More-individuals Hypothesis Research Articles

Regional and local patterns of soil arthropod diversity are explained by different processes in southwest China

Understanding the distribution patterns and underlying mechanisms of community assembly at different spatial scales is crucial for the ecology and conservation of organisms, yet little is known about soil arthropods. In this study, we tested the relative importance of the two hypotheses that could explain soil arthropod distribution patterns at local and regional scales: (1) the habitat heterogeneity hypothesis (spatial complexity and variability of a habitat allow the coexistence of many species), and (2) the more-individuals hypothesis (the amount of resources per se, regardless of its spatial heterogeneity, increases overall soil arthropod abundance and richness). We extensively collected soil arthropods (identified to families or subfamilies) and measured soil physicochemical properties in four forests distributed across latitudes in Yunnan Province, China. We found that soil arthropod abundance and taxon richness increased with increasing latitudes, contrasting with the generally known pattern for plants and aboveground animals. Litter biomass (resources) explained the taxon richness and abundance at the regional scale, which aligned with the more-individuals hypothesis. Other soil physicochemical properties (e.g., Ca, TP, Mg, Mn, and pH) were more important at the local scale, underpinning the habitat heterogeneity hypothesis. However, the relative importance of individual soil properties was highly location-specific, suggesting that the environmental factors operating in one location do not necessarily reflect those operating in others at different latitudes. These findings improve our understanding of the community assembly and ecological processes of soil arthropods at different spatial scales, which is vital for predicting the future of soil biodiversity.

Mechanisms determining the multi-diversity of carrion visiting species along a gradient of carrion body mass

Resource availability and habitat heterogeneity are essential drivers of biodiversity, but their individual roles often remain unclear since both factors are often correlated. Here, we tested the more-individuals hypothesis (MIH) and the habitat-heterogeneity hypothesis (HHH) for bacteria, fungi, dipterans, coleopterans, birds, and mammals on 100 experimentally exposed carcasses ranging by three orders of magnitude in body mass. At the level of each carcass we found marginal or significant support for the MIH for bacteria, fungi, and beetles in spring and significant support for fungi, dipterans, and mammals in summer. The HHH was supported only for bacteria in spring, while it was supported for all groups except mammals in summer. Overall multidiversity always increased with body mass, with a steeper increase in summer. Abundance based rarefaction-extrapolation curves for three classes of body mass showed the highest species richness for medium-sized carcasses, particular for dipterans and microbes, supporting the HHH also among carcasses. These findings complement existing necromass studies of deadwood, showing there are more niches associated with larger resource amounts and an increasing habitat heterogeneity between carcasses most pronounced for medium-sized species. Higher resource amount led to increased diversity of carrion-consuming organisms in summer, particularly due to the increasing number of niches with increasing size. Our findings underline the importance of distributed large carrion as well as medium-sized carrion in ecosystems supporting overall biodiversity of carrion-consumers. Furthermore, the different responses in spring and summer may inform strategies of carrion enrichment management schemes throughout the year.

Ecological drivers of carrion beetle (Staphylinidae: Silphinae) diversity on small to large mammals.

Silphinae (Staphylinidae; carrion beetles) are important contributors to the efficient decomposition and recycling of carrion necromass. Their community composition is important for the provision of this ecosystem function and can be affected by abiotic and biotic factors. However, investigations are lacking on the effects of carrion characteristics on Silphinae diversity. Carrion body mass may affect Silphinae diversity following the more individuals hypothesis (MIH). The MIH predicts a higher number of species at larger carrion because higher numbers of individuals can be supported on the resource patch. Additionally, biotic factors like carrion species identity or decomposition stage, and the abiotic factors elevation, season and temperature could affect Silphinae diversity. To test the hypotheses, we collected Silphinae throughout the decomposition of 100 carcasses representing 10 mammal species ranging from 0.04 to 124 kg. Experimental carcasses were exposed in a mountain forest landscape in Germany during spring and summer of 2021. We analysed Silphinae diversity using recently developed transformation models that considered the difficult data distribution we obtained. We found no consistent effect of carrion body mass on Silphinae species richness and, therefore, rejected the MIH. Carrion decomposition stage, in contrast, strongly influenced Silphinae diversity. Abundance and species richness increased with the decomposition process. Silphinae abundance increased with temperature and decreased with elevation. Furthermore, Silphinae abundance was lower in summer compared to spring, likely due to increased co-occurrence and competition with dipteran larvae in summer. Neither carrion species identity nor any abiotic factor affected Silphinae species richness following a pattern consistent throughout the seasons. Our approach combining a broad study design with an improved method for data analysis, transformation models, revealed new insights into mechanisms driving carrion beetle diversity during carrion decomposition. Overall, our study illustrates the complexity and multifactorial nature of biotic and abiotic factors affecting diversity.

Latitudinal richness patterns in a temperate forest region: disentangling the effects of evenness, density and aggregation

It is well known that species diversity declines with increasing latitude. However, the underlying causes and mechanisms remain ambiguous due to effects of scale, the shape of the species abundance distributions (SAD), the total number of individuals (N) and the spatial aggregation of individuals (AGG). In this study, we examine how SAD, N and AGG contribute to the latitudinal richness gradient. Our study is based on a dataset that includes tree, shrub, and herbaceous plants collected from 445 permanent forest plots scattered across an expansive temperate forest region, covering latitudes from 39.71°N to 53.37°N. We compared multiple diversity patterns along the latitude between three life forms (trees, shrubs and herbs) and analyzed the effects of SAD, N and AGG on the latitudinal richness gradient. Our results reveal a general decline in most diversity measures along the latitudinal gradient, with the exception of herbaceous plants. Furthermore, we observed consistent effects of AGG and SAD on the latitudinal richness gradient across all three life forms. Specifically, a decreasing species evenness with increasing latitude led to a reduction in richness, while the simultaneous decrease in spatial aggregation, along with the effects of total number of individuals, collectively contributed to the decline in species richness with increasing latitude. However, the effect of total abundance varies among life forms, with a negligible effect on tree richness, a negative effect on shrub richness, and a significantly positive effect on herbaceous richness. These findings highlight the dependency of the “more-individuals hypothesis” on specific life forms. Our study shows that decomposing species richness into community structure components provides an opportunity for linking specific processes to latitudinal diversity gradients.

Community-wide correlations between species richness, abundance and population genomic diversity in a freshwater biodiversity hotspot.

Understanding patterns of diversity across macro (e.g. species-level) and micro (e.g. molecular-level) scales can shed light on community function and stability by elucidating the abiotic and biotic drivers of diversity within ecological communities. We examined the relationships among taxonomic and genetic metrics of diversity in freshwater mussels (Bivalvia: Unionidae), an ecologically important and species-rich group in the southeastern United States. Using quantitative community surveys and reduced-representation genome sequencing across 22 sites in seven rivers and two river basins, we surveyed 68 mussel species and sequenced 23 of these species to characterize intrapopulation genetic variation. We tested for the presence of species diversity-abundance correlations (i.e. the more-individuals hypothesis, MIH), species-genetic diversity correlations (SGDCs) and abundance-genetic diversity correlations (AGDCs) across all sites to evaluate relationships between different metrics of diversity. Sites with greater cumulative multispecies density (a standardized metric of abundance) had a greater number of species, consistent with the MIH hypothesis. Intrapopulation genetic diversity was strongly associated with the density of most species, indicating the presence of AGDCs. However, there was no consistent evidence for SGDCs. Although sites with greater overall densities of mussels had greater species richness, sites with higher genetic diversity did not always exhibit positive correlations with species richness, suggesting that there are spatial and evolutionary scales at which the processes influencing community-level diversity and intraspecific diversity differ. Our work reveals the importance of local abundance as indicator (and possibly a driver) of intrapopulation genetic diversity.

Warming reverses directionality in the richness-abundance relationship in ephemeral Mediterranean plant communities.

Recent findings in forests worldwide have demonstrated how directionality in the richness-abundance causality shifts along global climate gradients: The so-called more-species hypothesis (richness determines abundance) prevails in Earth's most productive climates, whereas the opposite, the so-called more-individuals hypothesis (abundance determines richness), is more likely to prevail in climatically harsh conditions. Since temporal variability is the norm, a critical question is whether this directionality shift is also a function of temporal climatic fluctuations locally. Here, we analyze whether directionality in the richness-abundance relationship is contingent on temporal variability over 10 annual consecutive realizations in ephemeral plant assemblages. Our results support the idea that the more-species hypothesis prevailed in the most benign years, whereas the more-individuals hypothesis did so during less productive years, which were significantly linked to the warmest years. These results support the idea that rising temperatures can reverse directionality in the richness-abundance relationship in these annual plant communities, and therefore, climate warming can have a significant effect on the relationship between diversity and ecosystem functions, such as productivity, by altering the prevalence of primary mechanisms involved in species assembly.

More individuals or specialized niches? Distinguishing support for hypotheses explaining positive species–energy relationships

AbstractAimPositive productivity–richness relationships have been observed across taxa and ecosystems. We assessed support for two hypotheses explaining these relationships, the More Individuals Hypothesis (MIH) and the Niche Specialization Hypothesis (NSH) in two complementary datasets of avian communities in North America, the Breeding Bird Survey (BBS), which has been widely used to study productivity–richness relationships, and the Breeding Bird Census (BBC), which has been used rarely yet offers accurate population density estimates and more homogeneous landscapes, eliminating confounding variables that may make it more difficult to test predictions of the NSH and MIH in the BBS alone.LocationCoterminous United States, 1988–2009.TaxonBirds.MethodsWe first evaluated the strength of productivity–richness relationships in the BBS and BBC, and tested predictions from the MIH and NSH in the two datasets by examining patterns in community abundance, productivity niche breadth and number of foraging guilds along a gradient of normalized difference vegetation index (a measure of vegetation productivity). We also implemented a null model to examine the contribution of sampling effects due to increasing species richness in explaining patterns in number of foraging guilds in both datasets.ResultsWe found that the BBS had a much stronger productivity–richness relationship than the BBC, potentially as a result of increased landscape diversity with increasing productivity at BBS survey sites. Although the MIH and NSH may not be mutually exclusive, we found weaker support for the MIH in the BBC, and stronger support for the NSH in the BBC and BBS.Main ConclusionsResearchers should consider the role of confounding variables such as landscape diversity and focus on developing direct measurements of food resources at macroecological scales to determine the relative importance of mechanisms driving productivity–richness relationships.

Productivity, niche availability, species richness, and extinction risk: Untangling relationships using individual-based simulations.

It has often been suggested that the productivity of an ecosystem affects the number of species that it can support. Despite decades of study, the nature, extent, and underlying mechanisms of this relationship are unclear. One suggested mechanism is the “more individuals” hypothesis (MIH). This proposes that productivity controls the number of individuals in the ecosystem, and that more individuals can be divided into a greater number of species before their population size is sufficiently small for each to be at substantial risk of extinction. Here, we test this hypothesis using REvoSim: an individual‐based eco‐evolutionary system that simulates the evolution and speciation of populations over geological time, allowing phenomena occurring over timescales that cannot be easily observed in the real world to be evaluated. The individual‐based nature of this system allows us to remove assumptions about the nature of speciation and extinction that previous models have had to make. Many of the predictions of the MIH are supported in our simulations: Rare species are more likely to undergo extinction than common species, and species richness scales with productivity. However, we also find support for relationships that contradict the predictions of the strict MIH: species population size scales with productivity, and species extinction risk is better predicted by relative than absolute species population size, apparently due to increased competition when total community abundance is higher. Furthermore, we show that the scaling of species richness with productivity depends upon the ability of species to partition niche space. Consequently, we suggest that the MIH is applicable only to ecosystems in which niche partitioning has not been halted by species saturation. Some hypotheses regarding patterns of biodiversity implicitly or explicitly overlook niche theory in favor of neutral explanations, as has historically been the case with the MIH. Our simulations demonstrate that niche theory exerts a control on the applicability of the MIH and thus needs to be accounted for in macroecology.

Another rejection of the more-individuals-hypothesis: Carrion beetles (Silphidae, Coleoptera) in the Southern Rocky Mountains

Beetles are the most diverse animal clade on the planet, and understanding the mechanisms underlying their diversity patterns is critical to understanding animal biodiversity in general. Using carrion beetles (Silphidae; Coleoptera), I test the more-individuals hypothesis (MIH), consisting of positive climatic impacts on food resources leading to increased abundance and then diversity. I also test competing mechanistic hypotheses, including interacting effects of climate, local vegetation, habitat diversity, habitat heterogeneity, soil diversity, and elevational area. Carrion beetle species richness and abundances were estimated using 40 standardized pitfall traps set for 90 days at 30 survey sites on two elevational gradients in the Front Range and San Juan Mountains, Colorado, USA. Standardized measurements assessed 13 vegetative characteristics, food resources (mammal abundances), soil diversity, habitat diversity, elevational area, temperature, precipitation and net primary productivity at each site. Structural equation models were used to test competing diversity hypotheses and mechanisms. Species richness peaked at intermediate elevations on both gradients, whereas abundance was unimodal on one gradient and decreasing on the other. The MIH mechanism was rejected; all four potential SEM model constructions were unsupported and the majority of all SEM models did not support relationships between abundance and diversity or climate and food resources. The best SEM model included direct influences of temperature, vegetation biomass, and food resources but with separate effects on diversity and abundance. Carrion beetles were more diverse and abundant in sites with dense understory vegetation and warm temperatures, while higher abundances were also linked to more food resources. This climate-biotic relationship is likely due to a need for microclimates and microhabitats to mediate physiological tradeoffs of desiccation and thermoregulation with predation. This suggests a general hypothesis for beetle diversity and abundance, particularly on arid-based mountains globally.

A multiscale framework for disentangling the roles of evenness, density, and aggregation on diversity gradients.

Disentangling the drivers of diversity gradients can be challenging. The Measurement of Biodiversity (MoB) framework decomposes scale‐dependent changes in species diversity into three components of community structure: species abundance distribution (SAD), total community abundance, and within‐species spatial aggregation. Here we extend MoB from categorical treatment comparisons to quantify variation along continuous geographic or environmental gradients. Our approach requires sites along a gradient, each consisting of georeferenced plots of abundance‐based species composition data. We demonstrate our method using a case study of ants sampled along an elevational gradient of 28 sites in a mixed deciduous forest of the Great Smoky Mountains National Park, USA. MoB analysis revealed that decreases in ant species richness along the elevational gradient were associated with decreasing evenness and total number of species, which counteracted the modest increase in richness associated with decreasing spatial aggregation along the gradient. Total community abundance had a negligible effect on richness at all but the finest spatial grains, SAD effects increased in importance with sampling effort, and the aggregation effect had the strongest effect at coarser spatial grains. These results do not support the more‐individuals hypothesis, but they are consistent with a hypothesis of stronger environmental filtering at coarser spatial grains. Our extension of MoB has the potential to elucidate how components of community structure contribute to changes in diversity along environmental gradients and should be useful for a variety of assemblage‐level data collected along gradients.

Natural population variability may be masking the more-individuals hypothesis.

Species richness and productivity are correlated at global and regional scales, but the mechanisms linking them are inconclusive. The most commonly invoked mechanism, the more-individuals hypothesis (MIH), hypothesizes that increased productivity leads to increased food resource availability, which leads to an increased number of individuals supporting more species. Empirical evidence for the MIH remains mixed despite a substantial literature. Here we used simulations to determine whether interannual population variability could be masking a "true" MIH relationship. In each simulation, fixed linear relationships between productivity, richness, and 50-yr average abundance mimicked the MIH mechanism. Abundance was allowed to vary annually and sampled for 1-40yr. Linear regressions of richness on sampled abundance assessed the probability of detecting the fixed MIH relationship. Medium to high population variability with short-term sampling (1-3yr) led to poor detection of the fixed MIH relationship. Notably, this level of sampling and population variability describes nearly all MIH studies to date. Long-term sampling (5+yr) led to improved detection of the fixed relationship; thus it is necessary to detect support for the MIH reliably. Such sampling duration is nonexistent in the MIH literature. Robust future studies of the MIH necessitate consideration of interannual population variability.

Primary productivity and habitat diversity predict bird species richness and composition along urban-rural gradients of central Argentina

Although the role of primary productivity and habitat diversity in determining patterns of species richness and composition have been explored in urban areas, they have been rarely analysed in the context of species-energy relationships (SERs). Therefore, two hypotheses related to SERs were tested in bird communities: the more individuals hypothesis (MIH) and the niche position hypothesis (NPH). The MIH states that sites with more energy availability have more individuals, with extinction probability decreased thereby increasing the species richness. The NPH predicts that the amount of rare resources increases at sites with greater energy availability and environmental heterogeneity, increasing species richness. Birds were counted along three urban-rural gradients in the Pampas region of Argentina during the breeding season. Productive energy was measured using the NDVI and environmental heterogeneity via the diversity of different substrates. Although there was a positive relationship between bird richness and abundance, there was no positive relationship between bird abundance and primary productivity. Bird richness showed a positive relationship with habitat diversity and primary productivity, supporting the NPH. The exclusion of exotic species resulted in a tight relationship between bird richness and abundance, and between bird abundance and primary productivity, supporting the MIH. Bird composition varied with changes in habitat diversity and primary productivity. Path analysis revealed that habitat diversity increased bird richness through an increase of bird abundance. The NPH was the most consistent to predict bird richness along urban-rural gradients. Therefore, it is necessary to create areas with greater amount and variety of vegetation to increase bird richness.

Principal factors controlling biodiversity along an elevation gradient: Water, energy and their interaction

AbstractAimVariation in diversity is a well‐documented spatial pattern in biogeography, but an overarching climate‐based theory of diversity is lacking. We evaluate two models of species richness related to species‐richness‐energy theory. One is based on net primary production (NPP) or the more individuals hypothesis (MiH), and the other is a model based on water–energy dynamics (WED). We use taxa from three kingdoms along an extensive elevation‐temperature gradient. Both WED and NPP‐MiH are based on thermal energy, but the question is whether energy operates through regulating production and chemical (potential) energy only (NPP‐MiH), or if kinetic energy as regulator of available liquid water is needed (WED).LocationCentral Himalayas.MethodsThe biodiversity, that is, elevational gamma diversity, of 12 taxa containing animals, plants, and fungi was estimated from range data along an elevation gradient. Generalized linear models were fitted to the species richness data, and the Akaike information criterion (AIC) and deviance explained were used to evaluate the NPP‐MiH and WED models. In addition, we tested the relationships with precipitation and length of growing season (LGS) and their interaction.ResultsThe peaks in richness of the taxa are dispersed along the entire Himalayan bioclimatic gradient from the subtropics to the alpine zone. WED performs best for all taxa along the entire gradient. In the non‐tropical zone, NPP‐MiH is best for reptiles, and NPP‐MiH and WED are equally good for mammals and amphibians. Including the length of the growing season in the WED model improves the AIC for eight taxa, and WED is superior for combined cross‐taxon biodiversity along the entire gradient, but WED and NPP‐MiH are equally good in the non‐tropical zone.ConclusionWater–energy dynamics is able to predict peaks in species richness under different climate and primary production conditions; hence, WED is better and more general than NPP‐MiH. The interaction with precipitation and the length of the growing season, which also reflects primary production, improve the model for several organism groups. Hence, LGS may improve and unify future mechanistic first‐principle model of biodiversity.

Small mammal species richness is directly linked to regional productivity, but decoupled from food resources, abundance, or habitat complexity

AbstractAimSpecies richness is often strongly correlated with climate. The most commonly invoked mechanism for this climate‐richness relationship is the more‐individuals‐hypothesis (MIH), which predicts a cascading positive influence of climate on plant productivity, food resources, total number of individuals, and species richness. We test for a climate‐richness relationship and an underlying MIH mechanism, as well as testing competing hypotheses including positive effects of habitat diversity and heterogeneity, and the species‐area effect.LocationColorado Rocky Mountains, USA: two elevational gradients in the Front Range and San Juan Mountains.MethodsWe conducted standardized small mammal surveys at 32 sites to assess diversity and population sizes. We estimated vegetative and arthropod food resources as well as various aspects of habitat structure by sampling 20 vegetation plots and 40 pitfall traps per site. Temperature, precipitation and net primary productivity (NPP) were assessed along each gradient. Regressions and structural equation modelling were used to test competing diversity hypotheses and mechanistic links predicted by the MIH.ResultsWe detected 3,922 individuals of 37 small mammal species. Mammal species richness peaked at intermediate elevations, as did productivity, whereas temperature decreased and precipitation increased with elevation. We detected strong support for a productivity‐richness relationship, but no support for the MIH mechanism. Food and mammal population sizes were unrelated to NPP or mammal species richness. Furthermore, mammal richness was unrelated to habitat diversity, habitat heterogeneity, or elevational area.Main conclusionsSites with high productivity contain high mammal species richness, but a mechanism other than a contemporary MIH underlies the productivity–diversity relationship. Possibly a mechanism based on evolutionary climatic affiliations. Protection of productive localities and mid‐elevations are the most critical for preserving small mammal richness, but may be decoupled from trends in population sizes, food resources, or habitat structure.

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.

Multiple processes generate productivity–diversity relationships in experimental wood‐fall communities

Energy availability has long been recognized as a predictor of community structure, and changes in both terrestrial and marine productivity under climate change necessitate a deeper understanding of this relationship. The productivity-diversity relationship (PDR) is well explored in both empirical and theoretical work in ecology, but numerous questions remain. Here, we test four different theories for PDRs (More-Individuals Hypothesis, Resource-Ratio Theory, More Specialization Theory, and the Connectivity-Diversity Hypothesis) with experimental deep-sea wood falls. We manipulated productivity by altering wood-fall sizes and measured responses after 5 and 7 years. In November 2006, 32 Acacia sp. logs were deployed at 3203 m in the Northeast Pacific Ocean (Station Deadwood: 36.154098 degrees N, 122.40852 degrees W). Overall, we found a significant increase in diversity with increased wood-fall size for these communities. Increases in diversity with wood-fall size occurred because of the addition of rare species and increases of overall abundance, although individual species responses varied. We also found that limited dispersal helped maintain the positive PDR relationship. Our experiment suggests that multiple interacting mechanisms influence PDRs.

Causality between abundance and diversity is weak for wintering migratory waterbirds

Summary The species–area relationship, which is closely linked with the more general species‐energy theory, is one of the most well‐known patterns in geographical ecology, but the underlying causes remain contentious. The more individuals hypothesis (MIH) articulates a causal path from resource availability to population abundance to species richness. The MIH has been tested with a range of taxa including plants, invertebrates and land birds but never with migratory waterbirds. Using multiyear simultaneous survey data of wintering waterbirds in 10 lakes at Poyang Lake, China, and remotely sensed habitat condition measurements, we applied structural equation modelling (SEM) to test three causal paths: (A) good habitat conditions (e.g. habitat availability and heterogeneity) attract more species (high richness); (B) habitat conditions promote abundance (more individuals); and (C) habitat conditions promote abundance, which in turn increases richness. We also modelled responses of species richness and abundance to habitat conditions using generalised additive mixed modelling (GAMM) to assess their co‐variation. While our analysis confirmed the first two paths, we found no support for the third, which is the central postulate of the MIH. In addition, in agreement with GAMM, SEM indicated that species richness was more closely related to habitat quality than to abundance. Our findings suggest that wintering waterbird species richness and abundance are two intrinsic community indices that covary with environmental variables.

The decoupling of abundance and species richness in lizard communities

1. Patterns of species richness often correlate strongly with measures of energy. The more individuals hypothesis (MIH) proposes that this relationship is facilitated by greater resources supporting larger populations, which are less likely to become extinct. Hence, the MIH predicts that community abundance and species richness will be positively related. 2. Recently, Buckley & Jetz (2010, Journal of Animal Ecology, 79, 358-365) documented a decoupling of community abundance and species richness in lizard communities in south-west United States, such that richer communities did not contain more individuals. They predicted, as a consequence of the mechanisms driving the decoupling, a more even distribution of species abundances in species-rich communities, evidenced by a positive relationship between species evenness and species richness. 3. We found a similar decoupling of the relationship between abundance and species richness for lizard communities in semi-arid south-eastern Australia. However, we note that a positive relationship between evenness and richness is expected because of the nature of the indices used. We illustrate this mathematically and empirically using data from both sets of lizard communities. When we used a measure of evenness, which is robust to species richness, there was no relationship between evenness and richness in either data set. 4. For lizard communities in both Australia and the United States, species dominance decreased as species richness increased. Further, with the iterative removal of the first, second and third most dominant species from each community, the relationship between abundance and species richness became increasingly more positive. 5. Our data support the contention that species richness in lizard communities is not directly related to the number of individuals an environment can support. We propose an alternative hypothesis regarding how the decoupling of abundance and richness is accommodated; namely, an inverse relationship between species dominance and species richness, possibly because of ecological release.

Urbanization and the more‐individuals hypothesis

1. Urbanization is a landscape process affecting biodiversity world-wide. Despite many urban-rural studies of bird assemblages, it is still unclear whether more species-rich communities have more individuals, regardless of the level of urbanization. The more-individuals hypothesis assumes that species-rich communities have larger populations, thus reducing the chance of local extinctions. 2. Using newly collated avian distribution data for 1 km(2) grid cells across Florence, Italy, we show a significantly positive relationship between species richness and assemblage abundance for the whole urban area. This richness-abundance relationship persists for the 1 km(2) grid cells with less than 50% of urbanized territory, as well as for the remaining grid cells, with no significant difference in the slope of the relationship. These results support the more-individuals hypothesis as an explanation of patterns in species richness, also in human modified and fragmented habitats. 3. However, the intercept of the species richness-abundance relationship is significantly lower for highly urbanized grid cells. Our study confirms that urban communities have lower species richness but counters the common notion that assemblages in densely urbanized ecosystems have more individuals. In Florence, highly inhabited areas show fewer species and lower assemblage abundance. 4. Urbanized ecosystems are an ongoing large-scale natural experiment which can be used to test ecological theories empirically.

A Test of the Scale-dependence of the Species Abundance-People Correlation for Veteran Trees in Italy

The spatial correlation of the presence of people and species has been suggested to be scale-dependent. At local scales, large numbers of people often result in species impoverishment. At coarse scales, species-rich regions tend to be densely inhabited. Recently, broad-scale human presence has been shown to be correlated not only with numbers of species but also with their abundance, as predicted by the more-individuals hypothesis. However, it is not known whether the species abundance-human presence correlation could also be scale-dependent. This hypothesis was tested by use of a database of veteran trees in Italy. Veteran tree species richness and number of individuals were modelled as a function of human population size at two grains of analysis (provinces and regions), controlling for variations in area, latitude and spatial autocorrelation. A positive correlation was found between human presence and veteran tree species. As predicted, this correlation was stronger at a coarser resolution. However, only at the provincial but not regional level was there a positive correlation between human presence and veteran tree abundance when controlling for area and latitude. These results were confirmed for native and exotic trees. The present findings rule out the more-individuals hypothesis as an explanation of the scale-dependence of the species-people correlation for veteran trees in Italy. Potential mechanisms behind the observed spatial coincidence of high numbers of people and veteran tree species are discussed and implications for conservation are highlighted.