Niche versus neutrality in structuring the beta diversity of damselfly assemblages

1. Differences among communities in taxonomic composition – beta diversity – are frequently expected to result from taxon‐specific responses to spatial variation in ecological conditions, through niche partitioning. Such process‐derived patterns are in sharp contrast to arguments from neutral theory, where taxa are ecologically equivalent and beta diversity results primarily from dispersal limitation. 2. Here, we compared beta diversity among assemblages of damselflies (Odonata: Zygoptera), for which previous experiments have shown that niche differences maintain genera within a community, but patterns of relative abundance for species within each genus are shaped primarily by neutral dynamics. 3. Using null‐model and ordination‐based methods, we find that both genera and (in contrast to neutral theory) species assemblage composition vary across the landscape in a deterministic fashion, shaped by environmental and spatial factors. 4. While the observed patterns in species composition conflict with theory, we suggest that this a result of weak ecological filters acting to produce spatial variation in assemblages of ecologically similar species undergoing ecological drift within communities. Such patterns are especially likely in systems of relatively weak dispersers like damselflies.     

Intraspecific phenotypic variation in a fish predator affects multitrophic lake metacommunity structure

Contemporary insights from evolutionary ecology suggest that population divergence in ecologically important traits within predators can generate diversifying ecological selection on local community structure. Many studies acknowledging these effects of intraspecific variation assume that local populations are situated in communities that are unconnected to similar communities within a shared region. Recent work from metacommunity ecology suggests that species dispersal among communities can also influence species diversity and composition but can depend upon the relative importance of the local environment. Here, we study the relative effects of intraspecific phenotypic variation in a fish predator and spatial processes related to plankton species dispersal on multitrophic lake plankton metacommunity structure. Intraspecific diversification in foraging traits and residence time of the planktivorous fish alewife (Alosa pseudoharengus) among coastal lakes yields lake metacommunities supporting three lake types which differ in the phenotype and incidence of alewife: lakes with anadromous, landlocked, or no alewives. In coastal lakes, plankton community composition was attributed to dispersal versus local environmental predictors, including intraspecific variation in alewives. Local and beta diversity of zooplankton and phytoplankton was additionally measured in response to intraspecific variation in alewives. Zooplankton communities were structured by species sorting, with a strong influence of intraspecific variation in A. pseudoharengus. Intraspecific variation altered zooplankton species richness and beta diversity, where lake communities with landlocked alewives exhibited intermediate richness between lakes with anadromous alewives and without alewives, and greater community similarity. Phytoplankton diversity, in contrast, was highest in lakes with landlocked alewives. The results indicate that plankton dispersal in the region supplied a migrant pool that was strongly structured by intraspecific variation in alewives. This is one of the first studies to demonstrate that intraspecific phenotypic variation in a predator can maintain contrasting patterns of multitrophic diversity in metacommunities.     

Comparing primate communities: a multivariate approach

Although there have been many studies of the ecology of primates in communities throughout the world, there have been few attempts to compare community ecology within and among continents. In this study the ecological characteristics of the sympatric primate species at eight localities—two from each of the major biogeographic areas inhabited by primates today—South America, Africa, Madagascar, and Asia—were compared using a multivariate technique (principal components analysis of the correlation matrix) to summarize the ten dimensional ecological niche space. The most striking clustering of species in ecological multivariate space is according to phylogeny with closely related species showing similar ecological features. Likewise, the ecological characteristics of individual communities are determined by phylogenetic groups present at each locality or biogeographic region. As a result, communities within any biogeographical region are more similar ecologically to one another than to communities from other continental areas. In several measures of ecological diversity among the species comprising each community, the neotropical communities show lower overall diversity than do communities from other continents.     

Convergence, divergence, and homogenization in the ecological structure of emydid turtle communities: the effects of phylogeny and dispersal

Studies that have explored the origins of patterns of community structure from a phylogenetic perspective have generally found either convergence (similarity) in community structure between regions through adaptive evolution or lack of convergence (dissimilarity) due to phylogenetic conservatism in the divergent ecological characteristics of lineages inhabiting different regions. We used a phylogenetic approach to document a third pattern in the structure of emydid turtle communities. Emydid communities in southeastern North America tend to have a higher proportion of aquatic species than those in the northeast. This pattern reflects phylogenetic conservatism in the ecology and biogeography of two basal emydid clades, limiting convergence in community structure between these regions. However, differences in community structure between northeastern and southeastern North America have also been homogenized considerably by the dispersal of species with phylogenetically conserved ecological characteristics between regions. This pattern of ecologically conservative dispersal may be important in many continental and oceanic systems.     

Comparing classical community models: theoretical consequences for patterns of diversity

Abstract: Mechanisms proposed to explain the maintenance of species diversity within ecological communities of sessile organisms include niche differentiation mediated by competitive trade-offs, frequency dependence resulting from species-specific pests, recruitment limitation due to local dispersal, and a speciation-extinction dynamic equilibrium mediated by stochasticity (drift). While each of these processes, and more, have been shown to act in particular communities, much remains to be learned about their relative importance in shaping community-level patterns. We used a spatially-explicit, individual-based model to assess the effects of each of these processes on species richness, relative abundance, and spatial patterns such as the species-area curve. Our model communities had an order-of-magnitude more individuals than any previous such study, and we also developed a finite-size scaling analysis to infer the large-scale properties of these systems in order to establish the generality of our conclusions across system sizes. As expected, each mechanism can promote diversity. We found some qualitative differences in community patterns across communities in which different combinations of these mechanisms operate. Species-area curves follow a power law with short-range dispersal and a logarithmic law with global dispersal. Relative-abundance distributions are more even for systems with competitive differences and trade-offs than for those in which all species are competitively equivalent, and they are most even when frequency dependence (even if weak) is present. Overall, however, communities in which different processes operated showed surprisingly similar patterns, which suggests that the form of community-level patterns cannot in general be used to distinguish among mechanisms maintaining diversity there. Nevertheless, parameterization of models such as these from field data on the strengths of the different mechanisms could yield insight into their relative roles in diversity maintenance in any given community.     

Metacommunity speciation models and their implications for diversification theory

The emergence of new frameworks combining evolutionary and ecological dynamics in communities opens new perspectives on the study of speciation. By acknowledging the relative contribution of local and regional dynamics in shaping the complexity of ecological communities, metacommunity theory sheds a new light on the mechanisms underlying the emergence of species. Three integrative frameworks have been proposed, involving neutral dynamics, niche theory, and life history trade‐offs respectively. Here, we review these frameworks of metacommunity theory to emphasise that: (1) studies on speciation and community ecology have converged towards similar general principles by acknowledging the central role of dispersal in metacommunities dynamics, (2) considering the conditions of emergence and maintenance of new species in communities has given rise to new models of speciation embedded in the metacommunity theory, (3) studies of diversification have shifted from relating phylogenetic patterns to landscapes spatial and ecological characteristics towards integrative approaches that explicitly consider speciation in a mechanistic ecological framework. We highlight several challenges, in particular the need for a better integration of the eco‐evolutionary consequences of dispersal and the need to increase our understanding on the relative rates of evolutionary and ecological changes in communities.     

Host community similarity and geography shape the diversity and distribution of haemosporidian parasites in Amazonian birds

Identifying the mechanisms driving the distribution and diversity of parasitic organisms and characterizing the structure of parasite assemblages are critical to understanding host–parasite evolution, community dynamics, and disease transmission risk. Haemosporidian parasites of the genera Plasmodium and Haemoproteus are a diverse and cosmopolitan group of bird pathogens. Despite their global distribution, the ecological and historical factors shaping the diversity and distribution of these protozoan parasites across avian communities and geographic regions remain unclear. Here we used a region of the mitochondrial cytochrome b gene to characterize the diversity, biogeographical patterns, and phylogenetic relationships of Plasmodium and Haemoproteus infecting Amazonian birds. Specifically, we asked whether, and how, host community similarity and geography (latitude and area of endemism) structure parasite assemblages across 15 avian communities in the Amazon Basin. We identified 265 lineages of haemosporidians recovered from 2661 sampled birds from 330 species. Infection prevalence varied widely among host species, avian communities, areas of endemism, and latitude. Composition analysis demonstrated that both malarial parasites and host communities differed across areas of endemism and as a function of latitude. Thus, areas with similar avian community composition were similar in their parasite communities. Our analyses, within a regional biogeographic context, imply that host switching is the main event promoting diversification in malarial parasites. Although dispersal of haemosporidian parasites was constrained across six areas of endemism, these pathogens are not dispersal‐limited among communities within the same area of endemism. Our findings indicate that the distribution of malarial parasites in Amazonian birds is largely dependent on local ecological conditions and host evolutionary relationships.     

Ecological and community-wide character displacement: the next generation

Ecological character displacement, mostly seen as increased differences of size in sympatry between closely‐related or similar species, is a focal hypothesis assuming that species too similar to one another could not coexist without diverging, owing to interspecific competition. Thus, ecological character displacement and community‐wide character displacement (overdispersion in size of potential competitors within ecological guilds) were at the heart of the debate regarding the role of competition in structuring ecological communities. The debate has focused on the evidence presented in earlier studies and generated a new generation of rigorous, critical studies of communities. Character displacement research in the past two decades provides sound statistical support for the hypothesis in a wide variety of taxa, albeit with a phylogenetically skewed representation. A growing number of studies are strongly based in functional morphology, and some also demonstrate actual morphologically related resource partitioning. Phylogenetic models and experimental work have added to the scope and depth of earlier research, as have theoretical studies. However, many challenging ecological and evolutionary issues, regarding both selective forces (at the inter‐ and intraspecific level) and resultant patterns, remain to be addressed. Ecological character displacement and community‐wide character displacement are here to stay as the focus of much exciting research.     

Variability and convergence in benthic communities along the longitudinal gradients of four physically similar Rocky Mountain streams

1. High‐gradient mountain streams are ideal for studying longitudinal biological patterns, although the degree of similarity in the biological gradient among physically similar streams in a region is poorly known. Our first objective was to evaluate variability in benthic communities along four streams in the central Rocky Mountains of Colorado. We analysed the relative influence of longitudinal position versus reach‐scale physical variables on community structure and measured community similarity at comparable longitudinal positions on the four streams. 2. Our second objective was to evaluate the relative utility of taxonomically versus functionally defined communities to characterise assemblage structure: are taxonomic patterns more predictable along the gradient than are patterns of ecologically important species traits? 3. Redundancy analyses (RDA), including measures of both reach‐scale environmental variables (substratum properties, periphytic cover, local channel slope) and longitudinal position (altitude, stream size), confirmed that the longitudinal position of a site was most important in determining taxonomic composition. Functional community structure was also influenced by longitudinal position, but reach‐scale variables (especially periphyton and median particle size) were of greater importance. 4. Redundancy analyses explained 29.3% of total taxonomic variance and 26.0% of functional variance, indicating that defining assemblages functionally provides no greater understanding of community patterns given several known environmental variables. Strict longitudinal limits of taxa, the presumably identical regional species pool across our sites, and/or trade‐offs among different types of species traits probably explain this result. 5. Redundancy analyses did suggest, however, that traits related to longer life (semivoltinism, long‐lived adults, and slow larval development) were more common downstream, while long‐distance dispersal ability and high fecundity were associated with higher altitude and its associated harsher conditions. 6. When sampling sites were grouped into three ecological zones defined by altitude, mean community similarity (measured both taxonomically and functionally) was lowest across streams at the highest altitude. This pattern could be driven by increased insularity of alpine‐zone streams, resulting from a combination of harsh terrestrial environment, lack of hydrological connectivity, and limited species ranges along the longitudinal continuum.     

An intercontinental comparison of dung beetle diversity between two mediterranean-climatic regions: local versus regional and historical influences

The species richness of biological communities is influenced by both local ecological, regional ecological, and historical factors. The relative importance of these factors may be deduced by comparison between communities in climatically and ecologically equivalent, but geographically and historically separate regions of the world. This claim is based on the hypothesis that community processes driven by similar local ecological factors lead to convergence in species richness whereas those driven by differing regional or historical factors lead to divergence. An intercontinental comparison between the winter rainfall regions of South Africa and the Iberian Peninsula showed that overall species richness of dung beetles was dissimilar at local, subregional and regional scales in Scarabaeidae s. str. but similar at all scales in Aphodiinae. Removal of species widespread in the summer rainfall region of Africa or the temperate region of Europe (regional component) resulted in dissimilarity in species richness of mediterranean endemics at all scales in both dung beetle taxa. However, the lines joining each set of species richness values were parallel which may indicate similarities in processes between different mediterranean climatic regions despite slight differences in latitudinal range. The dominant pattern of dissimilarity or non-convergence may be related primarily to intercontinental differences in regional biogeographical and evolutionary history (faunal dispersal, glaciation effects in relation to geographical barriers to dispersal, speciation history, long-term disturbance history). The limited pattern of similarity or convergence in overall species richness of Aphodiinae may be a chance result or primarily related to intercontinental similarities in local ecological factors.     

A comparative hierarchical analysis of bacterioplankton and biofilm metacommunity structure in an interconnected pond system

It is unknown whether bacterioplankton and biofilm communities are structured by the same ecological processes, and whether they influence each other through continuous dispersal (known as mass effects). Using a hierarchical sampling approach we compared the relative importance of ecological processes structuring the dominant fraction (relative abundance ≥0.1%) of bacterioplankton and biofilm communities from three microhabitats (open water, Nuphar and Phragmites sites) at within‐ and among‐pond scale in a set of 14 interconnected shallow ponds. Our results demonstrate that while bacterioplankton and biofilm communities are highly distinct, a similar hierarchy of ecological processes is acting on them. For both community types, most variation in community composition was determined by pond identity and environmental variables, with no effect of space. The highest β‐diversity within each community type was observed among ponds, while microhabitat type (Nuphar, Phragmites, open water) significantly influenced biofilm communities but not bacterioplankton. Mass effects among bacterioplankton and biofilm communities were not detected, as suggested by the absence of within‐site covariation of biofilm and bacterioplankton communities. Both biofilm and plankton communities were thus highly structured by environmental factors (i.e., species sorting), with among‐lake variation being more important than within‐lake variation, whereas dispersal limitation and mass effects were not observed.     

Incorporating evolutionary history into conservation planning in biodiversity hotspots

There is increased evidence that incorporating evolutionary history directly in conservation actions is beneficial, particularly given the likelihood that extinction is not random and that phylogenetic diversity (PD) is lost at higher rates than species diversity. This evidence is even more compelling in biodiversity hotspots, such as Madagascar, where less than 10% of the original vegetation remains. Here, we use the Leguminosae, an ecologically and economically important plant family, and a combination of phylogenetics and species distribution modelling, to assess biodiversity patterns and identify regions, coevolutionary processes and ecological factors that are important in shaping this diversity, especially during the Quaternary. We show evidence that species distribution and community PD are predicted by watershed boundaries, which enable the identification of a network of refugia and dispersal corridors that were perhaps important for maintaining community integrity during past climate change. Phylogenetically clustered communities are found in the southwest of the island at low elevation and share a suite of morphological characters (especially fruit morphology) indicative of coevolution with their main dispersers, the extinct and extant lemurs. Phylogenetically over-dispersed communities are found along the eastern coast at sea level and may have resulted from many independent dispersal events from the drier and more seasonal regions of Madagascar.     

Spatial patterns and coexistence mechanisms in systems with unidirectional flow

River ecosystems are the prime example of environments where unidirectional flow influences the dispersal of individuals. Spatial patterns of community composition and species replacement emerge from complex interplays of hydrological, geochemical, biological, and ecological factors. Local processes affecting algal dynamics are well understood, but a mechanistic basis for large scale emerging patterns is lacking. To understand how these patterns could emerge in rivers, we analyze a reaction-advection-diffusion model for two competitors in heterogeneous environments. The model supports waves that invade upstream up to a well-defined "upstream invasion limit". We discuss how these waves are produced and present their key properties. We suggest that patterns of species replacement and coexistence along spatial axes reflect stalled waves, produced from diffusion, advection, and species interactions. Emergent spatial scales are plausible given parameter estimates for periphyton. Our results apply to other systems with unidirectional flow such as prevailing winds or climate-change scenarios.     

Aggregated seed arrival alters plant diversity in grassland communities

Aims Species aggregation is commonly seen in plant communities and may increase diversity by causing intraspecific competition to exceed interspecific competition. One potential source of this spatial aggregation is seed dispersal but it is unclear to what extent aggregated seed distributions affect plant diversity in real communities. Using a field experiment, I tested whether uniform or aggregated seed arrival alters community structure and whether these effects vary with sowing density.Methods The experiment consisted of two spatial seeding treatments (uniform and aggregated) that were fully crossed with three seed density treatments. Sixty, 3 × 4-m plots were arrayed in a low-diversity grassland located in Kansas, USA. Each plot was divided into forty-eight, 0.5 × 0.5-m patches. For aggregated seeding treatments, each of the 15 species was sown into three randomly selected patches within the plot (3×15 = 45). To create a uniform species arrival but control for the seed addition method, all 15 species were sown into 45 individual patches (with three patches remaining unsown) within each plot. Seed mass for each species was held constant at the plot scale between uniform or aggregated treatments within a given level of the sowing density treatment. After two growing seasons, plant density was quantified for all sown species in 15 randomly selected patches from each plot.Important findings I found evidence for shifts in community structure in response to the different spatial seeding patterns. The evenness of added species was higher under aggregated than uniform sowing patterns. There was no detectable effect of aggregated seed sowing on species richness at 3.75 m 2 scale. However, when species richness was extrapolated to larger scales (11.25 m 2), aggregated sowing was predicted to have greater richness than uniform sowing. Effects of seed aggregation on community structure were apparent only at moderate to high sowing rates, yet the latter are within the range of measured seed dispersal in similar grasslands. Additionally, as sowing density increased, seed mass became an increasingly effective predictor of relative abundances for added species, but only under uniform sowing patterns supporting the idea that aggregated dispersal may buffer weaker (smaller seeded) species from competition during colonization. This is the first experiment to show that aggregated seed dispersal patterns can increase at least some components of plant diversity in undisturbed grasslands and suggests that previous seed dispersal experiments, which utilize uniform seed sowing, may underestimate the potential effect of dispersal on plant community structure.     

The species pool concept as a framework for studying patterns of plant diversity

Co‐existence theories fail to adequately explain observed community patterns (diversity and composition) because they mainly address local extinctions. For a more complete understanding, the regional processes responsible for species formation and geographic dispersal should also be considered. The species pool concept holds that local variation in community patterns is dependent primarily on the availability of species, which is driven by historical diversification and dispersal at continental and landscape scales. However, empirical evidence of historical effects is limited. This slow progress can be attributed to methodological difficulties in determining the characteristics of historical species pools and how they contributed to diversity patterns in contemporary landscapes. A role of landscape‐scale dispersal limitation in determining local community patterns has been demonstrated by numerous seed addition experiments. However, disentangling general patterns of dispersal limitation in communities still requires attention. Distinguishing habitat‐specific species pools can help to meet both applied and theoretical challenges. In conservation biology, the use of absolute richness may be uninformative because the size of species pools varies between habitats. For characterizing the dynamic state of individual communities, biodiversity relative to species pools provides a balanced way of assessing communities in different habitats. Information about species pools may also be useful when studying community assembly rules, because it enables a possible mechanism of trait convergence (habitat filtering) to be explicitly assessed. Empirical study of the role of historic effects and dispersal on local community patterns has often been restricted due to methodological difficulties in determining habitat‐specific species pools. However, accumulating distributional, ecological and phylogenetic information, as well as use of appropriate model systems (e.g. archipelagos with known biogeographic histories) will allow the species pool concept to be applied effectively in future research.     

Phylogenetic plant community structure along elevation is lineage specific

The trend of closely related taxa to retain similar environmental preferences mediated by inherited traits suggests that several patterns observed at the community scale originate from longer evolutionary processes. While the effects of phylogenetic relatedness have been previously studied within a single genus or family, lineage‐specific effects on the ecological processes governing community assembly have rarely been studied for entire communities or flora. Here, we measured how community phylogenetic structure varies across a wide elevation gradient for plant lineages represented by 35 families, using a co‐occurrence index and net relatedness index (NRI). We propose a framework that analyses each lineage separately and reveals the trend of ecological assembly at tree nodes. We found prevailing phylogenetic clustering for more ancient nodes and overdispersion in more recent tree nodes. Closely related species may thus rapidly evolve new environmental tolerances to radiate into distinct communities, while older lineages likely retain inherent environmental tolerances to occupy communities in similar environments, either through efficient dispersal mechanisms or the exclusion of older lineages with more divergent environmental tolerances. Our study illustrates the importance of disentangling the patterns of community assembly among lineages to better interpret the ecological role of traits. It also sheds light on studies reporting absence of phylogenetic signal, and opens new perspectives on the analysis of niche and trait conservatism across lineages.     

Species interactions during diversification and community assembly in an island radiation of shrews

Background

Closely related, ecologically similar species often have adjacent distributions, suggesting competitive exclusion may contribute to the structure of some natural communities. In systems such as island archipelagos, where speciation is often tightly associated with dispersal over oceanic barriers, competitive exclusion may prevent population establishment following inter-island dispersal and subsequent cladogenesis.

Methodology/Principal Findings

Using a combination of tools, we test the hypothesis that the distributions of shrew (Crocidura) species in the Philippines are the result of competitive exclusion preventing secondary invasion of occupied islands. We first compare ecological niche models between two widespread, allopatric species and find statistical support for their ecological similarity, implying that competition for habitat between these species is possible. We then examine dispersion patterns among sympatric species and find some signal for overdispersion of body size, but not for phylogenetic branch length. Finally, we simulate the process of inter-island colonization under a stochastic model of dispersal lacking ecological forces. Results are dependent on the geographic scope and colonization probability employed. However, some combinations suggest that the number of inter-island dispersal events necessary to populate the archipelago may be much higher than the minimum number of colonization events necessary to explain current estimates of species richness and phylogenetic relationships. If our model is appropriate, these results imply that alternative factors, such as competitive exclusion, may have influenced the process of inter-island colonization and subsequent cladogenesis.

Conclusions/Significance

We interpret the combined results as providing tenuous evidence that similarity in body size may prevent co-occurrence in Philippine shrews and that competitive exclusion among ecologically similar species, rather than an inability to disperse among islands, may have limited diversification in this group, and, possibly other clades endemic to island archipelagos.     

Epiphytic bromeliad communities in secondary and mature forest in a tropical premontane area

We analyzed the differences in species richness, community composition, population structure and within-tree location of epiphytic bromeliads in contiguous secondary and mature forests in a premontane area in Costa Rica. Diversity in the mature forest was highest, and the communities differed in their composition as well as in the recruitment rates of the dominant species. Guzmania monostachia and Catopsis nutans dominated the secondary forests, whereas Tillandsia fasciculata and T. tricolor were more abundant in the mature forest. The secondary forest species showed high rates of seedling recruitment while the opposite was found for the mature forest species. Species presence and abundance among and within habitats did not correlate with their physiological (i.e. CAM vs. C₃ photosynthesis) or morphological attributes. The spatial distribution patterns were similar among habitats; bromeliads tended to aggregate on a few relatively large phorophytes. The species shared a similar vertical stratification within habitats, except for the two dominant species in the early and mid-successional stages, although its ecological implication is not clear. With some exceptions, conspecifics of different ages were located on similar substrate types (i.e. stems, primary, secondary, or tertiary branches) within the tree-crowns, which suggests limited within-tree dispersion. Differences in species composition and rates of seedling recruitment among secondary and mature forest may arise from ecophysiological differences among species; however, the combined effect of seed availability and dispersal differences may have a larger influence. Thus, epiphyte community assembly can only be understood when the differences in habitat conditions, the availability of propagules, their dispersal characteristics and requirements for seedling establishment are known.     

Population differentiation in Daphnia alters community assembly in experimental ponds

Most studies of community assembly ignore how genetic differentiation within species affects their colonization and extinction. However, genetic differentiation in ecologically relevant traits may be substantial enough to alter the colonization and extinction processes that drive community assembly. We measured significant molecular genetic and quantitative trait differentiation among three Daphnia pulex × pulicaria populations in southwestern Michigan ponds and investigated whether this differentiation could alter the assembly of pond zooplankton communities in experimental mesocosms. In this study, we monitored the invasion success of different D. pulex × pulicaria populations after their introduction into an established zooplankton community. We also monitored the invasion success of a diverse array of zooplankton species into different D. pulex × pulicaria populations. Zooplankton community composition depended on the D. pulex × pulicaria source population. Daphnia pulex × pulicaria from one population failed to invade zooplankton communities, while those from other populations successfully invaded similar communities. If population differentiation in other species plays a role in community assembly similar to that demonstrated in our study, assembly may be more sensitive to evolutionary processes than has been previously generally considered.     

THE MACROEVOLUTIONARY CONSEQUENCES OF ECOLOGICAL DIFFERENCES AMONG SPECIES

Abstract:  In this paper, I examine the dynamics of species richness in a model system in which multiple species compete in a metacommunity (multiple patches linked by dispersal). Patches lie along an environmental gradient, and new species are introduced into the system by speciation of existing species. This model is used to explore how the ecological similarity of species influences the patterns in community structure that result and to determine whether patterns in fossil and systematics data may be signatures for different types of community structure. Making species more similar overall along the entire gradient or making new species that have more similar optimal positions along the gradient to their progenitor both increase the time required to drive species extinction. As a result, making species more similar ecologically to one another increases overall species richness because of an increased frequency of transient species in the system. Having more transient species in systems shifted the longevity distributions of species in the fossil record towards having a greater frequency of shorter duration species, and the age distribution of extant species that would be estimated from molecular phylogenies also had a higher frequency of younger aged species. Comparisons of these results with species longevity distributions extracted from two data sets and with species ages derived from species-level molecular phylogenies strongly suggest that transient species are an important component of real biological communities.