Integration of local and regional species-area relationships from space-time species accumulation

A long-standing observation in community ecology is that the scaling of species richness, as exemplified by species-area curves, differs on local and regional scales. This decoupling of scales may be largely due to sampling processes (the increasing constraint imposed by sampling fewer individuals at fine scales), as distinct from ecological processes, such as environmental heterogeneity, that operate across scales. Removal of the sampling constraint from fine-scale richness estimates should yield species-area curves that behave like those of the regions in which they are embedded, but an effective method for this removal has not been available. We suggest an approach that incorporates the manner in which small areas accumulate species over time as a way to remove the signature of sampling processes from fine-scale species-area curves. We report for three species-rich grasslands from two continents how local plant species richness is distributed through time at multiple, nested spatial scales, and we ask whether sampling-corrected curves reflect the spatial scaling of richness of each larger floristic province. Our analysis suggests that fine-scale values of richness are highly constrained by sampling processes, but once these constraints are removed, the spatial scaling of species richness is consistent from the scale of individuals to that of an entire province.     

Climate and soil texture influence patterns of forb species richness and composition in big sagebrush plant communities across their spatial extent in the western U.S.

Big sagebrush (Artemisia tridentata Nutt.) plant communities are widespread in western North America and, similar to all shrub steppe ecosystems worldwide, are composed of a shrub overstory layer and a forb and graminoid understory layer. Forbs account for the majority of plant species diversity in big sagebrush plant communities and are important for ecosystem function. Few studies have explored geographic patterns of forb species richness and composition and their relationships with environmental variables in these communities. Our objectives were to examine the fine and broad-scale spatial patterns in forb species richness and composition and the influence of environmental variables. We sampled forb species richness and composition along transects at 15 field sites in Colorado, Idaho, Montana, Nevada, Oregon, Utah, and Wyoming, built species-area relationships to quantify differences in forb species richness at sites, and used Principal Components Analysis, non-metric multidimensional scaling, and redundancy analysis to identify relationships among environmental variables and forb species richness and composition. We found that species richness was most strongly correlated with soil texture, while species composition was most related to climate. The combination of climate and soil texture influences water availability, which our results indicate has important consequences for forb species richness and composition, and suggests that climate change-induced modification of soil water availability may have important implications for plant species diversity in the future.     

The species-area relationship in the hoverfly (Diptera, Syrphidae) communities of forest fragments in southern France

The effect of forest fragmentation was studied in hoverfly communities of 54 isolated forests (0.14–171 ha) in south west France. The positive relationship between species richness and wood patch area was investigated by testing the three hypotheses usually put forward to explain it: 1) the sampling effect hypothesis, 2) the patch heterogeneity hypothesis, 3) the hypothesis of equilibrium between distance from other patch (colonisation) and surface area of the patch (extinction). The syrphid species were divided into 3 ecological groups, based on larval biology as summarized in the "Syrph the Net" database: non forest species, facultative forest species and forest species. A total of 3317 adults belonging to 100 species, were captured in the 86 Malaise traps. Eight species were non forest (N=16), 65 facultative forest (N=2803) and 27 forest species (N=498).
Comparison of the slopes of the species-area curves for species richness and species density per forest patch showed a strong sampling effect in the species-area relationship. Wood patch heterogeneity increased with wood patch area and positively influenced hoverflies richness. Less isolated wood patches presented high richness of forest species and low richness of non forest species. Only forest species richness seemed to respond to the equilibrium between surface area and isolation. Depending on which hypothesis explained best the species-area relationship, management recommendations to mitigate fragmentation effects were formulated at various spatial scales and for different stakeholders.     

Native versus exotic community patterns across three scales: Roles of competition,environment and incomplete invasion

Three fundamental, interrelated questions in invasion ecology are: (1) to what extent do exotic species outcompete natives; (2) are native and exotic communities functionally similar or different; and (3) are differences in biogeographic patterns in native and exotic communities due to incomplete invasions among exotics? These questions are analogous to general questions in community ecology regarding the relative roles of competition, environmental response and dispersal limitation in community assembly. We addressed each of these questions for plant communities in discrete meadow patches, using analyses at three scales ranging from the landscape to microsites. A weak positive relationship between native and exotic species richness in microsites, and a predominance of positive correlations in abundance among native and exotic species pairs suggest that competition has been less important than other factors in determining native versus exotic abundance and community composition. In contrast, models of species richness and community compositional change across scales suggest native versus exotic community patterns are largely determined by a mix of scale-dependent concordant (shared positive or negative) and discordant relationships with environmental variables. In addition, detailed analyses of species-area and species-abundance relationships suggest ongoing expansion of exotic species populations, indicating that the assembly of the exotic community is in its early stages. Thus, while competition does not appear to strongly affect native versus exotic abundances and compositions at present, it may intensify in the future. Our results indicate that synoptic patterns in native versus exotic richness that have been previously attributed to a single cause may in fact be due to a complex mix of concordant and discordant responses to environmental factors across scales. They also suggest that conservation efforts aimed at promoting natives and reducing exotics should focus on the factors and scales for which such a response (i.e., promotion of high native and low exotic richness) can be expected.     

鄱阳湖湿地典型草洲植物群落种-面积关系

为了解鄱阳湖湿地草洲植物群落的结构,对其3种典型草洲植物群落种-面积关系进行了研究。采用巢式样方法调查植物物种数量,并用3种非饱和曲线拟合种-面积方程。结果表明,基于实测数据,3个草洲物种数随着取样面积的增加先快速增加后趋于平稳;群落最小取样面积均为30 m~2。幂函数模型对3个草洲群落的种-面积关系拟合效果均为最佳,3个草洲群落的RSE分别为0.35、0.35和0.56,AAD分别为0.23、0.17和0.35,AARD分别为0.06、0.02和0.07,而指数模型、Fisher模型的拟合效果一般。指数模型计算得到的不同比例因子下3个草洲最小取样面积与实际情况最为吻合。这为鄱阳湖区进行植物群落数据采集的样方设置提供了理论参考。     

SPECIES-AREA RELATIONSHIP DURING PRIMARY SUCCESSION IN GRANITE OUTCROP PLANT COMMUNITIES

The species-area relationship and its underlying explanatory mechanisms were investigated in a primary successional sere on a southeastern (United States) granite outcrop. There, plant communities occupy soil-filled depressions separated from one another by areas of bare rock. They have been termed “island communities.” Soil depth and area increase as succession proceeds from Sedum smallii, to lichen-annual, to annual-perennial, and to herb-shrub-tree stages. Although plant species richness is significantly and positively correlated with island area in the system studied (all successional stages considered), the relationship between species richness and island maximum soil depth is stronger. However, island maximum soil depth and area are significantly and positively correlated. The exponential function describes the speciesarea relationship better than the transformed power function. Within successional stage, species richness shows no significant relationship with area or depth, except for late-successional island communities. Processes related to community successional development may explain the species-area relationship that exists when islands of all stages are considered. However, mechanisms related to equilibrium between extinction and immigration may be responsible for the speciesarea relationship for late-successional island communities.     

Genetic structure of a foundation species: scaling community phenotypes from the individual to the region

Understanding the local and regional patterns of species distributions has been a major goal of ecological and evolutionary research. The notion that these patterns can be understood through simple quantitative rules is attractive, but while numerous scaling laws exist (e.g., metabolic, fractals), we are aware of no studies that have placed individual traits and community structure together within a genetics based scaling framework. We document the potential for a genetic basis to the scaling of ecological communities, largely based upon our long-term studies of poplars (Populus spp.). The genetic structure and diversity of these foundation species affects riparian ecosystems and determines a much larger community of dependent organisms. Three examples illustrate these ideas. First, there is a strong genetic basis to phytochemistry and tree architecture (both above- and belowground), which can affect diverse organisms and ecosystem processes. Second, empirical studies in the wild show that the local patterns of genetics based community structure scale up to western North America. At multiple spatial scales the arthropod community phenotype is related to the genetic distance among plants that these arthropods depend upon for survival. Third, we suggest that the familiar species-area curve, in which species richness is a function of area, is also a function of genetic diversity. We find that arthropod species richness is closely correlated with the genetic marker diversity and trait variance suggesting a genetic component to these curves. Finally, we discuss how genetic variation can interact with environmental variation to affect community attributes across geographic scales along with conservation implications.     

On the possible role of local effects on the species richness of acidic and calcareous rock grasslands in northern Hungary

Ewald (Folia Geobot. 38: 357–366, 2003, this issue) stated that in Central Europe the number of calcifrequent species is higher than the number of acidofrequent species, while the range of acidofrequent communities is larger than that of the calcifrequent ones. All the explanations considered in his paper are based on an evolutionary spatial and temporal scale. In this paper we are trying to prove that local effects might also be important. Five open rock grassland communities on different bedrocks, viz. rhyolite (acidic), andesite (slightly acidic), calcareous sandstone (slightly calcareous), limestone (calcareous) and dolomite (calcareous) were chosen for the analysis. Two parameters of the species-area curve (i.e., local richness and the slope of log area-species richness line) were estimated based on all species and on rock specialist species separately. With this method we could simultaneously study three attributes of diversity: local species number, the slope of log area-species richness line, and species pool size.     

Species-area relations in a New Zealand tussock grassland,with implications for nature reserve design and for community structure

Abstract. A tussock grassland, in Blackrock Reserve, New Zealand, was sampled thoroughly at scales ranging from 0.01 m x 0.01 m to 20 m x 20 m, to investigate species-area relations of relevance to plant community structure, and to offer a pointer to reserve design. In total, 96 species were found. Of the native vascular species among these, 20% were new records for the reserve. For the total and the native vascular flora the observed points fell midway between the Arrhenius and Gleason fitted curves. Cryptogams fitted the Arrhenius model well, with z close to the 0.26 value expected for isolates under Preston's Canonical hypothesis. Extrapolation of the Arrhenius curve to the whole of the New Zealand uplands gave values far too large; the Gleason curve gave values much too low. A General Root model is introduced. It fits the observed species richnesses at various quadrat sizes considerably better than previous models. When biogeographic limitations on species richness are included, extrapolation of the General Root curve accurately predicts the size of the upland flora. This fit, the lack of an asymptote, and the rarity structure, are compatible with a random and individualistic model of community structure. However, there are differences in species-area relation between vascular and cryptogamic plants, which cautions against expecting any universal type of community structure. Extrapolations using the General Root model suggest that if the ideal is a 10 km x 10 km reserve, a reserve one tenth that size would contain 81% of the native species in that ideal, and the present Blackrock Reserve contains 67%.     

Scale Dependency in the Species-Area Relationship of Plant Communities

The species-area relationship (SAR) is one of the most frequently described patterns in biodiversity; however, it is seldom applied in conservation practice. Awareness of some shortcomings such as scale dependency is the basis for a better understanding of biodiversity patterns and a way to cope with artifacts occurring in empirical data. We demonstrate effects of scale dependency within plant communities deriving from different scale ranges, extreme values and curve intersections on empirical data of forest and grassland communities and propose to reduce the effect of scale dependence and artifacts of the SAR within plant communities by using limited scale ranges. Due to scale dependence, an appropriate limited scale range should be chosen, depending on vegetation type and realized scale range of the considered data set. Species-area curves based on the standard Whittaker plots for plant communities could act as references to assess species richness of the large pool of vegetation records of different sizes and provide a tool for ranking species diversity for grassland and forest communities, where vegetation records differ considerably in plot size.     

Species-area curve,life history strategies,and succession: a field test of relationships

Changes of species richness along temporal and environmental gradients were investigated. Two data sets were used: a successional sere of old-field plant communities in the Bohemian Karst, and a set of plant communities under various intensities of disturbance in the Krkonoe (Giant) Mts, both in Czechoslovakia. The species richness of a plant community is a spatial phenomenon, and should be described by the species-area relationship (using e.g. the power function S=c\A ^z) rather than by a single number. In the old-field succession, the number of species in very small plots (0.1×0.1 m) tends to increase with successional age while the number of species in larger plots (4×4 m) decreases from the third year of succession. The plant community under the highest rate of disturbance of the Krkonoe Mts data set shows the lowest number of species on small plots and the highest number of species on large plots. The results may be explained using the distinction between founder-and dominance-controlled communities (Yodzis 1978, 1984). In accordance with this theory, the species-area relationship within a community is shaped mainly by the type of competitive interaction and may be predicted on the basis of life-history strategies of constituent species. Disturbance causes a shift from dominance to founder control. On the landscape scale, the species-area relationship is shaped by other factors, and so it is unjustified to extrapolate the relationship outside the range in which it was originally assessed.     

Are local patterns of anthropoid primate diversity related to patterns of diversity at a larger scale?

Aims (1) To determine the relationship between local and regional anthropoid primate species richness. (2) To establish the spatial and temporal scale at which the ultimate processes influencing patterns of primate species coexistence operate. Location Continental landmasses of Africa, South America and Asia (India to China, and all islands as far south as New Guinea). Methods The local–regional species richness relationship for anthropoid primates is estimated by regressing local richness against regional richness (independent variable). Local richness is estimated in small, replicate local assemblages sampled in regions that vary in total species richness. A strong linear relationship is taken as evidence that local assemblages are unsaturated and local richness results from proportional sampling of the regional pool. An asymptotic curvilinear relationship is interpreted to reflect saturated communities, where strong biotic interactions limit local richness and local processes structure the species assemblage. As a further test of the assumption of local assemblage saturation, we looked for density compensation in high‐density local primate assemblages. Results The local–regional species richness relationship was linear for Africa and South America, and the slope of the relationship did not differ between the two continents. For Asia, curvilinearity best described the relationship between local and regional richness. Asian primate assemblages appear to be saturated and this is confirmed by density compensation among Asian primates. However, density compensation was also observed among African primates. The apparent assemblage saturation in Asia is not a species–area phenomenon related to the small size of the isolated islands and their forest blocks, since similar low local species richness occurs in large forests on mainland and/or peninsular Asia. Main conclusions In Africa and South America local primate assemblage composition appears to reflect the influence of biogeographic processes operating on regional spatial scales and historical time scales. In Asia the composition of primate assemblages are by‐and‐large subject to ecological constraint operating over a relatively small spatial and temporal scale. The possible local influence of the El Niño Southern Oscillations on the evolution and selection of life‐history characteristics among Asian primates, and in determining local patterns of primate species coexistence, warrants closer inspection.     

Breakdown of the species-area relationship in exotic but not in native forest patches

We studied the pattern of bird species richness in native and exotic forest patches in Hungary. We hypothesized that species-area relationship will depend on forest naturalness, and on the habitat specialization of bird species. Therefore, we expected strong species-area relationship in native forest patches and forest bird species, and weaker relationship in exotic forest patches containing generalist species. We censused breeding passerine bird communities three times in 13 forest patches with only native tree species, and 14 with only exotic trees in Eastern Hungary in 2003. Although most bird species (92%) of the total of 41 species occurred in both exotic and native forests, the species-area relationship was significant for forest specialist, but not for generalist species in the native forests. No relationship between bird species and area was found for either species group in the forest with exotic tree species. The comparison of native versus exotic forest patches of similar sizes revealed that only large (>100 ha) native forests harbor higher bird species richness than exotic forests for the forest specialist bird species. There is no difference between small and medium forest patches and in richness of generalist species. Thus, the species-area relationship may diminish in archipelago of exotic habitat patches and/or for habitat generalist species; this result supports the warning that the extension of exotic habitats have been significantly contributing to the decline of natural community patterns.     

A unified mathematical framework for the measurement of richness and evenness within and among multiple communities

Biodiversity can be divided into two aspects: richness (the number of species or other taxa in a community or sample) and evenness (a measure of the distribution of relative abundances of different taxa in a community or sample). Sample richness is typically evaluated using rarefaction, which normalizes for sample size. Evenness is typically summarized in a single value. It is shown here that Hurlbert's probability of interspecific encounter (Δ₁), a commonly used sample-size independent measure of evenness, equals the slope of the steepest part of the rising limb of a rarefaction curve. This means that rarefaction curves provide information on both aspects of diversity. In addition, regional diversity (gamma) can be broken down into the diversity within local communities (alpha) and differences in taxonomic composition among local communities (beta). Beta richness is expressed by the difference between the composite rarefaction curve of all samples in a region with the collector's curve for the same samples. The differences of the initial slopes of these two curves reflect the beta evenness thanks to the relationship between rarefaction and Δ₁. This relationship can be further extended to help interpret species-area curves (SAC's). As previous authors have described, rarefaction provides the null hypothesis of passive sampling for SAC's, which can be interpreted as regional collector's curves. This allows evaluation of richness and evenness at local and regional scales using a single family of well-established, mathematically related techniques.     

Insular biogeography of mammals in Canadian parks: a re-analysis

Aim Glenn & Nudds (1989) compared mammal species richness in Canadian parks to estimated species-area relationships prior to European settlement to test if parks presently contain their historical compliment of species. However, the data they used to estimate the presettlement species-area relationships were not commensurate with the scale of the parks and were not independent. This uncertainty reduces the utility of Glenn and Nudds analysis to detect which, if any, parks are experiencing mammal extirpations and ultimately to direct conservation efforts to enhance mammal conservation in Canada. We improved Glenn and Nudds methods and re-assesed the conservation status of mammals in Canadian parks. Location Canada. Methods We constructed species-area curves for disturbance tolerant and intolerant species in five mammal regions of Canada by sampling historical range maps using plots of 10– 10,000 km². We compared these estimates of expected species richness to current species richness in thirty-six parks and contrast our results with those of Glenn and Nudds. Results All ten re-estimated species-area regressions had higher intercepts than those reported by Glenn and Nudds; four had lower slopes. Of seventy-two cases analysed, we found twenty in which parks had fewer species than expected and four in which parks had more species than expected, compared to fourteen and thirty-three, respectively, reported by Glenn and Nudds. Changes in both the regression parameters and increases in the prediction intervals accounted for the disparity between these results. Park residuals were not significantly different from zero in eight of ten analyses. Residuals for parks in the Alleghenian-Illinoian mammal province were negative and decreased with increasing park size. Main conclusions Improvement of Glenn and Nudds methods had an effect on both the parameter estimates and precison of the presettlement species-area curves. As a result, mammal conservation in some Canadian parks is worse than Glenn and Nudds reported. Six additional parks contained fewer disturbance intolerant species than predicted. However, Glenn and Nudds finding of systematic differences between expected and observed species richness in parks in the densely populated region of southern Ontario, southern Quebec and the maritime provinces was not an artefact of their methods. In this region, seven of ten parks appear to have lost disturbance-sensitive species of mammals. The pattern of species loss in this region was consistent with the idea that these parks have become isolated from mammal dispersal by surrounding habitat change and have experienced local species extirpations.     

Inferring Regional-Scale Species Diversity from Small-Plot Censuses

Estimation of the number of species at spatial scales too large to census directly is a longstanding ecological challenge. A recent comprehensive census of tropical arthropods and trees in Panama provides a unique opportunity to apply an inference procedure for up-scaling species richness and thereby make progress toward that goal. Confidence in the underlying theory is first established by showing that the method accurately predicts the species abundance distribution for trees and arthropods, and in particular accurately captures the rare tail of the observed distributions. The rare tail is emphasized because the shape of the species-area relationship is especially influenced by the numbers of rare species. The inference procedure is then applied to estimate the total number of arthropod and tree species at spatial scales ranging from a 6000 ha forest reserve to all of Panama, with input data only from censuses in 0.04 ha plots. The analysis suggests that at the scale of the reserve there are roughly twice as many arthropod species as previously estimated. For the entirety of Panama, inferred tree species richness agrees with an accepted empirical estimate, while inferred arthropod species richness is significantly below a previous published estimate that has been criticized as too high. An extension of the procedure to estimate species richness at continental scale is proposed.     

Analyzing tropical forest tree species abundance distributions using a nonneutral model and through approximate Bayesian inference

The neutral theory of biodiversity challenges the classical niche-based view of ecological communities, where species attributes and environmental conditions jointly determine community composition. Functional equivalence among species, as assumed by neutral ecological theory, has been recurrently falsified, yet many patterns of tropical tree communities appear consistent with neutral predictions. This may mean that neutral theory is a good first-approximation theory or that species abundance data sets contain too little information to reject neutrality. Here we present a simple test of neutrality based on species abundance distributions in ecological communities. Based on this test, we show that deviations from neutrality are more frequent than previously thought in tropical forest trees, especially at small spatial scales. We then develop a nonneutral model that generalizes Hubbell's dispersal-limited neutral model in a simple way by including one additional parameter of frequency dependence. We also develop a statistical method to infer the parameters of this model from empirical data by approximate Bayesian computation. In more than half of the permanent tree plots, we show that our new model fits the data better than does the neutral model. Finally, we discuss whether observed deviations from neutrality may be interpreted as the signature of environmental filtering on tropical tree species abundance distributions.     

Native communities determine the identity of exotic invaders even at scales at which communities are unsaturated

Aim To determine why some communities are more invasible than others and how this depends on spatial scale. Our previous work in serpentine ecosystems showed that native and exotic diversity are negatively correlated at small scales, but became positively correlated at larger scales. We hypothesized that this pattern was the result of classic niche partitioning at small scales where the environment is homogeneous, and a shift to the dominance of coexistence mechanisms that depend on spatial heterogeneity in the environment at large scales. Location Serpentine ecosystem, Northern California. Methods We test the above hypotheses using the phylogenetic relatedness of natives and exotics. We hypothesized that (1) at small scales, native and exotic species should be more distantly related than expected from a random assemblage model because with biotic resistance, successful invaders should have niches that are different from those of the natives present and (2) at large scales, native and exotic species should not be more distantly related than expected. Result We find strong support for the first hypothesis providing further evidence of biotic resistance at small scales. However, at large scales, native and exotic species were also more distantly related than expected. Importantly, however, natives and exotics were more distantly related at small scales than they were at large scales, suggesting that in the transition from small to large scales, biotic resistance is relaxed but still present. Communities at large scales were not saturated in the sense that more species could enter the community, increasing species richness. However, species did not invade indiscriminately. Exotic species closely related to species already established the community were excluded. Main conclusions Native communities determine the identity of exotic invaders even at large spatial scales where communities are unsaturated. These results hold promise for predicting which species will invade a community given the species present.     

Evaluation of species-area functions using Sonoran Desert plant data: not all species-area curves are power functions

Ecologists have been studying the relationship between species richness and area for about a century. As area increases, more species are typically observed. Many mathematical functions have been proposed to describe the pattern of increase. Numerous researchers have assumed that the relationship is a power function despite the fact that there are many possible alternatives. There has been limited work in evaluating which species-area functions are most appropriate for field data. This study examines which of a variety of functions best describe how Sonoran Desert plant species richness of remnant habitat patches in the Phoenix metropolitan area vary with sampled area and the area of entire patches. No single species-area function was adequate for describing all empirical datasets. Sample curves of woody species were most frequently best described by the sigmoid logistic, Hill, and Lomolino functions, whereas herbaceous datasets were best fit by the sigmoid logistic or convex rational functions. A curve depicting the relationship between patch-level woody species richness and patch area was best fit by the convex exponential function. The power function provided the best fit for only one case. This study demonstrates the utility of testing alternative functions for statistical fit rather than assuming that any particular equation adequately describes the species-area relationship.     

Diversity patterns of seasonal wetland plant communities mainly driven by rare terrestrial species

In cleared landscapes, wetlands can represent important reservoirs of native plant diversity, which include terrestrial species. Depending on study aims, non-wetland plants might be removed before analysis, affecting conclusions around biodiversity and community structure. We compared the native plant communities of seasonal wetlands in a predominately agricultural landscape as defined geographically (including all species) with that of the obligate wetland assemblage. We were primarily concerned with determining how this design decision affects ecological and conservation conclusions. We analysed a survey database containing >12,900 flora records from South Australia, developing a new area-based method to remove sampling bias to include only wetlands with a near-complete census. We modelled occupancy, species-area relationships, β-diversity and nestedness under our contrasting community definitions. Terrestrial species were 57.4 % of total richness. Removing these species reduced wetland α-diversity by 45 %, but did not affect the scaling of richness with area (power-law species-area relationship z = 0.21 ± 0.01). Occupancies for wetland plants were relatively uniform, but were heavily dominated by rare (satellite) species when terrestrial plants were included, and this also increased β-diversity. Nestedness for terrestrial species occupancies was marginally lower than predicted under null models, suggesting that rare species often do not co-occur with common species. An implication of these occupancy patterns is that twice as many wetlands (and 50 % more wetland area) would be needed to include every native species within at least one wetland compared with wetland-only species.