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.     

The carrying capacity for species richness

The idea that the number of species within an area is limited by a specific capacity of that area to host species is old yet controversial. Here, we show that the concept of carrying capacity for species richness can be as useful as the analogous concept in population biology. Many lines of empirical evidence indicate the existence of limits of species richness, at least at large spatial and phylogenetic scales. However, available evidence does not support the idea of diversity limits based on limited niche space; instead, carrying capacity should be understood as a stable equilibrium of biodiversity dynamics driven by diversity‐dependent processes of extinction, speciation and/or colonization. We argue that such stable equilibria exist even if not all resources are used and if increasing species richness increases the ability of a community to use resources. Evaluating the various theoretical approaches to modelling diversity dynamics, we conclude that a fruitful approach for macroecology and biodiversity science is to develop theory that assumes that the key mechanism leading to stable diversity equilibria is the negative diversity dependence of per‐species extinction rates, driven by the fact that population sizes of species must decrease with an increasing number of species owing to limited energy availability. The recently proposed equilibrium theory of biodiversity dynamics is an example of such a theory, which predicts that equilibrium species richness (i.e., carrying capacity) is determined by the interplay of the total amount of available resources, the ability of communities to use those resources, environmental stability that affects extinction rates, and the factors that affect speciation and colonization rates. We argue that the diversity equilibria resulting from these biodiversity dynamics are first‐order drivers of large‐scale biodiversity patterns, such as the latitudinal diversity gradient.     

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.     

Does energy availability influence classical patterns of spatial variation in exotic species richness?

Aim At macroecological scales, exotic species richness is frequently positively correlated with human population density. Such patterns are typically thought to arise because high human densities are associated with increased introduction effort and/or habitat modification and disturbance. Exotic and native species richness are also frequently positively correlated, although the causal mechanisms remain unclear. Energy availability frequently explains much of the variation in species richness and we test whether such species–energy relationships may influence the relationships of exotic species richness with human population density and native species richness. Location Great Britain. Methods We first investigate how spatial variation in the distributions of the 10 exotic bird species is related to energy availability. We then model exotic species richness using native avian species richness, human population density and energy availability as predictors. Species richness is modelled using two sets of models: one assumes independent errors and the other takes spatial correlation into account. Results The probability of each exotic species occurring, in a 10‐km quadrat, increases with energy availability. Exotic species richness is positively correlated with energy availability, human population density and native species richness in univariate tests. When taking energy availability into account, exotic species richness is negligibly influenced by human population density, but remains positively associated with native species richness. Main conclusions We provide one of the few demonstrations that energy availability exerts a strong positive influence on exotic species richness. Within our data, the positive relationship between exotic species richness and human population density probably arises because both variables increase with energy availability, and may be independent of the influence of human density on the probability of establishment. Positive correlations between exotic and native species richness remain when controlling for the influence of energy on species richness. The relevance of such a finding to the debate on the relationship between diversity and invasibility is discussed.     

Dissecting the species-energy relationship

Environmental energy availability can explain much of the spatial variation in species richness. Such species-energy relationships encompass a diverse range of forms, and there is intense debate concerning which of these predominate, and the factors promoting this diversity. Despite this there has been relatively little investigation of whether the form, and relative strength, of species-energy relationships varies with (i) the currency of energy availability that is used, and (ii) the ecological characteristics of the constituent species. Such investigations can, however, shed light on the causal mechanisms underlying species-energy relationships. We illustrate this using the British breeding avifauna. The strength of the species-energy relationship is dependent on the energy metric used, with species richness being more closely correlated with temperature than the Normalized Difference Vegetation Index, which is a strong correlate of net primary productivity. We find little evidence, however, for the thermoregulatory load hypothesis that high temperatures enable individuals to invest in growth and reproduction, rather than thermoregulation, increasing population sizes that buffer species from extinction. High levels of productive energy may also elevate population size, which is related to extinction risk by a negative decelerating function. Therefore, the rarest species should exhibit the strongest species-energy relationship. We find evidence to the contrary, together with little support for suggestions that high-energy availability elevates species richness by increasing the numbers of specialists or predators.     

Host population density and body mass as determinants of species richness in parasite communities: comparative analyses of directly transmitted nematodes of mammals

Epidemiological theory predicts positive correlations between host population density or body mass and species richness among parasite communities. Here I test these predictions by a comparative study of communities of directly transmitted mammalian parasites, gastrointestinal strongylid nematodes. I use data from 45 species of mammals, representing examination of 17 200 individual hosts. The variable studied was the average number of gastrointestinal strongylid nematode species per host population, and three different methods were used to obtain estimates of parasite species richness that are unbiased by number of host individuals examined. Analyses were done using the phylogenetically independent contrast method. Host population density and parasite species richness were strongly positively correlated when the effects of host body weight had been controlled for. Controlling for other variables did not change this, and the relationship was found regardless of method used to correct for uneven sampling effort among host species. A positive relationship between parasite species richness and host body weight was also found, but the effect of host densities had to be controlled for to see this. These relationships between host traits and species richness of directly transmitted parasites are stronger than patterns found using data on indirectly transmitted mammalian parasites, and suggests that links between host traits and parasite species richness are stronger than previously suggested. The results are consistent with parasite species richness being positively linked to pathogen transmission rates and reductions in transmission rates possibly increasing extinction probabilities in parasite populations. The results also suggest that parasites may exert a cost of increases in rate of population energy usage, and thus show that pathogens may be important in generating independence between body mass and rate of population energy usage among host species.     

Abundance, species richness and energy availability in the North American avifauna

Aim To determine how species richness, abundance, biomass, energy use and mean number of individuals per species scale with environmental energy availability in wintering and breeding avian assemblages, and to contrast assemblages of (i) common and rare species and (ii) breeding residents and migrants. To assess whether such patterns are compatible with the ‘more individuals hypothesis’ (MIH) that high‐energy areas are species‐rich because they support larger populations that are buffered against extinction. Location The North American continent (latitudinal range 23.4 °?48.1 °N; longitudinal range 124.2°?68.7° W). Methods Avian species richness, abundance, biomass and energy use were calculated for 295 Resident Bird Count plots. Environmental energy availability was measured using ambient temperature and the Normalized Difference Vegetation Index (NDVI), a close correlate of plant productivity. Analyses took plot area into account, and were conducted (with and without taking habitat type into account) using general linear models and spatial mixed models. Results Positive species–energy relationships were exhibited by both wintering and breeding assemblages, but were stronger in the former. The structure of winter assemblages responded more strongly to temperature than NDVI, while breeding assemblages tended to respond more strongly to NDVI. Breeding residents responded to annual measures of energy availability while breeding migrants and the winter assemblage responded more strongly to seasonal measures. In the winter assemblage, rare and common species exhibited species–energy relationships of a similar strength, but common breeding species exhibited a much stronger relationship than rare breeding species. In both breeding and wintering assemblages, abundance, biomass and energy use increased with energy availability and species richness. Energy availability was a poor predictor of the mean number of individuals per species. Main conclusions The nature of the species–energy relationship varies seasonally and with the manner in which energy availability is measured. Our data suggest that residents are less able to respond to seasonal fluxes in resource availability than long‐distance migrants. Increasing species richness and energy availability is associated with increasing numbers of individuals, biomass and energy use. While these observations are compatible with the MIH our data provide only equivocal support for this hypothesis, as the rarest species do not exhibit the strongest species–energy relationships.     

Ants and people: a test of two mechanisms potentially responsible for the large‐scale human population–biodiversity correlation for Formicidae in Europe

Aim: Recent coarse‐scale studies have shown positive relationships between the biodiversity of plants/vertebrates and the human population. Little is known about the generality of the pattern for invertebrates. Moreover, biodiversity and human population might correlate because they both covary with other factors such as energy availability and habitat heterogeneity. Here we test these two non‐mutually exclusive mechanisms with ant species‐richness data from the Fauna Europaea. Location Forty‐three European countries/regions. Methods We derived mixed models of total, native and exotic ant species richness as a function of human population size/density, controlling for country area, plant species richness (as a proxy for habitat heterogeneity), and mean annual temperature and precipitation (variables related to energy availability). Results Ant species richness increased significantly with increasing human population. This result was confirmed when controlling for variations in country area. Both for human population size/density and for ant species richness, there were positive correlations with temperature but not with precipitation. This finding is in agreement with the energy‐availability hypothesis. However, we observed a negative latitudinal gradient in ant and plant species richness, although not in human population size/density. Plant species richness was positively correlated with ant species richness but not with human population size/density. Thus, there is evidence that this type of habitat heterogeneity can play a role in the observed latitudinal gradient of ant species richness, but not in the positive correlation between ant species richness and human population. The results were confirmed for the 545 native and the 32 exotic ant species reported, and we observed a good correlation between exotic and native ant species richness. Main conclusions Ant species richness in European countries conforms to six macroecological patterns: (1) a negative latitudinal gradient; and a positive (2) species–energy relationship, (3) species–area relationship, (4) correlation with plant species richness, (5) exotic–native species richness correlation, and (6) species–people correlation. There is some evidence for the energy‐availability hypothesis, but little evidence for habitat heterogeneity as an explanation of the large‐scale human population–ant biodiversity correlation. This correlation has implications for the conservation of ant diversity in Europe.     

Plant extinction dynamics in an insular metacommunity

Changes in the composition of local communities through time (i.e. species turnover) is a common phenomenon in insular biology. However, the mechanisms promoting variation in species turnover, both among islands and among species, are poorly understood. In an effort to better understand the causes of variation in species turnover, we evaluated the colonization and extinction dynamics of plant populations on 18 small islands off the west coast of Canada. In 1997, we quantified total population sizes of 10 woody angiosperm species. A decade later, we resampled islands to test whether: 1) species turnover occurred, 2) colonization events were offset by extinction events, 2) variation in extinction rates among islands was associated with population sizes, average plant heights, island area, island isolation or each island's exposure to ocean-born disturbances, and 3) variation in extinction rates among species was associated with plant life history traits. Results showed that extinction events outnumbered colonization events, suggesting that the metacommunity is in 'disequilibrium'. Variation in extinction rates among islands was unrelated to island area and isolation. However, extinction rates increased with exposure to ocean-born disturbances and decreased with both initial population sizes and average plant heights. Species with thicker, tougher leaves (i.e. high leaf mass per area) were less prone to extinction than species with thinner, more papery leaves. Overall results indicate that species turnover is common and that it is generated primarily by extinction. Variation in extinction rates appears to result from an interaction between among-island effects (exposure, population size and plant stature) and among-species effects (leaf toughness), suggesting that ocean-born disturbances play a key role in determining metacommunity structure.     

Direct and indirect effects of area,energy and habitat heterogeneity on breeding bird communities

Aim To compare the ability of island biogeography theory, niche theory and species–energy theory to explain patterns of species richness and density for breeding bird communities across islands with contrasting characteristics. Location Thirty forested islands in two freshwater lakes in the boreal forest zone of northern Sweden (65°55′ N to 66°09′ N; 17°43′ E to 17°55′ E). Methods We performed bird censuses on 30 lake islands that have each previously been well characterized in terms of size, isolation, habitat heterogeneity (plant diversity and forest age), net primary productivity (NPP), and invertebrate prey abundance. To test the relative abilities of island biogeography theory, niche theory and species–energy theory to describe bird community patterns, we used both traditional statistical approaches (linear and multiple regressions) and structural equation modelling (SEM; in which both direct and indirect influences can be quantified). Results Using regression‐based approaches, area and bird abundance were the two most important predictors of bird species richness. However, when the data were analysed by SEM, area was not found to exert a direct effect on bird species richness. Instead, terrestrial prey abundance was the strongest predictor of bird abundance, and bird abundance in combination with NPP was the best predictor of bird species richness. Area was only of indirect importance through its positive effect on terrestrial prey abundance, but habitat heterogeneity and spatial subsidies (emerging aquatic insects) also showed important indirect influences. Thus, our results provided the strongest support for species–energy theory. Main conclusions Our results suggest that, by using statistical approaches that allow for analyses of both direct and indirect influences, a seemingly direct influence of area on species richness can be explained by greater energy availability on larger islands. As such, animal community patterns that seem to be in line with island biogeography theory may be primarily driven by energy availability. Our results also point to the need to consider several aspects of habitat quality (e.g. heterogeneity, NPP, prey availability and spatial subsidies) for successful management of breeding bird diversity at local spatial scales and in fragmented or insular habitats.     

Environmental correlates of leguminosae species richness in Mexico: Quantifying the contributions of energy and environmental seasonality

Explaining species richness patterns is a central issue in ecology, but a general explanation remains elusive. Environmental conditions have been proposed to be important drivers of these patterns, but we still need to better understand the relative contribution of environmental factors. Here, we aim at testing two environmental hypotheses for richness gradients: energy availability and environmental seasonality using diversity patterns of the family Leguminosae across Mexico. We compiled a data base of 502 species and 32,962 records. After dividing Mexico into 100 × 100 km grid cells, we constructed a map of variation in species richness that accounts for heterogeneity in sampling effort. We found the cells with the highest species richness of legumes are in the Neotropical region of Pacific coastal and southern Mexico, where the legume family dominates the tropical rain forests and seasonally dry tropical forests. Regression models show that energy and seasonality predictors can explain 25% and 49% of the variation in richness, respectively. Spatial autocorrelation analysis showed that richness has a strong spatial structure, but that most of this structure disappears when both energy and seasonality are used to account for richness gradient. Our study demonstrates multiple environmental conditions contribute complementarily to explain diversity gradients. Moreover, it shows that in some regions, environmental seasonality can be more important than energy availability, contradicting studies at coarser spatial scales. More basic taxonomic and floristic work is needed to help describe patterns of diversity for many groups to allow for testing the underlying mechanisms responsible for diversity gradients. Abstract in Spanish is available with online material.     

Relative contribution of abundant and rare species to species-energy relationships

A major goal of ecology is to understand spatial variation in species richness. The latter is markedly influenced by energy availability and appears to be influenced more by common species than rare ones; species-energy relationships should thus be stronger for common species. Species-energy relationships may arise because high-energy areas support more individuals, and these larger populations may buffer species from extinction. As extinction risk is a negative decelerating function of population size, this more-individuals hypothesis (MIH) predicts that rare species should respond more strongly to energy. We investigate these opposing predictions using British breeding bird data and find that, contrary to the MIH, common species contribute more to species-energy relationships than rare ones.     

Spatial scale, abundance and the species-energy relationship in British birds

1. The spatial scale of analysis may influence the nature, strength and underlying drivers of macroecological patterns, one of the most frequently discussed of which is the relationship between species richness and environmental energy availability. 2. It has been suggested that species-energy relationships are hump-shaped at fine spatial grains and consistently positive at larger regional grains. The exact nature of this scale dependency is, however, the subject of much debate as relatively few studies have investigated species-energy relationships for the same assemblage across a range of spatial grains. Here, we contrast species-energy relationships for the British breeding avifauna at spatial grains of 1 km x 1 km, 2 km x 2 km and 10 km x 10 km plots, while maintaining a constant spatial extent. 3. Analyses were principally conducted using data on observed species richness. While survey work may fail to detect some species, observed species richness and that estimated using nonparametric techniques were strongly positively correlated with each other, and thus exhibit very similar spatial patterns. Moreover, the forms of species-energy relationships using observed and estimated species richness were statistically indistinguishable from each other. 4. Positive decelerating species-energy relationships arise at all three spatial grains. There is little evidence that the explanatory power of these relationships varies with spatial scale. However, ratios of regional (large-scale) to local (small-scale) species richness decrease with increasing energy availability, indicating that local richness responds to energy with a steeper gradient than does regional richness. Local assemblages thus sample a greater proportion of regional richness at higher energy levels, suggesting that spatial turnover of species richness is lower in high-energy regions. Similarly, a crude measure of temporal turnover, the ratio of cumulative species richness over a 4-year period to species richness in a single year, is lower in high-energy regions. These negative relationships between turnover and energy appear to be causal as both total and mean occupancy per species increases with energy. 5. While total density in 1 km x 1 km plots correlates positively with energy availability, such relationships are very weak for mean density per species. This suggests that the observed association between total abundance and species richness may not be mediated by population extinction rates, as predicted by the more individuals hypothesis. 6. The sampling mechanism suggests that species-energy relationships arise as high-energy areas support a greater number of individuals, and that random allocation of these individuals to local areas from a regional assemblage will generate species-energy relationships. While randomized local species-energy relationships are linear and positive, predicted richness is consistently greater than that observed. The mismatch between the observed and randomized species-energy relationships probably arises as a consequence of the aggregated nature of species distributions. The sampling mechanism, together with species spatial aggregation driven by limited habitat availability, may thus explain the species-energy relationship observed at this spatial scale.     

Latitudinal gradients in North American avian species richness,turnover rates and extinction probabilities

A decline in species richness moving from equatorial regions to polar regions is a common, but not universal, macroecological pattern. Many studies have focused on this pattern, but few have focused on how the vital rates responsible for species richness patterns, local rates of species extinction and turnover, vary with latitude. We examine patterns of richness, turnover and extinction in North American avian communities inhabiting three ecoregions, using methods that account for failure to detect all species present. We use breeding bird point count data from > 1000 routes in the Breeding Bird Survey collected from 1982 to 2001 to estimate richness, extinction probability and turnover rates. Our analyses differ from others in 1) the use of annual estimates derived at specific locations rather than index data accumulated over numbers of years, 2) the use of estimators that incorporated detection probabilities and 3) a focus on dynamical processes (colonization, extinction) in addition to static patterns (species richness). We find average species richness estimates (48 to 135 species) increasing with latitude for all three regions, contradicting predictions based on the latitudinal diversity gradient. The estimated rates of extinction and turnover declined with latitude across the three ecoregions. We speculate that higher richness might be linked to periods of superabundant food supply in northern areas that support greater numbers of resident and migrant species. Our primary ecological conclusions are that the latitudinal gradient in species richness is reversed for North American birds in the studied ecoregions, and that both local extinction and turnover decrease from southern to northern latitudes. Thus, the vital rates that determine richness show evidence of greater stability and reduced dynamics in northern areas of higher richness. We recommend additional studies examining patterns of colonization, extinction and turnover in communities, that use clearly defined estimators that deal with detection probability.     

Variability in Energy Influences Avian Distribution Patterns Across the USA

Habitat transformations and climate change are among the most important drivers of biodiversity loss. Understanding the factors responsible for the unequal distribution of species richness is a major challenge in ecology. Using data from the North American Breeding Bird Survey to measure species richness and a change metric extracted from the MODerate resolution Imaging Spectroradiometer (MODIS), we examined the influence of energy variability on the geographic distribution of avian richness across the conterminous U.S. and in the different ecoregions, while controlling for energy availability. The analysis compared three groups of birds: all species, Neotropical migrants, and permanent residents. We found that interannual variability in available energy explained more than half of the observed variation in bird richness in some ecoregions. In particular, energy variability is an important factor in explaining the patterns of overall bird richness and of permanent residents, in addition to energy availability. Our results showed a decrease in species richness with increasing energy variability and decreasing energy availability, suggesting that more species are found in more stable and more productive environments. However, not all ecoregions followed this pattern. The exceptions might reflect other biological factors and environmental conditions. With more ecoclimatic variability predicted for the future, this study provides insight into how energy variability influences the geographical patterns of species richness. PR designed study, performed research, analyzed data and wrote paper. CAL, MAL, AMP, VCR, PDC, and EFL designed study and wrote paper.     

Is assemblage variability related to environmental variability? An answer for riverine fish

We examine the variability of riverine fish assemblages in terms of assemblage stability (i.e. variability of numbers of individuals within species over time and variability of assemblage total density), assemblage persistence, and assemblage species richness using data from a 9-yr survey of 27 sites within 18 coastal streams of North-western France. To do so, we test a hypothesized directional model for the expected relationships between environmental variability, assemblage variability, assemblage persistence, and assemblage species richness: 1) environmental variability within a given system is likely to generate variable local population size within this system, thus increasing local assemblages variability; 2) environmental variability should increase extinction rates (or, under constant colonization rates, decrease persistence), because the more population sizes vary within an assemblage, the more likely they are to become zero in some period of time; 3) assemblage variability should reduce assemblage species richness by increasing extinction rates within populations composing these assemblages. Results are compatible with our starting hypotheses and show that assemblage variability increased with environmental variability (i.e. discharge variability), that assemblage persistence decreased with environmental variability, and that species richness decreased with assemblage variability after environmental factors were controlled for. Thus, disturbance regimes, in our case, can alter the stability properties of assemblages and extrinsic determinants of assemblage variability may be an important determinant of assemblage species richness. These results have important conservation and management implications, due to the strong impact of river regulation on flow regimes.     

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.     

Separating the influence of resource 'availability' from resource 'imbalance' on productivity–diversity relationships

One of the oldest and richest questions in biology is that of how species diversity is related to the availability of resources that limit the productivity of ecosystems. Researchers from a variety of disciplines have pursued this question from at least three different theoretical perspectives. Species energy theory has argued that the summed quantities of all resources influence species richness by controlling population sizes and the probability of stochastic extinction. Resource ratio theory has argued that the imbalance in the supply of two or more resources, relative to the stoichiometric needs of the competitors, can dictate the strength of competition and, in turn, the diversity of coexisting species. In contrast to these, the field of Biodiversity and Ecosystem Functioning has argued that species diversity acts as an independent variable that controls how efficiently limited resources are utilized and converted into new tissue. Here we propose that all three of these fields give necessary, but not sufficient, conditions to explain productivity–diversity relationships (PDR) in nature. However, when taken collectively, these three paradigms suggest that PDR can be explained by interactions among four distinct, non-interchangeable variables: (i) the overall quantity of limiting resources, (ii) the stoichiometric ratios of different limiting resources, (iii) the summed biomass produced by a group of potential competitors and (iv) the richness of co-occurring species in a local competitive community. We detail a new multivariate hypothesis that outlines one way in which these four variables are directly and indirectly related to one another. We show how the predictions of this model can be fit to patterns of covariation relating the richness and biomass of lake phytoplankton to three biologically essential resources (N, P and light) in a large number of Norwegian lakes.     

The more‐individuals hypothesis revisited: the role of community abundance in species richness regulation and the productivity–diversity relationship

Species richness increases with energy availability, yet there is little consensus as to the exact processes driving this species–energy relationship. The most straightforward explanation is the more‐individuals hypothesis (MIH). It states that higher energy availability promotes a higher total number of individuals in a community, which consequently increases species richness by allowing for a greater number of species with viable populations. Empirical support for the MIH is mixed, partially due to the lack of proper formalisation of the MIH and consequent confusion as to its exact predictions. Here, we review the evidence of the MIH and evaluate the reliability of various predictions that have been tested. There is only limited evidence that spatial variation in species richness is driven by variation in the total number of individuals. There are also problems with measures of energy availability, with scale‐dependence, and with the direction of causality, as the total number of individuals may sometimes itself be driven by the number of species. However, even in such a case the total number of individuals may be involved in diversity regulation. We propose a formal theory that encompasses these processes, clarifying how the different factors affecting diversity dynamics can be disentangled.     

Urban biogeography

Summary Species richness and abundance of Diptera and Coleoptera were assessed in nine city parks in Cincinnati, Ohio, USA. Species richness for each park was related to the area of the park in a manner predicted by island biogeography theory. The z values for the Diptera and Coleoptera were 0.235 and 0.222 respectively. These values are somewhat higher than expected for continental islands and suggest that the Diptera and Coleoptera in these parks are as isolated as many species which occur on true oceanic islands.A stepwise multiple regression was conducted, regressing species richness against several aspects of habitat diversity in the parks. It was found that area alone was the best predictor of species richness. This result, coupled with data on population sizes, suggests that increased area acts primarily to reduce extinction rates rather than to provide new habitats for specialized species.