Drivers Of Species Research Articles

Species differ in their responses to wind: the underexplored link between species fine‐scale occurrences and variation in wind stress

AbstractQuestionsSpecies distribution models have traditionally relied heavily on temperature and precipitation, often ignoring other potentially important variables. However, recent advances have shown other climatic variables, including snow cover and solar radiation, may strongly improve predictions of species occurrence. Wind has long been known to have mechanical and physiological impacts on plants, but has not yet received adequate attention as a driver of species distributions.LocationMarion Island, sub‐Antarctic.MethodsUsing data from 1,440 plots in a chronically windy system, we test if wind stress (a combination of wind exposure and wind speed) improves species distribution models of vascular plant species, examining predictions for both species occurrence and cover.ResultsWind stress was a significant predictor of the occurrence of 12 of the 16 species, even after accounting for seven other ecophysiologically important abiotic variables. Species showed differential responses to wind, but wind stress was among the four most important drivers for the majority of species when modelling occurrence patterns (10 of 16) and variation in cover (12 of 16). Further, wind stress was more important than all temperature and precipitation variables in predicting the occurrence of six species (and three species’ cover).ConclusionsWind conditions were most influential for species that are characteristic of open, wet environments and for pteridophyte species, likely due to high wind speeds and exposure increasing the potential for moisture loss. This research highlights the value of incorporating wind metrics into species distribution models, particularly under changing wind patterns.

Drivers of species and genetic diversity within forest metacommunities across agricultural landscapes of different permeability

Drivers of species and genetic diversity within forest metacommunities across agricultural landscapes of different permeability

Abiotic stress and biotic factors mediate range dynamics on opposing edges

AbstractAimIn the face of global change, understanding causes of range limits are one of the most pressing needs in biogeography and ecology. A prevailing hypothesis is that abiotic stress forms cold (upper latitude/altitude) limits, whereas biotic interactions create warm (lower) limits. A new framework – Interactive Range‐Limit Theory (iRLT) – asserts that positive biotic factors such as food availability can ameliorate abiotic stress along cold edges, whereas abiotic stress can have a positive effect and mediate biotic interactions (e.g., competition) along warm limits.LocationNortheastern United StatesTaxonCarnivoraMethodsWe evaluated two hypotheses of iRLT using occupancy and structural equation modeling (SEM) frameworks based on data collected over a 6‐year period (2014–2019) of six carnivore species across a broad latitudinal (42.8–45.3°N) and altitudinal (3–1451 m) gradient.ResultsWe found that snow directly limits populations, but prey or habitat availability can influence range dynamics along cold edges. For example, bobcats (Lynx rufus) and coyotes (Canis latrans) were limited by deep snow and long winters, but the availability of prey had a strong positive effect. Conversely, snow had a strong positive effect on the warm limits of Canada lynx (Lynx canadensis), countering the negative effect of competition with the phylogenetically similar bobcat and with coyotes, highlighting how climate mediates competition between species.Main conclusionsWe used an integrated dataset that included competitors and prey species collected at the same spatial and temporal scale. As such, this design, along with a causal modeling framework (SEM), allowed us to evaluate community‐wide hypotheses at macroecological scales and identify coarse‐scale drivers of species' range limits. Our study supports iRLT and underscores the need to consider direct and indirect mechanisms for studying range dynamics and species' responses to global change.

Aligning quantitative vegetation classification and landscape scale mapping: updating the classification approach of the Regional Ecosystem classification system used in Queensland

Vegetation classification systems form a base for conservation management and the ecological exploration of the patterns and drivers of species’ distributions. A standardised system crossing administrative and geographical boundaries is widely recognised as most useful for broad-scale management. The Queensland Government, recognising this, uses the Regional Ecosystem (RE) classification system and accompanying mapping as a state-wide standardised vegetation classification system. This system informs legislation and policy at local, state and national levels, underpinning decisions that have wide-ranging implications for biodiversity and people’s livelihoods. It therefore needs to be robust from a scientific and legal perspective. The current approach in the RE system for identifying vegetation communities relies on expert-based class definition procedures. This is in contrast to best practice, which is based on quantitative procedures. This paper discusses the RE system in a global context and outlines the updated approach that incorporates quantitative class definition procedures, synthesises the research behind the updated approach and discusses its implications and implementation.

Drivers of species’ use for fuelwood purposes: A case study in the Brazilian semiarid region

Efforts have been taken to understand the criteria for selecting plants for various purposes. However, regarding fuelwood use, there is still disagreement in the literature concerning the processes that drive their use. As a result, wood quality and its availability in the environment have alternated in studies as the main determinants in terms of predictive power. The main goal of this study was to understand the influence of these factors on the behavior of fuelwood gatherers. For this purpose, perceptions of the fuel quality and its availability in the environment were accessed, as well as the availability measured by ecological tools. The study was conducted in a rural community in the Northeast of Brazil, and it was found that a small range of five native species is considered more important locally. We found that biological, as well as ecological aspects influence the use of fuelwood.

Spatial Patterns and Drivers of Angiosperm Sexual Systems in China Differ Between Woody and Herbaceous Species.

Plant sexual systems play an important role in the evolution of angiosperm diversity. However, large-scale patterns in the frequencies of sexual systems (i.e. dioecy, monoecy, and hermaphroditism) and their drivers for species with different growth forms remain poorly known. Here, using a newly compiled database on the sexual systems and distributions of 19780 angiosperm species in China, we map the large-scale geographical patterns in frequencies of the sexual systems of woody and herbaceous species separately. We use these data to test the following two hypotheses: (1) the prevalence of sexual systems differs between woody and herbaceous assemblies because woody plants have taller canopies and are found in warm and humid climates; (2) the relative contributions of different drivers (specifically climate, evolutionary age, and mature plant height) to these patterns differ between woody and herbaceous species. We show that geographical patterns in proportions of different sexual systems (especially dioecy) differ between woody and herbaceous species. Geographical variations in sexual systems of woody species were influenced by climate, evolutionary age and plant height. In contrast, these have only weakly significant effects on the patterns of sexual systems of herbaceous species. We suggest that differences between species with woody and herbaceous growth forms in terms of biogeographic patterns of sexual systems, and their drivers, may reflect their differences in physiological and ecological adaptions, as well as the coevolution of sexual system with vegetative traits in response to environmental changes.

Accounting for uncertainty in assessing the impact of climate change on biodiversity hotspots in Spain

Our limited understanding of the complexity of nature generates uncertainty in mathematical and cartographical models used to predict the effects of climate change on species’ distributions. We developed predictive models of distributional range shifts of threatened vertebrate species in mainland Spain, and in their accumulation in biodiversity hotspots due to climate change. We considered two relevant sources of climatological uncertainty that affect predictions of future climate: general circulation models and socio–economic scenarios. We also examined the relative importance of climate as a driver of species’ distribution and taxonomic uncertainty as additional biogeographical causes of uncertainty. Uncertainty was detected in all the forecasts derived from models in which climate was a significant explanatory factor, and in the species with taxonomic uncertainty. Uncertainty in forecasts was mainly located in areas not occupied by the species, and increased with time difference from the present. Mapping this uncertainty allowed us to assess the consistency of predictions regarding future changes in the distribution of hotspots of threatened vertebrates in Spain.

Identification of drivers of landscape distribution of forest orchids using germination experiment and species distribution models.

The family of orchids involves a number of critically endangered species. Understanding of drivers of their landscape distribution could provide a valuable insight into their decline. Our objectives were to develop models predicting distribution of selected orchid species-four co-occurring forest orchid species, Cephalanthera rubra, Epipactis atrorubens, E. helleborine, and Neottia nidus-avis-at a landscape scale using a wide range of habitat characteristics. Subsequently, we compared the model predictions with species occurrence and the results of the field germination experiment while considering two germination stages-asymbiotic (early stage) and symbiotic. And finally, we attempted to identify possible drivers of species' landscape distribution (i.e., dispersal, availability of habitat patches, or fungal associates). We have discovered that different habitat characteristics determined the distribution of different orchids. The species also differed in terms of availability of suitable habitat patches and patch occupancy (the highest being E. atrorubens with 80%). Landscape distribution of the species was primarily restricted by the availability of fungal associates (the most important factor for C. rubra) and by the availability of suitable habitat patches (the most important in case of N. nidus-avis). Despite expected easy dispersal of spores, orchid distribution seems to be limited by the availability of fungal associates in the landscape. In contrast, the availability of orchid seeds does not seem to limit their distribution. These results can provide useful guidelines for conservation of the studied species.

A habitat risk assessment and breeding site projection for Slackwater darter (Etheostoma boschungi) (Percidae) in Alabama and Tennessee USA

Catalysts for species decline are difficult to identify and are rarely single. The use of predictive models that incorporate multiple factors have proven useful in recognizing major drivers of species declines in multiple systems. In this study, a geographic information systems model was used to evaluate factors contributing to the decline of the Slackwater Darter throughout its distributional range. The species has suffered a precipitous decline, and it is essential to understand conservation threats and to prioritize habitat for restoration and protection. Variables incorporated into the habitat model included land use/land cover, soil and geographic descriptions, hydrologic variables, and others. Habitat where the species has become extirpated or undetectable by conventional sampling was compared to extant populations to identify factors contributing to population loss. Land cover and farm pond density were identified as potential drivers of extirpation. Finally, a model of suitable habitat was used to predict spawning areas in Limestone Creek, where the species was recently discovered. This model identified eight new spawning sites for Slackwater Darter in the system where breeding sites were unknown, and was 44% accurate in its predictive capabilities; however, positive detections were noticeably clustered and indicate the likely influence of some untested variable. With the information provided by this analysis, a framework for habitat protection can be created to assist with restoration of this species.

When phenotypes fail to illuminate underlying genetic processes in fish and fisheries science

Abstract Advances in genetic and genomic technologies have become widely available and have potential to provide novel insights into fish biology and fisheries science. In the present overview, we explore cases for which genomic analyses have proven instrumental in the rejection of hypotheses that have been well-motivated based on phenotypic and ecological properties of individuals and populations. We focus on study systems for which information derived using genomic tools contradicts conclusions drawn from traditional fisheries science methodologies and assumptions. We further illustrate the non-intuitive interplay of genomics and ecology in Atlantic salmon (Salmo salar L.) owing to the recently detected genetic architecture of age at maturity in this species. To this end, we explore a salmon management strategy applied in Québec, Canada and find that management measures aimed at protecting large, old individuals can lead to unexpected and undesired outcomes. Finally, we envisage ways in which genomic tools could be used more effectively in contemporary fisheries research and how their application could illuminate the ecological and evolutionary drivers of species and ecosystem dynamics.

Water Availability Influences Species Distributions on Serpentine Soils

Serpentine soils are highly variable environments, but how this variability contributes to maintaining distinct distributions of species at small scales is unclear. We studied parapatric populations of Layia jonesii A. Gray, a rare serpentine endemic, and L. platyglossa (Fisch. & C. A. Mey) A. Gray, a wide-spread species, to understand how edaphic variability and competitive interactions maintain co-occurring, somewhat interfertile, relatives within serpentine environments. We analyzed soil characteristics along a serpentine hillside where L. jonesii and L. platyglossa are restricted to the top and bottom of the hillside, respectively. We used reciprocal transplants to investigate fitness differences on the hillside and a competition experiment to determine if species interactions also restrict the distribution of L. jonesii. The soil analyses showed the top of the hill is drier than the bottom, yet both locations are chemically indistinguishable. Layia platyglossa is better adapted to the edaphic conditions at the bottom of the hillside and L. jonesii is sensitive to competition in the bottom region. Differences in soil water availability, and potentially its effect on the competitive environment, likely contributes to maintaining the distribution of these species. Ionic stress is often cited as a driver of species' distributions within serpentine environments, but our research suggests soil water availability can be an important factor influencing the distribution of interfertile species on serpentine at small scales. Annual variation in precipitation may drive local shifts in species distributions through alteration of the competitive environment.

Disentangling biotic interactions, environmental filters, and dispersal limitation as drivers of species co‐occurrence

A key focus in ecology is to search for community assembly rules. Here we compare two community modelling frameworks that integrate a combination of environmental and spatial data to identify positive and negative species associations from presence–absence matrices, and incorporate an additional comparison using joint species distribution models (JSDM).The frameworks use a dichotomous logic tree that distinguishes dispersal limitation, environmental requirements, and interspecific interactions as causes of segregated or aggregated species pairs. The first framework is based on a classical null model analysis complemented by tests of spatial arrangement and environmental characteristics of the sites occupied by the members of each species pair (Classic framework). The second framework, (SDM framework) implemented here for the first time, builds on the application of environmentally‐constrained null models (or JSDMs) to partial out the influence of the environment, and includes an analysis of the geographical configuration of species ranges to account for dispersal effects.We applied these approaches to examine plot‐level species co‐occurrence in plant communities sampled along a wide elevation gradient in the Swiss Alps. According to the frameworks, the majority of species pairs were randomly associated, and most of the non‐random positive and negative species associations could be attributed to environmental filtering and/or dispersal limitation. These patterns were partly detected also with JSDM. Biotic interactions were detected more frequently in the SDM framework, and by JSDM, than in the Classic framework. All approaches detected species aggregation more often than segregation, perhaps reflecting the important role of facilitation in stressful high‐elevation environments.Differences between the frameworks may reflect the explicit incorporation of elevational segregation in the SDM framework and the sensitivity of JSDM to the environmental data. Nevertheless, all methods have the potential to reveal general patterns of species co‐occurrence for different taxa, spatial scales, and environmental conditions.

Understanding predation risk and individual variation in risk avoidance for threatened boreal caribou.

Predation risk is a driver of species’ distributions. Animals can increase risk avoidance in response to fluctuations in predation risk, but questions remain regarding individual variability and the capacity to respond to changes in spatial risk across human‐altered landscapes. In northeast British Columbia, Canada, boreal caribou populations declined as roads and seismic lines have increased, which are theorized to increase gray wolf predation. Our goal was to model risk and to evaluate individual variability and the development of risk perception by examining individual risk avoidance in response to reproductive status and age. We used locations from collared caribou and wolves to identify landscape features associated with the risk of a potential wolf‐caribou encounter and risk of being killed given an encounter. We built resource selection functions to estimate individual responses to risk. We used general linear regressions to evaluate individual risk and linear feature avoidance as a function of age and reproductive status (calf or no calf). Linear features increased the risk of encounter. Older caribou and caribou with calves demonstrated stronger avoidance of the risk of encounter and roads, but weaker avoidance in late summer to the risk of being killed relative to younger and calf‐less individuals. Mechanisms explaining the inverse relationships between the risk of encounter and risk of being killed are uncertain, but it is conceivable that caribou learn to avoid the risk of encounter and roads. Responses by females with vulnerable calves to the risk of encounter and risk of being killed might be explained by a trade‐off between these two risk types and a prioritization on the risk of encounter. Despite the capacity to alter their responses to risk, the global decline in Rangifer populations (caribou and wild reindeer) suggests these behaviors are insufficient to mitigate the impacts of anthropogenic disturbances.

Landscape context explains changes in the functional diversity of regenerating forests better than climate or species richness

AbstractAimA rich literature on forest succession provides general expectations for the steps forests go through while reassembling after disturbance, yet we still have a surprisingly poor understanding of why the outcomes of forest recovery after logging (or other disturbances) vary so extensively. In this paper, we test the hypothesis that regional species pool, system productivity, climate and landscape structure are important drivers of forest reassembly outcomes.LocationTransect 1,500 km in length along the east coast of Australia.Time periodSurvey of 50‐ to 60‐year‐old rain forest regrowth and primary forest conducted in 2012 and 2013.Major taxa studiedRain forest plants.MethodsIn this study, we compare species and functional diversity patterns in pairs of remnant and regrowth (‘secondary’) rain forests spread across a 1,500 km climate and productivity gradient along the east coast of Australia. Our controlled natural experiment was designed to test the importance of regional species pool, system productivity, climate and landscape structure as drivers of species and functional diversity in regenerating forests. Notably, our study design allowed us to hold soil type, general forest type and disturbance history relatively constant in order to test our hypotheses effectively.ResultsCounter to expectations, few tested factors were strongly related to the recovery of species or functional diversity in regenerating Australian rain forests. The extent of local forest fragmentation was the only factor strongly related to differences between regrowth forests and primary forest remnants, and then only for functional diversity. We found no evidence that species diversity is a reasonable proxy for, or potential driver of, functional diversity patterns.Main conclusionsOur findings suggest that forest functional recovery over decades is influenced more by regional landscape context than distinct assembly processes operating across climate and productivity gradients.

Drivers of species' role in avian seed-dispersal mutualistic networks.

The mutualistic interaction between frugivore birds and the fruiting plants they disperse presents an asymmetric interaction pattern, with some species having a more important role (i.e. being essential) for maintaining the structure and functioning of the interaction network. The identification of the biological characteristics of these species is of major importance for the understanding and conservation of seed-dispersal interactions. In this study, I use a network approach and avian seed-dispersal networks from 23 different geographical areas to test five hypotheses about species characteristics determining the structure of the assemblage. I expected (i) large birds to forage on a large number of fruits and (ii) large fruits to be dispersed by few bird species (because of morphological constraints), and (iii) highly energetic fruits to be dispersed by more bird species (in accordance with optimal foraging theory). Besides the number of interacting partners, I also expected (iv) large birds and (v) small energetic fruits to be important for the maintenance of the structure of the interactions in seed-dispersal networks. As species that are closely related are more likely to be similar to each other, I performed phylogenetically corrected analyses to account for this data dependence. Although bird size was not associated to species important in the maintenance of the structure of the seed-dispersal community, I identified that bird species whose diet was strongly dependent on fruits were important for the structure of the network. Regarding the plants, I found that large fruits were dispersed by fewer species, but the most important attribute to predict the role of a fruit was its energy content (higher energy, more bird species dispersing the plant, but low-energy fruits being of conservation concern because they are dispersed by specific species). The results of this study suggest that the role of the species in seed-dispersal assemblages seems to be determined by the role of the species as consumers (frugivory degree for animals) or by their nutritional inputs (energy content for fruits) rather than by morphological constrains.

Community transcriptomics, genomics and the problem of species co‐occurrence

Summary Genomic and transcriptomic information has been largely leveraged in ecological investigations of primarily model species and their relatives. The amount of infrastructure and financing necessary for such investigations has meant that analyses of assemblages full of non‐model species were impossible. These barriers have rapidly eroded over the past few years to the point where community ecology will soon see a large influx of genomic and transcriptomic investigations. The hope is that such studies will greatly refine or even transform our inferences regarding the distribution and co‐occurrence of plant species. This special feature presents five studies that use cutting‐edge ‘omic concepts and approaches in assemblages of non‐model species. All studies focus on the drivers of species co‐occurrence. The work blends functional phylogenomic, differential gene expression and metagenomic approaches in observational and experimental frameworks. Two studies demonstrate of functional phylogenomic approaches that can be used in conjunction with neighbourhood demographic models to understand the role of neighbouring heterospecific species on focal tree demography. Two additional papers demonstrate how genes are differentially expressed in different competitive contexts and how this is related to species coexistence. The final study uses a metabarcoding approach to document whether the incredible levels of soil fungal and tree biodiversity in the tropics are linked. Synthesis. The articles presented in this special feature clearly demonstrate that ‘omic toolkits can now be readily integrated into community ecology. While there are still many obstacles facing this integration, it seems clear that the amounts and types of data provided by such approaches will lead to a rapid transformation in how we study plant communities.

Global patterns and drivers of species and trait composition in diatoms

AbstractAimTo examine species and trait composition in stream diatoms along environmental, climatic and spatial gradients and to ascertain if the use of different levels of biological organization is beneficial for investigating global environmental changes and the role of history in structuring communities.LocationGlobal cover with datasets from the Antilles, France, Finland, New Zealand, La Réunion and the United States.MethodsWe related diatom species composition, guild composition, total richness and richness across guilds to environmental, climatic and spatial variables. We used non‐metric multidimensional scaling (NMDS) with environmental variable fitting, redundancy analysis (RDA) with variation partitioning, analysis of similarities and linear mixed models as statistical tools.ResultsSpecies composition differed significantly among the study regions, while the differences in guild composition were less pronounced. US and French streams shared a large number of species, whereas islands shared only a few species with continents. For species composition, all predictors showed significant relationships with diatoms but pH, longitude, annual temperature and precipitation had the strongest impact. Variation partitioning revealed that the local environment outperformed climatic and spatial variables. For guild composition, there was a substantial overlap across regions in NMDS. The results from RDA demonstrated, however, that guild composition was better explained than species composition, especially by environmental variables. Both species and guild richness were significantly correlated with most predictors. Notably, species richness scaled positively with latitude.Main conclusionsDiatom species and guild composition varied substantially in response to local environment and climatic and spatial variables indicating both environmental and historical effects. Species composition discriminated the geographical regions better, while guild composition detected the environmental gradients better. This emphasizes the need to examine different levels of organization to gain a deeper understanding of the roles of environment versus history in structuring communities. These findings suggest that diatom species distributions are under strong microevolutionary constraints. Conversely, guild distributions are less dependent on historic factors and are driven primarily by the environment, which makes them better suited for research on global environmental change.

Fine‐scale hydrological niche differentiation through the lens of multi‐species co‐occurrence models

Summary Theory suggests spatial heterogeneity can facilitate species co‐occurrence at fine scales, but environmental data are rarely collected at sufficiently high resolution to test this empirically. Whilst there is emerging evidence that subtle variation in soil hydrology represents a fundamental fine‐scale niche axis within plant communities, this is largely derived from studies of soil hydrology in isolation from other environmental factors. We assessed the comparative importance of fine‐scale hydrological niche differentiation for species co‐occurrence using a high‐resolution study of soil hydrology and other edaphic variables, coupled with a long‐term (24 years) data set of herbaceous plant plots in a heathland community in south‐east Australia. For the analysis, we employed novel latent variable models (LVMs), which offer an explicit, model‐based approach to partitioning out the different drivers of species co‐occurrence patterns. Whilst the regression component of an LVM models the species‐specific environmental responses, the latent variable component can be used to identify residual patterns of co‐occurrence, which may be attributable to unmeasured factors and/or biotic interactions. Relative to a host of plant resources, non‐resource factors and ‘unmeasured’ latent variables, soil hydrology emerged as the best predictor of negative co‐occurrences within the community, with the dominant species exhibiting strongly differentiated responses across a comparatively narrow moisture gradient. Nevertheless, strong species‐specific responses to environmental variability only emerged at scales greater than those at which plants may be expected to compete for resources, throwing doubt on the direct role of spatial heterogeneity as a mechanism for local‐scale coexistence. Synthesis. This study confirms the vital role of hydrological niches for the maintenance of within‐community plant diversity, but also highlights the need for more rigorous analysis of scale dependencies to better understand the underlying coexistence mechanisms at play. In addition, it illustrates the inferential gains made possible with model‐based approaches to the analysis of species co‐occurrence. r code illustrating model fitting and inference is provided as a supplement.

Separating sensitivity from exposure in assessing extinction risk from climate change.

Predictive frameworks of climate change extinction risk generally focus on the magnitude of climate change a species is expected to experience and the potential for that species to track suitable climate. A species' risk of extinction from climate change will depend, in part, on the magnitude of climate change the species experiences, its exposure. However, exposure is only one component of risk. A species' risk of extinction will also depend on its intrinsic ability to tolerate changing climate, its sensitivity. We examine exposure and sensitivity individually for two example taxa, terrestrial amphibians and mammals. We examine how these factors are related among species and across regions and how explicit consideration of each component of risk may affect predictions of climate change impacts. We find that species' sensitivities to climate change are not congruent with their exposures. Many highly sensitive species face low exposure to climate change and many highly exposed species are relatively insensitive. Separating sensitivity from exposure reveals patterns in the causes and drivers of species' extinction risk that may not be evident solely from predictions of climate change. Our findings emphasise the importance of explicitly including sensitivity and exposure to climate change in assessments of species' extinction risk.

Effects of variable selection on modelling habitat and potential distribution of the Andean bear in Bolivia

Species distribution models are used in ecology and conservation biology to draw inferences about the drivers of species' ranges. However, poor conceptual background, environmental variable selection, and algorithm selection can contribute to misleading model predictions. We assessed the effects of environment variable selection and compared statistical performance and output maps of correlative resource- and biotope-based models for estimating the habitat and potential distribution of the Andean bear (Tremarctos ornatus) in Bolivia's Tropical Andes. The resource-based approach estimated bear habitat using 7 resources associated with 3 ecological functions: feeding, shelter, and access to water. In contrast, the biotope model described the habitat by applying 11 environmental predictors related to topography, vegetation, and human activities. Both models performed equally well overall and better than random, with shelter as the most influential variable for the resource model and Yunga forest for the biotope model. However, discrepancies in the extent and arrangement of predicted bear distribution between models differed and emphasized the effect of variable selection, which could influence the delineation of conservation areas for this species. We suggest using a resource-based approach when modelling species distribution because of the more direct relationship to the species investigated and greater ease of interpreting results.