Habitat heterogeneity drives bird species richness,nestedness and habitat selection by individual species in fluvial wetlands of the Paraná River,Argentina

We assessed the relationship between habitat heterogeneity and bird species richness and composition within wetlands of the floodplain of the Middle Paraná River, Argentina. Given the high habitat heterogeneity in these wetland systems, we sought to determine whether (i) there was a positive relationship between bird species richness and habitat heterogeneity; (ii) whether bird species richness was associated with certain types of individual habitat types; (iii) whether there was a pattern of species nestedness and turnover between sites as a function of habitat heterogeneity and composition, respectively; and (iv) whether individual species exhibited associations with habitat heterogeneity. Point counts were used to survey birds at 60 sites. We estimated the area of eight habitat types found within a 200‐m radius from the centre of each site and calculated number and Pielou's evenness of habitat types. These indices, together with area proportion of each habitat type, were used as explanatory factors of bird species richness in linear regression models. Habitat heterogeneity per se rather than area of individual habitat types was a more important predictor of species richness in these fluvial wetlands. Sites with more habitat types supported more bird species. Results showed that individual bird species were associated with different habitat types and, therefore, sites that contained more habitat types contained more species. Number of habitat types accounted for species nestedness between sites whereas composition of habitat types accounted for species turnover between sites. Results suggest that selection of heterogeneous sites by individual species could help explain the positive heterogeneity–species richness relationship. Our findings highlight the importance of habitat heterogeneity per se resulting from flood disturbances in maintaining bird richness in fluvial systems.     

The effect of energy and seasonality on avian species richness and community composition

We analyzed geographic patterns of richness in both the breeding and winter season in relation to a remotely sensed index of seasonal production (normalized difference vegetation index [NDVI]) and to measures of habitat heterogeneity at four different spatial resolutions. The relationship between avian richness and NDVI was consistent between seasons, suggesting that the way in which available energy is converted to bird species is similar at these ecologically distinct times of year. The number and proportion of migrant species in breeding communities also increased predictably with the degree of seasonality. The NDVI was a much better predictor of seasonal richness at finer spatial scales, whereas habitat heterogeneity best predicted richness at coarser spatial resolutions. While we find strong support for a positive relationship between available energy and species richness, seasonal NDVI explained at most 61% of the variation in richness. Seasonal NDVI and habitat heterogeneity together explain up to 69% of the variation in richness.     

Energy density and its variation in space limit species richness of boreal forest birds

Aim An area’s ability to support species may be dependent not only on the total amount of available energy it contains but also on energy density (i.e. available energy per unit area). Acknowledging these two aspects of energy availability may increase mechanistic understanding of how increased energy availability results in increased species richness. We studied the relationship between energy density, its variation in space and boreal forest bird species richness and investigated two possible mechanisms: (1) metabolic constraints of organisms, and (2) increased resource availability for specialists. Location Protected areas in Finland’s boreal forest. Methods We tested whether bird species richness was best determined by total energy availability in an area or by energy density and its variation within the area, before and after including bird abundance in the models. We evaluated two main explanatory variables: tree growth reflecting the rate of energy production and tree volume as a measure of biomass. In addition, we modelled individual species’ responses to energy density and its variation, and evaluated the prediction of the metabolic constraints hypothesis that small species are limited by energy density whereas large species are limited by total energy availability in the area. Results Energy density and its variation were good predictors of species richness: together with abundance they explained 84% of variation in species richness (compared with 74% for abundance alone). Pure metabolic constraints were unlikely to explain this relationship. Instead, the mechanism probably involved increased habitat heterogeneity benefiting specialist species. Total energy availability was also an important factor determining species richness but its effect was indirect via abundance. Main conclusions Our results corroborate the importance of energy availability as a driver of species richness in forest bird communities, and they indicate that energy density and its variation in the landscape strongly influence species richness even after accounting for abundance.     

Environmental correlates of mammal species richness in South America: effects of spatial structure, taxonomy and geographic range

Although some consensus exists regarding the positive synergism between energy and heterogeneity in increasing species diversity, the role of environmental variability remains controversial. We examine how these factors interact to explain spatial variation in mammal species richness in South America. After taking into account the effects of spatial autocorrelation and area, elevation variability and energy mainly drive spatial variation in mammal species richness. The effect of environmental variability is less important. When different taxonomic groups of mammals are analyzed separately, three ways emerge whereby energy and heterogeneity interact to promote species richness. Heterogeneity may have no effect on species richness, habitat heterogeneity and energy availability contribute independently to species richness, or heterogeneity increases in importance with an increase in energy availability. The partition of species into range size quartiles shows that habitat heterogeneity and temporal instability in the resource supply account for the species richness pattern in the narrowest- ranging species. Habitat heterogeneity is significant also for intermediate ranging species but not for the widest-ranging species. Energy alone drives the species richness pattern in the latter species. The interplay between ecology and biogeographic history may ultimately explain these differences given that narrow- and wide-ranging species show distinct biogeographic patterns, and different taxonomic groups also unequally represent them.     

How,and how much,natural cover loss increases species richness

Aim The species–area relationship has been applied in the conservation context to predict monotonic species richness declines as natural area is converted to human‐dominated land covers. However, some conversion of natural cover could introduce new habitat types and allow new open habitat species to occur. Moreover, decelerating richness–area relationships suggest that, as natural area is converted to human‐dominated covers, more species will be added to the rare habitat than are lost from the common one. Area effects and increased habitat diversity could each lead to a peaked relationship between species richness and the relative amount of natural area. The purpose of this study is to quantify the effect on avian species richness of conversion of natural area to human‐dominated land cover. Location Ontario, Canada. Methods We evaluated the responses of total avian richness, forest bird richness and open habitat bird richness to remaining natural area within 993 quadrats, each of 100 km². We quantified the amount of natural land cover and land‐cover heterogeneity using remote sensing data. We used structural equation modelling (SEM) to disentangle the relationships among avian richness, natural area and land‐cover heterogeneity. Results Spatial variation in avian richness was a peaked function of remaining natural area, such that losses of up to 44% of the natural area increased avian richness. This partly reflects increased variety of land cover; however, SEM suggests that much of the increase in richness is due to pure area effects. Richness of forest species declined by two species over this range of natural cover loss while open habitat bird richness increased by approximately 20 species. The effect of natural area on species richness is consistent with the sum of species–area curves for natural habitat species and human‐dominated habitat species. Main conclusions At least in northern temperate forests, almost half of the natural land cover can be converted to human‐dominated forms before avian richness declines. Conversion of < 50% of regional natural area to human‐dominated land cover can benefit open‐area species richness with relatively few losses of forest obligate species. However, with > 50% natural area conversion, species begin to drop out of regional assemblages.     

Vascular plant species richness on wetland remnants is determined by both area and habitat heterogeneity

There is an ongoing ecological debate on whether area per se or habitat heterogeneity is the main driver for species richness. The wetland remnants in the Sanjiang Plain, NE China harbor a high biodiversity and play an important role for local ecosystems. Fifty-one wetland remnants were sampled to examine the effect of area and habitat heterogeneity on vascular plant species richness. Number of community types, elevation, water heterogeneity and soil resource heterogeneity were employed as habitat heterogeneity variables, but only water heterogeneity was identified as the proper surrogate for habitat heterogeneity. Compared with the classic species-area model, the choros model achieved better fitness when water heterogeneity and elevation were employed as habitat heterogeneity variables. Nevertheless, elevation was poorly correlated with species richness. It suggests, without a comprehensive analysis of habitat heterogeneity variables, the choros model might result in a misleading result. In this study, species richness was significantly influenced by water heterogeneity, area and number of community types. Water heterogeneity and area both controlled the number of community types, and they were the two main determinants of species richness. As area was significantly and positively correlated with water heterogeneity, the variance in species richness was mainly related to the mutual effect of area and water heterogeneity. The results of this study confirmed that the relationship between the area per se hypothesis and the habitat heterogeneity hypothesis was conjunct rather than mutually exclusive. In addition, it is critical that both area and water heterogeneity should be taken into account for biodiversity conservation and management in wetland remnants.     

The unusual suspect: Land use is a key predictor of biodiversity patterns in the Iberian Peninsula

Although land use change is a key driver of biodiversity change, related variables such as habitat area and habitat heterogeneity are seldom considered in modeling approaches at larger extents. To address this knowledge gap we tested the contribution of land use related variables to models describing richness patterns of amphibians, reptiles and passerines in the Iberian Peninsula. We analyzed the relationship between species richness and habitat heterogeneity at two spatial resolutions (i.e., 10 km × 10 km and 50 km × 50 km). Using both ordinary least square and simultaneous autoregressive models, we assessed the relative importance of land use variables, climate variables and topographic variables. We also compare the species–area relationship with a multi-habitat model, the countryside species–area relationship, to assess the role of the area of different types of habitats on species diversity across scales. The association between habitat heterogeneity and species richness varied with the taxa and spatial resolution. A positive relationship was detected for all taxa at a grain size of 10 km × 10 km, but only passerines responded at a grain size of 50 km × 50 km. Species richness patterns were well described by abiotic predictors, but habitat predictors also explained a considerable portion of the variation. Moreover, species richness patterns were better described by a multi-habitat species-area model, incorporating land use variables, than by the classic power model, which only includes area as the single explanatory variable. Our results suggest that the role of land use in shaping species richness patterns goes beyond the local scale and persists at larger spatial scales. These findings call for the need of integrating land use variables in models designed to assess species richness response to large scale environmental changes.     

Food plant diversity as broad-scale determinant of avian frugivore richness

The causes of variation in animal species richness at large spatial scales are intensively debated. Here, we examine whether the diversity of food plants, contemporary climate and energy, or habitat heterogeneity determine species richness patterns of avian frugivores across sub-Saharan Africa. Path models indicate that species richness of Ficus (their fruits being one of the major food resources for frugivores in the tropics) has the strongest direct effect on richness of avian frugivores, whereas the influences of variables related to water-energy and habitat heterogeneity are mainly indirect. The importance of Ficus richness for richness of avian frugivores diminishes with decreasing specialization of birds on fruit eating, but is retained when accounting for spatial autocorrelation. We suggest that a positive relationship between food plant and frugivore species richness could result from niche assembly mechanisms (e.g. coevolutionary adaptations to fruit size, fruit colour or vertical stratification of fruit presentation) or, alternatively, from stochastic speciation-extinction processes. In any case, the close relationship between species richness of Ficus and avian frugivores suggests that figs are keystone resources for animal consumers, even at continental scales.     

Patterns of species richness for vascular plants in China's nature reserves

Explaining the heterogeneous distribution of biodiversity across the Earth has long been a challenge to ecologists and biogeographers. Here, we document the patterns of plant species richness for different taxonomic groups in China's nature reserves, and discuss their possible explanations at national and regional scales, using vascular plant richness data coupled with information on climate and topographical variables. We found that water deficit, energy and elevation range (a surrogate of habitat heterogeneity) represent the primary explanations for variation in plant species richness of the nature reserves across China. There are consistent relationships between species richness and climate and habitat heterogeneity for different taxonomic vascular plant groups at the national scale. Habitat heterogeneity is strongly associated with plant richness in all regions, whereas climatic constraints to plant diversity vary regionally. In the regions where energy is abundant or water is scarce, plant richness patterns were determined by water and habitat heterogeneity, whereas in the region with low energy inputs, water interacting with energy, and habitat heterogeneity determined its species richness pattern. Our results also suggest that energy variables alone do not represent the primary predictor of plant richness.     

宏生态尺度上景观破碎化对物种丰富度的影响

生物多样性的地理格局及其形成机制是宏生态学与生物地理学的研究热点。大量研究表明,景观尺度上的生境破碎化对物种多样性的分布格局具有重要作用,但目前尚不清楚这种作用是否足以在宏生态尺度上对生物多样性地理格局产生显著影响。利用中国大陆鸟类和哺乳动物的物种分布数据,在100 km×100 km网格的基础上生成了这两个类群生物的物种丰富度地理格局,进一步利用普通最小二乘法模型和空间自回归模型研究了物种丰富度与气候、生境异质性、景观破碎化的相关关系。结果表明,景观破碎化因子与鸟类和哺乳动物的物种丰富度都具有显著的关联关系,其方差贡献率可达约30%—50%(非空间模型)和60%—80%(空间模型),略低于或接近于气候和生境异质性因子。方差分解结果显示,景观破碎化因子与气候和生境异质性因子的方差贡献率的重叠部分达20%—40%。相对鸟类而言,景观破碎化对哺乳动物物种丰富度的地理格局具有更高的解释率。     

Ecological biogeography of Mexican bats: the relative contributions of habitat heterogeneity,beta diversity,and environmental gradients to species richness and composition patterns

Mexico has higher mammalian diversity than expected for its size and geographic position. High environmental hetero geneity throughout Mexico is hypothesized to promote high turnover rates (β‐diversity), thus contributing more to observed species richness and composition than within‐habitat (α) diversity. This is true if species are strongly associated with their environments, such that changes in environmental attributes will result in changes in species composition. Also, greater heterogeneity in an area will result in greater species richness. This hypothesis has been deemed false for bats, as their ability to fly would reduce opportunities for habitat specialization. If so, we would expect no significant relationships between 1) species composition and environmental variables, 2) species richness and environmental heterogeneity, 3) β‐diversity and environmental heterogeneity. We tested these predictions using 31 bat assemblages distributed across Mexico. Using variance partitioning we evaluated the relative contribution of vegetation, climate, elevation, horizontal heterogeneity (a variate including vegetation, climate, and elevational heterogeneity), spatial variation (lat‐long), and vertical hetero geneity (of vegetation strata) to variation in bat species composition and richness. Variation in vegetation explained 92% of the variation in species composition and was correlated with all other variables examined, indicating that bats respond directly to habitat composition and structure. Beta‐diversity and vegetational heterogeneity were significantly correlated. Bat species richness was significantly correlated with vertical, but not horizontal, heterogeneity. Nonetheless, neither horizontal nor vertical heterogeneity were random; both were related to latitude and to elevation. Variation in bat community composition and richness in Mexico were primarily explained by local landscape heterogeneity and environmental factors. Significant relationships between β‐diversity and environmental variation reveal differences in habitat specialization by bats, and explain their high diversity in Mexico. Understanding mechanisms acting along environmental or geographic gradients is as important for understanding spatial variation in community composition as studying mechanisms that operate at local scales.     

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

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

Landscape‐ and small‐scale determinants of grassland species diversity: direct and indirect influences

Species richness is influenced both by mechanisms occurring at landscape scales, such as habitat availability, and local‐scale processes, that are related to abiotic conditions and plant–plant interactions. However, it is rarely tested to what extent local species richness can be explained by the combined effect of factors measured at multiple spatial scales. In this study, we quantified the simultaneous influence of historical landscape‐scale factors (past human population density, and past habitat availability – an index combining area and connectivity) and small‐scale environmental conditions (shrub cover, and heterogeneity of light, soil depth, and other soil environmental variables) on plant species richness in dry calcareous grasslands (alvars). By applying structural equation modelling (SEM) we found that both landscape conditions and local environmental factors had significant direct and indirect (i.e. through the modification of another factor), effects on species richness. At the landscape scale, we found a direct positive influence of historical habitat availability on species richness, and indirect positive influence of past human population (via its effects on historical habitat availability). At small scales, we found a positive direct influence of light heterogeneity and shrub cover on species richness. Conversely, we found that small‐scale soil environmental heterogeneity, which was mainly determined by soil depth heterogeneity, had a negative effect on species richness. Our study indicates that patterns of species richness in alvar grasslands are positively influenced by the anthropogenic management regime that maintained the landscape habitat conditions in the past. However, the abandonment of management, leading to shrub invasion and increased competition for light resources also influenced species richness. In contrast to the positive heterogeneity–diversity relationship we found that soil heterogeneity reduced species richness. Environmental heterogeneity, occurring at the plant neighbourhood scale (i.e. centimetres), can increase the isolation among suitable soil patches and thus hinder the normal functioning of populations. The combination of previous knowledge of the system with new ecological theories facilitates disentangling how species richness responds to complex relationships among factors operating at multiple scales.     

The area-independent effects of habitat complexity on biodiversity vary between regions

Potential explanatory variables often co‐vary in studies of species richness. Where topography varies within a survey it is difficult to separate area and habitat‐diversity effects. Topographically complex surfaces may contain more species due to increased habitat diversity or as a result of increased area per se. Fractal geometry can be used to adjust species richness estimates to control for increases in area on complex surfaces. Application of fractal techniques to a survey of rocky shores demonstrated an unambiguous area‐independent effect of topography on species richness in the Isle of Man. In contrast, variation in species richness in south‐west England reflected surface availability alone. Multivariate tests and variation in limpet abundances also demonstrated regional variation in the area‐independent effects of topography. Community composition did not vary with increasing surface complexity in south‐west England. These results suggest large‐scale gradients in the effects of heterogeneity on community processes or demography.     

Environmental correlates of species and genetic richness in lungless salamanders (family plethodontidae)

Biological diversity is distributed across the planet in non-random, organised ways. At the species level, numerous environmental variables have been proposed to explain this non-random distribution with available energy and habitat heterogeneity receiving the most empirical support. With regard to genetic organisation, environmental stress and habitat heterogeneity have been widely supported. However, few studies have addressed if these two scales of biological organisation are structured via similar processes. Here, we tested whether or not the distributional organisation of genetic and species richness were driven by similar environmental variables for salamanders of the family Plethodontidae across North America. In general, we found that those environmental variables related to energy, particularly energy made accessible to salamanders via the actions of available moisture, were the primary determinants of both genetic and species richness. This finding is consistent with both the “more individuals hypothesis” of species richness and neutral-theory expectations for genetic richness. Additionally, greater habitat heterogeneity, as measured by increased topographic variance, was of secondary importance in positively influencing species richness, although its effects on genetic richness were far more variable. In total, our results suggest that both of these scales of biological organisation are influenced by similar environmental variables, even though increased genetic richness at the population-level does not always translate into greater species richness.     

Effects of habitat area, isolation, and landscape diversity on plant species richness of calcareous grasslands

Calcareous grasslands harbour a high biodiversity, but are highly fragmented and endangered in central Europe. We tested the relative importance of habitat area, habitat isolation, and landscape diversity for species richness of vascular plants. Plants were recorded on 31 calcareous grasslands in the vicinity of the city of Göttingen (Germany) and were divided into habitat specialist and generalist species. We expected that habitat specialists were more affected by area and isolation, and habitat generalists more by landscape diversity. In multiple regression analysis, the species richness of habitat specialists (n = 66 species) and habitat generalists (n = 242) increased with habitat area, while habitat isolation or landscape diversity did not have significant effects. Contrary to predictions, habitat specialists were not more affected by reduced habitat area than generalists. This may have been caused by delayed extinction of long-living plant specialists in small grasslands. Additionally, non-specialists may profit more from high habitat heterogeneity in large grasslands compared to habitat specialists. Although habitat isolation and landscape diversity revealed no significant effect on local plant diversity, only an average of 54% of habitat specialists of the total species pool were found within one study site. In conclusion, habitat area was important for plant species conservation, but regional variation between habitats contributed also an important 46% of total 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.     

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

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

Habitat heterogeneity overrides the species–area relationship

Aim The most obvious, although not exclusive, explanation for the increase of species richness with increasing sample area (the species–area relationship) is that species richness is ultimately linked to area-based increases in habitat heterogeneity. The aim of this paper is to examine the relative importance of area and habitat heterogeneity in determining species richness in nature reserves. Specifically, the work tests the hypothesis that species–area relationships are not positive if habitat heterogeneity does not increase with area. Location Sixteen nature reserves (area range 89–11,030 ha) in central Hungary. Methods Four-year faunistic inventories were conducted in the reserves involving c. 70 fieldworkers and 65 taxonomists. CORINE 50,000 land-cover maps were used for calculating the heterogeneity of the reserve landscape (number of habitat types, number of habitat patches and total length of edges). Results Large reserves were less heterogeneous than small reserves, probably because large reserves were established in large blocks of unproductive land whereas small reserves tended to be in more fertile land. In total, 3975 arthropod species were included in the analysis. The slope of the species–area relationship was positive only for Neuroptera and Trichoptera. There was no significant relationship in the other nine taxa examined (Collembola, Acari, Orthoptera, Thysanoptera, Coleoptera, Araneae, Diplopoda, Chilopoda, Diptera). The density (number of species ha⁻¹) of all species, however, showed a positive correlation with heterogeneity. Main conclusions The general lack of fit of species–area relationships in this study is inconsistent with most previous published studies. Importantly, and unlike many other studies, habitat heterogeneity was not correlated with reserve area in the studied system. In the absence of this source of covariation, stronger relationships were identified that suggested a fundamental link between species richness and habitat heterogeneity. The results indicate that habitat heterogeneity rather than area per se is the most important predictor of species richness in the studied system.     

Determinants of species richness in patches of grassland and heathland in Himmerland (Denmark)

Island biogeographical theory predict species richness to increase with habitat area and to decrease with isolation from colonisation sources. This theory has been applied to habitat fragments, and the predictions tested and found valid in empirical studies on fragments of deciduous woodland in northern Europe. However, previous results on fragments of grassland have been ambiguous. In the present study the species richness of vascular plants was investigated in 63 patches of grassland and heathland scattered in an agricultural and forested landscape using multiple linear regression. The relationship between species richness and patch area, isolation and local habitat conditions including heterogeneity were examined. Area was an important determinant of species richness, both in the full data set and in a subset of small habitat patches. In contrast, spatial isolation and habitat heterogeneity were not important factors determining species richness. Differences in soil acidity were accounting for a large proportion of the variance in species richness. This result is probably due to differences in the size of the regional species pools of grasslands with different levels of soil pH.