88 P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98Fig. 1. Provisional domain tree of biodiversity based on the survey of 125 text documents in English (Kaennel, 1998). Concepts used by various authors to deﬁne biodiversityare in square boxes, related concepts in rounded boxes. Type and direction of conceptual relationships are indicated by arrows. Synonyms and quasi-synonyms are in italics.
P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98 892. Why is it so difﬁcult to reach a consensus on biodiversity has sometimes been used to allude to orthe use of biodiversity indicators? indicate some aspect of environmental quality. If a species or a group of species is a good indicator The complexity of all the aspects of the term bio- for lead contamination, it may not indicate biodiver-diversity is illustrated in Fig. 1. It is obvious that sity, i.e. there may not be a linear correlate to biodi-no single indicator for biodiversity can be devised. versity. It is fundamentally a contamination indicator,Each aspect of biodiversity requires its own indicator. or an environmental indicator (McGeoch, 1998) ratherThe difﬁculties for reaching a consensus on the use than a biodiversity indicator.of biodiversity indicators are manifold. They imply However, “real” biodiversity indicators may bediffering choices for values and measures, which will needed to measure the impact of e.g. lead contami-be discussed here more in detail. nation on biodiversity itself (indicator FOR biodiver- Terms such as biodiversity, indicator or index are sity). Such an assessment is different from measuringnot well deﬁned and their use varies between different the impact of lead on a selected taxonomic group,countries and disciplines. Dismissing research ﬁndings which had been chosen because it is especially sensi-or scientiﬁc reports simply on the grounds of differing tive to lead poisoning (indicator FROM biodiversity).views on the use of particular terms (semantic discrim-ination) would be counterproductive, but study reportsmust clearly state what is meant by the terms used. A 4. Alpha-diversity, or contribution to higherhelpful review on indicator categories for bioindica- scale biodiversity?tion is given by McGeoch (1998). In this paper, the term indicator is used in the sense A second major dichotomy in the value system forof any measurable correlate to the entity to be as- biodiversity indicators is the question of whether thesessed: a particular aspect of biodiversity. species (or allele, or higher taxon unit) diversity of a The most promising and convincing indicators of given area is to be indicated (local, regional or nationalbiodiversity are measurable portions of the entity level), or if the contribution of the biodiversity of thatthat we consider to represent a target aspect of bio- area to a higher scale surface area (regional, national,diversity. The term index is used here in the sense global) is important.of a scaled measure for one or several concordant In the ﬁrst case (alpha-diversity, e.g. species rich-indicators. ness of an ecological compensation area), an indicator ideally has to be a linear correlate to the biodiversity aspect or entity of the surface area in question. Each3. Indicator FOR or FROM biodiversity? species has the same value. In the second case, the value of the measurable units A ﬁrst major source of misunderstanding is, whether of biodiversity (alleles, species, ecosystems) dependsbiodiversity itself is to be indicated, or whether cer- on their rarity or uniqueness with regard to a highertain components of biodiversity are used as indica- level area. A nationally rare or threatened species intors for something else. Until 1990, the search for a local assessment has a higher conservation valuebioindicators had focussed on indicators of “envi- than a common species, because it contributes moreronmental health” or ecological processes such as to regional or national biodiversity than the ubiqui-disturbance, human impact, environmental or global tous species. Thus a biodiversity indicator in the latterchange (Hellawell, 1986; Spellerberg, 1991; Meffe case not only has to count the units (alleles, species,and Carroll, 1994; Dufrene and Legendre, 1997). ecosystems), but it has to value them differently andAfter the world-wide launch of the term biodiversity add the values.at the Rio Convention in 1992, there was a sudden The best known examples are red list species. Forand drastic shift in the published literature towards measuring alpha-diversity, they are not given a valuethe search for indicators of biodiversity itself (Noss, that is greater than any other species in a plot or trap1990; Gaston and Williams, 1993; Gaston, 1996a; sample, but for measuring the conservation value of aPrendergast, 1997). Since then, however, the term plot, their higher contribution to regional, national, or
90 P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98even global biodiversity has to be recognised. Raised dex, the Simpson index and Fisher’s alpha (Magurran,bogs are notorious for their poor species richness, 1988). Recent observations (Duelli, unpubl.) havebut if only a few raised bogs are left within a coun- shown that when undergraduate biodiversity studentstry, the few characteristic species present in a “good in entomology lectures have to choose which of thebog” are of very high national importance. The prob- two communities shown in Fig. 2 (without seeing thelems of estimating complementarity or distinctness text below them) they consider to be more diverse,are addressed e.g. by Colwell and Coddington (1994) more than half of them decide for the left popula-and Vane-Wright et al. (1991), endemism and spatial tion, because they consider evenness to be of greaterturnover by Harte and Kinzig (1997). importance than species numbers. When individuals This dichotomy between “species richness” and from other disciplines were asked during lectures and“conservation value” is the most fervently debated seminars, particularly conservationists and extensionissue among applied biologists concerned with biodi- workers in agriculture and forestry, species numbersversity indicators, and a recurrent source of misunder- are decisive. In recent years, indices involving even-standings. It will be elaborated further in the chapter ness have essentially fallen out of favour, mostlyon value systems. because they are difﬁcult to interpret (Gaston, 1996c). Particularly in agriculture or forestry, single species are often collected in huge numbers with standardised5. Indicator for what aspect of biodiversity? methods, which results in a drastic drop of evenness and hence yields low diversity values, in spite of After agreement on indicators FOR biodiversity, comparatively high species richness.and a decision between “alpha-diversity” and “con- The deﬁnition of biodiversity given in the interna-tribution to higher scale biodiversity”, there is still tional Convention on Biological Diversity (Johnson,potential for disagreement on “what is biodiversity?” 1993) encompasses the genetic diversity within(Gaston, 1996c). In practice, in a majority of cases, species, between species, and of ecosystems. Fur-species are “the units of biodiversity” (Claridge et al., thermore, Noss (1990) distinguished three sets of1997). However, species diversity can be measured as attributes: compositional, structural and functionalsimple number of species, usually of selected groups biodiversity (see also Fig. 1). The most common ap-of organisms, or species richness may be combined proach is to measure compositional biodiversity. Pre-with the evenness of the abundance distribution of the sumably, both structural and functional biodiversityspecies. The best known indices are the Shannon in- are either based on or lead to higher compositionalFig. 2. “Which of the two populations do you consider to have a higher biodiversity?” A choice test for biodiversity evaluation regularlyoffered by the ﬁrst author to students and at public lectures. For the vote, only the upper part without text is shown.
P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98 91diversity. We are convinced that ecosystem diver- better quantiﬁable measures of biodiversity, suchsity, as well as structural and functional diversity, is as species richness (Gaston, 1996b; Claridge et al.,somehow reﬂected in the number of species present. 1997).If they are not correlated with species richness, they The aspect of intraspeciﬁc diversity is a differentmust be special cases and not representative as biodi- case. To our knowledge there is no published exampleversity indicators. More trophic levels will normally of a tested correlation between inter- and intraspeciﬁcinclude more species, and a higher structural diversity diversity.will harbour more ecological niches. In fact, there isincreasing evidence that at least for some taxonomicgroups, species numbers are correlated with habitat 6. Value systemsheterogeneity (Moser et al., 2002), but not in others(Rykken and Capen, 1997). People involved in developing or using biodiversity For all these hierarchical separations or entities indicators are inﬂuenced by their personal and/or pro-within the huge concept of biodiversity, separate fessional goals. They all may want to measure or mon-comprehensible indicators can be researched and de- itor biodiversity, but they address different aspects ofveloped. In many cases, however, a rigorous scientiﬁc it. Their focus depends on their motivation for deal-test may show that the conceptual entities are difﬁcult ing with biodiversity. In an agricultural context, andto quantify (Prendergast, 1997; Lindenmayer, 1999; in an industrialised country in Europe, the three mostNoss, 1999), or they are basically reﬂected in other, important motivations to enhance biodiversity areFig. 3. Illustration of the hypothesis that abundant species usually are of higher ecological but lower conservation value, in contrast torare and threatened species. Stars indicate red list species collected with pitfall traps, yellow water pans and window interception traps ina semidry meadow (Duelli and Obrist, 1998). Number of individuals (N Ind(log)) are plotted versus number of species (N species).
92 P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–981. Species conservation (focus on rare and endangered of “species conservation” and “ecological resilience” species). is illustrated in Fig. 3.2. Ecological resilience (focus on genetic or species Prendergast et al. (1993) found low coincidence of diversity). species-rich areas and areas harbouring rare species3. Biological control of potential pest organisms (fo- for either plants, birds, butterﬂies or dragonﬂies. An cus on predatory and parasitoid arthropods). investigation of carabid beetles in Scotland (Foster et al., 1997) showed that neither the number of red There are additional motivations, of course, but list species nor the number of stenotopic (faunisticallyeither they are closely related to the ones mentioned interesting) species are correlated with the mean totalhere, or their causal link to biodiversity is less clear number of carabid species in a variety of habitats such(e.g. sustainability, landscape protection, cultural as moorland, grassland, heathland, peat, saltmarsh,heritage). bracken and swamps (Fig. 4). In an intense investiga- Each of these three aspects of biodiversity requires tion with 51 trap stations and standardised samplingits own indicators. They often do not correlate with methods in ﬁeld and forest habitats in Switzerland,each other or even show a negative correlation. Con- the number of red list species of all identiﬁed arthro-sequently, simply adding up different indicators may pod groups was not signiﬁcantly correlated to overalllead to misinterpretations, as long as they do not ad- species richness per trap station (Fig. 5), while e.g.dress the same aspect of biodiversity. Species con- the numbers of aculeate Hymenoptera species corre-servation focusses on rare and threatened species and lated well (R2 = 0.88; Fig. 6). In an assessment ofoften regards more common species in a derogatory the effects of ecological compensation measures inway as ubiquists of little interest. Ecologists, on the Swiss crop ﬁelds and grassland, the number of but-other hand, focus more on abundant species, because a terﬂy species did not show any correlation with thespecies on the verge of extinction is likely to have less species numbers of spiders (Jeanneret, pers. comm.).signiﬁcant ecological inﬂuence. The hypothesis of an In an effort to test the suitability of Collembola asalmost vicarious relationship between the motivations indicators of the conservation value of AustralianFig. 4. Neither red list carabid species nor stenotopic carabid species are correlated signiﬁcantly with the average number of carabid speciescollected in 18 types of habitats using pitfall traps. Data from Foster et al. (1997).
P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98 93Fig. 5. No signiﬁcant correlation exists between the number of red list species (from numerous arthropod taxa) and the “overall” number ofarthropods collected with ﬂight traps, pitfall traps and yellow water pans at the same 51 locations (Araneae, Coleoptera, Diplopoda, Diptera(Syrphidae only), Heteroptera, Hymenoptera (Aculeata only), Isopoda, Mecoptera, Megaloptera, Neuroptera, Raphidioptera, Thysanoptera).Data from agricultural areas (Duelli and Obrist, 1998) and forest edges (Flückiger, 1999).grasslands, Greenslade (1997) found no correlation Jones index for the stock exchange. The measuredwith species numbers of ants and carabid beetles. indicators within one basket have to be fairly con- The optimal approach is to select a “basket” of cordant and are pooled to form an index. The re-indicators for each motivation, similar to the Dow sult is a set of three separate indices for the threeFig. 6. Species numbers of aculeate Hymenoptera (bees, wasps and ants) show excellent correlation with the overall number of arthropodspecies at 51 locations (for details of data sources see Fig. 5).
94 P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98basic motivations “conservation”, “ecology” and “pest species (Duelli, 1994). Inevitably, the choice of thecontrol”. groups of organisms used for an inventory depends strongly on the red lists available, and on the avail- ability of specialists to identify the listed organisms.7. How to select indicators for the three main Lacking the information on the second step (fullmotivations account of the conservation value of an area), it is not currently possible to come up with a scientiﬁcally7.1. Several steps are necessary tested indicator for that value. Nevertheless, a correla- tion between the cumulated conservation values of all The most accurate indicators of biodiversity are presently available red listed species per habitat withproven linear correlates of the entity or aspect of biodi- the conservation values of single taxonomic groups,versity being evaluated. McGeoch (1998) proposed a such as birds, butterﬂies or carabids, would at leastnine-step approach for selecting bioindicators among give greater credibility to the red list species approach.terrestrial insects. Basically, the whole procedure can In addition to red list status (degree of threat of ex-be separated into three steps. The ﬁrst step is to de- tinction), species values have been calculated on theﬁne the aspect or entity in as quantiﬁable a way as bases of national or global rarity (Mossakowski andpossible. The second step is to actually quantify that Paje, 1985) or endemism. The rationale in the contextaspect or entity in a statistically reliable number of of habitat evaluation is that the presence of a nation-cases. The third step is a rigorous test for linear cor- ally or globally rare species increases the biodiversityrelation in a set of proposed indicators. The urgent value of that habitat, because it contributes more toneed to perform a scientiﬁcally solid test has been ad- the conservation of national or global biodiversity thanvocated repeatedly (Balmford et al., 1996; McGeoch, the presence of a ubiquitous species.1998; Niemelä, 2000). Only after a reliable basket of indicators for con- Starting with the ﬁrst step, the three mayor motiva- servation value has been established, are further stepstions for protecting or enhancing biodiversity in agri- possible to test the correlative power of potential in-cultural landscapes are differentiated. dicators such as length of hedgerows, amount of dead wood, or the surface of ecological compensation ar-7.2. Conservation (an index based on the motivation eas per unit area. Environmental diversity (ED) as ato protect or enhance threatened species) surrogate measure of the conservation value was pro- posed by Faith and Walker (1996), but so far there are For assessing the value of a given habitat, e.g. no empirical data to test their proposal.an ecological compensation area, for species con-servation, the entity to indicate is the accumulated 7.3. An index for the motivation “pest control”conservation values (e.g. red list status) of all speciespresent in that area. The highest values are contributed For the biodiversity aspect of biological control ofby species of national or even global importance, potential pest organisms, the ﬁrst step may be to de-while the so-called ubiquists are of little value. The ﬁne the measurable entity as the species diversity ofsecond step thus is a comprehensive measurement of all predators or parasites of potential pest organisms.the conservation values in a number of ecosystems or For short-term interests, the number of individuals ofhabitat types. beneﬁcial organisms may appear more important than The third step would be to ﬁnd and test the best species richness, because prey and hosts are reducedlinear correlate to that otherwise elusive entity “con- by the number of antagonistic individuals rather thanservation value”. The standard indicators for the by species numbers (Kromp et al., 1995; Wrattenconservation basket are numbers of red list species of and Van Emden, 1995). However, with a longer-termselected taxa, weighed according to their category of perspective on maintaining a high diversity of antag-threat. However, only very few of the tens of thou- onist species of potential pest organisms is certainlysands of species present in a country are listed; in more important. While the species richness of preda-Switzerland they are a mere 7% of all known animal tors in a small area can be assessed with reasonable
P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98 95accuracy and effort, the diversities of parasitoids are et al., 1996; Balmford et al., 1996; Cranston andmuch harder to quantify. Trueman, 1997; Duelli and Obrist, 1998). The second step is therefore to test inventory meth-ods, and selected taxa for their correlation with theabove biodiversity aspect of biological control. At 8. Effort and costs, the limiting factors for thepresent species numbers of carabid and staphylinid choice of measuresbeetles, as well as spiders, are often used as indica-tors because of established standardised collecting 8.1. The dilemma of indicating complexity withmethods (Duffey, 1974; Desender and Pollet, 1988; simple measuresHalsall and Wratten, 1988) and readily available keysfor identiﬁcation and interpretation. Specialised aphi- Large environmental monitoring programmes usu-dophaga among the syrphid ﬂies, coccinellids and ally avoid using invertebrates for their indicators,Neuroptera are another option, but so far the meth- although these constitute by far the largest portion ofods are not fully standardised. Parasitoid wasps and measurable biodiversity. To cut down on effort andﬂies are promising, but so far there is no easy way to costs, measurement of the immense richness and quan-identify them to the species level. Other possibilities tity of invertebrates has to be reduced to an optimisedfor indicators to test are ratios between herbivores selection of taxa. The proposed three-step approachand predators, or parasitoids and a range of other allows for testing all kinds of indicators for their cor-arthropods (see e.g. Denys and Tscharntke, 2002). relation with aspects of biodiversity. The search for linear correlates of quantiﬁed entities or aspects of7.4. An index for ecological resilience biodiversity is not limited to taxonomic units. Instead of choosing birds or grasshoppers as indicators, the For the basket of indicators for the motivation eco- spectrum of taxa considered can be determined by anlogical resilience (“Balance of Nature”, Pimm, 1991), inventory method such as Berlese soil samples or ﬂightthe entire genetic and taxonomic spectrum of biodi- interception traps. The broader the taxonomic spec-versity is the entity to be indicated. The assumption is trum of the samples, the higher the chance of obtain-that the higher the number of alleles and species, the ing a good correlation with the entity to be assessed.higher is the ecological potential of an ecosystem to Furthermore, indicators, which are not part of the or-react adequately to environmental change. ganismic spectrum, can also be tested in the three-step Here again, a ﬁrst step requires quantiﬁcation of approach: habitat diversity and heterogeneity, distur-a measurable proportion of local organismic diver- bance by trafﬁc, neighbourhood or percentage of pro-sity, which can be trusted to represent total species tected areas, etc. At present, various indicators are inrichness of animals and plants (alpha-diversity). Re- use, but few of them have been tested for their correla-alistically, only few and small areas will ever be tion with aspects of biodiversity. At least in Neotropi-fully assessed. For the second and third steps, ap- cal butterﬂies, a positive correlation of species richnessproximations with large, measurable proportions was found with composite environmental indices ofof alpha-diversity have to be used to test potential heterogeneity and natural disturbance (Brown, 1997).indicators. These “ecological” indicators can be seen as indica- 8.2. Plots and transectstors for ecosystem functioning (Schläpfer et al., 1999)and are representing a very basic notion of wholesale Plots (for plants) and transects (for birds and in-biodiversity. Most studies claiming to measure or in- sects such as butterﬂies, dragonﬂies and grasshoppers)dicate biodiversity assume that the group of organisms are widely used relative assessment methods for thethey investigate is somehow representative of biodi- species richness of a selected group of organisms (e.g.versity. However, in only very few cases has the cor- Pollard and Yates, 1993; Wagner et al., 2000). Therelation between a group or several groups of species main advantages are that the specimens survive the in-with a more or less representative sample of all organ- ventory (important for indicating conservation value),isms been measured and published (Abensperg-Traun and that large areas can be searched in a relatively
96 P. Duelli, M.K. Obrist / Agriculture, Ecosystems and Environment 98 (2003) 87–98short time. Scientiﬁcally, the drawback is that usually 9. Conclusionsthere are no voucher specimens kept for verifying theidentiﬁcation. Also, these popular groups (except for There is no single indicator for biodiversity. Thevascular plants) have only few species in agricultural choice of indicators depends on the aspect or entity ofhabitats, so their species richness, even if cumulated, biodiversity to be evaluated and is guided by a valuenever reaches 1% of the local species diversity of all system based on personal and/or professional moti-organisms. Their correlation power with local species vation. Each biodiversity index for a particular valuediversity has never been tested. Vascular plants, on system should consist of a basket of methods with onethe other hand, seem to correlate reasonably well with or several concordant indicators. In order to achieveoverall organismic diversity (Duelli and Obrist, 1998). greater reliability and a broader acceptance, indicatorsPlots and transects are low budget measures and worth have to be tested for their linear correlation with a sub-testing for their correlation power in the conservation stantial and quantiﬁable portion of the entity to assess.and ecology baskets of indicators. The challenge now is to assign all the presently used or proposed indicators to a basket with a declared value8.3. Standardised trapping methods for arthropods system—and to test them with empirical measures. Pitfall traps for surface dwelling arthropods and var-ious kinds of ﬂight traps for insects are often used Referencesfor biodiversity assessment in agricultural areas. Ei-ther one or a few taxonomic groups are collected over Abensperg-Traun, M., Arnold, G.W., Steven, D.E., Smith, G.T., Atkins, L., Viveen, J.J., Gutter, M., 1996. Biodiversity indicatorslonger periods, or a larger number of taxa are sampled in semiarid, agricultural Western Australia. Paciﬁc Conserv.within a shorter collecting period. In both cases, suit- Biol. 2, 375–389.able correlates have been found for the indicator bas- Balmford, A., Green, M.J.B., Murray, M.G., 1996. Usingket of ecological resilience (Duelli and Obrist, 1998). higher-taxon richness as a surrogate for species richness. I.Bugs (Heteroptera), and wild bees and wasps (ac- Regional tests. Proc. R. Soc. Lond. B 263, 1267–1274. Brown, K.S., 1997. Diversity, disturbance and sustainable useuleate Hymenoptera; see also Fig. 6) collected during of Neotropical forests: insects as indicators for conservationan entire vegetation period, where highly correlated monitoring. J. Insect Conserv. 1, 25–42.with overall species richness, while carabids and spi- Claridge, M.F., Dawah, H.A., Wilson, M.R. (Eds.), 1997. Species:ders in pitfall traps were not. Reducing the collecting The Units of Biodiversity. Chapman & Hall, London.time to ﬁve carefully selected weeks, but extending Cohen, S., Burgiel, S.W. (Eds.), 1997. Exploring Biodiversity Indicators and Targets under the Convention on Biologicalthe spectrum of identiﬁed taxa (Duelli et al., 1999), Diversity. BIONET and IUCN, Washington, DC and Gland.yielded correlation values comparable to those of sea- Colwell, R.K., Coddington, J.A., 1994. Estimating terrestrialsonal collections of bugs or bees. Tests are under way biodiversity through extrapolation. Phil. Trans. R. Soc. Lond.to further reduce the effort required for collecting and B 345, 101–118.identifying through an adaptation of the Australian Cranston, P.S., Hillman, T., 1992. Rapid assessment of biodiversity using biological diversity technicians. Aust. Biol. 5, 144–154.method of Rapid Biodiversity Assessment (Cranston Cranston, P.S., Trueman, J.W.H., 1997. Indicator taxa inand Hillman, 1992; Oliver and Beattie, 1996). With invertebrate biodiversity assessment. Mem. Mus. Victoria 56,that method, the whole taxonomic spectrum collected 267–274.within a few selected weeks in a standardised trap Denys, C., Tscharntke, T., 2002. Plant–insect communities andcombination is considered, but only at the level of predator–prey ratios in ﬁeld margin strips, adjacent crop ﬁelds, and fallows. Oecologia 130, 315–324.morphospecies, i.e. without identifying the catches Desender, K., Pollet, M., 1988. Sampling pasture carabids withto the species level (Duelli et al., unpubl.). Obvi- pitfalls: evaluation of species richness and precision. Med. Fac.ously, the resulting indicator will not be useful for Landbouww. Rijksuniv. Gent 53, 1109–1117.the indicator baskets of conservation or pest control, Duelli, P., 1994. Rote Listen der gefährdeten Tierarten der Schweiz.where identiﬁcation of the species is essential. How- Bundesamt für Umwelt Wald und Landschaft. BUWAL-Reihe Rote Listen. 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