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Spatial patterns in the species richness of birds in the New World

Spatial patterns in the species richness of birds in the New World Blackburn, T M and Gaston, K J 1996 Spatial pattems in the species nchness of birds in the New World - Ecography 19 369-376 Spatial pattems m species nchness of the complete New World avifauna were analysed, using data previously employed to examme spatial trends in geographic range size This allowed vanation in the pattems to be compared Species richness was highest around the equator, and decreased towards higher latitudes in both hemispheres This decrease was asymmetncal, at equivalent latitudes, richness was higher in the southem than in the northem hemisphere, although the reverse was true for a measure of endemism Controlling for latitude, species nchness was higher in the west than m the east The net primary productivity of, and solar radiation received by an area were both correlated with species nchness However, neither explained more variation in richness than did latitude No single mechanism developed to explain spatial pattems in species nchness satisfactonly explains the pattems observed in the New World avifauna We discuss, reasons why this might be the case Finally, we point out that species nchness at low latitudes is not simply a multiplication of nchness at high latitudes, species found at high and low latitudes are unlikely to be ecologically equivalent Any mechanism that is proposed to explain nchness pattems in New World birds will need also to account for this observation T M Blackburn (correspondence), NERC Centre for Population Biologv, Imperial College at Silwood Park, Ascot, Berkshire, U K SL5 7PY (t blackburn@ic ac uk) ~ K J Gaston, Dept of Animal and Plant Sciences, Umv of Sheffield, Sheffield, U K SIO 2TN In a previous paper (Blackburti atid Gaston 1996) we started a macroecological examination of one of the two defining vanables tn ecology, species distribution We demonstrated relationships between the size of a species' geographic range and its body size, migratory behaviour, probabihty of extinction and latitudinal position, usmg the complete (as currently known) avifauna of the New World as our study assemblage Several of the patterns shown were not expected from existing macroecological theory In particular, the tendency for species geographic ranges to decrease with latitude ('Rapoport's rule', Stevens 1989) was found not to hold below ca 17°N. In this paper, we develop our study of the distribution of birds m the New World from consideration of patterns in geographic range size to consideration of pattems in the coincidence of geographic ranges or, put more conventionally, patterns in species richness Species nchness and geographic range size are related aspects of the spatial distnbution of species, and a causal link between the two has even been hypothesised (Stevens 1989) It IS clear that the species richness of a region (we are not concerned here with local pattems) is not spatially homogenous Although there are exceptions (e g Owen and Owen 1974, Janzen 1981, Rabenold 1993, Williams 1993, Smith et al 1994), most taxa exhibit highest species richness m the tropics, with nchness decreasing as latitude increases (for birds see Dobzhansky 1950, Accepted 16 February 1996 Copynght © ECOGRAPHY 1996 ISSN 0906-7590 Pnnted in Ireland - all nghts reserved ECOORAPHY 194 (19%) to produce plausible hypotheses that can explain a reverse trend of higher richness towards the Poles The common mechanism argument seems implicitly to assume that It should be just as easy to produce reversed richness trends this may not be the case If the common mechanism yardstick for judging richness pattems was relaxed, some of the hypotheses rejected as circular by Rohde (1992) would require reconsideration The third possibility is that richness pattems in individual taxa have multiple causes (which may differ between taxa) For example, the pattem of nchness in New World birds cannot simply be explained by greater evolutionary speed in the tropics, because this does not predict a longitudinal nchness gradient However, the longitudinal gradient might be caused by greater habitat diversity in western America, in tum caused by a greater range of elevations there The longitudinal pattern could be overlaid on a tendency for faster evolution m the tropics If the species nchness of a group in an area does have multiple causes, it will be difficult to differentiate between altemative hypotheses for what determines the pattem, the failure of any one hypothesis to explain part of any observed patterns can then always be attributed to the confounding effect of a second mechanism If species nchness does indeed have multiple causes, as seems likely to us, far more rigorous tests of the altemative hypotheses than have so far been attempted will be required Finally, any explanation of species nchness will need to address the fact that richness at low latitudes is not simply a multiplication of nchness at high latitudes Rather, the latitudinal pattems in endemism and nchness indicate that the latter is higher at low latitudes because of the accrual of species of low range size This IS a pattem that is also partly reflected m the trend towards decreasing mean geographic range size at low latitudes (Blackbum and Gaston 1996), although the two trends are quantitatively different, they may serve to reinforce each other Thus, endemism patterns indicate that species found at high and low latitudes are unlikely to be ecologically equivalent, an inference for which there is some evidence For example, the migratory habits of New World bird species vary with latitude (Blackbum and Gaston 1996), while Bergmann's rule suggests that species at high latitudes are more likely to be large-bodied (Bergmann 1847, cited in James 1970) If so, since body size is correlated with many aspects of avian life history (Bennett and Harvey 1987, Saether 1987, 1989, Trevelyan et al 1990), it implies that species nchness may interact with species biologies in a complicated fashion This is, of course, speculation, but the life histones of tropical and temperate sjjecies are known to be quantitatively different in some respects (e.g MacAnhur 1972), and it is not impossible that there might be feed-back into pattems of species nchness Analyses of spatial pattems m body size m New World birds, to complement those m ECOGRAPHY 19 4 (1996) distribution, are clearly desirable, and are addressed in a subsequent paper (Blackbum and Gaston in press) Acknowledgements - We thank L Birch, E Warr, J Lawton and N Loder for assistance with data sources, P Williams for the WORLDMAP program, C Thebaud for suggesting and performing the boot-strap analysis, and G Graves, R Gregory, J Kouki, J Lawton, J Prendergast and P Williams for comments on this work T M B was supported by a NERC grant K J G is a Royal Society Umv Research Fellow http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Ecography Wiley

Spatial patterns in the species richness of birds in the New World

Blackburn, Tim M.; Gaston, Kevin J.
Ecography , Volume 19 (4) – Dec 1, 1996
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References (61)

Publisher
Wiley
Copyright
Copyright © 1996 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0906-7590
eISSN
1600-0587
DOI
10.1111/j.1600-0587.1996.tb00001.x
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See Article on Publisher Site

Abstract

Blackburn, T M and Gaston, K J 1996 Spatial pattems in the species nchness of birds in the New World - Ecography 19 369-376 Spatial pattems m species nchness of the complete New World avifauna were analysed, using data previously employed to examme spatial trends in geographic range size This allowed vanation in the pattems to be compared Species richness was highest around the equator, and decreased towards higher latitudes in both hemispheres This decrease was asymmetncal, at equivalent latitudes, richness was higher in the southem than in the northem hemisphere, although the reverse was true for a measure of endemism Controlling for latitude, species nchness was higher in the west than m the east The net primary productivity of, and solar radiation received by an area were both correlated with species nchness However, neither explained more variation in richness than did latitude No single mechanism developed to explain spatial pattems in species nchness satisfactonly explains the pattems observed in the New World avifauna We discuss, reasons why this might be the case Finally, we point out that species nchness at low latitudes is not simply a multiplication of nchness at high latitudes, species found at high and low latitudes are unlikely to be ecologically equivalent Any mechanism that is proposed to explain nchness pattems in New World birds will need also to account for this observation T M Blackburn (correspondence), NERC Centre for Population Biologv, Imperial College at Silwood Park, Ascot, Berkshire, U K SL5 7PY (t blackburn@ic ac uk) ~ K J Gaston, Dept of Animal and Plant Sciences, Umv of Sheffield, Sheffield, U K SIO 2TN In a previous paper (Blackburti atid Gaston 1996) we started a macroecological examination of one of the two defining vanables tn ecology, species distribution We demonstrated relationships between the size of a species' geographic range and its body size, migratory behaviour, probabihty of extinction and latitudinal position, usmg the complete (as currently known) avifauna of the New World as our study assemblage Several of the patterns shown were not expected from existing macroecological theory In particular, the tendency for species geographic ranges to decrease with latitude ('Rapoport's rule', Stevens 1989) was found not to hold below ca 17°N. In this paper, we develop our study of the distribution of birds m the New World from consideration of patterns in geographic range size to consideration of pattems in the coincidence of geographic ranges or, put more conventionally, patterns in species richness Species nchness and geographic range size are related aspects of the spatial distnbution of species, and a causal link between the two has even been hypothesised (Stevens 1989) It IS clear that the species richness of a region (we are not concerned here with local pattems) is not spatially homogenous Although there are exceptions (e g Owen and Owen 1974, Janzen 1981, Rabenold 1993, Williams 1993, Smith et al 1994), most taxa exhibit highest species richness m the tropics, with nchness decreasing as latitude increases (for birds see Dobzhansky 1950, Accepted 16 February 1996 Copynght © ECOGRAPHY 1996 ISSN 0906-7590 Pnnted in Ireland - all nghts reserved ECOORAPHY 194 (19%) to produce plausible hypotheses that can explain a reverse trend of higher richness towards the Poles The common mechanism argument seems implicitly to assume that It should be just as easy to produce reversed richness trends this may not be the case If the common mechanism yardstick for judging richness pattems was relaxed, some of the hypotheses rejected as circular by Rohde (1992) would require reconsideration The third possibility is that richness pattems in individual taxa have multiple causes (which may differ between taxa) For example, the pattem of nchness in New World birds cannot simply be explained by greater evolutionary speed in the tropics, because this does not predict a longitudinal nchness gradient However, the longitudinal gradient might be caused by greater habitat diversity in western America, in tum caused by a greater range of elevations there The longitudinal pattern could be overlaid on a tendency for faster evolution m the tropics If the species nchness of a group in an area does have multiple causes, it will be difficult to differentiate between altemative hypotheses for what determines the pattem, the failure of any one hypothesis to explain part of any observed patterns can then always be attributed to the confounding effect of a second mechanism If species nchness does indeed have multiple causes, as seems likely to us, far more rigorous tests of the altemative hypotheses than have so far been attempted will be required Finally, any explanation of species nchness will need to address the fact that richness at low latitudes is not simply a multiplication of nchness at high latitudes Rather, the latitudinal pattems in endemism and nchness indicate that the latter is higher at low latitudes because of the accrual of species of low range size This IS a pattem that is also partly reflected m the trend towards decreasing mean geographic range size at low latitudes (Blackbum and Gaston 1996), although the two trends are quantitatively different, they may serve to reinforce each other Thus, endemism patterns indicate that species found at high and low latitudes are unlikely to be ecologically equivalent, an inference for which there is some evidence For example, the migratory habits of New World bird species vary with latitude (Blackbum and Gaston 1996), while Bergmann's rule suggests that species at high latitudes are more likely to be large-bodied (Bergmann 1847, cited in James 1970) If so, since body size is correlated with many aspects of avian life history (Bennett and Harvey 1987, Saether 1987, 1989, Trevelyan et al 1990), it implies that species nchness may interact with species biologies in a complicated fashion This is, of course, speculation, but the life histones of tropical and temperate sjjecies are known to be quantitatively different in some respects (e.g MacAnhur 1972), and it is not impossible that there might be feed-back into pattems of species nchness Analyses of spatial pattems m body size m New World birds, to complement those m ECOGRAPHY 19 4 (1996) distribution, are clearly desirable, and are addressed in a subsequent paper (Blackbum and Gaston in press) Acknowledgements - We thank L Birch, E Warr, J Lawton and N Loder for assistance with data sources, P Williams for the WORLDMAP program, C Thebaud for suggesting and performing the boot-strap analysis, and G Graves, R Gregory, J Kouki, J Lawton, J Prendergast and P Williams for comments on this work T M B was supported by a NERC grant K J G is a Royal Society Umv Research Fellow

Journal

Ecography – Wiley

Published: Dec 1, 1996

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