Species Diversity in Biological Communities: Patterns and Mechanisms
Spatial patterns of biodiversity are results of contemporary climate, disturbance, and geological history. In this paper, we review the historical hypothesis which explains historical importance in shaping biodiversity patterns, focusing on the recent development in its studies on mechanisms, parameter selection, and relative importance of historical factors versus contemporary climate. Based on literature research, we conclude that, (1) the historical events significantly affect the present patterns of biodiversity, and that these effects are masked by the strong collinearity between historical processes and contemporary climate; (2) historical processes are more significant in influencing distributional patterns of species with small ranges (or endemic species) than those of wide-spread species; (3) measurement of historical processes is a challenge in testing historical hypothesis, as the surrogates currently used are strongly collinear with contemporary climates. Phylogenetic analysis may be help assess the importance of historical hypothesis in controlling spatial patterns of biodiversity.
A plant community is an assemblage of plant populations that live in certain area, and interact with and adapt to one another in the context of long-term environmental changes. Plant communities maintain global ecosystem functions, and provide food and habitats for animals and other organisms. Plant communities also provide primary resources for human survival and development, and are therefore indispensable to human societies. China is among the countries with the most diverse plant communities in the world. However, no systematic national inventory has been conducted for Chinese plant communities. This fact obstructs exploitation and protection of China’s plant resources, and also hampers the development of the fields of Chinese ecology and geography. There is an urgent need to survey Chinese plant communities using consistent methods and protocols. In this paper, we review major concepts in plant community ecology, and propose a framework for developing plant community inventories based on recent progress in community ecology and our own experience with long-term field surveys. Our framework provides protocols for site selection and plot design, items to be measured in a plot, and measurements of functional traits of dominant species. We also review protocols for field surveys of large, long-term plots. The protocols proposed in this paper are expected to be a base for standardizing methodology for inventory of Chinese plant communities.
Rapoport’s rule is a hypothesis about the relationship between species latitudinal locations and their latitudinal range widths. By stimulating a wide range of empirical tests and theoretical discussions, Rapoport’s rule has been recognized as one of a few crucial hypotheses in macroecology, and has been extended to distributional patterns across gradients in elevation and ocean depth. A variety of algorithms have been applied to test the rule, and several interpretations, in addition to climatic variability, have been proposed as underlying mechanisms. Studies have also bridged the Rapoport’s effect with other important biogeographic phenomenon, e.g. species richness pattern, species-area relationship, and boundary limit to distribution. However, the universality of Rapoport’s rule is still debatable, and tests of the rule are greatly affected by data collection and analysis methodology. Hence, methodological improvements are key to settling the debate on the rule. Our objectives are to provide an overview on Rapoport’s rule, including conceptual changes, changes in methodology and mechanistic interpretations, the debate on the universality of the rule, and the relationship between Rapoport’s rule and latitudinal/altitudinal patterns of species richness.
Understanding macro-scale spatial patterns in species diversity and their underlying mechanisms is central to macroecology and biogeography. In this study, we explored geographic patterns of species richness and their environmental determinants for overall terrestrial mammals and each major mammalian order in China, using datasets of species distribution, climate, topography and vegetation. Species richness of terrestrial mammals exhibited significant latitudinal gradients, decreasing from south to north. High species richness generally occurred in tropical and subtropical mountains, whereas low species richness was found in the eastern plains, the arid areas of northwest regions, and central areas of the Qinghai-Tibet Plateau. Geographic patterns of species richness varied among mammalian orders. The best model, which included a remote sensing-based vegetation index (NDVI), number of ecosystems, and annual range of temperature, accounted for 66.2% of variation in overall mammal species richness, with NDVI being the most important determinant. This suggests that patterns of mammal richness in China are governed by the integrated effects of different environmental predictors, with vegetation productivity playing a major role. The best models for various orders of mammals identified different combinations of determinants, possibly reflecting differences in evolutionary history and physiological tolerances.
The Rapoport’s rule predicts that altitudinal range size of species increases with altitude. Using a dataset on range size of vascular plant species across an altitudinal gradient at Mt. Shennongjia, central China, we analyzed altitudinal patterns of all species richness, and four quarters of species classified by an order of increasing range size. Previous four methods and the midpoint method modified by this study were applied to explore the relationship between range size and range midpoint along an altitudinal gradient to test the Rapoport’s rule. Altitudinal variation in vascular plant species richness on Mt. Shennongjia was hump-shaped with a peak at 1,100-1,200 m a.s.l. A similar pattern was found for the four quarters of species, and peak value of the species richness shifted to lower elevations with decreased range size of quarters. Stevens’ method, Pagel’s method, and cross-species method all supported the Rapoport effect, while the midpoint method generated a quadratic pattern, suggesting a mid-domain effect. The sectionalized midpoint method presented inconsistent results, even after removing the mid-domain effect. Our results imply that tests of Rapoport’s rule are limited by methodological considerations, and that comparisons among more empirical tests are needed in order to generate conclusive insight into altitudinal patterns of species range size, and the mechanism shaping such patterns.
Geographic patterns of species diversity and their underlying mechanisms have long been the focus of macro-ecology and biogeography. Recently, the mid-domain effect (MDE) hypothesis has been proposed to explain geographical diversity patterns. The hypothesis states that if the ranges of the species are randomly distributed within a bounded domain then more ranges will overlap near the middle of the domain than at the edges, and thus decreasing species richness will be observed from the mid-domain to the edges. Many studies have shown that the MDE is an important mechanism affecting geographic richness pattern. However, its relative role in such patterns differs markedly depending on many factors. In this paper, we introduced the assumptions and basic models of the hypothesis and illustrate that the models differ in their predictions as a result of different assumptions. We also review the debate on the MDE hypothesis, and discuss the limitations of present mid-domain models. Although the hypothesis has improved our understanding of the effects of geometric constraints and random process on geographic richness gradients, current MDE models are too simplistic to describe biodiversity patterns in the real world. Improvements to mid-domain models should be based on a better understanding of the mechanisms underlying species distribution.
The manner in which species richness increases with increasing sampling area is among the important laws of ecology. However, forms and parameters of the species-area relationships vary depending on sampling methods, climate, and spatial scales. Because the species-area relationships connect biodiversity at different scales, they are used for estimating species richness at local or regional scales, and for assessing regional biodiversity losses. Here, we review recent developments in the forms, spatial variation in parameters, applications, and scale-dependence of species-area relationships. As a case study, we used species-area relationships to estimate number of plant species in different regions of the Qinling Mountains, and found that the well-consistent estimated and recorded numbers of species were achieved for different nature reserves in the Qinling Mountains.
Understanding species coexistence and the maintenance of biodiversity has long been the central interest of ecologists. The niche-based theory of community assembly has dominated community ecology for nearly a century, yet understanding of the mechanisms of species coexistence has remained elusive. The newly developed neutral theory of biodiversity has offered a promising alternative to the niche paradigm. The analytical elegance and simplicity of the neutral theory and its predictive power have made the theory widely popular. However, it is the very same simplicity of the theory (e.g. the symmetric assumption) that makes the theory vulnerable to stark criticisms. Widespread empirical evidence has shown that species in communities are not functionally symmetric; ecological equivalence is more a conceptual simplicity than a biological realism. Recognizing that niche and neutral processes do not have to diametrically oppose each other and a community is likely determined by the interplay of the two processes, ecologists currently are searching to reconcile the two theories by either incorporating drift into niche theory or niche into the neutral framework. However, this reconciliation process is still at its very early stage, we expect this direction will lead to a more complete understanding of community assembly mechanisms. In this paper, we provide a review on the brief histories of the niche and neutral theories, with the focus on comparing the distinct importance of the two theories in explaining community assembly. We discuss in details several integrated models that attempt to unify the niche and neutral theories. We argue that it is an essential step for any successful theory to withstand substantial experimental and field tests. The experimental tests of neutral theories are an important direction that has currently not received due attention.
The negative density-dependent hypothesis focuses mainly on conspecific interactions to explain the coexistence of diverse species in natural communities. The hypothesis describes the impairment of performance among conspecific individuals due to resource competition, predation of pests (e.g., pathogen, herbivore) and so on. Impairment of conspecific individuals decreases growth and increases mortality, thereby freeing space for other species, and thus promotes coexistence of diverse species. There are three main kinds of density dependent effects including distance-dependence of mortality and abundance of offspring near parents (Janzen-Connell hypothesis), density dependent thinning (random-mortality hypothesis), and community compensatory trends (CCT). Research has shown that density dependence among phylogenetically closely-related species results partially from competition for similar resources. This fact led to the proposal of species herd protection and phylodiversity dependence models. Density dependence has long history of study and the recent establishment of a global network of large-scale forest dynamic plots facilitates the detection of density dependence in natural communities. However, there are many challenges when testing for density dependence. For example, some previous studies can not disentangle density dependence from other confounding effects, and most studies focus exclusively on the tropical zone, seldom considering other zones. Therefore, though strong evidence to contrary does not exist, debate continues on the importance of density dependence in maintaining diverse-species coexistence.
Exploring the mechanisms underlying community species richness is a key issue in ecology and conservation biology, and many hypotheses based on small-scale, local processes have traditionally been used as explanations. The species pool hypothesis developed by Zobel et al. suggests that the variation in community species richness is not only associated with contemporary environmental factors and ecological processes (e.g. competition and predation), but also limited by the regional species pool. The regional species pool is the set of species in a certain region that are capable of coexisting in a target community, which is shaped by historical (e.g. glaciation and geological age) and regional processes (e.g. speciation, immigration, dispersion, and extinction). The species pool hypothesis suggests that the larger the area of a habitat type and the greater its geological age, the greater the opportunity for speciation and hence the larger the number of available species adapted to that particular habitat, which will in turn lead to higher community diversity. The species pool is generally studied at two spatial scales: the regional and the actual scales. While the regional species pool is primarily determined by biogeographic processes, the actual species pool (species present in the target community) is determined by both ecological processes (e.g. competition) and the regional pool. In this review, we introduce and discuss the concepts relating to, and evidence for the species pool hypothesis, together with methods for estimating the species pool.
Metabolisms are fundamental processes of organisms. The recently-proposed Metabolic Theory of Ecology (MTE) seeks to explain ecological patterns and processes in terms of the effects of body size and temperature on the metabolic processes of organisms and using the scaling approach. James Brown and his colleagues extended the MTE to explain large-scale patterns of species diversity, and proposed a potential mechanism for species richness-temperature relationships, which predicted that the number of species increases exponentially with increasing environmental temperature. More quantitatively, they predicted that (1) log-transformed number of species varies linearly with the reciprocal of absolute temperature, and (2) the slope of the relationship ranges between -0.70 and -0.60. The MTE has generated widespread attention and controversy and has been tested by a number of empirical observations, but no agreement has yet been reached. Although several issues need to be resolved, the theory is quite different from conventional regression-based methods because of its biological mechanism-sound approach. Previous empirical tests of the theory may ignore two important assumptions (i.e. unconstrained environments except temperature and equilibrium communities) that are the basis for understanding the MTE. This article reviews the frame-work, predictions, and biological meanings of the MTE and examines previous empirical tests of the theory. We also comment on criticisms raised by previous studies and prospect some aspects for the further study.
Large-scale patterns of species diversity are one of the most important and attractive issues for ecology and biogeography. Many hypotheses have been proposed to understand the mechanisms that shape and maintain the diversity patterns. Among them, the energy hypothesis, which focuses on the influence of energy on species diversity, has generated the most attention. Based on the forms of energy and the mechanisms of energy effects on diversity patterns, five versions of the energy hypothesis have been recognized, i.e. productivity hypothesis, water-energy dynamic hypothesis, ambient energy hypothesis, freezing tolerance hypothesis, and metabolic theory of ecology. The current paper reviews the development of the energy hypothesis, and then presents the context, energy forms, variables, predictions, and underlying mechanisms for the five versions of the energy hypothesis. Furthermore, we discuss the advantages, shortcomings, and challenges of each version of the energy hypothesis.
Assessing the distribution of nature reserves is an important step for conserving biodiversity. We used geographic information system (GIS) to assess the conservation status of vegetation types, endangered plant and animal species, and biodiversity hotspots in China, based on the area, endangered species list and geographic position of 2,047 nature reserves in China. The results showed that, while the proportion of total area protected as nature reserves is higher in China than the world average, of the 47 natural vegetation types in China, 21 (45% of the total) were deficiently protected, with less than 10% of their area included in nature reserves, suggesting that these vegetation types have not been perfectly protected. According to the Dobson complementary algorithm, among 216 nature reserves, the top five priority nature reserves, i.e., Xishuangbanna, Mt. Wuyi, Mt. Changbai, Mt. Gaoligong and Mt. Qilian, contained 381 protected species (~ 50% of the total), and the top 21 priority nature reserves contained 590 protected species (~75% of the total). Nature reserves covered nearly all the hotspots selected by different approaches. However, as there are several areas lacking proper protection, e.g., Northern Xinjiang, Sichuan and South of the Yangtze River, the distribution of Chinese nature reserves needs further improvement.
Biodiversity Committee, CAS
Botanical Society of China
Institute of Botany, CAS
Institute of Zoology, CAS
Institute of Microbiology, CAS
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