Species diversity in deep-sea communities

Recent extensive quantitative sampling of the deep-sea bottom has revealed communities much richer in species than previously thought. In situ experiments and more precise sampling using free-vehicle instruments and submersibles have provided a more accurate assessment of spatial and temporal variation on the sea floor. These studies have demonstrated the importance of small patches (millimeters to meters) of biogenic disturbance and food input separated on spatial scales of meters to kilometers. In this respect, the processes maintaining deep-sea diversity are similar to those in other species-rich environments such as rain forests.     

Network isolation and local diversity in neutral metacommunities

Biologists seek an understanding of the biological and environmental factors determining local community diversity. Recent advances in metacommunity ecology, and neutral theory in particular, highlight the importance of dispersal processes interacting with the spatial structure of a landscape for generating spatial patterns and maintaining biodiversity. The relative spatial isolation of a community is traditionally thought to have a large influence on local diversity. However, isolation remains an elusive concept to quantify, particularly in metacommunities with complex spatial structure. We represent the metacommunity as a network of local communities, and use network centrality measures to quantify the isolation of a local community. Using spatially explicit neutral theory, we examine how node position predicts variation in alpha diversity across a metacommunity. We find that diversity increases with node centrality in the network, but only when centrality is measured on a given scale in the network that widens with increasing dispersal rates and narrows with increasing evolutionary rates. More generally, complex biodiversity patterns form only when the underlying geography has structure on this critical scale. This provides a framework for understanding the influence of spatial geographic structure on global biodiversity patterns.     

Species diversity in postglacial diatom lake communities of Finland

Species diversity in three sections of Flandrian sediment from the Finnish Lake District was studied, together with orthodox diatom stratigraphy, to analyse the pattern of change in diatom communities over the postglacial time.The general agreement of the two approaches is clear, and in particular there is a positive direct relationship between the temporal development of these aquatic systems and changes in the diatom diversity.As indicated by both the quantitative inventory and the species diversity, these diatom communities consisted of few species immediately after glaciation at 10 000 radiocarbon years B.P. The minimum diversity at 9 000 B.P. coincides with the maximum of Melosira islandica ssp. helvetica. A trend to development of a larger number of equally abundant species, and higher total diversity followed isolation of the lake from the Baltic Sea at 8 000 B.P. This is consistent with earlier results of Duthie and Sreenivasa that the number of individuals and total diversity of the samples increase over the postglacial time.     

松嫩平原南部主要群落植物多样性的比较研究

应用Ｍａｒｇａｌｅｆ（Ｒ１）和Ｍｅｎｈｉｎｉｃｋ（Ｒ２）丰富度指数、Ｈｉｌｌ多样性指数（Ｎ１,Ｎ２）和Ｐｉｅｌｏｕ均匀度指数（Ｅ）比较研究了松嫩平原南部１４个主要群落的植物多样性、生活型多样性及植物多样性与群落演替的关系结果表明,分布于低平原的羊草－蔓萎陵菜群落的植物多样性和丰富度指数最大,Ｒ１为１１．９６,Ｒ２为５．２４,Ｎ１为１３．９２,Ｎ２为８．７６,分布于固定砂丘的榆树疏林和贝加尔针茅等４个群落有较高的植物多样性,而分布于碱化草甸的角碱蓬等群落植物多样性最小,Ｒ１为１．３１,Ｒ２为０７１,Ｎ１为１．８４,Ｎ２为１．４６．具有较高植物多样性的群落其植物生活型多样性指数也较高．在演替系列中,羊草群落的植物多样性指数介于演替初期和中期阶段之间．     

Species diversity of grasses promotes genotypic diversity of clover populations in simulated communities

Recent studies in community genetics have demonstrated strong effects of intraspecific genetic variation on the diversity of interacting species but largely ignored the potential for effects of species diversity on genetic diversity, which could also create a positive correlation between these two levels of biodiversity. Here we investigated the role that species diversity of competitors could play in shaping the genotypic diversity of a focal species, using a modified version of an existing model of grass–clover dynamics. We simulated communities in which clover genotypes varied in their relative competitive ability when growing adjacent to different grass species. Under many parameter combinations, communities with greater species richness of grasses retained greater genotypic richness within the clover population. Increasing grass species richness effectively increased biotic environmental heterogeneity with respect to clover growth, thereby promoting the maintenance of genotypic diversity. This result depended on three conditions being met: (1) a sufficiently strong tradeoff among genotypes in their fitness when growing with different grass species, (2) partial de‐coupling of competition and facilitation, with grass and clover capable of sharing microsites rather than strictly excluding one another, and (3) sufficiently high rates of clover growth and clonal spread, which allowed clover genotypes to ‘track’ the shifting mosaic of grass species. Our results demonstrate that species diversity can act as an important promoter of genotypic diversity, and they provide testable predictions concerning the conditions that promote this outcome in nature.     

三江平原湿地典型植物群落物种多样性研究

以三江平原分布最广的毛果苔草(Carex lasiocarpa)群落和小叶章(Calamagrostis angustifolia)群落为研究对象,研究2个群落的植物物种及其组成特征。结果表明,毛果苔草群落以毛果苔草为优势种,以漂筏苔草(C.pseudocuraica)为亚优势种,二者的重要值之和为56.4%;小叶章群落以小叶章为优势种,其重要值为50.8%。物种多样性分析表明,毛果苔草群落和小叶章群落的Simpson指数(D)、Shannon-Wiener指数(H)和Pielou均匀度指数(J)均有较大波动,且H与J呈显著正相关,与D呈显著负相关,与S无显著相关关系;毛果苔草群落的H、D、J和S均高于小叶章群落,但差异不显著;2群落的相似性指数为46.5%,表明二者间存在一定的联系。     

Species diversity reduces invasion success in pathogen‐regulated communities

The loss of natural enemies is thought to explain why certain invasive species are so spectacularly successful in their introduced range. However, if losing natural enemies leads to unregulated population growth, this implies that native species are themselves normally subject to natural enemy regulation. One possible widespread mechanism of natural enemy regulation is negative soil feedbacks, in which resident species growing on home soils are disadvantaged because of a build‐up of species‐specific soil pathogens. Here we construct simple models in which pathogens cause resident species to suffer reduced competitive ability on home soils and consider the consequences of such pathogen regulation for potential invading species. We show that the probability of successful invasion and its timescale depend strongly on the competitive ability of the invader on resident soils, but are unaffected by whether or not the invader also suffers reduced competitive ability on home soils (i.e. pathogen regulation). This is because, at the start of an invasion, the invader is rare and hence mostly encounters resident soils. However, the lack of pathogen regulation does allow the invader to achieve an unusually high population density. We also show that increasing resident species diversity in a pathogen‐regulated community increases invasion resistance by reducing the frequency of home‐site encounters. Diverse communities are more resistant to invasion than monocultures of the component species: they preclude a greater range of potential invaders, slow the timescale of invasion and reduce invader population size. Thus, widespread pathogen regulation of resident species is a potential explanation for the empirical observation that diverse communities are more invasion resistant.     

Species diversity and functional assessment of macroinvertebrate communities in Austrian rivers

We applied an extensive data set from 211 locations along Austrian rivers to assess community structure and the ratios of functional feeding groups of benthic macroinvertebrates. A total of 569 taxa have been identified. At the catchment scale, the Enns, Salzach, and Traun Rivers exhibited the highest taxa richness whereas the Inn River showed the lowest richness. Beta-diversity was highest along the impounded and fragmented Enns and Drau Rivers. Consequently, high corridor diversity corresponded to a low degree of nestedness. Overall, scrapers and gathering-collectors dominated the benthic community. Further, the relationship between habitat conditions and metrics based on functional feeding groups were statistically analyzed to validate the potential of these metrics as indicators of ecosystem attributes. We examined four major ecosystem attributes: species diversity, material cycling, longitudinal material transport, and lateral material input. Multiple regression analyses for midorder rivers demonstrated that metrics were significantly related to habitat conditions. For example, the metric set indicating primary production was positively correlated with periphyton cover, dissolved oxygen, dominant sediment size, and average annual discharge. Overall, most metrics exhibited unique responses to habitat conditions, implying that they are useful proxies of ecosystem attributes. Thus, a function-based approach based on macroinvertebrates has the potential to become an effective tool for the assessment of river ecosystems.     

Species identity, diversity and microbial carbon flow in reassembling macrobenthic communities

Intense disturbance may locally destroy patches of habitat and shape the landscape into a mosaic of reassembling communities. The development of ecosystem properties during such community reassembly is poorly understood. In intertidal bare sediments, trophic relations between microphytobenthos or heterotrophic bacteria and macrofauna invertebrates may guarantee fundamental ecosystem properties such as carbon flow through the food web. We studied the dynamic relation between reassembling macrofauna communities and such microbial carbon flow during recovery after severe disturbance. We deliberately induced prolonged hypoxia in winter and early summer and allowed recolonisation for periods of two and five months. Carbon flow was quantified from basal resources (microphytobenthos and bacteria) to intermediate consumers using ¹³C as a tracer. Within the period of study (5 months), microbial carbon flow fully recovered, although macrofauna diversity was still very low compared to the natural communities (ranging from 6 to 17 species). More than 90% of microbial carbon flow to macrofauna was due to the consumers that recolonised within two months. Two of these species were dominant contributors to microphytobenthos carbon transfer to fauna. Furthermore, at an early stage of reassembly, this ecosystem property was remarkably similar when disturbance took place at different times of the year (winter or early summer), although there were differences in assemblage composition and functional diversity. We conclude that species assemblages and ecosystem function developed relatively independently in this benthic system. We discuss which ecological factors may have caused such non-parallel development of macrofaunal communities and carbon flow.     

Variation in the degree of specialization can maintain local diversity in model communities

We hypothesize that the continuum between generalist and specialist adaptations is an important general trade-off axis in the maintenance of local diversity, and we explore this idea with a simple model in which there are patch types to which species arrive as propagules and compete. Each patch type is defined by a competitive ranking of all species. A highly specialist species is the top competitor in one patch type but has a relatively low average ranking across different patch types, while a generalist species has a high average rank across patch types but is not the top competitor in any patch type. We use random dispersal and vary the fecundity of all species together to vary total propagule density and therefore recruitment limitation and density-dependent mortality. When fecundity is very high, each patch becomes occupied by its specialist species and generalists go extinct, so the number of species at equilibrium is equal to the number of patch types. If fecundity is very low, generalists dominate and specialists go extinct. There is a range of fecundity levels in which specialists, generalists, and intermediates coexist, and the number of species is substantially greater than the number of patch types. While coexistence of specialists and generalists has been considered a problem in evolutionary ecology, our results suggest to the contrary that this trade-off contributes to the maintenance of local diversity.     

Species diversity, invasion, and alternative community states in sequentially assembled communities

The relationship between resident species diversity and invasion is generally negative in experimental studies but takes various forms in observational studies of natural communities. We hypothesized that stochastic species colonization, which applies to natural communities but not to experimental communities generally assembled through simultaneous species introduction, may lead to nonnegative diversity-invasion relationships via incurring priority effects. To test this hypothesis, we manipulated both resident species diversity and colonization history in sequentially assembled communities of bacterivorous protist species. We found that, despite a significant effect of assembly history on invader abundance, invader abundance decreased with diversity. This result was largely driven by positive selection effects associated with the dominant influence of an invasion-resistant species, which shared the most similar resource use pattern with the invader, and by the overall weak priority effects observed for the resident communities. Increasing species diversity, however, significantly strengthened priority effects, providing the first experimental support for the idea that larger species pools promote alternative community states. We suggest that elucidating mechanisms regulating the strength of priority effects may help in understanding variation in diversity-invasion relationships among natural communities.     

Invasibility of neutral communities

Comparative data in invasion ecology show that (i) disturbance enhances community invasibility, (ii) there is a positive relationship between residence time of an invader and its success, (iii) there are broadly constant proportions of newly arrived species to those that become established and dominant (“tens rule”), and (iv) invasive species have higher growth rates in comparison with non-invasive species. I use a simple neutral model to test whether these patterns occur in communities with all species identical and no species-specific interactions. In the model, local communities are grouped into continents with immigration rates smaller between than within the continents. Species coming from the other continent are considered to be alien and their fates are recorded. In the model, disturbance predictably increases species numbers and numbers of individuals of aliens. However, the model makes different predictions on effects of disturbance on three processes involved in alien species spreading: establishment (positive effect of disturbance), naturalization (negative effect) and dominance (positive effect). The predictions do not change if variation of growth rates is incorporated into the model. The model also predicts positive relationship between residence time and abundance. Total community size had little effect on success of alien species. The broad agreement of the predictions of the neutral model with the patterns from the field suggests that some of these general patterns of community invasibility are to some degree fully independent of any specific biological assumptions and by themselves may not provide many insights on underlying biological processes. Aggregate data should therefore be used with great caution and statistical patterns must be removed by means of generating null model predictions.     

Taxa-area relationship and neutral dynamics influence the diversity of fungal communities on senesced tree leaves

This study utilized individual senesced sugar maple and beech leaves as natural sampling units within which to quantify saprotrophic fungal diversity. Quantifying communities in individual leaves allowed us to determine if fungi display a classic taxa–area relationship (species richness increasing with area). We found a significant taxa–area relationship for sugar maple leaves, but not beech leaves, consistent with Wright's species‐energy theory. This suggests that energy availability as affected plant biochemistry is a key factor regulating the scaling relationships of fungal diversity. We also compared taxa rank abundance distributions to models associated with niche or neutral theories of community assembly, and tested the influence of leaf type as an environmental niche factor controlling fungal community composition. Among rank abundance distribution models, the zero‐sum model derived from neutral theory showed the best fit to our data. Leaf type explained only 5% of the variability in community composition. Habitat (vernal pool, upland or riparian forest floor) and site of collection explained > 40%, but could be attributed to either niche or neutral processes. Hence, although niche dynamics may regulate fungal communities at the habitat scale, our evidence points towards neutral assembly of saprotrophic fungi on individual leaves, with energy availability constraining the taxa–area relationship.     

Species sorting and neutral processes are both important during the initial assembly of bacterial communities

Many studies have shown that species sorting, that is, the selection by local environmental conditions is important for the composition and assembly of bacterial communities. On the other hand, there are other studies that could show that bacterial communities are neutrally assembled. In this study, we implemented a microcosm experiment with the aim to determine, at the same time, the importance of species sorting and neutral processes for bacterial community assembly during the colonisation of new, that is, sterile, habitats, by atmospheric bacteria. For this we used outdoor microcosms, which contained sterile medium from three different rock pools representing different environmental conditions, which were seeded by rainwater bacteria. We found some evidence for neutral assembly processes, as almost every 4th taxon growing in the microcosms was also detectable in the rainwater sample irrespective of the medium. Most of these taxa belonged to widespread families with opportunistic growth strategies, such as the Pseudomonadaceae and Comamonadaceae, indicating that neutrally assembled taxa may primarily be generalists. On the other hand, we also found evidence for species sorting, as one out of three media selected a differently composed bacterial community. Species sorting effects were relatively weak and established themselves via differences in relative abundance of generalists among the different media, as well as media-specific occurrences of a few specific taxa. In summary, our results suggest that neutral and species sorting processes interact during the assembly of bacterial communities and that their importance may differ depending on how many generalists and specialists are present in a community.     

Finding local communities in protein networks

Background  

Protein-protein interactions (PPIs) play fundamental roles in nearly all biological processes, and provide major insights into the inner workings of cells. A vast amount of PPI data for various organisms is available from BioGRID and other sources. The identification of communities in PPI networks is of great interest because they often reveal previously unknown functional ties between proteins. A large number of global clustering algorithms have been applied to protein networks, where the entire network is partitioned into clusters. Here we take a different approach by looking for local communities in PPI networks.     

Recognizing diversity in coral symbiotic dinoflagellate communities

A detailed understanding of how diversity in endosymbiotic dinoflagellate communities maps onto the physiological range of coral hosts is critical to predicting how coral reef ecosystems will respond to climate change. Species-level taxonomy of the dinoflagellate genus Symbiodinium has been predominantly examined using the internal transcribed spacer (ITS) region of the nuclear ribosomal array (rDNA ITS2) and downstream screening for dominant types using denaturing gradient gel electrophoresis (DGGE). Here, ITS2 diversity in the communities of Symbiodinium harboured by two Hawaiian coral species was explored using direct sequencing of clone libraries. We resolved sixfold to eightfold greater diversity per coral species than previously reported, the majority of which corresponds to a novel and distinct phylogenetic lineage. We evaluated how these sequences migrate in DGGE and demonstrate that this method does not effectively resolve this diversity. We conclude that the Porites spp. examined here harbour diverse assemblages of novel Symbiodinium types and that cloning and sequencing is an effective methodological approach for resolving the complexity of endosymbiotic dinoflagellate communities harboured by reef corals.     

Species size distributions in Arctic benthic communities

In most subtidal soft-sediment faunal assemblages there is a trough in the species size distribution separating the macrofauna and meiofauna. To examine the hypothesis that this may be maintained by the avoidance of competition between macrofaunal adults and settling larvae, samples were taken from around Svalbard, an area where most macrofaunal species have either lecithotrophic or direct development. At three contrasting sites, bimodality of the species size spectrum was conserved. However, it is proposed that the adult-larval interaction hypothesis should not be rejected as low endemism in the fauna of the study area suggests its species size spectrum probably reflects evolutionary processes outside rather than within the Arctic. Received: 13 May 1996/Accepted: 26 July 1996     

Species size distributions in marine benthic communities

Summary Species body size distributions from eight temperate benthic communities show a highly conservative pattern with two separate lognormal distributions, corresponding to the traditional categories of meiofauna and macrofauna. The meiofaunal mode occurs at a dry body weight of 0.64 g and the macrofaunal mode at 3.2 mg, with a trough between them at 45 g. It is suggested that there is a particular body size at which meiofaunal life-history and feeding traits can be optimised, and another for macrofaunal traits. As size departs in either direction (larger or smaller) from these optima, fewer species of the same size are able to co-exist. The split occurs at 45 g because many life history and feeding characteristics switch more or less abruptly at about this body size, compromise traits being either non-viable or disadvantageous. Meiofauna and macrofauna therefore comprise two separate evolutionary units each with an internally coherent set of biological characteristics.The expression of this conservative pattern is modified by water depth: the proportion of macrofauna species increases from intertidal situations to deeper water, and it is suggested that mechanisms of resource partitioning and diversity maintainence in the meiofauna and macrofauna are affected differentially by sediment disturbance. Salinity does not affect this proportionality, and so does not differentially affect mechanisms for maintaining species diversity in any particular size category of animals. Meiofauna species size distributions may be modified in sandy sediments because of physical impositions on interstitial or burrowing lifestyles.Brief discussion of some implications of these observations includes speculations on the larval ecology of macrofauna, on gigantism in Antarctic invertebrates, and on the benthic Sheldon spectrum.     

Species diversity patterns in Mediterranean grasslands

Abstract. A field experiment involving herbivore exclusion, ploughing, and the combination of both was carried out over a period of 4 - 5 yr in Mediterranean grasslands located along an elevational gradient. The empirical results provide a general hierarchical framework for understanding patterns of plant species diversity in thesegrasslands. In grazed grasslands, plant species density decreased as altitude increased, and this pattern was maintained through time. The reduced seasonality along the climatic gradient is suggested as the extrinsic, indirect control factor. Ploughing caused species loss, but after 4 yr the original diversity was recovered in most grasslands. Our hypothesis is that a negative feedback mechanism regulates species increase towards a characteristic level. A trend of species density reduction was observed in ungrazed grasslands. Plant-herbivore interaction is considered to be essential for maintaining species diversity in grazed grasslands and for the recovery of diversity in mechanically disturbed grasslands.