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Species richness covariance in higher taxa: empirical tests of the biodiversity indicator concept

Species richness covariance in higher taxa: empirical tests of the biodiversity indicator concept The distribution of much of the world's biodiversity is poorl> known. It is suggested that the species richness of certain Indicator taxa may reflect that of other, poorly studied taxa, making the mapping of individual taxa uneccessary and providing a guide to conservationists. In this paper we examine this proposition at a spatial scale relevant to practical conservation We show that the relationship between the species richnesses of certain higher taxa in Britain is spatially highly variable and unpredictable. Britain may not be representative of other areas of the world but our results indicate that considerable further analysis is required before indicator taxa can be recruited as a reliable short cut to conservation planning The mapping of higher taxon diversity is a crucial first stage in planning conservation strategies However, most taxa are poorly studied and are likely to remain so, and so conservation biologists are forced to develop indirect methods for estimating their diversity The use of biodiversity indicator taxa is one such, and relies on the premise that, across large areas, the number of species in one well-studied taxon is well correlated with the number of species in other less well-known taxa (Schall and Pianka 1987, Crowe 1990, Gaston 1996a). By locating conservation areas to protect as many species as possible in the well-studied taxon, protection IS automatically conferred on the other(s). Hard data are, however, hard to come by, and there has been no spatially explicit exploration of the pattem in the predictive power which one taxon might have for the diversity of another across the full range of diversities for both taxa. Here we show that it is highly vanable and spatially complex, and calls into question the use of biodiversity indicator taxa as a conservation tool The proposition that spwcies richness m one taxon might exhibit spatial covariance with that in another seems to be scale-dependent It is undoubtedly true m a global sense, for most higher taxa there is a well documented diversity gradient, with speaes nchness increasing from the poles to the equator (Rhode 1992) At large geographic scales there is evidence that some pairs of taxa do indeed exhibit spatial covanance in species nchness (Pearson and Cassola 1992) But m most parts of the world large scales are mappropnate for conservation planning and the indicator taxon concept needs to be tested at much finer spatial scales In Bntain the numbers of species in different taxa vary markedly over fairly small distances (tens of km (Heath et al 1984, Turner et al 1987, 1988, Gibbons et al 1994)) in response to geological, topographical and climatic variation, and to habitat features, of both natural and human ongin In graphical plots of the number of species in one taxon with that in another, either on a site or grid cell basis, the subtleties of any relationship are usually obscured by this local noise (Schall and Pianka 1978, Gaston 1996a) (Fig. 1) At best they indicate that the levels of sjjecies nchness in two taxa may correspond tn some areas but not m others (Prendergast et al 1993, Gaston 1996b) •.! ;=: Fig 1 Scatter plot of butterfly and breeding bird species nchness in 100 contiguous 10 x 10 km squares in England and Wales ECOORAPHY 20J (1997) The underlying trends can, however, be revealed using smoothing techniques to suppress small scale differences (Lawton et al 1994) Here we use smoothed species nchness information to examine geographical vanation in the spatial covanance of pairwise combina- tions of butterflies (Lepidoptera), dragonflies and damselflies (Odonata) and breeding birds (Aves) at the 10 km scale This is the smallest scale at which species nchness data for these taxa can be considered consistently reliable across Bntain Maps plotted using the • 0.751 -a9 • OJMI -0.75 • OJOl - 0 4 5 • 0.151 -OJ • OJOOI • 0451 - O J 6 • -ai5 -•0.15 •OJOOI • •0.151 -•03 • •OJOl -•045 '"OS ECOGRAPHY 202 (1997) Fig 3 Correlations of smoothed butterfly and dragonfly/damselfly species nchness data and technique descnbed lti Fig 2 portray a stnoothed pattem of correlation coefficients, the values for adjacent squares are spatially autocorrelated, each square shanng an average of 56% of its input data with the other 24 squares in its block (The precise results depend on the size of the smoothing block lmpxjsed, but the same fundamental pattems are evident at several smoothing scales) Correlation coefficients between smoothed species nchness values in blocks of 5 x 5 10 km squares range widely, birds/butterflies vary from —0.84 to -(-0 96, butterflies/dragonflies from - 0 . 6 3 to + 0 93 and dragonflies/birds between —0.65 and +0.96 For predictive purposes, a significant negative correlation is as useful as a positive one, so areas where valid predictions might be made are m fact more common than is immediately apparent The most problematic areas are those where absolute correlation coefficients are weak Two major features of the maps (Figs 2, 3 and 4) reveal severe obstacles to the uncntical use of biodiversity indicators as conservation tools First, the high spatial vanability in the correlation of speaes nchness of one taxon with that of another is seen in all three maps The inconsistent nature of this value implies that any predictive procedure which might be applied to a poorly known taxon is likely to fail in many areas Second, the vanability has no consistent pattem across the three maps, and areas of positive correlation on one map may well show negative, or zero, correlation in the other two As far as these taxa are concemed, it is not possible to identify geographic areas where prediction may consistently succeed or fail Fig 2 Data for this and subsequentfiguresare speaes nchnesses for the best-recorded Bntish taxa, butterflies and dragonflies, summarised in 10 x 10 km squares, and drawn from The Biological Records Centre database (Harding and Sheail 1992) The number of native species recorded since 1960 m each taxon was counted in each 10 km square of the Ordnance Survey gnd Breeding bird data are from Sharrock (1976) Records from the BRC database and the (1976) bird atlas are broadly contemporaneous Local vanation was smoothed by replacmg the value in each square with the mean number of speaes of the 3 x 3 square block of which it forms the centre In coastal squares, only land-containing cells were included m the averaging The Pearson product moment correlation coefBaent for each pair of taxa was then calculated for all possible 5 x 5 blocks of squares, and the value inserted into the central square of each block Squares were excluded, and appear on the maps as blank areas, if their surrounding 24 squares did not all contain land (l e coastal squares), or did not all contain records for both of the taxa, thus the values in all squares are denved from equal sample sizes (n = 25) This map illustrates the distnbution of correlation coefficients for smoothed butterfly and breeding bird speaes nchness Positive correlaUons, of varying strength, appear mainly in eastern England, south Wales and western Scotland Negative values occur m clusters across much of the rest of the country. Clustenng is to be expected and is due to data smoothing The key to Fig 2 applies to all maps 212 ECXWRAPHY 20-2 (1997) Fig 4 Correlations of smoothed dragonfly/damselfly and breeding bird species nchness It IS, however, reasonable to ask why the species nchness of very difTerent taxa should covary Why, for example, should a small subgroup of one order of phytophagous insect ectotherms (butterflies) be expected to covary in its speaes nchness with that of breeding birds, an entire class of homoiothemuc vertebrates embraang three trophic levels (Figs 1 and 2)'' Might the covanance between potentially trophically linked groups be more geographically stable'' The data for butterflies and two families of insectivorous birds, the tits (Pandae) and warblers (Sylviidae), suggest not (Fig 5) Species nchnesses are no better correlated than the data in Figs 2-4, and differ m several important details from Fig 2 Beccaloni and Gaston (1995) suggested that the species nchness of one easily sampled butterfly subgroup might act as a good indicator of overall butterfly species nchness m Central and South Amenca Again, we counsel caution, across Bntain the species nchnesses of individual butterfly families do not correlate m any consistent way with overall butterfly species nchness (Fig 6 a-d) For all famihes, pockets of low, or negative correlation exist, and the spatial arrangement of these areas differs between famihes Correlation coefficients for the four examples in Fig 6, are Piendae -0.61 to 0 98, Hespendae 0 01 to 0 99, Lycaemdae -0,11 to 0 99, Nymphalidae 0 2 to 0 99 The structure seen in each of these maps suggests that no single third factor is responsible for the vanation m higher taxon correlations, since no known macro-vanable reflects the patterns seen in Figs 2-5 Given existECOGRAPHY 20-2 0 » 7 ) ing knowledge of the vanables which control individual species distnbutions, and generate patterns of species replacement along environmental gradients, the prospects of elucidating the causes of this extensive vanation m the sign and strength of the correlations between the species nchness of higher taxa seem slim at present There is no reason to exjsect correlations between pairwise combinations of other Bntish taxa to be any less vanable There are however instances where vanability might be lower First, Bntain has a highly fragmented and complex semi-natural landscape, in other geographical areas where landscapes may be simpler, so may be the patterns of species nchness covanance Second, the species nchness covariance of taxon subsets which are sjjecific or more generally found in particular habitat types (Beccaloni and Gaston's (1995) study focused on forest butterflies) may be more consistent than for the entire higher taxa, or taxonomically defined subgroups used here Testing of both these hypotheses awaits the availability of reliable species richness data at the appropnate scale, collected over entire landscapes The general message, however, is clear At spatial scales relevant to the existing Bntish nature reserve network (10 km and below), our understanding of the relationship between the sp)eaes nchnesses of different taxa is currently insufficient to support the use of individual higher taxa as biodiversity indicators for conservation planning Aeknowledgment - We thank John Lawton for helpful comments on the manuscript 213 Fig 5 CorrelaUons of butterflies and smoothed, summed species nchness of Pandae and Sylviidae (see text) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ecography Wiley

Species richness covariance in higher taxa: empirical tests of the biodiversity indicator concept

Prendergast, J. R.; Eversham, B. C.
Ecography , Volume 20 (2) – Apr 1, 1997
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Publisher
Wiley
Copyright
Copyright © 1997 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0906-7590
eISSN
1600-0587
DOI
10.1111/j.1600-0587.1997.tb00363.x
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Abstract

The distribution of much of the world's biodiversity is poorl> known. It is suggested that the species richness of certain Indicator taxa may reflect that of other, poorly studied taxa, making the mapping of individual taxa uneccessary and providing a guide to conservationists. In this paper we examine this proposition at a spatial scale relevant to practical conservation We show that the relationship between the species richnesses of certain higher taxa in Britain is spatially highly variable and unpredictable. Britain may not be representative of other areas of the world but our results indicate that considerable further analysis is required before indicator taxa can be recruited as a reliable short cut to conservation planning The mapping of higher taxon diversity is a crucial first stage in planning conservation strategies However, most taxa are poorly studied and are likely to remain so, and so conservation biologists are forced to develop indirect methods for estimating their diversity The use of biodiversity indicator taxa is one such, and relies on the premise that, across large areas, the number of species in one well-studied taxon is well correlated with the number of species in other less well-known taxa (Schall and Pianka 1987, Crowe 1990, Gaston 1996a). By locating conservation areas to protect as many species as possible in the well-studied taxon, protection IS automatically conferred on the other(s). Hard data are, however, hard to come by, and there has been no spatially explicit exploration of the pattem in the predictive power which one taxon might have for the diversity of another across the full range of diversities for both taxa. Here we show that it is highly vanable and spatially complex, and calls into question the use of biodiversity indicator taxa as a conservation tool The proposition that spwcies richness m one taxon might exhibit spatial covariance with that in another seems to be scale-dependent It is undoubtedly true m a global sense, for most higher taxa there is a well documented diversity gradient, with speaes nchness increasing from the poles to the equator (Rhode 1992) At large geographic scales there is evidence that some pairs of taxa do indeed exhibit spatial covanance in species nchness (Pearson and Cassola 1992) But m most parts of the world large scales are mappropnate for conservation planning and the indicator taxon concept needs to be tested at much finer spatial scales In Bntain the numbers of species in different taxa vary markedly over fairly small distances (tens of km (Heath et al 1984, Turner et al 1987, 1988, Gibbons et al 1994)) in response to geological, topographical and climatic variation, and to habitat features, of both natural and human ongin In graphical plots of the number of species in one taxon with that in another, either on a site or grid cell basis, the subtleties of any relationship are usually obscured by this local noise (Schall and Pianka 1978, Gaston 1996a) (Fig. 1) At best they indicate that the levels of sjjecies nchness in two taxa may correspond tn some areas but not m others (Prendergast et al 1993, Gaston 1996b) •.! ;=: Fig 1 Scatter plot of butterfly and breeding bird species nchness in 100 contiguous 10 x 10 km squares in England and Wales ECOORAPHY 20J (1997) The underlying trends can, however, be revealed using smoothing techniques to suppress small scale differences (Lawton et al 1994) Here we use smoothed species nchness information to examine geographical vanation in the spatial covanance of pairwise combina- tions of butterflies (Lepidoptera), dragonflies and damselflies (Odonata) and breeding birds (Aves) at the 10 km scale This is the smallest scale at which species nchness data for these taxa can be considered consistently reliable across Bntain Maps plotted using the • 0.751 -a9 • OJMI -0.75 • OJOl - 0 4 5 • 0.151 -OJ • OJOOI • 0451 - O J 6 • -ai5 -•0.15 •OJOOI • •0.151 -•03 • •OJOl -•045 '"OS ECOGRAPHY 202 (1997) Fig 3 Correlations of smoothed butterfly and dragonfly/damselfly species nchness data and technique descnbed lti Fig 2 portray a stnoothed pattem of correlation coefficients, the values for adjacent squares are spatially autocorrelated, each square shanng an average of 56% of its input data with the other 24 squares in its block (The precise results depend on the size of the smoothing block lmpxjsed, but the same fundamental pattems are evident at several smoothing scales) Correlation coefficients between smoothed species nchness values in blocks of 5 x 5 10 km squares range widely, birds/butterflies vary from —0.84 to -(-0 96, butterflies/dragonflies from - 0 . 6 3 to + 0 93 and dragonflies/birds between —0.65 and +0.96 For predictive purposes, a significant negative correlation is as useful as a positive one, so areas where valid predictions might be made are m fact more common than is immediately apparent The most problematic areas are those where absolute correlation coefficients are weak Two major features of the maps (Figs 2, 3 and 4) reveal severe obstacles to the uncntical use of biodiversity indicators as conservation tools First, the high spatial vanability in the correlation of speaes nchness of one taxon with that of another is seen in all three maps The inconsistent nature of this value implies that any predictive procedure which might be applied to a poorly known taxon is likely to fail in many areas Second, the vanability has no consistent pattem across the three maps, and areas of positive correlation on one map may well show negative, or zero, correlation in the other two As far as these taxa are concemed, it is not possible to identify geographic areas where prediction may consistently succeed or fail Fig 2 Data for this and subsequentfiguresare speaes nchnesses for the best-recorded Bntish taxa, butterflies and dragonflies, summarised in 10 x 10 km squares, and drawn from The Biological Records Centre database (Harding and Sheail 1992) The number of native species recorded since 1960 m each taxon was counted in each 10 km square of the Ordnance Survey gnd Breeding bird data are from Sharrock (1976) Records from the BRC database and the (1976) bird atlas are broadly contemporaneous Local vanation was smoothed by replacmg the value in each square with the mean number of speaes of the 3 x 3 square block of which it forms the centre In coastal squares, only land-containing cells were included m the averaging The Pearson product moment correlation coefBaent for each pair of taxa was then calculated for all possible 5 x 5 blocks of squares, and the value inserted into the central square of each block Squares were excluded, and appear on the maps as blank areas, if their surrounding 24 squares did not all contain land (l e coastal squares), or did not all contain records for both of the taxa, thus the values in all squares are denved from equal sample sizes (n = 25) This map illustrates the distnbution of correlation coefficients for smoothed butterfly and breeding bird speaes nchness Positive correlaUons, of varying strength, appear mainly in eastern England, south Wales and western Scotland Negative values occur m clusters across much of the rest of the country. Clustenng is to be expected and is due to data smoothing The key to Fig 2 applies to all maps 212 ECXWRAPHY 20-2 (1997) Fig 4 Correlations of smoothed dragonfly/damselfly and breeding bird species nchness It IS, however, reasonable to ask why the species nchness of very difTerent taxa should covary Why, for example, should a small subgroup of one order of phytophagous insect ectotherms (butterflies) be expected to covary in its speaes nchness with that of breeding birds, an entire class of homoiothemuc vertebrates embraang three trophic levels (Figs 1 and 2)'' Might the covanance between potentially trophically linked groups be more geographically stable'' The data for butterflies and two families of insectivorous birds, the tits (Pandae) and warblers (Sylviidae), suggest not (Fig 5) Species nchnesses are no better correlated than the data in Figs 2-4, and differ m several important details from Fig 2 Beccaloni and Gaston (1995) suggested that the species nchness of one easily sampled butterfly subgroup might act as a good indicator of overall butterfly species nchness m Central and South Amenca Again, we counsel caution, across Bntain the species nchnesses of individual butterfly families do not correlate m any consistent way with overall butterfly species nchness (Fig 6 a-d) For all famihes, pockets of low, or negative correlation exist, and the spatial arrangement of these areas differs between famihes Correlation coefficients for the four examples in Fig 6, are Piendae -0.61 to 0 98, Hespendae 0 01 to 0 99, Lycaemdae -0,11 to 0 99, Nymphalidae 0 2 to 0 99 The structure seen in each of these maps suggests that no single third factor is responsible for the vanation m higher taxon correlations, since no known macro-vanable reflects the patterns seen in Figs 2-5 Given existECOGRAPHY 20-2 0 » 7 ) ing knowledge of the vanables which control individual species distnbutions, and generate patterns of species replacement along environmental gradients, the prospects of elucidating the causes of this extensive vanation m the sign and strength of the correlations between the species nchness of higher taxa seem slim at present There is no reason to exjsect correlations between pairwise combinations of other Bntish taxa to be any less vanable There are however instances where vanability might be lower First, Bntain has a highly fragmented and complex semi-natural landscape, in other geographical areas where landscapes may be simpler, so may be the patterns of species nchness covanance Second, the species nchness covariance of taxon subsets which are sjjecific or more generally found in particular habitat types (Beccaloni and Gaston's (1995) study focused on forest butterflies) may be more consistent than for the entire higher taxa, or taxonomically defined subgroups used here Testing of both these hypotheses awaits the availability of reliable species richness data at the appropnate scale, collected over entire landscapes The general message, however, is clear At spatial scales relevant to the existing Bntish nature reserve network (10 km and below), our understanding of the relationship between the sp)eaes nchnesses of different taxa is currently insufficient to support the use of individual higher taxa as biodiversity indicators for conservation planning Aeknowledgment - We thank John Lawton for helpful comments on the manuscript 213 Fig 5 CorrelaUons of butterflies and smoothed, summed species nchness of Pandae and Sylviidae (see text)

Journal

Ecography – Wiley

Published: Apr 1, 1997

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