Community-based processes behind species richness gradients: contrasting abundance–extinction dynamics and sampling effects in areas of low and high productivity

Aim  To consider the role of local colonization and extinction rates in explaining the generation and maintenance of species richness gradients at the regional scale.
Location  A Mediterranean biome (oak forests, deciduous forests, shrublands, pinewoods, firwoods, alpine heathlands, crops) in Catalonia, Spain.
Methods  We analysed the relative importance of direct and indirect effects of community size in explaining species richness gradients. Direct sampling effects of community size on species richness are predicted by Hubbell's neutral theory of biodiversity and biogeography. The greater the number of individuals in a locality, the greater the number of species expected by random direct sampling effects. Indirect effects are predicted by the abundance–extinction hypothesis, which states that in more productive sites increased population densities and reduced extinction rates may lead to high species richness. The study system was an altitudinal gradient of forest bird species richness.
Results  We found significant support for the existence of both direct and indirect effects of community size in species richness. Thus, both the neutral and the abundance–extinction hypotheses were supported for the altitudinal species richness gradient of forest birds in Catalonia. However, these mechanisms seem to drive variation in species richness only in low-productivity areas; in high-productivity areas, species richness was uncorrelated with community size and productivity measures.
Main conclusions  Our results support the existence of a geographical mosaic of community-based processes behind species richness gradients, with contrasting abundance–extinction dynamics and sampling effects in areas of low and high productivity.     

Neutral community dynamics, the mid-domain effect and spatial patterns in species richness

The mid‐domain effect (MDE) aims to explain spatial patterns in species richness invoking only stochasticity and geometrical constraints. In this paper, we used simulations to show that its main qualitative prediction, a hump‐shaped pattern in species richness, converges to the expectation of a spatially bounded neutral model when communities are linked by short‐distance migration. As these two models can be linked under specific situations, neutral theory may provide a mechanistic population level basis for MDE. This link also allows establishing in which situations MDE patterns are more likely to be found. Also, in this situation, MDE models could be used as a first approximation to understand the role of both stochastic (ecological drift and migration) and deterministic (adaptation to environmental conditions) processes driving the spatial structure of species richness.     

The positive correlation between avian species richness and human population density in Britain is not attributable to sampling bias

Aim  To assess whether spatial variation in sampling effort drives positive correlations between human population density and species richness.
Location  British 10 × 10 km squares.
Methods  We calculated three measures of species richness from atlas data of breeding birds in Britain: total species richness, species richness standardised for sampling effort, and the number of species only recorded in supplementary casual records in a manner not standardised for survey effort. We then assessed the form of the relationship between these richness estimates and human population density, both with and without taking spatial autocorrelation into account.
Results  Both total and standardised species richness exhibit similar species richness–human population density relationships; species richness generally increases with human population density, but decreases at the very highest densities. Supplementary species richness is very weakly correlated with human population density.
Main conclusions  In this example, sampling effort only slightly influences the form of species richness–human population density relationships. The positive correlation between species richness and human population density and any resultant conservation conflicts are thus not artefactual patterns generated by confounding human density and sampling effort.     

People, energy and avian species richness

Aim To investigate the inter‐relationships between energy availability, species richness and human population density, particularly whether human population density influences the manner in which species richness responds to energy availability. Location British 10‐km grid cells. Methods Using regressions, we investigate how human population density varies with energy availability and the nature of relationships between the numbers of species, classified by abundance and threat categories, and human population density. We then assess whether the relationships between these species richness measures and energy availability are altered when accounting for human population density. We conduct analyses using both independent error models and ones that control for spatial autocorrelation. Results Human population density was strongly and positively correlated with energy availability. Total species richness, and that of unthreatened, threatened, common and moderately common species, increases in a positive decelerating manner with human density. When human population density was taken into account, these species groups exhibited similar species–energy relationships, but the slopes of these relationships were significantly reduced in independent error models and, in the case of total richness, in spatial models. Main conclusions Positive correlations between human density and species richness probably arise as both increase with energy availability. Our data are compatible with the suggestion that high human population densities reduce the rate at which species richness increases with energy availability, but additional research is required before causality can be confirmed.     

Relative roles of ecological and energetic constraints,diversification rates and region history on global species richness gradients

Regions worldwide differ markedly in species richness. Here, for birds and mammals worldwide, we directly compare four sets of hypotheses regarding geographical richness gradients: (1) evolutionary, emphasising heterogeneity in diversification rates, (2) historical, related to differences in region ages and sizes, (3) energetic, associated with variation in productive or ambient energy and (4) ecological, reflecting differences in ecological niche diversity. Among highly independent regions, or ‘evolutionary arenas’, we find that richness is weakly influenced by richness‐standardised ecological niche diversity, questioning the significance of ecological constraints for producing large‐scale diversity gradients. In contrast, we find strong evidence for the importance of region area and its changes over time, together with a role for temperature. These predictors affect richness predominately directly without concomitant positive effects on diversification rates. This suggests that regional richness is governed by historical and evolutionary processes, which promote region‐specific accumulation of diversity through time or following asymmetrical dispersal.     

Different evolutionary histories underlie congruent species richness gradients of birds and mammals

Aim The global species richness patterns of birds and mammals are strongly congruent. This could reflect similar evolutionary responses to the Earth’s history, shared responses to current climatic conditions, or both. We compare the geographical and phylogenetic structures of both richness gradients to evaluate these possibilities. Location Global. Methods Gridded bird and mammal distribution databases were used to compare their species richness gradients with the current environment. Phylogenetic trees (resolved to family for birds and to species for mammals) were used to examine underlying phylogenetic structures. Our first prediction is that both groups have responded to the same climatic gradients. Our phylogenetic predictions include: (1) that both groups have similar geographical patterns of mean root distance, a measure of the level of the evolutionary development of faunas, and, more directly, (2) that richness patterns of basal and derived clades will differ, with richness peaking in the tropics for basal clades and in the extra‐tropics for derived clades, and that this difference will hold for both birds and mammals. We also explore whether alternative taxonomic treatments for mammals can generate patterns matching those of birds. Results Both richness gradients are associated with the same current environmental gradients. In contrast, neither of our evolutionary predictions is met: the gradients have different phylogenetic structures, and the richness of birds in the lowland tropics is dominated by many basal species from many basal groups, whereas mammal richness is attributable to many species from both few basal groups and many derived groups. Phylogenetic incongruence is robust to taxonomic delineations for mammals. Main conclusions Contemporary climate can force multiple groups into similar diversity patterns even when evolutionary trajectories differ. Thus, as widely appreciated, our understanding of biodiversity must consider responses to both past and present climates, and our results are consistent with predictions that future climate change will cause major, correlated changes in patterns of diversity across multiple groups irrespective of their evolutionary histories.     

Effects of herbivore species richness on the niche dynamics and distribution of blue sheep in the Trans-Himalaya

Aim To understand the community structure of mountain ungulates by exploring their niche dynamics in response to sympatric species richness. Location Ladakh and Spiti Regions of the Western Indian Trans‐Himalaya. Methods We used the blue sheep Pseudois nayaur, a relatively widely distributed mountain ungulate, as a model species to address the issue. We selected three discrete valleys in three protected areas with similar environmental features but varying wild ungulate species richness, and studied blue sheep’s diet and habitat utilization in them. Habitat variables such as slope angle, distance to cliff and elevation at blue sheep locations were recorded to determine the habitat width of the species. Faecal pellets were collected and microhistological faecal analysis was carried out to determine the diet width of blue sheep in the three areas with different ungulate species richness. Blue sheep’s niche width in terms of habitat and diet was determined using the Shannon’s Index. Results The habitat width of blue sheep had a negative relationship with the number of sympatric species. However, contrary to our expectation, there was a hump‐shaped relationship between blue sheep’s diet width and the sympatric species richness, with the diet width being narrower in areas of allopatry as well as in areas with high herbivore species richness, and the greatest in areas with moderate species richness. Main conclusions We suspect that the narrow diet width in allopatry is out of choice, whereas it is out of necessity in areas with high herbivore species richness because of resource partitioning that enables coexistence. We suggest that interactions with sympatric species lead to niche adjustment of mountain ungulates, implying that competition may play a role in structuring Trans‐Himalayan mountain ungulate assemblages. Given these results, we underscore the importance of including biotic interactions in species distribution models, which have often been neglected.     

Multiregional comparison of the ecological and phylogenetic structure of butterfly species richness gradients

Aim To examine butterfly species richness gradients in seven regions/countries and to quantify geographic mean root distance (MRD) patterns. My primary goal is to determine the extent to which an explanation for butterfly richness patterns based on tropical niche conservatism and the evolution of cold tolerance, proposed for the fauna of Canada and the USA, applies to other parts of the world. Location USA/Canada, Mexico, Europe/NW Africa, Transbaikal Siberia, Chile, South Africa and Australia. Methods Digitized range maps for butterfly species in each region were used to map richness patterns in summer (for all areas) and winter (for USA/Canada, Europe/NW Africa and Australia). A phylogeny resolved to subfamily was used to map the geographic MRD patterns. Regression trees and general linear models examined climatic and vegetation correlates of species richness and MRD within and among regions. Results Various combinations of climate and vegetation were strong predictors of species richness gradients within regions, but unresolved ‘regional’ factors contributed to the multiregional pattern. Regionally based differences in phylogenetic structure also exist, but MRD is negatively correlated with temperature both within and across areas. MRD patterns consistent with tropical niche conservatism occur in most areas. With a possible partial exception of Mexico, faunas in cold climates and in mountains are more derived than faunas in lowlands and tropical/subtropical climates. In USA/Canada, Europe and Australia, winter faunas are more derived than summer faunas. Main conclusions The phylogenetic pattern previously found in the USA and Canada is widespread in both the Northern and Southern Hemispheres, and niche conservatism and the evolution of cold tolerance is the likely explanation for the development of the global butterfly species richness gradient over evolutionary time. Contemporary climate also influences species richness patterns but is unlikely to be a complete explanation globally. The importance of climate is also manifested in the seasonal loss of more basal butterfly elements outside the tropics in winter.     

The effect of 50 years of landscape change on species richness and community composition

We analysed a 50-year dataset of avian species observations to determine how richness and community composition varied over a period of landscape-scale environmental change. Our study area, northern lower Michigan, has experienced substantial land-use and land-cover change over time. Like much of the northern Midwest, it has shifted from a largely unpopulated, post-logging shrubland to a moderately populated closed-canopy forest. Such changes are generally expected to influence overall richness and community composition. We found that regional richness per year remained virtually unchanged over the study period. Year-to-year variation in species number was surprisingly low. Richness totals included vastly different species groups as the composition of the regional bird community changed substantially over time. Changes in the types of species present appear to reflect deterministic changes in habitat. The number of grassland and open-habitat species decreased, for example, while species associated with older forests and urban habitats increased. Our results suggest that habitat changes at the landscape scale do not necessarily lead to changes in the number of species a region can support. Such changes, however, do appear to influence the types of species that will occupy a region, and can lead to substantial changes in community composition.     

Presence, richness and extinction of birds of prey in the Mediterranean and Macaronesian islands

Aim We analyse modern patterns of richness, presence and extinction of birds of prey (Accipitriforms and Falconiforms) in the Mediterranean and Macaronesian islands, using an integrated approach involving both biogeographical and human‐induced factors. Location Forty‐three islands grouped into nine Mediterranean and Macaronesian archipelagos. Methods Information about 25 species breeding during the past century and their fate (permanence or extinction) was compiled from the literature and regional reports. Jaccard's similarity index and cluster analyses were applied to define island assemblages. In order to detect the factors driving richness, presence and extinction, generalized linear models (GLM) were applied to 32 explanatory variables, evaluating location, physiography, isolation of island, taxonomic affinities and life‐history patterns of the raptor species. Results Islands belonging to the same archipelago clustered when raptor assemblages were compared, revealing a marked biogeographical signal. Species richness was influenced by island area and accessibility from the continent (explained deviance of 51% in the GLM). Models of the probability of presence (explained deviance of 32%) revealed positive influences of migratory patterns (maximum for partial migrants), size of distribution areas and proximity to main migration routes. The model for probability of extinction explained only 8% of the deviance. It revealed that populations living on islands with a high density of human population were more prone to disappear. Also, raptors depending on human resources had more risk of extinction. Main conclusions Basic predictions of island biogeography can explain current patterns of raptor richness in the study area despite millennia of intense humanization processes. Colonization success appears to depend on life‐history traits linked to migratory and dispersal strategies, whereas body‐size constraints are not influential. Additionally, our results reveal the importance of species‐based analyses in studies of island biogeography.     

Plant species richness and spatial organization at different small scales in western Mediterranean landscapes

The relationship between plant species richness and the space organization of the community at different small scales was studied. The study was based on 51 sites distributed along a belt from Central Spain to Portugal. Each site was analyzed with a transect cutting across the boundary between two neighboring patches of shrubland and grassland. Local spatial organization of vegetation was analyzed at different levels of detail and each transect was divided into successively smaller portions. The first division coincides with a physiognomic perception of the site in two patches (shrubland and grassland). The average spatial niche width of the species was used to calculate the spatial organization of the vegetation of each division in each site. The correlation between species richness and spatial organization depended on the block size under consideration. A physiognomic criterion, sectorizing the sites into patches of shrubland and grassland, determines noteworthy floristic changes but does not enable us to express satisfactorily the variability in plant richness. In order to account for this variation, other factors must be taken into account which act at a more detailed small-scale and which determine the internal variability of these patches. In the case studied, the species richness of the sites increases along with an increase in the percentage of species whose occupation of the space is relatively restricted within the site. Many of these species are, however, frequent within the whole of the territory studied. The results highlight the importance of the level of local scale at which the factors influencing occupation of the space, and consequentially, plant richness, preferentially act. This circumstance ought to be taken into consideration in strategies for the conservation of biological diversity, and based on the delimitation of protected spaces with criteria frequently linked to the physiognomy of the vegetation.Nomenclature: Follows T.G. Tutin et al. 1964-1980. Flora Europaea. Cambridge University Press, Cambridge     

Bayesian estimation of species richness from quadrat sampling data in the presence of prior information

We consider the problem of estimating the number of species of an animal community. It is assumed that it is possible to draw up a list of species liable to be present in this community. Data are collected from quadrat sampling. Models considered in this article separate the assumptions related to the experimental protocol and those related to the spatial distribution of species in the quadrats. Our parameterization enables us to incorporate prior information on the presence, detectability, and spatial density of species. Moreover, we elaborate procedures to build the prior distributions on these parameters from information furnished by external data. A simulation study is carried out to examine the influence of different priors on the performances of our estimator. We illustrate our approach by estimating the number of nesting bird species in a forest.     

Species richness, species range size and ecological specialisation among African primates: geographical patterns and conservation implications

The geographical distribution of species richness and species range size of African anthropoid primates (catarrhines) is investigated and related to patterns of habitat and dietary niche breadth. Catarrhine species richness is concentrated in the equatorial regions of central and west Africa; areas that are also characterised by low average species range sizes and increased ecological specificity. Species richness declines with increasing latitude north and south of the equator, while average species range size, habitat and dietary breadth increase. Relationships between species richness, species range size and niche breadth remain once latitudinal and longitudinal effects have been removed. Among areas of lowest species richness, however, there is increased variation in terms of average species range size and niche breadth, and two trends are identified. While most such areas are occupied by a few wide-ranging generalists, others are occupied by range-restricted specialist species. That conservation efforts increasingly focus on regions of high species richness may be appropriate if these regions are also characterised by species that are more restricted in both their range size and their ecological versatility, although special consideration may be required for some areas of low species richness.     

People, species richness and human population growth

Aim To investigate how the magnitude of conservation conflicts arising from positive relationships between human population size and species richness is altered during a period of marked human population growth (2% year^?1). Location South Africa. Methods Anuran and avian species richness were calculated from atlas distribution maps, and human population was measured in 1996 and 2001, all at a quarter‐degree resolution. We investigated the relationships between human population size in, and its change during, these two periods and environmental energy availability. We then investigated the nature of relationships between species richness and human population size in both time periods, and its change during them; these analyses were conducted both with and without taking environmental energy availability into account. Finally, we investigated the nature of the relationships between human population size, and its change, and the proportion of protected land. Analyses were conducted both without and with taking spatial autocorrelation into account; the latter was achieved using mixed models that fitted a spatial covariance structure to the data. Results Change in human population size between 1996 and 2001 exhibited marked spatial variation, with both large increases and decreases, but was poorly correlated with environmental energy availability. The nature of the relationship between human population size and environmental energy availability did not, however, exhibit statistically significant differences regardless of whether the former was measured in 1996 or 2001. Similarly, relationships between species richness and human population size did not exhibit significant differences between the two periods. The strengths of the species–human relationships were markedly reduced when energy availability was taken into account. Change in human population size was poorly correlated with species richness. The proportion of protected land was negatively, albeit rather weakly, correlated with human population size in 1996 and 2001, and with its change between these two periods. Main conclusions Positive species–human relationships arise largely, but not entirely, because both species richness and human population size exhibit similar responses to environmental energy availability. During a period of rapid human population growth, and marked changes in the spatial variation in human population size, positive correlations remained between human population size and both anuran and avian species richness. The slope of these correlations did not, however, alter, and the most species‐rich areas are not those with the largest increases in human population. Despite marked population growth, the magnitude of conservation conflicts arising from positive species–human relationships thus appears to have remained largely unchanged.     

Estimating species richness and catch per unit effort from boat electro‐fishing in a lowland river in temperate Australia

Abstract Biodiversity estimates are typically a function of sampling effort and in this regard it is important to develop an understanding of taxon‐specific sampling requirements. Northern hemisphere studies have shown that estimates of riverine fish diversity are related to sampling effort, but such studies are lacking in the southern hemisphere. We used a dataset obtained from boat electro‐fishing the fish community along an essentially continuous 13‐km reach of the Murrumbidgee River, Australia, to investigate sampling effort effects on fish diversity estimates. This represents the first attempt to investigate relationships between sampling effort and the detection of fish species in a large lowland river in Australia. Seven species were recorded. Species‐specific patterns in catch per unit effort were evident and are discussed in terms of solitary and gregarious species, recreational fishing and the monitoring of rare and threatened species. There was a requirement to sample substantial lengths of river to describe total species richness of the fish community in this river reach. To this end, randomly allocated sampling effort and use of species richness estimators produced accurate estimates of species richness without the requirement for excessive levels of effort. Twenty operations were required to estimate species richness at this site, highlighting the need for comparable studies of river fish communities in lowland rivers elsewhere in Australia and the southern hemisphere.