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The elevational gradient of species richness: a uniform pattern?

The elevational gradient of species richness: a uniform pattern? Carsten Rahbek, Centre for Tropical Biodiversity, Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen 0, Denmark. The decline m species nchtiess with increasing elevation IS widely accepted as a general pattern (Table 1) In as much as the elevational gradient is often claimed to mirror the latitudinal gradient, spectes richness is assumed to decrease monotonically (l e because of reduced temperature and consequent decrease in producUvity) Perhaps because such a relation is intuitive, biologists have readily generalized the results of a few early studies of tropical birds as supporting a general biogeographic Table 1 Typical examples of statements atwut the relationship between species nchness and elevation from books and papers in major journals "For all of these reasons, we expect the number of species to decrease with altitude and, m fact, it does " (MacArthur 1972, p 107) "In terrestrial habitats, vanation in species diversity along gradients of elevauon and available soil moisture are [sic] almost as general and striking as latitudinal vanation " (Brown and Gibson 1983, p 502) "In terrestnal environments, a decrease in species with altitude IS a phenomenon almost as widespread as a decrease with latitude " (Begon et al 1990, p 805) "Just as change of physical conditions with altitude resembles in many respects the vanauon with latitude, so the decreasing diversity of most organisms with increasing elevation mirrors in most respects the latitudinal gradient of species nchness" (Brown 1988, p 62) "biologists have long recogmzed that elevational and latitudinal species-nchness gradients mirror each other " (Stevens 1992, p 899) "In terrestnal ecosystems, diversity generally decreases with increasing altitude there appear to be no substantiating data for [the] 'mid-altitudinal bulge' as a general phenomenon" (World Conservation Momtonng Centre 1992, pp 43,45) "Decrease in the number of species with decreasing temperatures at higher altitudes is as conspicuous as the decrease with latitude (e g Brown and Gibson 1983), although exceptions occur" (Rohde 1992, p 522) pattern This has resulted in "citation inbreeding" Here, I outline the supporting evidence for the generalization and discuss the influence of samplmg regime and the often Ignored influence of area I then present a quantitative review of the data already present, although often ignored, in the literature Altogether 97 papers (with 163 examples) have been reviewed. The analysis of these empincal data support the view that species nchness declines with elevation, but not the view that this decline IS necessanly monotonic Some possible reasons for variation in the exact shape of the relationship between species nchness and elevation for different taxa and zoogeographic areas are commented, but our understanding of the relation between elevation and species nchness suU appears to be immature The empirical basis for the "general pattern" The generalization (Table 1) grew mamly from two studies dealing with tropical birds one from the Peruvian Andes (Terborgh 1977), and the other from New Guinea (Kikkawa and Williams 1971) The textbook example from New Guinea was published as a short note based on compilation of the published distnbutional data (Kikkawa and Williams 1971) It was conducted at a Ume when the knowledge of the elevational distnbution of New Guinean btrds was still somewhat rudimentary Another more detailed study on birds of New Guinea has also been cited as proof of the general pattern (Diamond 1972, cited in MacArthur 1972) However, Diamond's New Guinea data actually show a small peak tn species nchness at 1100 m with a marked decline m species nchness only above this level The second textbook example was based on a carefully conducted field survey and cntical data analysis (Terborgh 1977). Based on mist-netting and opportunisuc ECXX3RAPHY 18 2 (1995) A 200 B 70 ELEVATION (M) 1000 2000 3000 ELEVATION (M) Fig I Species nchness of syntopic birds versus elevation on an Amazonian slope of the Andes in Peru Figure 1A is based on data not standardized for area and sampling effort, whereas Fig IB is based on standardized samples of 300 mist-netted birds (data from Terborgh 1977) I have fitted the lines by distance-weighted least-squares smoothing field observattons at camps situated along ati elevational gradient on the humid east slope of the Peruvian Andes, Terborgh showed that species nchness declined monotonically with elevation if the number of species is simply plotted against elevation (Fig lA) However, when Terborgh tned to eliminate the effect of sampling time by standardizmg his mist-netting data, a different pattern emerged (Fig IB) Terborgh explained the emerging "hump-shaped" curve, which had a peak in species nchness around 1400 m, as the result of a local "hot-spot" in resources (Terborgh 1977) Although this unexpected pattern is addressed through most of his discussion, only the first non-standardized graph is usually cited in the literature (e.g Brown 1988) Though the Ube of underA 2500 in storey mist-netting data limited the scope of this study, it serves to demonstrate the strong effect that sampling effort can exert - especially in species-nch tropical forest where most species occur at low densities The importance of area The effect of area on the relationship between spwcies nchness and elevation has rarely been considered, although we would expect area to have a significant impact on the form of the elevational pattern, as the relationship between area and species nchness seems as universal as ffl ELEVATION (M) ELEVATION (M) Fig 2 Species nchness of South Amencan tropical landbirds versus elevation Figure 2A is based on data not standardized for area, whereas Fig 2B is based on data standardized for area set to 50000 km^ using equations for species-area curves (log S/log A transformation) of each elevauonal zone (based on data from Rahbek unpubl) Area is set to 50000 km' because it is within the range of the ongmal data upon which the equaUons are based, and is a size within the geographical regional scale (e g Wiens 1981, Rosenzweig and Abramsky 1993) ECOGRAPHY 18 2 (1995) Table 2 Number of data sets (n = 163) found in the literature with data on the variation of species nchness with elevation summanzed by biomes (NT = non-tropical biomes, T = tropical biomes) and scale (R = regional, L = local), listed for studies conducted on mainland and island, respectively, and subdivided for whether the researcher(s) have attempted to standardize for the effect of area and sampling regime and/or effort, only one of these factors, or none ("non-standardized") Invertebrates Vertebrates NT(R/L) T(R/L) Plants NT (R/L) T(R/L) Totals NT (R/L) T (R/L) NT(R/L) T(R/L) Mainland Area and sampling Area Sampling Non-standardized Island Area and sampling Area Sampling Non-standardized Total 0/2 0/0 4/1 9/11 1/0 0/0 0/0 0/5 14/19 0/0 0/0 3/4 4/6 3/2 1/0 0/5 2/5 13/22 0/6 0/0 0/6 3/0 0/0 0/0 0/0 0/0 3/12 0/1 1/0 3/1 17/4 0/0 0/3 0/1 5/2 26/12 0/10 0/0 0/1 1/0 0/4 0/0 0/0 0/0 1/15 0/6 4/3 0/0 8/3 0/0 0/0 0/0 2/0 14/12 0/18 0/0 4/8 13/11 1/4 0/0 0/0 0/5 18/46 0/7 5/3 6/5 29/13 3/2 1/3 0/6 9/7 53/46 the latitudinal gradient To understand the relationship between area and species nchness along elevational gradients, especially on a regional scale, the effect of area must be considered since areas of equal-sized elevational belts may vary with elevation Thus, areas often decrease with elevation because of generally steeper terrain toward the highest peaks When landbird data from tropical South Amenca, compiled at a regional scale using countries as units, are standardized for area, the relationship between species nchness and elevation gives a humpshaped curve (Fig 2) However, area alone is unlikely to explain any global pattern of species nchness, as close couplings can be expected to exist between biological diversification and habitat complexity (see also Rosenzweig 1992) A quantitative review of the literature The repeated citation of the same few studies provides a false picture of the amount and diversity of data published on the issue I have been able to find the surpnsmgly high number of 97 papers, with 163 examples that give data on the vanation in number of species with elevation. It is highly probable that additional data exist, as many data sets are published in httle known journals, or in the "gray" literature. Table 2 summanzes some charactenstics of each data set (taxonomic group, region, scale and data treatment) The mfluences of sampling regime/effort and the effect of area are among the most influential biasing factors in most field studies of species-nchness patterns, and, unfortunately, equally difficult to eliminate successfully I have thus only judged whether an attempt was made to deal with these two factors, either in the design of the survey or afterward, during the data analyses. Remaricably, many of the pajjers reviewed do not give any details 202 on this subject These data sets are classified as "Nonstandardized" together with those with no attempt to standardize data (Table 2) Unlike the traditional histoncal trend within most fields of biological research, most data sets are from the tropics (99 out of 163) The focus on the tropics is presumably related to the circumstance that tropical elevational gradients compnse a wider range of climatic vanation than temjserate elevational gradients Out of 163 data sets, 68 are on invertebrates, 53 on vertebrates (including 36 on birds), and 42 on plants The majonty of the data sets comes from mainland biota (122), whereas 41 data sets are fi-om islands As shown, standardizing for sampling effort or effect of area can have a significant influence on the emerging shape of the relationship between sjjecies richness and elevation In 87 of the 163 data sets, the data have not been standardized for area or sampling effort (corresponding figures for tropical and non-tropical biomes are 59% and 45%) Only in 35 (21 %) cases has a standardization been attempted for area as well as sampling effort (figures for tropical and non-tropical biomes are 12% and 36%, respectively) Considenng the high mobility of birds compared to other groups, the reliance by most reviewers on primanly bird examples to illustrate a universal relationship seems ill-founded, especially when the bulk of data in the hterature actually denves from invertebrates and plants Table 2 also provides an overview for which combmations of, for example, taxonomic groups, scale, and region we lack studies - especially those that consider the effect of area and samphng regime on data. Methodological problems and patterns In many studies, a stated decline in species nchness with ECOGRAPHY 18 2 (1995) Table 3 The relationship between species nchness and elevation summarized by type of pattern Only the 90 data sets (of 163) that provide data points spanning from below 500 m to above 1500 m are included (see text) The classification of each pattern is based on a visual examination of bivanate plots NT= non-tropical biomes and T = tropical biomes Scale Monotomcally decreasing Horizontal, then decreasing NT T Hump-shaped Increasing Other NT NT Regional Invertebrates Vertebrates Plants Total regional Local Invertebrates Vertebrates Plants Total local Total 1 1 NT NT elevation was restncted to only a part of the elevational gradient In other cases, mid-elevational data were lacking Others used correlation tests on data sets that include few stations from low- and mid-elevation, but many from higher elevations, thereby biasing their findings toward a strong negative correlation In such instances, the data are inapplicable to support a monotonic relationship Conclusions based on correlation tests sometimes ignore that stations at mid-elevation actually have more species than stations at low elevation To analyze the general vanation of species richness with elevation, a minimum requirement for any data set is that It includes data spanning the entire gradient, albeit it becomes lncreasmgly difficult to find appropnate gradients with continuous natural habitat along the entire gradient This is es{)ecially a problem with respect to lowland stations, as lowlands and foothills often are the most disturbed elevational zone(s) Of the 163 data sets, 47 do not include data from below 500 m. In the descriptive analysis of the elevational pattern of species nchness, I have only included data sets which are based on a gradient from below 500 m to above 1500 m (see Table 3) This hmits the analysis to 90 data sets of the onginal 163 37 on invertebrates, 26 on vertebrates (lncludmg 19 on birds), and 27 on plants. As for the entire data set, this subsample of data sets is biased toward the tropics with 73 data sets compared to only 17 from non-tropical biomes It also has more data sets from mainland (n = 71) than islands (n = 19) Non-standardized data sets are dominant (n = 49) In only 13 data sets (14%) have attempts been made to take the effect of area and sampling into account. The corresponding figure for tropical data sets, which have been the main source of generalizations on the elevational gradient, is only 7% (5 of 73 tropical data sets), in contrast to 47% of the nontropical data sets ECOGRAPHY 18 2 (1995) To conduct proper descnpuve statistical analyses of the vanation of species nchness with elevation, the stations (l e the data points) must be reasonably equally distnbuted over the gradient, and the number of data points sufficient to refiect any marked changes in habitats/ biomes over the analyzed gradient. Unfortunately these two requirements for an optimal data set are rarely fulfilled As It IS difficult to judge especially the latter cntenon for most of the published data no attempt has been made to select or exclude data on this basis. Compansons of elevational patterns between taxa, latitudinal climatic zones, biogeographic regions or mainland versus islands could be misleading without correction for the area effect and differences in sampling regime Compansons of studies are also biased by differences in the species included, and sometimes further by limitations to Sjjecific trophic levels, guilds or habitat These problems and the pronounced heterogeneity of the quality of data sets make it difficult to conduct proper cntical statistical tests for each data set of the relationship between sp>ecies nchness and elevation that are mutually comparable Still, disregarding these biases, a rough companson based on a classification of the pattern in each data set by visual examination of bivanate plots serves to illustrate how ambiguous the pattern is, both withm region, spatial scale and crude taxonomic groupmgs (Table 3). A decline in species nchness with elevation seems to be a general trend Yet, a pattern where the speciesrichness curve IS almost honzontal up to a certain elevaUon before declining, or is hump-shaped, seems more typical than a monotomc declme (Table 3) ELEVATION (M) Fig 3 Patterns of species nchness versus elevation may vary within the same area for different taxa, here illustrated by New Guinean bats (*) and rodents (•) (data compiled ', T 1990 Mammals of New Guinea - Robert Brown the position of humps or bend of curves vanes between and Ass . Sydney different sets of data Vanous taxa are also differently Hutchinson. G E 1959 Homage to Santa Rosalia, or "why are there so many kinds of animals''" - A m Nat 93 145-159 affected by abiotic factors, such as, for example, humidKikkawa. J and Williams, E E 1971 Altitude distnbution of ity land birds in New Guinea - Search 2 64-65 Further studies, including analyses of pnmary-level MacArthur. R H 1972 Geographical ecology - Harper and Rowe New York processes that could influence the pattern, are needed to Mares. M Publ ,1992 Neotropical mammals and the myth of A reveal whether general patterns exit within biogeographic Amazonian biodiversity - Science 25 976-979 regions, taxa, spatial scale, mainland versus islands, etc , Preston, F W 1962 The canonical distribution of commonness and rarity - Ecology 43 185-215, 410-^32 or various combinations of these Appropriate null-models should be considered before explaining elevational Rohde, K 1992 Latitudinal gradients in species diversity the search for the pnmary cause - Oikos 65 514-527 gradient patterns as results of climatic, biological and Rosenzweig, M L 1971 Paradox of ennchment destabilization histoncal processes (Colwell and Hurtt 1994) Although a of exploitation ecosystem in ecological time - Science 171 385-387 difficult task, the development of testable hypotheses is - 1992 Species diversity gradients we know more and less important to achieve significant progress within this field than we thought - J Mammal 73 715-730 and to contnbute to our general understanding of di- and Abramsky. Z 1993 How are diversity and productivity versity patterns This would be preferable to founding related"' - In Ricklefs, R and Schluter, D (eds). Species diversity in ecological communities Histoncal and geonew generalizations on simple accumulations of case graphical perspectives Univ of Chicago Press, Chicago, pp studies Much is still to be learned about this topic, for 52-65 now, we have to accept the unsatisfactory realization that Stevens. G C 1992 The elevational gradient in altitudmal we do not know whether a general relationship exists range an extension of Rapoport's latitudinal rule to altitude between species nchness and elevation, or whether an - A m Nat 140 893-911 Terborgh. J 1977 Bird species diversity on an Andean elevauniversal explanation or model can be given tional gradient - Ecology 58 1007-1019 Wiens, J A 1981 Scale problems in avian censusing - S t u d Avian Biol 6 513-521 Acknowledgements - 1 thank G Graves and J Fjeldsa for many discussions and cnUcally reading the manuscnpt Also thanks to World Conservation Monitonng Centre 1992 Global diversity status of the Earth's living resources - Chapman and Hall, J Lovett,J M CardosadaSilva,R Zusi,D ThurberandM E London Petersen for their cntical comments to the manuscnpt The study was conducted dunng a stay at The National Museum of Natural Wnght, D H 1983 Species-energy theory an extension of species-area theory - Oikos 41 496-506 History, Smithsonian Instituuon supported by a Fulbnght grant ECOGRAPHY 18 2(199'>) physical causes, including climatic factors, the narrow width of the elevational gradient, and histoncal perturbations that shape the available species pool Biotic mteractions are secondary mechamsms that can be reflected in the emergent patterns The vanation of sjjecies nchness with elevation might be connected to the reduction of temjjerature with elevation and the assumed corresfwndlng reduction in productivity However, other factors such as vanation in steepness, geological perturbations, alterations of precipitation patterns, etc might also be involved, probably with varying impact from case to case For any correlation of species nchness with variables measured over an elevational gradient, we need to examine whether the correlation reflects a direct coupling, or if It could be a result of interactions of several other factors The complete list of papers which this Forum contnbution is based on is available from the author http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ecography Wiley

The elevational gradient of species richness: a uniform pattern?

Ecography , Volume 18 (2) – Jun 1, 1995

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Publisher
Wiley
Copyright
Copyright © 1995 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0906-7590
eISSN
1600-0587
DOI
10.1111/j.1600-0587.1995.tb00341.x
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Abstract

Carsten Rahbek, Centre for Tropical Biodiversity, Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen 0, Denmark. The decline m species nchtiess with increasing elevation IS widely accepted as a general pattern (Table 1) In as much as the elevational gradient is often claimed to mirror the latitudinal gradient, spectes richness is assumed to decrease monotonically (l e because of reduced temperature and consequent decrease in producUvity) Perhaps because such a relation is intuitive, biologists have readily generalized the results of a few early studies of tropical birds as supporting a general biogeographic Table 1 Typical examples of statements atwut the relationship between species nchness and elevation from books and papers in major journals "For all of these reasons, we expect the number of species to decrease with altitude and, m fact, it does " (MacArthur 1972, p 107) "In terrestrial habitats, vanation in species diversity along gradients of elevauon and available soil moisture are [sic] almost as general and striking as latitudinal vanation " (Brown and Gibson 1983, p 502) "In terrestnal environments, a decrease in species with altitude IS a phenomenon almost as widespread as a decrease with latitude " (Begon et al 1990, p 805) "Just as change of physical conditions with altitude resembles in many respects the vanauon with latitude, so the decreasing diversity of most organisms with increasing elevation mirrors in most respects the latitudinal gradient of species nchness" (Brown 1988, p 62) "biologists have long recogmzed that elevational and latitudinal species-nchness gradients mirror each other " (Stevens 1992, p 899) "In terrestnal ecosystems, diversity generally decreases with increasing altitude there appear to be no substantiating data for [the] 'mid-altitudinal bulge' as a general phenomenon" (World Conservation Momtonng Centre 1992, pp 43,45) "Decrease in the number of species with decreasing temperatures at higher altitudes is as conspicuous as the decrease with latitude (e g Brown and Gibson 1983), although exceptions occur" (Rohde 1992, p 522) pattern This has resulted in "citation inbreeding" Here, I outline the supporting evidence for the generalization and discuss the influence of samplmg regime and the often Ignored influence of area I then present a quantitative review of the data already present, although often ignored, in the literature Altogether 97 papers (with 163 examples) have been reviewed. The analysis of these empincal data support the view that species nchness declines with elevation, but not the view that this decline IS necessanly monotonic Some possible reasons for variation in the exact shape of the relationship between species nchness and elevation for different taxa and zoogeographic areas are commented, but our understanding of the relation between elevation and species nchness suU appears to be immature The empirical basis for the "general pattern" The generalization (Table 1) grew mamly from two studies dealing with tropical birds one from the Peruvian Andes (Terborgh 1977), and the other from New Guinea (Kikkawa and Williams 1971) The textbook example from New Guinea was published as a short note based on compilation of the published distnbutional data (Kikkawa and Williams 1971) It was conducted at a Ume when the knowledge of the elevational distnbution of New Guinean btrds was still somewhat rudimentary Another more detailed study on birds of New Guinea has also been cited as proof of the general pattern (Diamond 1972, cited in MacArthur 1972) However, Diamond's New Guinea data actually show a small peak tn species nchness at 1100 m with a marked decline m species nchness only above this level The second textbook example was based on a carefully conducted field survey and cntical data analysis (Terborgh 1977). Based on mist-netting and opportunisuc ECXX3RAPHY 18 2 (1995) A 200 B 70 ELEVATION (M) 1000 2000 3000 ELEVATION (M) Fig I Species nchness of syntopic birds versus elevation on an Amazonian slope of the Andes in Peru Figure 1A is based on data not standardized for area and sampling effort, whereas Fig IB is based on standardized samples of 300 mist-netted birds (data from Terborgh 1977) I have fitted the lines by distance-weighted least-squares smoothing field observattons at camps situated along ati elevational gradient on the humid east slope of the Peruvian Andes, Terborgh showed that species nchness declined monotonically with elevation if the number of species is simply plotted against elevation (Fig lA) However, when Terborgh tned to eliminate the effect of sampling time by standardizmg his mist-netting data, a different pattern emerged (Fig IB) Terborgh explained the emerging "hump-shaped" curve, which had a peak in species nchness around 1400 m, as the result of a local "hot-spot" in resources (Terborgh 1977) Although this unexpected pattern is addressed through most of his discussion, only the first non-standardized graph is usually cited in the literature (e.g Brown 1988) Though the Ube of underA 2500 in storey mist-netting data limited the scope of this study, it serves to demonstrate the strong effect that sampling effort can exert - especially in species-nch tropical forest where most species occur at low densities The importance of area The effect of area on the relationship between spwcies nchness and elevation has rarely been considered, although we would expect area to have a significant impact on the form of the elevational pattern, as the relationship between area and species nchness seems as universal as ffl ELEVATION (M) ELEVATION (M) Fig 2 Species nchness of South Amencan tropical landbirds versus elevation Figure 2A is based on data not standardized for area, whereas Fig 2B is based on data standardized for area set to 50000 km^ using equations for species-area curves (log S/log A transformation) of each elevauonal zone (based on data from Rahbek unpubl) Area is set to 50000 km' because it is within the range of the ongmal data upon which the equaUons are based, and is a size within the geographical regional scale (e g Wiens 1981, Rosenzweig and Abramsky 1993) ECOGRAPHY 18 2 (1995) Table 2 Number of data sets (n = 163) found in the literature with data on the variation of species nchness with elevation summanzed by biomes (NT = non-tropical biomes, T = tropical biomes) and scale (R = regional, L = local), listed for studies conducted on mainland and island, respectively, and subdivided for whether the researcher(s) have attempted to standardize for the effect of area and sampling regime and/or effort, only one of these factors, or none ("non-standardized") Invertebrates Vertebrates NT(R/L) T(R/L) Plants NT (R/L) T(R/L) Totals NT (R/L) T (R/L) NT(R/L) T(R/L) Mainland Area and sampling Area Sampling Non-standardized Island Area and sampling Area Sampling Non-standardized Total 0/2 0/0 4/1 9/11 1/0 0/0 0/0 0/5 14/19 0/0 0/0 3/4 4/6 3/2 1/0 0/5 2/5 13/22 0/6 0/0 0/6 3/0 0/0 0/0 0/0 0/0 3/12 0/1 1/0 3/1 17/4 0/0 0/3 0/1 5/2 26/12 0/10 0/0 0/1 1/0 0/4 0/0 0/0 0/0 1/15 0/6 4/3 0/0 8/3 0/0 0/0 0/0 2/0 14/12 0/18 0/0 4/8 13/11 1/4 0/0 0/0 0/5 18/46 0/7 5/3 6/5 29/13 3/2 1/3 0/6 9/7 53/46 the latitudinal gradient To understand the relationship between area and species nchness along elevational gradients, especially on a regional scale, the effect of area must be considered since areas of equal-sized elevational belts may vary with elevation Thus, areas often decrease with elevation because of generally steeper terrain toward the highest peaks When landbird data from tropical South Amenca, compiled at a regional scale using countries as units, are standardized for area, the relationship between species nchness and elevation gives a humpshaped curve (Fig 2) However, area alone is unlikely to explain any global pattern of species nchness, as close couplings can be expected to exist between biological diversification and habitat complexity (see also Rosenzweig 1992) A quantitative review of the literature The repeated citation of the same few studies provides a false picture of the amount and diversity of data published on the issue I have been able to find the surpnsmgly high number of 97 papers, with 163 examples that give data on the vanation in number of species with elevation. It is highly probable that additional data exist, as many data sets are published in httle known journals, or in the "gray" literature. Table 2 summanzes some charactenstics of each data set (taxonomic group, region, scale and data treatment) The mfluences of sampling regime/effort and the effect of area are among the most influential biasing factors in most field studies of species-nchness patterns, and, unfortunately, equally difficult to eliminate successfully I have thus only judged whether an attempt was made to deal with these two factors, either in the design of the survey or afterward, during the data analyses. Remaricably, many of the pajjers reviewed do not give any details 202 on this subject These data sets are classified as "Nonstandardized" together with those with no attempt to standardize data (Table 2) Unlike the traditional histoncal trend within most fields of biological research, most data sets are from the tropics (99 out of 163) The focus on the tropics is presumably related to the circumstance that tropical elevational gradients compnse a wider range of climatic vanation than temjserate elevational gradients Out of 163 data sets, 68 are on invertebrates, 53 on vertebrates (including 36 on birds), and 42 on plants The majonty of the data sets comes from mainland biota (122), whereas 41 data sets are fi-om islands As shown, standardizing for sampling effort or effect of area can have a significant influence on the emerging shape of the relationship between sjjecies richness and elevation In 87 of the 163 data sets, the data have not been standardized for area or sampling effort (corresponding figures for tropical and non-tropical biomes are 59% and 45%) Only in 35 (21 %) cases has a standardization been attempted for area as well as sampling effort (figures for tropical and non-tropical biomes are 12% and 36%, respectively) Considenng the high mobility of birds compared to other groups, the reliance by most reviewers on primanly bird examples to illustrate a universal relationship seems ill-founded, especially when the bulk of data in the hterature actually denves from invertebrates and plants Table 2 also provides an overview for which combmations of, for example, taxonomic groups, scale, and region we lack studies - especially those that consider the effect of area and samphng regime on data. Methodological problems and patterns In many studies, a stated decline in species nchness with ECOGRAPHY 18 2 (1995) Table 3 The relationship between species nchness and elevation summarized by type of pattern Only the 90 data sets (of 163) that provide data points spanning from below 500 m to above 1500 m are included (see text) The classification of each pattern is based on a visual examination of bivanate plots NT= non-tropical biomes and T = tropical biomes Scale Monotomcally decreasing Horizontal, then decreasing NT T Hump-shaped Increasing Other NT NT Regional Invertebrates Vertebrates Plants Total regional Local Invertebrates Vertebrates Plants Total local Total 1 1 NT NT elevation was restncted to only a part of the elevational gradient In other cases, mid-elevational data were lacking Others used correlation tests on data sets that include few stations from low- and mid-elevation, but many from higher elevations, thereby biasing their findings toward a strong negative correlation In such instances, the data are inapplicable to support a monotonic relationship Conclusions based on correlation tests sometimes ignore that stations at mid-elevation actually have more species than stations at low elevation To analyze the general vanation of species richness with elevation, a minimum requirement for any data set is that It includes data spanning the entire gradient, albeit it becomes lncreasmgly difficult to find appropnate gradients with continuous natural habitat along the entire gradient This is es{)ecially a problem with respect to lowland stations, as lowlands and foothills often are the most disturbed elevational zone(s) Of the 163 data sets, 47 do not include data from below 500 m. In the descriptive analysis of the elevational pattern of species nchness, I have only included data sets which are based on a gradient from below 500 m to above 1500 m (see Table 3) This hmits the analysis to 90 data sets of the onginal 163 37 on invertebrates, 26 on vertebrates (lncludmg 19 on birds), and 27 on plants. As for the entire data set, this subsample of data sets is biased toward the tropics with 73 data sets compared to only 17 from non-tropical biomes It also has more data sets from mainland (n = 71) than islands (n = 19) Non-standardized data sets are dominant (n = 49) In only 13 data sets (14%) have attempts been made to take the effect of area and sampling into account. The corresponding figure for tropical data sets, which have been the main source of generalizations on the elevational gradient, is only 7% (5 of 73 tropical data sets), in contrast to 47% of the nontropical data sets ECOGRAPHY 18 2 (1995) To conduct proper descnpuve statistical analyses of the vanation of species nchness with elevation, the stations (l e the data points) must be reasonably equally distnbuted over the gradient, and the number of data points sufficient to refiect any marked changes in habitats/ biomes over the analyzed gradient. Unfortunately these two requirements for an optimal data set are rarely fulfilled As It IS difficult to judge especially the latter cntenon for most of the published data no attempt has been made to select or exclude data on this basis. Compansons of elevational patterns between taxa, latitudinal climatic zones, biogeographic regions or mainland versus islands could be misleading without correction for the area effect and differences in sampling regime Compansons of studies are also biased by differences in the species included, and sometimes further by limitations to Sjjecific trophic levels, guilds or habitat These problems and the pronounced heterogeneity of the quality of data sets make it difficult to conduct proper cntical statistical tests for each data set of the relationship between sp>ecies nchness and elevation that are mutually comparable Still, disregarding these biases, a rough companson based on a classification of the pattern in each data set by visual examination of bivanate plots serves to illustrate how ambiguous the pattern is, both withm region, spatial scale and crude taxonomic groupmgs (Table 3). A decline in species nchness with elevation seems to be a general trend Yet, a pattern where the speciesrichness curve IS almost honzontal up to a certain elevaUon before declining, or is hump-shaped, seems more typical than a monotomc declme (Table 3) ELEVATION (M) Fig 3 Patterns of species nchness versus elevation may vary within the same area for different taxa, here illustrated by New Guinean bats (*) and rodents (•) (data compiled ', T 1990 Mammals of New Guinea - Robert Brown the position of humps or bend of curves vanes between and Ass . Sydney different sets of data Vanous taxa are also differently Hutchinson. G E 1959 Homage to Santa Rosalia, or "why are there so many kinds of animals''" - A m Nat 93 145-159 affected by abiotic factors, such as, for example, humidKikkawa. J and Williams, E E 1971 Altitude distnbution of ity land birds in New Guinea - Search 2 64-65 Further studies, including analyses of pnmary-level MacArthur. R H 1972 Geographical ecology - Harper and Rowe New York processes that could influence the pattern, are needed to Mares. M Publ ,1992 Neotropical mammals and the myth of A reveal whether general patterns exit within biogeographic Amazonian biodiversity - Science 25 976-979 regions, taxa, spatial scale, mainland versus islands, etc , Preston, F W 1962 The canonical distribution of commonness and rarity - Ecology 43 185-215, 410-^32 or various combinations of these Appropriate null-models should be considered before explaining elevational Rohde, K 1992 Latitudinal gradients in species diversity the search for the pnmary cause - Oikos 65 514-527 gradient patterns as results of climatic, biological and Rosenzweig, M L 1971 Paradox of ennchment destabilization histoncal processes (Colwell and Hurtt 1994) Although a of exploitation ecosystem in ecological time - Science 171 385-387 difficult task, the development of testable hypotheses is - 1992 Species diversity gradients we know more and less important to achieve significant progress within this field than we thought - J Mammal 73 715-730 and to contnbute to our general understanding of di- and Abramsky. Z 1993 How are diversity and productivity versity patterns This would be preferable to founding related"' - In Ricklefs, R and Schluter, D (eds). Species diversity in ecological communities Histoncal and geonew generalizations on simple accumulations of case graphical perspectives Univ of Chicago Press, Chicago, pp studies Much is still to be learned about this topic, for 52-65 now, we have to accept the unsatisfactory realization that Stevens. G C 1992 The elevational gradient in altitudmal we do not know whether a general relationship exists range an extension of Rapoport's latitudinal rule to altitude between species nchness and elevation, or whether an - A m Nat 140 893-911 Terborgh. J 1977 Bird species diversity on an Andean elevauniversal explanation or model can be given tional gradient - Ecology 58 1007-1019 Wiens, J A 1981 Scale problems in avian censusing - S t u d Avian Biol 6 513-521 Acknowledgements - 1 thank G Graves and J Fjeldsa for many discussions and cnUcally reading the manuscnpt Also thanks to World Conservation Monitonng Centre 1992 Global diversity status of the Earth's living resources - Chapman and Hall, J Lovett,J M CardosadaSilva,R Zusi,D ThurberandM E London Petersen for their cntical comments to the manuscnpt The study was conducted dunng a stay at The National Museum of Natural Wnght, D H 1983 Species-energy theory an extension of species-area theory - Oikos 41 496-506 History, Smithsonian Instituuon supported by a Fulbnght grant ECOGRAPHY 18 2(199'>) physical causes, including climatic factors, the narrow width of the elevational gradient, and histoncal perturbations that shape the available species pool Biotic mteractions are secondary mechamsms that can be reflected in the emergent patterns The vanation of sjjecies nchness with elevation might be connected to the reduction of temjjerature with elevation and the assumed corresfwndlng reduction in productivity However, other factors such as vanation in steepness, geological perturbations, alterations of precipitation patterns, etc might also be involved, probably with varying impact from case to case For any correlation of species nchness with variables measured over an elevational gradient, we need to examine whether the correlation reflects a direct coupling, or if It could be a result of interactions of several other factors The complete list of papers which this Forum contnbution is based on is available from the author

Journal

EcographyWiley

Published: Jun 1, 1995

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