Species richness vs evenness: independent relationship and different responses to edaphic factors

The main components of species diversity are richness and evenness. Species richness has been the subject of biodiversity studies more often than species evenness. Some simulated models suggest that the relationship between these two components is equal and predictable in a stable environment. This study was conducted in a human-shaped and unstable environment in arable field boundaries. A total of 30 sampling plots were laid out at random in the boundaries. Vegetation at the sites was of meadow type. The relationship between species richness and evenness, and their responses to edaphic nutrient factors in the field boundaries, were examined by correlation analysis and an ordination method. Correlation analyses demonstrated that no consistent pattern was present in the relationship between species evenness and species richness in this human-shaped ecosystem. Species richness and evenness also had different responses to edaphic factors, with species richness being negatively correlated with phosphorus, and species evenness negatively correlated with the ratio of organic carbon to total nitrogen in soil. The results indicate that different and independent ecological processes determine species richness and evenness. The relationship between these two components may be site-specific, reflecting variation in resource utilization by plant species. The pattern thus reflects the spatial heterogeneity of disturbances or 'patchiness' of resources between sites in a semi-natural ecosystem.     

The species-area-energy relationship

Area and available energy are major determinants of species richness. Although scale dependency of the relationship between energy availability and species richness (the species-energy relationship) has been documented, the exact relationship between the species-area and the species-energy relationship has not been studied explicitly. Here we show, using two extensive data sets on avian distributions in different biogeographic regions, that there is a negative interaction between energy availability and area in their effect on species richness. The slope of the species-area relationship is lower in areas with higher levels of available energy, and the slope of the species-energy relationship is lower for larger areas. This three-dimensional species-area-energy relationship can be understood in terms of probabilistic processes affecting the proportions of sites occupied by individual species. According to this theory, high environmental energy elevates species' occupancies, which depress the slope of the species-area curve.     

Energy, range dynamics and global species richness patterns: reconciling mid-domain effects and environmental determinants of avian diversity

Spatial patterns of species richness follow climatic and environmental variation, but could reflect random dynamics of species ranges (the mid-domain effect, MDE). Using data on the global distribution of birds, we compared predictions based on energy availability (actual evapotranspiration, AET, the best single correlate of avian richness) with those of range dynamics models. MDE operating within the global terrestrial area provides a poor prediction of richness variation, but if it operates separately within traditional biogeographic realms, it explains more global variation in richness than AET. The best predictions, however, are given by a model of global range dynamics modulated by AET, such that the probability of a range spreading into an area is proportional to its AET. This model also accurately predicts the latitudinal variation in species richness and variation of species richness both within and between realms, thus representing a compelling mechanism for the major trends in global biodiversity.     

Determinants of species evenness in a neotropical bat ensemble

Evenness is an important property of communities. Species richness alone does not capture the fact that one or a few species may dominate total abundance and biomass of a community. This in turn has important consequences for ecosystem functioning and species interactions. Evenness has been observed to vary systematically along environmental and productivity gradients. However, a truly general theory about which factors control evenness in a community has yet to emerge. Prior research on evenness has suggested that high richness, biomass and abundance should lead to lower community evenness in our study system of bats in Panama. However, only few empirical studies examine the simultaneous effects of species richness, biomass or abundance on evenness. For the first time, we applied path analysis in the study of evenness to tease apart the relative importance and direction (positive or negative) of causality among these three factors. As predicted, we found that evenness decreases with increasing species richness, abundance and biomass. The negative effect of abundance was mediated by the positive joint effect of biomass and richness. The selected models varied in the strength of the correlation between the three variables with evenness but their direction was consistent. Overall, we argue that rarity, high mobility and differences in resource availability at sites with lower environmental stress can explain the negative effects of richness on evenness.     

Zooplankton species richness–productivity relationship: Confronting monotonic positive and hump‐shaped models from a local perspective

This study examined two models that are most frequently used to describe the relationship between species richness and productivity (SPR): monotonic positive and hump‐shaped models. We assessed zooplankton community diversity in response to algal productivity. The relationship between net primary productivity (NPP) and rarefied species richness was examined by fitting the data to two models and comparing them using the Akaike information criterion (AICc). Macrophyte banks with the highest net primary productivity had the highest zooplankton abundance. Our results pointed to a hump‐shaped model as the best fit to describe the relationship between zooplankton species richness and primary productivity (ΔAICc > 4). Thus, the diversity was lower at the extremes of productivity and higher at intermediate levels of productivity. We suggest that this relationship might occur because when the resource supply rates are low, environmental conditions are stressful, whereas a high availability of resources enhances competitive exclusion. Two observations supported this statement: (i) the total abundance of the community positively correlated with NPP (P < 0.05), indicating that less productive sites had few consumers and the raised productivity tended to favour the total abundance; (ii) NPP was negatively correlated with evenness (P < 0.05), indicating that productivity increased the dominance of certain species in the communities. Therefore, we challenged two of the models most frequently used to explain SPR, and discuss some mechanisms underlying a hump‐shaped SPR.     

Relationships between arthropod richness,evenness, and diversity are altered by complementarity among plant genotypes

Biodiversity is quantified via richness (e.g., the number of species), evenness (the relative abundance distribution of those species), or proportional diversity (a combination of richness and evenness, such as the Shannon index, H′). While empirical studies show no consistent relationship between these aspects of biodiversity within communities, the mechanisms leading to inconsistent relationships have received little attention. Here, using common evening primrose (Oenothera biennis) and its associated arthropod community, we show that relationships between arthropod richness, evenness, and proportional diversity are altered by plant genotypic richness. Arthropod richness increased with O. biennis genotypic richness due to an abundance-driven accumulation of species in response to greater plant biomass. Arthropod evenness and proportional diversity decreased with plant genotypic richness due to a nonadditive increase in abundance of a dominant arthropod, the generalist florivore/omnivore Plagiognathas politus (Miridae). The greater quantity of flowers and buds produced in polycultures—which resulted from positive complementarity among O. biennis genotypes—increased the abundance of this dominant insect. Using choice bioassays, we show that floral quality did not change in plant genotypic mixtures. These results elucidate mechanisms for how plant genotypic richness can modify relationships between arthropod richness, evenness, and proportional diversity. More broadly, our results suggest that trophic interactions may be a previously underappreciated factor controlling relationships between these different aspects of biodiversity.     

Impacts of ecosystem engineers on community attributes: effects of cushion plants at different elevations of the Chilean Andes

Ecosystem engineers are organisms able to modulate environmental forces and, hence, may change the habitat conditions for other species. In so doing, ecosystem engineers may affect both species richness and evenness of communities and, in consequence, change species diversity. If these changes in community attributes are related to the magnitude of the habitat changes induced by the engineers, it seems likely that engineer species will have greater effects on diversity in sites where they cause larger habitat changes. We addressed this issue by evaluating the effects of three alpine cushion plants on species richness, evenness, and diversity of high-Andean plant communities. Given that the difference in microclimatic conditions between cushions and the external environment increases with elevation, we proposed that these organisms should have greater effects on community attributes at higher than at lower elevation sites. Results showed that the three cushion species had positive effects on species richness, diversity, and evenness of plant communities. It was also observed that the magnitude of these effects changed with elevation: positive effects on species richness and diversity increased towards upper sites for the three cushions species, whereas positive effects on evenness increased with elevation for one cushion species but decreased with elevation for other two cushion species. These results suggest that the presence of cushions is important to maintain plant diversity in high-Andean communities, but this positive effect on diversity seems to increase as the difference in environmental conditions between cushions and the external environment increases with elevation.     

Plio-Pleistocene climate change and geographic heterogeneity in plant diversity–environment relationships

Plio‐Pleistocene climate change may have induced geographic heterogeneity in plant species richness–environment relationships in Europe due to greater in situ species survival and speciation rates in southern Europe. We formulate distinct hypotheses on how Plio‐Pleistocene climate change may have affected richness–topographic heterogeneity and richness–water‐energy availability relationships, causing steeper relationships in southern Europe. We investigated these hypotheses using data from Atlas Florae Europaeae on the distribution of 3069 species and geographically weighted regression (GWR). Our analyses showed that plant species richness generally increased with topographic heterogeneity (ln‐transformed altitudinal range) and actual evapotranspiration (AET). We also found evidence for strong geographic heterogeneity in the species richness–environment relationship, with a greater increase in species richness with increasing topographic heterogeneity in southern Europe (mean standardized local slope 0.610±0.245 SD in southern Europe, but only 0.270±0.175 SD in northern Europe). However, the local AET slopes were, at most, weakly different between the two regions, and their pattern did not conform to predictions, as there was a band of high local slopes across southern‐central northern Europe. This band broadly matches the transition between the temperate and boreal zones and may simply reflect the fact that few species tolerate the boreal climate. We discuss the potential explanations for the contrasting findings for the two richness–environment relationships. In conclusion, we find support for the idea that Plio‐Pleistocene climate change may sometimes affect current species richness–environment relationships via its effects on regional species pools. However, further studies integrating information on species ages and clade differentiation rates will be needed to substantiate this interpretation. On a general level, our results indicate that although strong richness–environment relationships are often found in macroecological studies, these can be contingent upon the historical constraints on the species pool.     

Biodiversity and tallgrass prairie decomposition: the relative importance of species identity, evenness, richness, and micro-topography

Biodiversity has been declining in many areas, and there is great interest in determining whether this decline affects ecosystem functioning. Most biodiversity—ecosystem functioning studies have focused on the effects of species richness on net primary productivity. However, biodiversity encompasses both species richness and evenness, ecosystem functioning includes other important processes such as decomposition, and the effects of richness on ecosystem functioning may change at different levels of evenness. Here, we present two experiments on the effects of litter species evenness and richness on litter decomposition. In the first experiment, we varied the species evenness (three levels), identity of the dominant species (three species), and micro-topographic position (low points [gilgais] or high points between gilgais) of litter in three-species mixtures in a prairie in Texas, USA. In a second experiment, we varied the species evenness (three levels), richness (one, two, or four species per bag), and composition (random draws) of litter in a prairie in Iowa, USA. Greater species evenness significantly increased decomposition, but this effect was dependent on the environmental context. Higher evenness increased decomposition rates only under conditions of higher water availability (in gilgais in the first experiment) or during the earliest stages of decomposition (second experiment). Species richness had no significant effect on decomposition, nor did it interact with evenness. Micro-topographic position and species identity and composition had larger effects on decomposition than species evenness. These results suggest that the effects of litter species diversity on decomposition are more likely to be manifested through the evenness component of diversity than the richness component, and that diversity effects are likely to be environmentally context dependent.     

山西翅果油树群落的多样性研究

用丰富度指数、多样性指数和均匀度指数对山西翅果油树群落的多样性进行了研究 ,并用相关分析研究了海拔与多样性指数及多样性指数间的关系 ,结果表明 :1 )在干扰强烈的生境中 ,翅果油树群落具有较低的丰富度指数、多样性指数的均匀性 ,而接近顶极群落阶段 ,多样性指数和丰富度指数也较低 ,但具有较高的均匀性 ;干扰强度较小的生境中 ,群落具有较高的丰富度指数、多样性指数和均匀性。2 )灌木层和草本层的丰富度、多样性指数和均匀度指数呈现多元化的趋势。 3)海拔对山西翅果油树群落多样性的影响不显著。     

Tree heterogeneity,resource availability,and larger scale processes regulating arboreal ant species richness

Abstract Processes acting on different spatial and temporal scales may influence local species richness. Ant communities are usually described as interactive and therefore determined by local processes. In this paper we tested two hypotheses linked to the question of why there is local variation in arboreal ant species richness in the Brazilian savanna (‘cerrado’). The hypotheses are: (i) there is a positive relationship between ant species richness and tree species richness, used as a surrogate of heterogeneity; and (ii) there is a positive relationship between ant species richness and tree density, used as a surrogate of resource availability. Arboreal ants were sampled in two cerrado sites in Brazil using baited pitfall traps and manual sampling, in quadrats of 20 m × 50 m. Ant species richness in each quadrat was used as the response variable in regression tests, using tree species richness and tree density as explanatory variables. Ant species richness responded positively to tree species richness and density. Sampling site also influenced ant species richness, and the relationship between tree density and tree species richness was also positive and significant. Tree species richness may have influenced ant species richness through three processes: (i) increasing the variety of resources and allowing the existence of a higher number of specialist species; (ii) increasing the amount of resources to generalist species; and (iii) some other unmeasured factor may have influenced both ant and tree species richness. Tree density may also have influenced ant species richness through three processes: (i) increasing the amount of resources and allowing a higher ant species richness; (ii) changing habitat conditions and dominance hierarchies in ant communities; and (iii) increasing the area and causing a species–area pattern. Processes acting on larger scales, such as disturbance, altitude and evolutionary histories, as well as sampling effect may have caused the difference between sites.     

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.     

What Factors Affect Diversity and Species Composition of Endangered Tumbesian Dry Forests in Southern Ecuador?

This paper reports a study on species richness and composition of Tumbesian dry forest communities. We tested two alternative hypotheses about species assemblage processes in tropical dry forests: (1) species assemblage is determined by the filtering effect of environmental conditions and (2) species assemblage is determined by facilitative processes along the gradient of water availability, and thus, species richness and evenness increase as water becomes limited. In addition, we also explored the effect of climate and soil conditions on species composition in tropical dry forests. Species composition was sampled in 109 plots in terms of cover and tree diameter at breast height. Climatic, edaphic, topographic and anthropogenic degradation variables were obtained for each plot. We used generalized linear models and canonical correspondence analyses to evaluate the effect of environmental variables on species composition, richness and evenness. Water availability negatively affected richness and significantly determined the species assemblage. Species richness increased from ridges to valleys and evenness increased at higher altitudes. Soil characteristics showed no effect on richness and evenness but soil moisture, nitrogen concentration and soil temperature explained significant fractions of species composition. Although timber extraction and livestock in our study area were of low intensity, it negatively affected richness but had only a minor effect on species composition. Our results suggest that species composition in these endangered tropical dry forests may be at least partially explained by the stress‐gradient hypothesis, with higher species richness at drier conditions probably induced by facilitation processes.     

Species richness and evenness respond in a different manner to propagule density in developing prairie microcosm communities

Diversity has two basic components: richness, or number of species in a given area, and evenness, or how relative abundance or biomass is distributed among species. Previously, we found that richness and evenness can be negatively related across plant communities and that evenness can account for more variation in Shannon’s diversity index (H′) than richness, which suggests that relationships among diversity components can be complex. Non-positive relationships between evenness and richness could arise due to the effects of migration rate or local species interactions, and relationships could vary depending on how these two processes structure local communities. Here we test whether diversity components are equally or differentially affected over time by changes in seed density (and associated effects on established plant density and competition) in greenhouse communities during the very early stages of community establishment. In our greenhouse experiment, we seeded prairie microcosms filled with bare field soil at three densities with draws from a mix of 22 grass and forb species to test if increased competition intensity or seedling density would affect the relationships among diversity components during early community establishment. Increased seed density treatments caused diversity components to respond in a different manner and to have different relationships with time. Richness increased linearly with seed density early in the experiment when seedling emergence was high, but was unrelated to density later in the experiment. Evenness decreased log-linearly with seed densities on all sampling dates due to a greater dominance by Rudbeckia hirta with higher densities. Early in the experiment, diversity indices weakly reflected differences in richness, but later, after the competitive effects of Rudbeckia hirta became more intense, diversity indices more strongly reflected differences in evenness. This suggests that species evenness and diversity indices do not always positively covary with richness. Based on these results, we suggest that evenness and richness can be influenced by different processes, with richness being more influenced by the number of emerging seedlings and evenness more by species interactions like competition. These results suggest that both diversity components should be measured in plant diversity studies whenever it is possible.     

Why do mountains support so many species of birds?

Although topographic complexity is often associated with high bird diversity at broad geographic scales, little is known about the relative contributions of geomorphologic heterogeneity and altitudinal climatic gradients found in mountains. We analysed the birds in the western mountains of the New World to examine the two‐fold effect of topography on species richness patterns, using two grains at the intercontinental extent and within temperate and tropical latitudes. Birds were also classified as montane or lowland, based on their overall distributions in the hemisphere. We estimated range in temperature within each cell and the standard deviation in elevation (topographic roughness) based on all pixels within each cell. We used path analysis to test for the independent effects of topographic roughness and temperature range on species richness while controlling for the collinearity between topographic variables. At the intercontinental extent, actual evapotranspiration (AET) was the primary driver of species richness patterns of all species taken together and of lowland species considered separately. In contrast, within‐cell temperature gradients strongly influenced the richness of montane species. Regional partitioning of the data also suggested that range in temperature either by itself or acting in combination with AET had the strongest “effect” on montane bird species richness everywhere. Topographic roughness had weaker “effects” on richness variation throughout, although its positive relationship with richness increased slightly in the tropics. We conclude that bird diversity gradients in mountains primarily reflect local climatic gradients. Widespread (lowland) species and narrow‐ranged (montane) species respond similarly to changes in the environment, differing only in that the richness of lowland species correlates better with broad‐scale climatic effects (AET), whereas mesoscale climatic variation accounts for richness patterns of montane species. Thus, latitudinal and altitudinal gradients in species richness can be explained through similar climatic‐based processes, as has long been argued.     

Relationship Between Aboveground Biomass and Multiple Measures of Biodiversity in Subtropical Forest of Puerto Rico

Anthropogenic activities have accelerated the rate of global loss of biodiversity, making it more important than ever to understand the structure of biodiversity hotspots. One current focus is the relationship between species richness and aboveground biomass (AGB) in a variety of ecosystems. Nonetheless, species diversity, evenness, rarity, or dominance represent other critical attributes of biodiversity and may have associations with AGB that are markedly different than that of species richness. Using data from large trees in four environmentally similar sites in the Luquillo Experimental Forest of Puerto Rico, we determined the shape and strength of relationships between each of five measures of biodiversity (i.e., species richness, Simpson's diversity, Simpson's evenness, rarity, and dominance) and AGB. We quantified these measures of biodiversity using either proportional biomass or proportional abundance as weighting factors. Three of the four sites had a unimodal relationship between species richness and AGB, with only the most mature site evincing a positive, linear relationship. The differences between the mature site and the other sites, as well as the differences between our richness–AGB relationships and those found at other forest sites, highlight the crucial role that prior land use and severe storms have on this forest community. Although the shape and strength of relationships differed greatly among measures of biodiversity and among sites, the strongest relationships within each site were always those involving richness or evenness.     

Diversity patterns from sequentially restored grasslands support the ‘environmental heterogeneity hypothesis’

The ‘environmental heterogeneity hypothesis’ (EHH) has been proposed as a mechanism that enables species coexistence through resource partitioning. In accordance with this hypothesis, plant diversity is predicted to increase with variability in resources, but there has been weak support for this hypothesis from experimental studies. The objectives of this research were to 1) characterize how resource availability and heterogeneity (coefficient of variation) change as plant communities develop using sequentially restored grasslands, 2) determine if resource heterogeneity relates to plant diversity (effective number of species, richness and evenness) and 3) reveal if the strength of resource heterogeneity–diversity relationships is different among levels of resource availability. We quantified means and coefficients of variation in soil nitrate and light availability in grasslands established on former agricultural lands for different times and their relationship to plant diversity using a geostatistically‐informed design. Nitrate availability decreased exponentially with restoration age, but no directional change in nitrate heterogeneity across the chronosequence occurred due to high resource variability in some restorations. Light availability also decreased exponentially across the chronosequence, but there was no directional change in light heterogeneity. Nitrate heterogeneity was positively correlated with both plant richness and plant effective number of species at high levels of nitrate availability. However, no nitrate heterogeneity correlation was detected at low levels of nitrate availability. Light heterogeneity was positively correlated with plant effective number of species at low levels of light availability. However, no light heterogeneity correlation was detected at high levels of light availability. Plant evenness was not correlated with resource heterogeneity at any resource availability level. These results support the positive heterogeneity–diversity relationship predicted by EHH, and uniquely that this relationship develops within a decade of plant community development, but can be obscured by resource availability.     

Species–energy relationships and habitat complexity in bird communities

Species–energy theory is a commonly invoked theory predicting a positive relationship between species richness and available energy. The More Individuals Hypothesis (MIH) attempts to explain this pattern, and assumes that areas with greater food resources support more individuals, and that communities with more individuals include more species. Using a large dataset for North American birds, I tested these predictions of the MIH, and also examined the effect of habitat complexity on community structure. I found qualitative support for the relationships predicted by the MIH, however, the MIH alone was inadequate for fully explaining richness patterns. Communities in more productive sites had more individuals, but they also had more even relative abundance distributions such that a given number of individuals yielded a greater number of species. Richness and evenness were also higher in structurally complex forests compared to structurally more simple grasslands when controlling for available energy.     

Seasonal changes in the relationship between plant species richness and community biomass in early succession

We analysed the relationship between plant species richness and productivity on first-year-old fields at two similar sites in central Europe. At both sites, a wide range of productivity levels was available resulting from different long-term fertilisation. In order to identify underlying mechanisms of the species richness–productivity relationship we included the seasonal dynamics and the number of individuals of each species in our analysis. We sampled 10 and 21 plots, respectively, at the two sites in May, June and July by harvesting all aboveground parts of vascular plants in 0.25 m² subplots. Species richness, number of individuals of each species and community biomass as a surrogate of productivity were recorded in each sample.At one site, the relationship between species richness and biomass was significantly positive in the May and June harvest. This relationship disappeared in the July harvest due to a reduction in species richness at high productivity levels. The relations between species richness and number of individuals and between number of individuals and biomass paralleled the species richness–productivity relation but the individual number–biomass relationship remained positive until the last harvest. Between-species differences in individual number–community biomass relationships and their seasonal dynamics revealed “interspecific competitive exclusion” even though the species richness–biomass relationships were not negative or hump-shaped. At the second site, species richness was not related to productivity or to number of individuals. Our study demonstrated the importance of temporal dynamics and regional processes in understanding species richness–productivity patterns.     

Reductions in grassland species evenness increase dicot seedling invasion and spittle bug infestation

Previous experiments that tested whether diverse plant communities have lower invasibility have all varied species richness. We experimentally varied evenness of four grassland species (three grasses and one forb) by planting a field experiment in Texas, and monitored the number of unplanted dicot and monocot species that invaded plots for two growing seasons. By varying evenness, we eliminated any sampling effect in our diversity treatment, because all plots contained the same plant species. Experimentally reducing evenness led to a greater number of dicot invaders, which emerged in plots throughout the growing season, but had less of an effect on monocot invaders, which emerged in flushes when experimental plants were semi‐dormant. Frequency of Solidago canadensis (altissima) stems with spittle bugs significantly increased with reductions in evenness during the first year, apparently because the greater number of Solidago stems in high evenness plots diluted the spittle‐bug effect. These results support the view that higher diversity plant communities are more resistant to dicot invaders and insect herbivores.