Environment and dispersal influence changes in species composition at different scales in woody plants of the Western Ghats, India (original) (raw)
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Journal of Biogeography, 2001
Aim Current weaknesses of diversity theory include: a failure to distinguish different biogeographical response variables under the general heading of diversity; and a general failure of ecological theory to deal adequately with geographical scale. Our aim is to articulate the case for a top-down approach to theory building, in which scale is addressed explicitly and in which different response variables are clearly distinguished.
Biological Communications, 2018
This paper seeks to shed light on the primary causes of the latitudinal cline in species diversity, the asymmetry in species richness between the northern and southern hemispheres, and various patterns of species richness along mountain and continental slopes, which are at present of central interest in ecology. To that end, we restate, further develop and unify Janzen's ideas about the higher fidelity of tropical organisms to their habitats; the notions of Sanders on temporal and spatial variations of environment and their impact on the breadth of species adaptations; the hypothesis of latitude-niche breadth and niche overlap; the theories of climatic stability, competitive exclusion and competitive divergence with the incorporation of some elements of the gradient model of diversification. We argue that during adaptation to a wide range of the same environmental factors in time, the high latitude species also become adapted to a wide range of those factors in space. As a result, they form not only very wide, but also widely overlapping ecological niches. This eventually leads to the competitive extinction of many species and a general impoverishment of biota. In contrast, relatively stable environments allow species to move more and more towards specialization with a simultaneous narrowing of their ecological niches that in turn leads to a reduction of niche overlap and greater species packing in communities. In tropical mountains and on the continental slope, where the environment is stable enough, the degree of its differentiation depends mainly on the steepness of slope. And since the steepest slopes tend to be located at intermediate elevations and intermediate bathyal depths, it is there that there are conditions for the highest specialization. These ideas can provide a framework for new approaches to biodiversity conservation of different regions.
Disentangling the Role of Climate, Topography and Vegetation in Species Richness Gradients
Plos One, 2016
Environmental gradients (EG) related to climate, topography and vegetation are among the most important drivers of broad scale patterns of species richness. However, these different EG do not necessarily drive species richness in similar ways, potentially presenting synergistic associations when driving species richness. Understanding the synergism among EG allows us to address key questions arising from the effects of global climate and land use changes on biodiversity. Herein, we use variation partitioning (also know as commonality analysis) to disentangle unique and shared contributions of different EG in explaining species richness of Neotropical vertebrates. We use three broad sets of predictors to represent the environmental variability in (i) climate (annual mean temperature, temperature annual range, annual precipitation and precipitation range), (ii) topography (mean elevation, range and coefficient of variation of elevation), and (iii) vegetation (land cover diversity, standard deviation and range of forest canopy height). The shared contribution between two types of EG is used to quantify synergistic processes operating among EG, offering new perspectives on the causal relationships driving species richness. To account for spatially structured processes, we use Spatial EigenVector Mapping models. We perform analyses across groups with distinct dispersal abilities (amphibians, non-volant mammals, bats and birds) and discuss the influence of vagility on the partitioning results. Our findings indicate that broad scale patterns of vertebrate richness are mainly affected by the synergism between climate and vegetation, followed by the unique contribution of climate. Climatic factors were relatively more important in explaining species richness of good dispersers. Most of the variation in vegetation that explains vertebrate richness is climatically structured, supporting the productivity hypothesis. Further, the weak synergism between topography and vegetation urges caution when using topographic complexity as a surrogate of habitat (vegetation) heterogeneity.
Individual-scale variation, species-scale differences: inference needed to understand diversity
Ecology Letters, 2011
As ecological data are usually analysed at a scale different from the one at which the process of interest operates, interpretations can be confusing and controversial. For example, hypothesised differences between species do not operate at the species level, but concern individuals responding to environmental variation, including competition with neighbours. Aggregated data from many individuals subject to spatio-temporal variation are used to produce species-level averages, which marginalise away the relevant (process-level) scale. Paradoxically, the higher the dimensionality, the more ways there are to differ, yet the more species appear the same. The aggregate becomes increasingly irrelevant and misleading. Standard analyses can make species look the same, reverse species rankings along niche axes, make the surprising prediction that a species decreases in abundance when a competitor is removed from a model, or simply preclude parameter estimation. Aggregation explains why niche differences hidden at the species level become apparent upon disaggregation to the individual level, why models suggest that individual-level variation has a minor impact on diversity when disaggregation shows it to be important, and why literature-based synthesis can be unfruitful. We show how to identify when aggregation is the problem, where it has caused controversy, and propose three ways to address it.
Regional and local impact on species diversity - from pattern to processes
Oecologia, 2002
The impact of regional factors (such as speciation or dispersal) on the species richness in local communities (S L ) has received increasing attention. A prominent method to infer the impact of regional factors is the comparison of species richness in local assemblages (S L ) with the total number of species in the region (S R ). Linear relations between S R and S L have been interpreted as an indication of strong regional influence and weak influence of interactions within local communities. We propose that two aspects bias the outcome of such comparisons: (1) the spatial scale of local and regional sampling, and (2) the body size of the organisms. The impact of the local area reflects the scales of ecological interactions, whereas the ratio between local and regional area reflects the inherent moment of autocorrelation. A proposed impact of body size on the relation is based on the high dispersal and high abundance of small organisms. We predict strongest linearity between S R and S L for large organisms, for large local areas (less important ecological interactions) and for sampling designs where the local habitat area covers a high proportion of the regional area (more important autocorrelation). We conducted a meta-analysis on 63 relations obtained from the literature. As predicted, the linearity of the relationship between S L and S R increased with the proportion of local to regional sampling area. In contrast, neither the body size of the organisms nor the local area itself was significantly related to the relation between S L and S R . This indicated that ecological interactions played a minor role in the shape of local to regional richness plots, which instead was mainly influenced by the sampling design. We found that the studies published so far were highly biased towards larger organisms and towards high similarity between the local and regional area. The proposed prevalence of linear relationships may thus be an artefact and plots of S L to S R are not a suitable tool with which to infer the strength of local interactions.
PLOS ONE, 2020
Understanding the determinants of range location and size is fundamental to our understanding of spatial patterns in species richness. Here, we aimed to test the role of 'climatic stability' in determining latitudinal trends in range size and as a consequence on species richness of tropical woody plants. Using primary data from 156 (0.06 ha) plots comprising 20,400 occurrences of more than 400 species of tropical woody plants, we built a biomewide species database that covers the entire latitudinal extent of the wet-evergreen forests of the Western Ghats (8 o to 20 o N), India. We consolidated this database using secondary data from other published species inventories. We then calculated the range sizes and climatic niche width of woody plants to test the predictions of the climatic stability hypothesis and examined the relationship between range position and climatic tolerance of species. Our results show a significant latitudinal gradient in species richness and turnover where local and regional species richness increase monotonically from higher latitudes to lower latitudes of the Western Ghats. We found strong support for Rapoport's Rule with an increase in range size from lower to higher latitudes; our results are consistent with the predictions of the climatic stability hypothesis, where species at higher latitudes exhibited greater tolerance to temperature and rainfall seasonality. Contrary to earlier work, our findings suggest that Rapoport's Rule and the climatic stability hypothesis can operate over regional scales, and even at lower latitudes. We suggest that latitude associated climatic seasonality through its influence on species ranges, can influence latitudinal patterns in species turnover as well as species richness.