Scale dependence of species-energy relationships: evidence from fishes in thousands of lakes

Variation in the shape of relationships between species richness and different measures of energy may be linked to variation in the spatial scale on which such relationships are measured. We examine scale dependence in the relationship between potential evapotranspiration and the species richness of fishes in 7,885 postglacial lakes. The strength of this relationship is weak across lake communities but strong and positive across groups of lakes or regions. In addition, the strength and slope of this relationship increase significantly as the regional scale of analysis is increased. We interpret the observed patterns in terms of a simple model whereby energy influences the linear character of the species-energy relationship through its influence on spatial turnover in the species composition (beta diversity). Our results suggest that if energy is strongly tied to patterns of site occupancy or abundance, the parameters of species-energy relationships will depend, to a considerable extent, on the scale of measurement. Furthermore, the ability of high-energy regions to accommodate relatively large numbers of rare or infrequent species may underlie any general tendency for the strength or shape of species-energy relationships to change with scale.     

Species traits and the form of individual species-energy relationships

Environmental energy availability explains much of the spatial variation in species richness at regional scales. While numerous mechanisms that may drive such total species-energy relationships have been identified, knowledge of their relative contributions is scant. Here, we adopt a novel approach to identify these drivers that exploits the composite nature of species richness, i.e. its summation from individual species distributions. We construct individual species-energy relationships (ISERs) for each species in the British breeding avifauna using both solar (temperature) and productive energy metrics (normalized difference vegetation index) as measures of environmental energy availability. We use the slopes of these relationships and the resultant change in deviance, relative to a null model, as measures of their strength and use them as response variables in multiple regressions that use ecological traits as predictors. The commonest species exhibit the strongest ISERs, which is counter to the prediction derived from the more individuals hypothesis. There is no evidence that predatory species have stronger ISERs, which is incompatible with the suggestion that high levels of energy availability increase the length of the food chain allowing larger numbers of predators to exist. We find some evidence that species with narrow niche breadths have stronger ISERs, thus providing one of the few pieces of supportive evidence that high-energy availability promotes species richness by increasing the occurrence of specialist species that use a narrow range of resources.     

The relationships between local and regional species richness and spatial turnover

Aim To determine the empirical relationships between species richness and spatial turnover in species composition across spatial scales. These have remained little explored despite the fact that such relationships are fundamental to understanding spatial diversity patterns. Location South‐east Scotland. Methods Defining local species richness simply as the total number of species at a finer resolution than regional species richness and spatial turnover as turnover in species identity between any two or more areas, we determined the empirical relationships between all three, and the influence of spatial scale upon them, using data on breeding bird distributions. We estimated spatial turnover using a measure independent of species richness gradients, a fundamental feature which has been neglected in theoretical studies. Results Local species richness and spatial turnover exhibited a negative relationship, which became stronger as larger neighbourhood sizes were considered in estimating the latter. Spatial turnover and regional species richness did not show any significant relationship, suggesting that spatial species replacement occurs independently of the size of the regional species pool. Local and regional species richness only showed the expected positive relationship when the size of the local scale was relatively large in relation to the regional scale. Conclusions Explanations for the relationships between spatial turnover and local and regional species richness can be found in the spatial patterns of species commonality, gain and loss between areas.     

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.     

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.     

Breeding bird species richness in Spain: assessing diversity hypothesis at various scales

The variation of passerine species richness in Spain was studied at various spatial scales. Presence-absence data was resampled to construct three species richness maps in lattices of 10×10, 30×30, and 50×50 km UTM cells. The importance of habitat, species-energy, climatic variability, disturbance, history and geometric constraints hypotheses was assessed using geographical data. Stochastic, range-based models were used to simulate neutral colonization events from Europe or from Africa. The importance of small scale processes remained after the inclusion of environmental covariates, indicating a possible role of ecological interactions that was represented in the models by a conditional spatial autoregressive term. Historical effects and energy related measures explained most of the variation in regional species richness. Local and regional habitat structure measures explained the pattern only after large scale trends were considered. The differences when species richness was analyzed at each scale reveal the importance of spatial issues in diversity studies. The possible role of post glacial migration in shaping the observed patterns, and implications for conservation are discussed.     

Local-scale species-energy relationships in fish assemblages of some forested streams of the Bolivian Amazon

Productivity (trophic energy) is one of the most important factors promoting variation in species richness. A variety of species-energy relationships have been reported, including monotonically positive, monotonically negative, or unimodal (i.e. hump-shaped). The exact form of the relationship seems to depend, among other things, on the spatial scale involved. However, the mechanisms behind these patterns are still largely unresolved, although many hypotheses have been suggested. Here we report a case of local-scale positive species-energy relationship. Using 14 local fish assemblages in tropical forested headwater streams (Bolivia), and after controlling for major local abiotic factors usually acting on assemblage richness and structure, we show that rising energy availability through leaf litter decomposition rates allows trophically specialized species to maintain viable populations and thereby to increase assemblage species richness. By deriving predictions from three popular mechanistic explanations, i.e. the 'increased population size', the 'consumer pressure', and the 'specialization' hypotheses, our data provide only equivocal support for the latter.     

Species-energy relationships at the macroecological scale: a review of the mechanisms

Correlations between the amount of energy received by an assemblage and the number of species that it contains are very general, and at the macro-scale such species-energy relationships typically follow a monotonically increasing curve. Whilst the ecological literature contains frequent reports of such relationships, debate on their causal mechanisms is limited and typically focuses on the role of energy availability in controlling the number of individuals in an assemblage. Assemblages from high-energy areas may contain more individuals enabling species to maintain larger, more viable populations, whose lower extinction risk elevates species richness. Other mechanisms have, however, also been suggested. Here we identify and clarify nine principal mechanisms that may generate positive species-energy relationships at the macro-scale. We critically assess their assumptions and applicability over a range of spatial scales, derive predictions for each and assess the evidence that supports or refutes them. Our synthesis demonstrates that all mechanisms share at least one of their predictions with an alternative mechanism. Some previous studies of species-energy relationships appear not to have recognised the extent of shared predictions, and this may detract from their contribution to the debate on causal mechanisms. The combination of predictions and assumptions made by each mechanism is, however, unique, suggesting that, in principle, conclusive tests are possible. Sufficient testing of all mechanisms has yet to be conducted, and no single mechanism currently has unequivocal support. Each may contribute to species-energy relationships in some circumstances, but some mechanisms are unlikely to act simultaneously. Moreover, a limited number appear particularly likely to contribute frequently to species-energy relationships at the macro-scale. The increased population size, niche position and diversification rate mechanisms are particularly noteworthy in this context.     

Dispersal ability links to cross‐scale species diversity patterns across the Eurasian Arctic tundra

Aim The role of dispersal in structuring biodiversity across spatial scales is controversial. If dispersal controls regional and local community assembly, it should also affect the degree of spatial species turnover as well as the extent to which regional communities are represented in local communities. Here we provide the first integrated assessment of relationships between dispersal ability and local‐to‐regional spatial aspects of species diversity across a large geographical area. Location Northern Eurasia. Methods Using a cross‐scale analysis covering local (0.64 m²) to continental (the Eurasian Arctic biome) scales, we compared slope parameters of the dissimilarity‐to‐distance relationship in species composition and the local‐to‐regional relationship in species richness among three plant‐like groups that differ in dispersal ability: lichens with the highest dispersal ability; mosses and moss allies with intermediate dispersal ability; and seed plants with the lowest dispersal ability. Results Diversity patterns generally differed between the three groups according to their dispersal ability, even after controlling for niche‐based processes. Increasing dispersal ability is linked to decreasing spatial species turnover and an increasing ratio of local to regional species richness. All comparisons supported our expectations, except for the slope of the local‐to‐regional relationship in species richness for mosses and moss allies which was not significantly steeper than that of seed plants. Main conclusions The negative link between dispersal ability and spatial species turnover and the corresponding positive link between dispersal ability and the ratio of local‐to‐regional species richness support the idea that dispersal affects community structure and diversity patterns across spatial scales.     

Diversity patterns of stygobiotic crustaceans across multiple spatial scales in Europe

1. Using species distribution data from 111 aquifers distributed in nine European regions, we examined the pairwise relationships between local species richness (LSR), dissimilarity in species composition among localities, and regional species richness (RSR). In addition, we quantified the relative contribution of three nested spatial units – aquifers, catchments and regions – to the overall richness of groundwater crustaceans.
2. The average number of species in karst and porous aquifers (LSR) varied significantly among regions and was dependent upon the richness of the regional species pool (RSR). LSR–RSR relationships differed between habitats: species richness in karstic local communities increased linearly with richness of the surrounding region, whereas that of porous local communities levelled off beyond a certain value of RSR.
3. Dissimilarity in species composition among aquifers of a region increased significantly with increasing regional richness because of stronger habitat specialisation and a decrease in the geographic range of species among karst aquifers. Species turnover among karst aquifers was positively related to RSR, whereas this relationship was not significant for porous aquifers.
4. The contribution of a given spatial unit to total richness increased as size of the spatial unit increased, although 72% of the overall richness was attributed to among-region diversity. Differences in community composition between similar habitats in different regions were typically more pronounced than between nearby communities from different habitats.
5. We conclude by calling for biodiversity assessment methods and conservation strategies that explicitly integrate the importance of turnover in community composition and habitat dissimilarity at multiple spatial scales.     

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.     

Human impacts, energy availability and invasion across Southern Ocean Islands

Aim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues. Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia. Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale. Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate‐matching, were responsible for differences in the extent of invasion. Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.     

Effect of sampling grain on patterns of species richness and turnover in Amazonian forests

Grain (size of sampling units) affects the spatial resolution at which ecological patterns can be observed and analysed, and potentially has an important effect on the results of broad‐scale studies on diversity gradients. Here we examine the effect of grain on patterns of species richness and turnover in lowland rainforests of western Amazonia (Peru and Colombia). We inventoried pteridophytes (ferns and lycophytes), melastomes (Melastomataceae) and palms (Arecaceae) in four line transects of 22–29 km length. Different grains were obtained by aggregating original 100‐m‐long sampling units into larger segments up to 19.2 km long. With any given grain and plant group, local species richness varied considerably both within and among transects, and a transect segment that was species‐rich with one grain could be relatively species‐poor with another. Which transect had the highest vs lowest mean species richness per sampling unit (α richness) differed among plant groups. It also varied to some degree with grain, as transects differed in how rapidly local species richness increased with increasing grain. Patterns of species turnover were more consistently correlated among plant groups than patterns of species richness were, and NMDS ordinations were rather similar with all grains and plant groups. Floristic heterogeneity within the Amazonian terra firme rainforest seems to contain a general compositional pattern that is sufficiently robust to be detectable with various sampling schemes, but patterns of species richness appear more case‐specific. Therefore, using one plant group as an indicator for patterns in other plant groups can be expected to work better for species turnover than for species richness.     

Plant species coexistence at local scale in temperate swamp forest: test of habitat heterogeneity hypothesis

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.     

Tree growth indicates resource quality for foliage-feeding insects: Pattern and structure of herbivore diversity in response to productivity

In forests, local site conditions can affect both trees and herbivores and hence site-related factors act indirectly on herbivores mediated by tree growth rates. Here, tree foliage represents a fundamental prerequisite for insect herbivore development providing energy in the form of plant tissue quality. Resource-based theories, on the other hand, assume that the synthesis of defensive compounds is a trade-off with growth and peaks at low resource availability. However, the extent to which plant tissue quality in response to site productivity is relevant in the species-energy relationship is unknown. Therefore, we aimed at a better understanding of the form and structure of the species-energy relationship in forest insects. We used census data of foliage-feeding insects along a productivity gradient of Scots pine forests defined by relative growth rates of trees (RGR). As a result, diversity monotonically increases with decreasing RGR (as a proxy for energy) during almost two decades of sampling. Herbivore assemblages become more similar with available energy as species turnover linearly decreases and proportions of sites occupied by individual species rise. The results suggest that tree growth rate influences herbivore dynamics in this system by altering the chemical composition of needles, without necessarily affecting the form in the relationship. The site-specific resource availability requires trees to adjust their allocation to synthesis of carbon-based secondary metabolites or growth, which then results in fundamental differences in herbivore dynamics at low vs. lowest RGR (regular cycles (dominance) vs. dampened cycles (evenness)). However, these differences inevitably demonstrate that species richness is not necessarily a result of more individuals and implicate that different mechanisms are involved (facilitation vs. competition/temporal heterogeneity). The resulting pattern and structure of foliage-feeding insects advance our understanding of herbivore dynamics in response to site quality and tree growth, which may ultimately improve our knowledge of plant-insect interactions in the face of environmental change.     

Regional species pools control community saturation in lake phytoplankton

Recent research has highlighted that positive biodiversity–ecosystem functioning relationships hold for all groups of organisms, including microbes. Yet, we still lack understanding regarding the drivers of microbial diversity, in particular, whether diversity of microbial communities is a matter of local factors, or whether metacommunities are of similar importance to what is known from higher organisms. Here, we explore the driving forces behind spatial variability in lake phytoplankton diversity in Fennoscandia. While phytoplankton biovolume is best predicted by local phosphorus concentrations, phytoplankton diversity (measured as genus richness, G) only showed weak correlations with local concentrations of total phosphorus. By estimating spatial averages of total phosphorus concentrations on various scales from an independent, spatially representative lake survey, we found that close to 70 per cent of the variability in local phytoplankton diversity can be explained by regionally averaged phosphorus concentrations on a scale between 100 and 400 km. Thus, the data strongly indicate the existence of metacommunities on this scale. Furthermore, we show a strong dependency between lake productivity and spatial community turnover. Thus, regional productivity affects beta-diversity by controlling spatial community turnover, resulting in scale-dependent productivity-diversity relationships. As an illustration of the interaction between local and regional processes in shaping microbial diversity, our results offer both empirical support and a plausible mechanism for the existence of common scaling rules in both the macrobial and the microbial worlds. We argue that awareness of regional species pools in phytoplankton and other unicellular organisms may critically improve our understanding of ecosystems and their susceptibility to anthropogenic stressors.     

Estimated plant richness pattern across northwest South America provides similar support for the species-energy and spatial heterogeneity hypotheses

Explaining geographic variation in plant species richness at broad spatial scales has long been a major challenge. Many hypotheses have been proposed during the last 200 yr, but recent work has focused on a few major alternatives. Among these, two hypotheses contend that plant species richness reflects 1) variation in energy and water availability among sampling units (the species-energy hypothesis) and 2) habitat and topographic heterogeneity within sampling units (the spatial heterogeneity hypothesis). We used a large botanical database and regression models to simultaneously confront the predictions from both hypotheses against an estimate of vascular plant richness across northwest South America. This estimate provided similar support for both hypotheses, a result that may be seen as contrasting with the notion that variation in energy and water availability among sampling units is the main determinant of plant species richness. We discuss potential explanations for this apparent discrepancy. Regression models that incorporated the relative contributions of both hypotheses predicted that the highest plant species richness in northwest South America is found in topographically complex areas. In contrast to several of the most recent mapping efforts, lowland Amazonia was predicted to be a plant richness trough in the study region. We suggest that diverging portrayals of plant richness across northwest South America result from differences in estimates of the relative importance of the species-energy and the spatial heterogeneity hypotheses.     

Environmental heterogeneity among lakes promotes hyper β‐diversity across phytoplankton communities

相似文献   

Structure of the species--energy relationship

The relationship between energy availability and species richness (the species-energy relationship) is one of the best documented macroecological phenomena. However, the structure of species distribution along the gradient, the proximate driver of the relationship, is poorly known. Here, using data on the distribution of birds in southern Africa, for which species richness increases linearly with energy availability, we provide an explicit determination of this structure. We show that most species exhibit increasing occupancy towards more productive regions (occurring in more grid cells within a productivity class). However, average reporting rates per species within occupied grid cells, a correlate of local density, do not show a similar increase. The mean range of used energy levels and the mean geographical range size of species in southern Africa decreases along the energy gradient, as most species are present at high productivity levels but only some can extend their ranges towards lower levels. Species turnover among grid cells consequently decreases towards high energy levels. In summary, these patterns support the hypothesis that higher productivity leads to more species by increasing the probability of occurrence of resources that enable the persistence of viable populations, without necessarily affecting local population densities.