic homogenization.     

Species diversity and population density affect genetic structure and gene dispersal in a subtropical understory shrub

Aims The dispersal of pollen and seeds is spatially restricted and may vary among plant populations because of varying biotic interactions, population histories or abiotic conditions. Because gene dispersal is spatially restricted, it will eventually result in the development of spatial genetic structure (SGS), which in turn can allow insights into gene dispersal processes. Here, we assessed the effect of habitat characteristics like population density and community structure on small-scale SGS and estimate historical gene dispersal at different spatial scales.Methods In a set of 12 populations of the subtropical understory shrub Ardisia crenata, we assessed genetic variation at 7 microsatellite loci within and among populations. We investigated small-scale genetic structure with spatial genetic autocorrelation statistics and heterogeneity tests and estimated gene dispersal distances based on population differentiation and on within-population SGS. SGS was related to habitat characteristics by multiple regression.Important findings The populations showed high genetic diversity (H e = 0.64) within populations and rather strong genetic differentiation (F ′ ST = 0.208) among populations, following an isolation-by-distance pattern, which suggests that populations are in gene flow–drift equilibrium. Significant SGS was present within populations (mean Sp = 0.027). Population density and species diversity had a joint effect on SGS with low population density and high species diversity leading to stronger small-scale SGS. Estimates of historical gene dispersal from between-population differentiation and from within-population SGS resulted in similar values between 4.8 and 22.9 m. The results indicate that local-ranged pollen dispersal and inefficient long-distance seed dispersal, both affected by population density and species diversity, contributed to the genetic population structure of the species. We suggest that SGS in shrubs is more similar to that of herbs than to trees and that in communities with high species diversity gene flow is more restricted than at low species diversity. This may represent a process that retards the development of a positive species diversity–genetic diversity relationship.     

Effect of landscape context on fish metacommunity structuring in stream networks

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Metacommunity organisation,spatial extent and dispersal in aquatic systems: patterns,processes and prospects

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The effects of species pool,dispersal and competition on the diversity–productivity relationship

Aim The diversity–productivity relationship is a controversial issue in ecology. Diversity is sometimes seen to increase with productivity but a unimodal relationship has often been reported. Competitive exclusion was cited initially to account for the decrease of diversity at high productivity. Subsequently, the roles of evolutionary history (species pool size) and dispersal rate have been acknowledged. We explore how the effects of species pool, dispersal and competition combine to produce different diversity–productivity relationships. Methods We use a series of simulations with a spatially explicit, individual‐based model. Following empirical expectations, we used four scenarios to characterize species pool size along the productivity gradient (uniformly low and high, linear increase and unimodal). Similarly, the dispersal rate varied along the productivity gradient (uniformly low and high, and unimodal). We considered both neutral communities and communities with competitive exclusion. Results and main conclusions Our model predicts that competitive interactions will result in unimodal diversity–productivity relationships. The model often predicts unimodal patterns in neutral communities as well, although the decline in richness at high productivity is less than in competing communities. A positive diversity–productivity relationship is simulated for neutral communities when the species pool size increases with productivity and the dispersal rate is high. This scenario is probably more widespread in nature than the others since positive diversity–productivity relationships have been observed more frequently than previously expected, especially in the tropics and for woody species. Our simulated effects of species pool, dispersal and competition on diversity patterns can be linked to empirical observations to uncover mechanisms behind the diversity–productivity relationship.     

Accounting for dispersal and biotic interactions to disentangle the drivers of species distributions and their abundances

Although abiotic factors, together with dispersal and biotic interactions, are often suggested to explain the distribution of species and their abundances, species distribution models usually focus on abiotic factors only. We propose an integrative framework linking ecological theory, empirical data and statistical models to understand the distribution of species and their abundances together with the underlying community assembly dynamics. We illustrate our approach with 21 plant species in the French Alps. We show that a spatially nested modelling framework significantly improves the model's performance and that the spatial variations of species presence-absence and abundances are predominantly explained by different factors. We also show that incorporating abiotic, dispersal and biotic factors into the same model bring new insights to our understanding of community assembly. This approach, at the crossroads between community ecology and biogeography, is a promising avenue for a better understanding of species co-existence and biodiversity distribution.     

Post-settlement dispersal: the neglected link in maintenance of soft-sediment biodiversity

Seafloor integrity is threatened by disturbances owing to human activities. The capacity of the system to recover from disturbances, as well as maintain resilience and function, depends on dispersal. In soft-sediment systems, dispersal continues after larval settlement, but there are very few measurements of how far the post-settlers disperse in nature. Spatial scales of post-settlement dispersal are, however, likely to be similar to pelagic larval dispersal because of continued, frequent, small-scale dispersal over longer periods. The consequences of this dispersal may be more important for the maintenance of biodiversity and metacommunity dynamics than is pelagic larval dispersal, because of the greater size and competency of the dispersers. We argue that an increased empirical understanding of post-settlement dispersal processes is key for predicting how benthic communities will respond to local disturbances and shrinking regional species pools, with implications for monitoring, managing and conserving biodiversity.     

A process‐based metacommunity framework linking local and regional scale community ecology

The metacommunity concept has the potential to integrate local and regional dynamics within a general community ecology framework. To this end, the concept must move beyond the discrete archetypes that have largely defined it (e.g. neutral vs. species sorting) and better incorporate local scale species interactions and coexistence mechanisms. Here, we present a fundamental reconception of the framework that explicitly links local coexistence theory to the spatial processes inherent to metacommunity theory, allowing for a continuous range of competitive community dynamics. These dynamics emerge from the three underlying processes that shape ecological communities: (1) density‐independent responses to abiotic conditions, (2) density‐dependent biotic interactions and (3) dispersal. Stochasticity is incorporated in the demographic realisation of each of these processes. We formalise this framework using a simulation model that explores a wide range of competitive metacommunity dynamics by varying the strength of the underlying processes. Using this model and framework, we show how existing theories, including the traditional metacommunity archetypes, are linked by this common set of processes. We then use the model to generate new hypotheses about how the three processes combine to interactively shape diversity, functioning and stability within metacommunities.     

Evolution of dispersal in metacommunities of interacting species

Theoretical studies on the evolution of dispersal in metacommunities are rare despite empirical evidence suggesting that interspecific interactions can modify dispersal behaviour of organisms. To understand the role of species interactions for dispersal evolution, we utilize an individual‐based model of a metacommunity where local population dynamics follows a stochastic version of the Nicholson–Bailey model and dispersal probability is an evolving trait. Our results show that in comparison with a neutral system (commensalism), parasitism promotes dispersal of hosts and parasites, while mutualism tends to reduce dispersal in both partners. Search efficiency of guests (only in the case of parasitism), dispersal mortality and external extinction risk can influence the evolution of dispersal of all partners. In systems composed of two host and two guest species, lower dispersal probabilities evolve under parasitism as well as mutualism than in one host and one guest species systems. This is because of frequency‐dependent modulations of dispersal benefits emerging in such systems for all partners.     

Condition‐dependent movement and dispersal in experimental metacommunities

Dispersal and the underlying movement behaviour are processes of pivotal importance for understanding and predicting metapopulation and metacommunity dynamics. Generally, dispersal decisions are condition‐dependent and rely on information in the broad sense, like the presence of conspecifics. However, studies on metacommunities that include interspecific interactions generally disregard condition‐dependence. Therefore, it remains unclear whether and how dispersal in metacommunities is condition‐dependent and whether rules derived from single‐species contexts can be scaled up to (meta)communities. Using experimental protist metacommunities, we show how dispersal and movement depend on and are adjusted by the strength of interspecific interactions. We found that the predicting movement and dispersal in metacommunities requires knowledge on behavioural responses to intra‐ and interspecific interaction strengths. Consequently, metacommunity dynamics inferred directly from single‐species metapopulations without taking interspecific interactions into account are likely flawed. Our work identifies the significance of condition‐dependence for understanding metacommunity dynamics, stability and the coexistence and distribution of species.     

Dispersal and diversity in experimental metacommunities: linking theory and practice

There has been a recent rise in the number of experiments investigating the effect of dispersal on diversity, with many of the predictions for these tests derived from metacommunity theory. Despite the promise of linking observed relationships between dispersal and diversity to underlying metacommunity processes, empirical studies have faced challenges in providing robust tests of theory. We review experimental studies that have tested how dispersal affects metacommunity diversity to determine why shortcomings emerge, and to provide a framework for empirical tests of theory that capture the processes structuring diversity in natural metacommunities. We first summarize recent experimental work to outline trends in results and to highlight common methods that cause a misalignment between empirical studies and the processes described by theory. We then identify the undesired implications of three widely used experimental methods that homogenize metacommunity structure or species traits, and present alternative methods that have been used to successfully integrate experiments and theory in a biologically relevant way. Finally, we present methodological and theoretical insights from three related ecological fields (coexistence, food web and priority effects theory) that, if integrated into metacommunity experiments, could help isolate the independent and joint effects of local interactions and dispersal on diversity, and reveal the mechanisms underlying observed dispersal–diversity patterns. Together, these methods can provide stronger tests of existing theory and stimulate new theoretical explorations. Synthesis Although metacommunity experiments offer a unique opportunity to test classic and emerging theory on the relationship between dispersal and diversity, several common challenges have hindered robust tests of theory. We outline how emerging theory on the invasion criterion, food webs and priority effects could be help clarify when and how dispersal affects metacommunity diversity, and identify when experimental approaches that homogenize metacommunities fail to test existing theory. By forging better links between theoretical and empirical work, we hope to motivate novel and improved experimental approaches to understanding the joint effects of local and regional processes on diversity.     

Dissecting NDVI–species richness relationships in Hawaiian dry forests

Aim A growing body of research has used the normalized difference vegetation index (NDVI) as a proxy for productivity to predict species richness. Yet the mechanisms that produce the relationship between NDVI and species richness remain unclear because of correlated biotic and abiotic factors that influence NDVI. In this study we investigated different biotic and abiotic effects that potentially drive plant species richness–productivity relationships. Location Hawaiian Islands, USA. Methods We quantified woody plant species richness, structure (density, basal area and canopy height), and species composition along a precipitation gradient of 14 Hawaiian dry forest plots. We then used structural equation models combined with 10 years of satellite data to disentangle the effects of precipitation, structure and NDVI‐estimated productivity on species richness. Results Underlying the simple correlation between NDVI and species richness was the indirect effect of precipitation and direct effect of forest structure. The best‐fit model showed there was no direct effect of NDVI on species richness. Main conclusions Our results demonstrate that complex relationships drive simple correlations between species richness and productivity. Considering the mechanisms and underlying factors driving NDVI–species richness relationships could improve predictions of species diversity as satellite measures of productivity have an increasingly important role in habitat mapping, species distribution modelling and predictions for global change.     

Dispersal and diversity – unifying scale‐dependent relationships within the neutral theory

The response of species diversity to dispersal capability is inherently scale‐dependent: increasing dispersal capability is expected to increase diversity at the local scale, while decreasing diversity at the metacommunity scale. However, these expectations are based on model formulations that neglect dispersal limitation and species segregation at the local scale. We developed a unifying framework of dispersal–diversity relationships and tested the generality of these expectations. For this purpose we used a spatially‐explicit neutral model with various combinations of survey area (local scale) and landscape size (metacommunity scale). Simulations were conducted using landscapes of finite and of conceptually infinite size. We analyzed the scale‐dependence of dispersal‐diversity relationships for exponentially‐bounded versus fat‐tailed dispersal kernels, several levels of speciation rate and contrasting assumptions on recruitment at short dispersal distances. We found that the ratio of survey area to landscape size is a major determinant of dispersal–diversity relationships. With increasing survey‐to‐landscape area ratio the dispersal–diversity relationship switches from monotonically increasing through a U‐shaped pattern (with a local minimum) to a monotonically decreasing pattern. Therefore, we provide a continuous set of dispersal–diversity relationships, which contains the response shapes reported previously as extreme cases. We suggest the mean dispersal distance with the minimum of species diversity (minimizing dispersal distance) for a certain scenario as a key characteristic of dispersal–diversity relationships. We show that not only increasing mean dispersal distances, but also increasing variances of dispersal can enhance diversity at the local scale, given a diverse species pool at the metacommunity scale. In conclusion, the response of diversity to variations of dispersal capability at spatial scales of interest, e.g. conservation areas, can differ more widely than expected previously. Therefore, land use and conservation activities, which manipulate dispersal capability, need to consider the landscape context and potential species pools carefully.     

Metacommunity structuring in stream networks: roles of dispersal mode,distance type,and regional environmental context

Within a metacommunity, both environmental and spatial processes regulate variation in local community structure. The strength of these processes may vary depending on species traits (e.g., dispersal mode) or the characteristics of the regions studied (e.g., spatial extent, environmental heterogeneity). We studied the metacommunity structuring of three groups of stream macroinvertebrates differing in their overland dispersal mode (passive dispersers with aquatic adults; passive dispersers with terrestrial adults; active dispersers with terrestrial adults). We predicted that environmental structuring should be more important for active dispersers, because of their better ability to track environmental variability, and that spatial structuring should be more important for species with aquatic adults, because of stronger dispersal limitation. We sampled a total of 70 stream riffle sites in three drainage basins. Environmental heterogeneity was unrelated to spatial extent among our study regions, allowing us to examine the effects of these two factors on metacommunity structuring. We used partial redundancy analysis and Moran's eigenvector maps based on overland and watercourse distances to study the relative importance of environmental control and spatial structuring. We found that, compared with environmental control, spatial structuring was generally negligible, and it did not vary according to our predictions. In general, active dispersers with terrestrial adults showed stronger environmental control than the two passively dispersing groups, suggesting that the species dispersing actively are better able to track environmental variability. There were no clear differences in the results based on watercourse and overland distances. The variability in metacommunity structuring among basins was not related to the differences in the environmental heterogeneity and spatial extent. Our study emphasized that (1) environmental control is prevailing in stream metacommunities, (2) dispersal mode may have an important effect on metacommunity structuring, and (3) some factors other than spatial extent or environmental heterogeneity contributed to the differences among the basins.     

Life‐history syndromes: Integrating dispersal through space and time

Recent research has highlighted interdependencies between dispersal and other life‐history traits, i.e. dispersal syndromes, thereby revealing constraints on the evolution of dispersal and opportunities for improved ability to predict dispersal by considering suites of dispersal‐related traits. This review adds to the growing list of life‐history traits linked to spatial dispersal by emphasising the interdependence between dispersal through space and time, i.e. life‐history diversity that distributes individuals into separate reproductive events. We reviewed the literature that has simultaneously investigated spatial and temporal dispersal to examine the prediction that traits of these two dispersal strategies are negatively correlated. Our results suggest that negative covariation is widely anticipated from theory. Empirical studies often reported evidence of weak negative covariation, although more complicated patterns were also evident, including across levels of biological organisation. Existing literature has largely focused on plants with dormancy capability, one or two phases of the dispersal process (emigration and/or transfer) and a single level of biological organisation (theory: individual; empirical: species). We highlight patterns of covariation across levels of organisation and conclude with a discussion of the consequences of dispersal through space and time and future research areas that should improve our understanding of dispersal‐related life‐history syndromes.     

Population synchrony decreases with richness and increases with environmental fluctuations in an experimental metacommunity

Fluctuations of local but connected populations may show correlation or synchrony whenever they experience significant dispersal or correlated environmental biotic and abiotic variability. Synchrony may be an important variable in multispecies systems, but its nature and implications have not been explicitly examined. Because the number of locally coexisting species (richness) affects the population variability of community members, we manipulated richness under different regimes of environmental fluctuation (EF). We predicted that the temporal synchrony of populations in a species should decline with increasing richness of the metacommunity they live in. Additionally, we predicted that specialist species that are sensitive to a specific environmental factor would show higher synchronization when EF increases. We thus created experimental communities with varied richness, EF, and species specialization to examine the synchronizing effects of these factors on three aquatic invertebrate species. We created four levels of richness and three levels of EF by manipulating the salinity of the culture media. Monocultures exhibited higher population synchrony than metacommunities of 2–4 species. Furthermore, we found that species responded differently to EF treatments: high EF enhanced population synchrony for the specialist and intermediate species, but not for the generalist species. Our findings emphasize that the magnitude of EF and species richness both contribute to determine population synchrony, and importantly, our results suggest that biotic diversity may actually stabilize metacommunities by disrupting synchrony.     

Which plant traits predict species loss in calcareous grasslands with extinction debt?

Aim Habitat loss and degradation pose a major threat to biodiversity, which can result in the extinction of habitat characteristic species. However, many species exhibit a delayed response to environmental changes because of the slow intrinsic dynamics of populations, resulting in extinction debt. We assess directly the changes in habitat characteristic species composition by comparing historical (1923) and current inventories in highly fragmented grasslands. We aim to characterize the species that constitute extinction debt in European calcareous grasslands. Location Europe, Estonia, 59–60° N, 24–25° E. Methods We related eleven life‐history traits and selected habitat preferences to local extinctions of populations in grasslands where extinction debt has been largely paid. Traits were chosen to describe species dispersal and persistence abilities and were quantified from databases. Results The studied grasslands have lost 90% of their area and 30% of their characteristic plant populations in 90 years. Species more prone to local population extinction were characterized by shorter life span, self‐pollination, a lack of clonal growth, fewer seeds per shoot, lower average height, lower soil nitrogen preference and higher requirements for light, indicating a limited ability to tolerate the range of changes in biotic and abiotic conditions of the sites. Locally extinct populations were also characterized by wind‐dispersed seeds, lower seed weight and lower terminal velocity of seeds, suggesting that species strategies for long‐distance dispersal are not favoured in highly fragmented landscapes. Thus, both increased habitat isolation and decreased habitat quality are important in determining local population extinction. Main conclusions Populations more prone to local extinction were characterized by a number of life‐history traits, demonstrating a greater extinction risk for species with poorer abilities for local persistence and competition. Our results can be applied to less degraded grasslands where the extinction debt is not yet paid to determine those species most susceptible to future extinction.