Biological Diversity: Frontiers in Measurement and Assessment

Ecological integrity assessment as a metric of biodiversity: are we measuring what we say we are?
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Biological Diversity - Frontiers In Measurement, Assessment - A. Magurran, B. M...

New Releases. Biological Diversity : Frontiers in Measurement and Assessment. Description Biological Diversity provides an up to date, authoritative review of the methods of measuring and assessing biological diversity, together with their application. The book's emphasis is on quantifying the variety, abundance, and occurrence of taxa, and on providing objective and clear guidance for both scientists and managers. This is a fast-moving field and one that is the focus of intense research interest. However the rapid development of new methods, the inconsistent and sometimes confusing application of old ones, and the lack of consensus in the literature about the best approach, means that there is a real need for a current synthesis.

Biological Diversity covers fundamental measurement issues such as sampling, re-examines familiar diversity metrics including species richness, diversity statistics, and estimates of spatial and temporal turnover , discusses species abundance distributions and how best to fit them, explores species occurrence and the spatial structure of biodiversity, and investigates alternative approaches used to assess trait, phylogenetic, and genetic diversity.

The final section of the book turns to a selection of contemporary challenges such as measuring microbial diversity, evaluating the impact of disturbance, assessing biodiversity in managed landscapes, measuring diversity in the imperfect fossil record, and using species density estimates in management and conservation.

Review quote probably the best single-volume book on biodiversity available to date. About Anne E. Magurran Anne Magurran is a professor at the University of St Andrews, Scotland and an ecologist interested in the measurement, conservation and evolution of biological diversity. Brian McGill is a professor at the University of Arizona. He is interested in understanding and measuring how human-caused global change especially global warming and land cover change affect communities of organisms.

He works with large datasets from many different types of organisms and locations. While it is certain that animal species assemblages are influenced by vegetation, among other drivers, what is not so clear is the nature of the relationship between vegetation and diversity, and the degree to which the former can serve as a surrogate for the latter. The published evidence suggests that the relationship is at best scale-specific, taxon-specific, and of limited value at the scale at which habitat management is typically practiced.

A third question is whether focal biodiversity is representative of biological diversity in an area. The use of so-called focal biodiversity to focus conservation planning efforts is intertwined with the issue of biodiversity indicators by Parrish et al. But is the assumption that one or a small number of focal species reliably represents much of the regional biota empirically supported? Andelman and Fagan evaluated patterns of spatial co-occurrence between different biodiversity indicator species and regional biota in three conservation databases representing different scales and regions, and found that none of the various schemes e.

Prendergast et al. Cushman et al. The assumption that the response of indicator species will be typical of the response of many other species is not supported by evidence Lindenmayer et al. In order to use an indicator species or taxon as a reliable proxy for the presence, abundance, or richness of other taxa or for particular environmental conditions, it is essential to quantify the relationship between the indicator and what it is supposed to represent Lindenmayer and Likens No such relationship has been demonstrated for vascular plants or other taxa at sites evaluated by ecological integrity assessment methods.

Finally, there remains the question of whether multi-metric indices of site condition are correlated with diversity of a range of taxa. Site-condition multi-metric indices standardize, weight, and combine a variety of habitat-based variables in a single score, as distinct from habitat-based biological diversity surrogates comprising individual habitat measures Lindenmayer et al. There have been few tests of how diversity Oliver et al.

Kwok et al. Index values from both ecological scorecards were weakly and inconsistently related to arthropod diversity in all three orders. Similarly, Oliver et al. McGoff et al.

Biological Diversity

Although in some cases selected macroinvertebrate metrics were significantly correlated with habitat scores, there was no relationship between overall lakeshore index scores and macroinvertebrate diversity metrics across all the regions McGoff et al. Taken together, these findings suggest that a scorecard approach is likely to be of limited use in representing patterns of biological diversity across multiple taxonomic groups, perhaps because biological processes operate at different scales for the different species using a site, and that empirical validation is essential.

According to Parrish et al. Substantial bias or high statistical variability could limit their usefulness, irrespective of their linkage to biodiversity. In this section we examine two issues related to statistical robustness, namely systematic bias and statistical power. Systematic bias is a concern with multi-metric indices, such as the ecological scorecard, that convert raw measurements of diverse variables into scores and weight and combine them into a single score.

The indices can be subject not only to measurement error and observer bias in collecting the raw data Gorrod et al.

In a study of uncertainty associated with site-condition scores for two Australian multi-metric indices based on vegetation composition and structure and used to predict value of sites for diversity of a range of taxa, Gorrod et al. The resulting bias in site scores clearly could have significant implications for conservation outcomes, especially in a market-based context Gorrod et al.

Dolph et al. In addition, as pointed out for multi-metric indices in general Suter ; Efroymson et al. Thus, the inherent statistical properties of multi-metric indices in general tend to confound their results. A second issue concerns the statistical power needed to detect trends over time. These statements notwithstanding, distinguishing real change from natural variation e. A critical issue is the level of survey effort required to achieve enough precision to identify quantify trends MacKenzie et al.

For a single site, a key is the length of the time series and the precision of measurement at each point Magurran et al. Power calculations or determination of confidence intervals can be used to estimate the level of survey effort needed Magurran et al. Consideration of statistical power is not mentioned as part of ecological integrity assessment methodology in Faber-Langendoen et al. Without explicit design for adequate statistical power, there is no assurance that the ecological integrity index can distinguish among directional change in diversity, natural variation over time, and measurement error.

In sampling plants and animals alike, detectability and spatial variation are well known issues in statistical methodology Yoccoz et al. These metrics have been the core of the approach from its earliest development by Wilhelm , Taft et al. In this section we examine concerns about various sources of error in the vegetation methodology of ecological integrity assessments.

One concern is bias in estimates of plant species richness. Species richness measures are sensitive to the number of individuals sampled, and to the number, size, and spatial arrangement of samples Gotelli and Colwell The single-plot sampling of vascular plant species richness at a wetland site in Faber-Langendoen et al. Without multiple plots, it is impossible to determine the mean and variance of index values among plots Magurran and thus to examine whether the sample data are actually representative.

Any individual plot cannot characterize site variation in plant communities accurately Bourdaghs et al. However, increasing the area sampled has its own pitfalls, because increasing the size of the sampling area also increases species richness estimates Krebs as well as the mean coefficient of conservatism and floristic quality index Matthews et al.

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Species richness of an entire site, rather than one plot in the site, is often used in floristic quality assessment Nichols , following the protocols defined by Wilhelm and Ladd and cited by Faber-Langendoen et al. In such a case, variation in site size can lead to variation in estimates of species richness and related measures such as mean coefficient of conservatism.

A second concern is sampling error in detecting plant species presence. Vegetation measures are highly susceptible to detectability problems. Numerous studies have empirically tested detectability of plants Chen et al. In surveys of plant species presence and distribution, detection error is the norm rather than the exception: in some surveys a high percentage of species e. If survey designs do not explicitly incorporate detection probability, serious bias in estimates of species distributions and species richness can result.

Accommodation for sampling error is not mentioned by Faber-Langendoen et al. Another concern is subjectivity observer variability in assigning coefficients of conservatism. Coefficients of conservatism assigned to plant species are qualitative and subjectively determined on the basis of professional judgment, rather than objectively assigned. Because of this subjectivity, the metric is highly susceptible to inter-observer variability and bias.


Land and Chiarucci specifically tested inter-observer variation in coefficients of conservatism, and found that scores given by different experts were not consistent and resulted in derived floristic quality indices that were statistically different. Inter-observer variation within NatureServe species databases that contain pre-assigned coefficients of conservatism apparently has not been examined by Faber-Langendoen et al. There are other potential problems with the coefficient of conservatism in addition to those mentioned above.

The metric is computed by averaging predetermined conservatism values for all the native species observed at a site, and then assigning the resulting average value to one of a few ordinal categories see Appendix 3. One problem is that the coefficient of conservatism is based on species occurrence at a single site, which may or may not represent the larger area to which inference is made. The absence of any accounting of spatial variability associated with it Magurran limits its usefulness for broader assessments.

In addition, the metric is based on observed rather than actual occurrence, with no accommodation for sampling error in detecting plant species presence see above discussion of this topic. Also, sensitivity of the metric to cut-off points used to distinguish categories is a concern that is yet to be investigated.

Finally, even under ideal circumstances, the metric would be at best an ambiguous measure of native plant diversity: a large mean C at a site can be obtained when just one or a few high-value species are observed, and a much smaller mean C can be obtained from the range of conservatism values resulting when numerous species are observed.

Community ecology: Diversity, stability, function

Among other things this means the coefficient of conservatism can actually be inversely related to diversity of native plants at a site. The foregoing issues affect the usefulness of ecological integrity assessment in decision making.

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Assessments are held by some to be useful in land-use planning, in the prioritization of sites for mitigation Faber-Langendoen et al. One example would be a comparative assessment of sites for land-use decisions in environmental markets and trading schemes, such as those described by Salzman and Ruhl It is unclear how an ecological integrity assessment could play these important roles.

Whatever else is involved, decision making is based on a comparison of alternative management choices against value-based criteria, so as to allow one to recognize value differences among alternatives. From the regulatory point of view, it is important that objectives are clear and acceptable, and measures allow discrimination among alternative outcomes on the basis of the scientific method, including hypothesis testing.

For example, any metric used by federal agencies in a context of the U. In the ESA context, 25 of 32 listing decisions reviewed by the court in were set aside, many because the U. It is unlikely that an ecological integrity assessment as currently described and justified would meet the necessary criteria to withstand such scrutiny.

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Magurran and Stuart E. IV Alternative measures of diversity The book's emphasis is on quantifying the variety, abundance, and occurrence of taxa, and on providing objective and clear guidance for both scientists and managers. Species density as a topic of study Artenreichtum -- Methoden und Technik. The University of Sydney.

Biodiversity measures can indeed provide valuable empirical metrics for assessing change Buckland et al. But one size definitely does not fit all—there is no universally appropriate generic monitoring program, and an optimal design must always be tailored to a particular situation and purpose Ferraz et al.

Different indices measure different aspects of diversity, so it is essential to define the objectives of any monitoring program clearly in order to choose an appropriate index Yoccoz et al. Recent biometric research on how to measure and monitor biological diversity has proliferated, and includes work on appropriate statistical criteria, new indices and methods, design of monitoring programs, and methodological comparisons Yoccoz et al.

While it is beyond the scope of this paper to review the statistical literature, here we cover some important considerations in designing statistically robust surveys of biological diversity. The size of the area and the number of taxa of interest affect the design of a monitoring program. For monitoring change in biological diversity over time in a wide heterogeneous region, Buckland et al. Yoccoz et al. Lamb et al. They evaluated 13 diversity indices of 3 types traditional, community [species] intactness based on occurrence, community [species] intactness based on abundance against several criteria such as sensitivity to detection error and power to detect trends, in six ecological scenarios.

Monitoring of single sites with unique habitats or rare species could be treated by collecting data only on the specialist species of interest Buckland et al. Any serious attempt to monitor biological diversity should address the two main sources of error, detectability and spatial variation or environmental heterogeneity Yoccoz et al.

Detection error results when some individual organisms or species evade detection during a survey. Distance sampling and capture—recapture are two types of methodology that can be used to estimate detection probabilities associated with count statistics Yoccoz et al. While these methods may be more appropriate for one or a few sites, Pollock et al.

Survey errors result when inferences about a larger area are not based on an appropriate spatial design for sampling smaller sites. A particular problem is the use of subjectively chosen sampling sites, which can lead to biased estimates of diversity at the larger scale Yoccoz et al.

A survey that is properly designed can quantify the uncertainty and precision of diversity measures, and thus their reliability Buckland et al. Determining the survey effort needed for sufficient precision to quantify change over time is key to producing unbiased results Buckland et al. The effort needed for a given level of precision can be estimated in advance by power calculations. How long the time series is, and how precise the measurements are at each time, are important factors at a single site Magurran et al.

In addition, the number of plots, plot size, and frequency of sampling also determine precision for multi-site sampling of a larger area Magurran et al. In a case study, Nielsen et al. Dornelas et al. Broad applicability and cost-effectiveness are both important considerations in implementing large-scale biological diversity monitoring programs.

Use of common monitoring designs across global regions was recommended by Buckland et al. They suggested designing surveys such that entry at various levels is possible, thus allowing nations with fewer resources to take part, perhaps with design modifications such as a subset of species, lower sampling rates, and simpler methods Buckland et al. If the monitoring program and sampling protocols are well-designed and well-coordinated, professionals are not necessarily needed to collect all data Magurran et al.

With adequate training and supervision, non-professional volunteers can be as proficient as professionals in many tasks, as shown by a number of quantitative evaluations of volunteer-collected data [e. The investigation of ecological integrity addresses a critical need for usable information to help stem the accelerating loss of biological diversity. But we believe there are serious and unaddressed concerns about the suitability of ecological integrity assessment as described by Faber-Langendoen et al.

For example, the vegetation sampling methods—including protocols for estimating plant species richness—are susceptible to sampling error and observer bias. Vascular plant diversity, which is used as a key proxy for biological diversity, is not a reliable indicator of diversity of other taxa and has no demonstrated relationship to measures of cross-taxon diversity at a site.

The empirical studies discussed earlier point to serious difficulties in using these indicators as surrogates for biological diversity.

In fact, patterns of biological diversity in landscapes, however they are represented, are too complex to be represented effectively with these indicators. And there is no evidence that the ecological integrity assessment protocols as currently designed can resolve problems of detectability and environmental heterogeneity in distinguishing natural variation from ecological change over time.

A related issue that merits discussion is whether expansion of the current index to include taxa other than vascular plants would improve its suitability for measuring biological diversity. Proponents hold that the ecological integrity index can be extended as necessary to include other components of the biota such as birds or amphibians Faber-Langendoen et al. In our opinion, the inclusion of additional taxa would still leave ecological integrity assessment in its present form an inadequate measure of biological diversity.

First, none of the current sampling protocols described in Faber-Langendoen et al. Second, as discussed previously, no taxon or group has been shown empirically to be a reliable proxy for diversity of a range of taxa. Third, the process of converting raw data to numerical scores and then weighting them in an assessment can introduce bias Gorrod et al. A noteworthy point is that in the various publications describing ecological integrity assessment, proponents of the methodology Parrish et al.

Measuring diversity of biota and estimating the conservation importance of any given site requires more than a few proxy variables or indicator species. There is a large body of recent literature on diversity theory reviewed by Bestelmeyer et al. Many factors interact to determine animal diversity patterns, including competition, territoriality, dispersal, predation, physical environmental variation especially landscape-scale gradients and patchiness , and historical variation in biogeography Bestelmeyer et al.

For individual species, distribution patterns across scales are determined by habitat requirements, dispersal capabilities, and the size and location of the geographic range. Thus, measuring habitat variables at a given site is not sufficient for monitoring population viability or abundance of even a single species MacKenzie et al. By extension, scorecards at individual sites are not sufficient to explain distribution patterns across sites.

For biological diversity as a whole, patterns of species diversity are strongly influenced by spatial heterogeneity in a scale-dependent way Williams et al. However, ecological integrity assessment largely ignores these issues, and assumes instead that habitat and landscape features at individual sites fully account for diversity of biota in a predictable way. Inaccurate estimates of the conservation value of a site could easily result from this unproven assumption.

While we applaud efforts to address important environmental issues with an ecological integrity assessment, further development is needed, especially in the areas of technical refinements and validation. In its current form the methodology is of limited use in providing meaningful metrics of biological diversity, and lacks a foundation in ecological and statistical principles. At a minimum, sources of sampling error—especially organism detectability and spatial variation—should be investigated and accounted for, along with the potential for bias and loss of essential information due to condensing such a large amount of disparate data into a single index.

Further empirical investigation is needed to quantify how well the indicator variables and metrics are correlated with the particular ecological processes or environmental conditions they are supposed to represent Lindenmayer and Likens ; Lindenmayer et al. Relationships between indicator variables and biological diversity attributes should be quantified to determine the transferability of a given indicator to the biotic component for which it is used as a proxy Lindenmayer and Likens More effective development of a metric will require greater collaboration of statisticians, landscape ecologists, and theoretical ecologists.

A broader acceptance will necessitate evidence from the refereed scientific literature that ecological integrity assessment actually measures biological diversity. In particular, the methodology should be subjected to ongoing critical review in the literature to a much greater extent than it has been to date. Empirical studies such as those suggested in the previous paragraph should be undertaken, with results published in peer-reviewed journals. In addition to a more thorough investigation of the assessment in relation to biological diversity than this paper permits, the linkage between ecological integrity assessment and ecological structure and function should also be investigated, for example by identifying criteria with respect to the use of various proxy variables, by which to assess index performance; conducting a comprehensive literature review to identify evidence for each criterion; and subjecting the evidence to formal analysis.

With improvements in methodology and thoughtful choices, measuring biological diversity can produce unbiased results that reflect real change rather than sources of error, and provide the accurate assessments necessary for effective conservation decisions. John Sauer provided an incisive review of the manuscript, and two anonymous reviewers provided useful comments. Skip to main content Skip to sections.

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Advertisement Hide. Download PDF. Ecological integrity assessment as a metric of biodiversity: are we measuring what we say we are? Open Access. First Online: 27 April Introduction The dynamics and functioning of ecosystems, and hence the ability of ecosystems to provide humans with essential goods and services, depends to a great extent on the diversity of life Cardinale ; Hector ; Tscharntke et al. Ecological integrity assessment An ecological integrity assessment is a multi-metric index in the form of an ecological scorecard Faber-Langendoen et al.

For example, Parrish et al.