- Email: [email protected]
Patterns in the distribution and abundance of grassland species
NEWS
&
COMMENT
Oceanic foraging flights The Crozet wandering albatrosses forage in all directions from the breeding island, irrespective of prevailing wind direction235Jl. The shape of the route can vary considerably, and the albatrosses do not use a search mechanism when returning to the island, but instead they usually approach the breeding island along a straight course even in crosswinds. Based on satellite trackings reported in the literature, Papi and Luschi concluded that the green turtles and the albatrosses probably rely on a mechanism of true navigation to find their way on these transoceanic travels5. With such a mechanism, the sea turtles and albatrosses can, based on local cues, calculate a specific direction that is then located based on compass information. True navigation can be based on a mosaic map or two physical and/or chemical gradients8J3X14.The relative location of the individual features contributing to a mosaic map, for example, topographical landmarks or local deviations of the geomagnetic field parameters, has to be learned by individual exploration and such a map is therefore limited in its extension. Because the sea turtles are myopic out of water and so, for example, are unable to see stars, and because they experience changing directions of ocean currents that are comparable with the changing wind conditions that birds experience during longdistance travels, visual and chemical cues might not prove to be as reliable on ocean migrations as a position-fixing mechanism. Instead an oceanwide position-finding mechanism based on the Earth’s magnetic field seems more plausible. The results reported by Lohmann and Lohmann on magnetic orientation by
hatchling loggerhead sea turtles is the first evidence that an animal can perceive two parameters that potentially contribute to a geomagnetic bicoordinate map. These data and the satellite trackings of adult sea turtles and foraging albatrosses, however, provide only circumstantial evidence for a geographic position-system based on the Earth’s magnetic field. Furthermore, one cannot rule out that the animals use a combination of different cues, for example, visual, magnetic and cheniical, to guide them. Such cues might be important in different phases of their journey. That animals performing longdistance travels in the ocean can use geomagnetic information for orientation and navigation is now quite clear (for reviews, see Refs 5,9; also see Ref. 15), but how they perceive the magnetic field remains a mystery. To solve the physiological function of the magnetic sense must be one of the greatest challenges that lies ahead. Acknowledgements Thanks to T. Ale&am, A. Hedenstram and R. Wehner for helpful comments on an earlier draft of this manuscript. Susanne Akesson Dept of Zoology, Ziirich University Winterthurerstrasse 190, CH-8057 Ziirich, Switzerland
References Lohmann, K.J. and Lohmann, C.M.F.(1996) Detection of magnetic field intensity by sea turtles, Nature 380,59-61 Jouventin, P. and Wiemerskirch, H. (1990) Satellite tracking of wandering albatrosses, Nature 343, 746-748
Patterns in the distributionand abundance of grasslandspecies
A
ndrewartha’sl definition of ecology as the study of the ‘distribution and abundance of organisms’ also serves well to define one of the central missions of ecology: to understand spatial and temporal patterns in the relative densities of species. Preston2 referred to such patterns as the ‘commonness and rarity of species’ and his early studies were instrumental in motivating ecologists to quantify such patterns rigorously and to find the mechanisms and processes that account for them. The search for mechanism and process in ecology, however, has 400
0
1996, Elsevier
Science
Ltd
often led to advances in theoretical and experimental knowledge, while our understanding of what governs patterns in relative abundance distributions (RADs) has lagged behind. That is, there is a tendency to get the theoretical and experimental ‘cart’in front of the RAD‘horse’that should be driving community ecology. The recent study by Wilson et a13 is an attempt to get the horse back in front of the cart. As Wilson et al. note at the outset of their paper, it is remarkable what little empirical information exists on how well patterns in RADs fit the predictions of
3 Luschi, P. et al. (1996) Long-distance migration and homing after displacement in the green turtle (Chelonia my&s): a satellite tracking study, J. Camp.Physiol.A 178, 447-452
4 Papi, P., Liew, H. and Luschi, P. (1995) Long-range migratory travel of a green turtle tracked by satellite: evidence for navigation ability in the open sea, Mar. Biol. 122, 171-175 5 Papi, F. and Luschi, P. (1996) Pinpointing ‘Isla Meta’:the case of sea turtles and albatrosses, J. @p. Biol. 199,65-71 6 Lohmann, K.J. and Lohmann, C.M.F.(1994) Detection of magnetic inclination angle by sea turtles: a possible mechanism for detection of latitude, J Exp. Biol. 194, 23-32 7 Lohmann, K.J. and Lohmann, C.M.F.(1996) Orientation and open-sea navigation in sea turtles, J Exp. Biol. 199, 73-81 8 Wallralf, H.G. (1991) Conceptual approaches to avian navigation systems, in Orientation in Birds (Berthold, P., ed.), pp. 38-51, Birkhauser 9 Wiltschko, R. and Wiltschko, W. (1995) Magnetic Orientationin Animals, Springer-Verlag 10 Alerstam, T. (1996) The geographical scale factor in orientation of migrating birds, J. Exp. BioL 199,9-19 11 Wiemerskirch, H. et al (1993) Foraging strategy of wandering albatrosses through the breeding season: a study using satellite telemetry, Auk 110,325-342 12 Prince, P. et al. (1992) Satellite tracking of wandering albatrosses (Diomedea exulans) in the South Atlantic, Antarct. Sci. 4, 31-36
13 Walcott, C. (1991) Magnetic maps in pigeons, in Orientation in Birds (Berthold, P., ed.), pp. 38-51, Birkhluser 14 Walcott, C. (1996) Pigeon homing: observations, experiments and confusions, J. Exp. Biol. 199,21-27 15 Quinn, T.P. (1994) How do sharks orient at sea? Trends Ecol. Evol. 9,277-278
major theories in community ecology. These authors focus on resource-based competition theory and successional theory, two important areas in ecology that predict how RADsshould vary in response to changing nutrient levels or aging of a community. Other areas in ecology, however, such as food-web theory4, competition-based theory5, Lotka-Volterrabased community theory6, and islandbiogeographic theory7, also exhibit the same shortcomings. Wilson et a/. examine plant species abundances in three, well-studied grassland communities in the UK. These communities are the experimental plots at Monk’s Wood in Huntingdonshire, the Park Grass experimental plots in Hertfordshire, and the Compton grassland plots at Wolverhampton. They examine how grassland RADs vary in response to resource
PII: SO169-5347(96)30039-s
TREE
vol.
I I, no.
I0 October
1996
NEWS manipulations at Park Grass and at Compton as a way of testing the predictions of resource-based community theory. They also examine how RADsvary over time at Monks Wood and at Compton in order to test the predictions of successional theory. The authors focus on three aspects of RADs:(1) evenness, (2) the fit of observed distributions to biologically-based RADs, and (3) rank-abundance consistency (C’J among temporal or spatial replicates. Evenness is a measure of uniformity in a proportional representation of species (either by biomass or by density) within a community. There are many indices, each with its pros and con+10, but Wilson et al. settle for one used by Camargo because it is insensitive to variation in species richness (the number of species in a community). The authors also examine the fit of observed RADs to four well-known, biologically-based, theoretical distributions: the Broken Stick, Geometric, General Lognormal, and Zipf-Mandelbrot distributionslz. The theoretical distributions that best fit the observed RADs are compared with patterns predicted from theory. Finally, C, is quantified by measuring variation in the pattern of rankings of densities of species from one place to another, or one time to another, using a method devised by Watkins and Wilsonl3. High spatial or temporal consistency of rank abundances supports intrinsic, biotic processes as being important processes regulating RADs.Alternatively, low consistency supports extrinsic environmental factors, like weather or variation in local abiotic conditions, as being important factors regulating RADs. The authors essentially describe a three-axis space describing (1) patterns in distribution and abundance comprising evenness, (2) the fit to biologically-based theoretical RADs, and (3) C,. If communities reside near the origin, where evenness is low, the observed RADfits no biologically-based theoretical RAD,and there is no spatial or temporal consistency in rank abundance, then the communities are likely to be regulated by extrinsic, abiotic processes. At the other extreme, if evenness is high, the observed RADclosely fits biologically-based theoretical RADs, and rank-abundances are highly consistent irrespective of space or time, then the community is more likely to be structured by intrinsic or biotic processes. Wilson et al. found that the observed response of RAD to resource manipulations did not fit predictions of resourcebased competition theory but did fit their model for succession. RAD responses to phosphorus treatments in Park Grass showed some changes in agreement with resource-based theory, however, few consistent responses to other manipulations TREE
uol. I I, no. IO October
1996
were observed. The Compton plots showed no constant responses at all. For successional theory, however, temporal trends in evenness, in the fit of observed RADs to the Broken Stick and Geometric RADs, and in C,.,were largely consistent with those predicted by Gitay and Wilson’s three-phase model of successionl4. How does one interpret these findings of Wilson et al.? Although the fit of observed RADs to theoretical RADs cannot tell us about biological processesaJ5, if communities shift within Wilson et d’s three-axis space then such a response qualitatively provides evidence to reject a biology-free, random, or null explanation for variation in RAD.If, for example, nutrient-based competition were regulating these systems, then manipulations of nutrients should have yielded declining evenness (increasing dominance of common species), predictable changes in bestfit theoretical RAD,and low C,among plots. It seems clear (with the possible exception of phosphorus in the Park Grass plots) that nutrient-based competition is not sup ported in this study as the factor responsible for grassland community composition and structure because there is very little support for patterns produced by biotic or intrinsic factors. Conversely, if succession was largely an abiotic process, then the predicted patterns in changing evenness, fit to the Broken Stick and Geometric RADs, and patterns in C,.,should not have been observed. Should we accept their conclusions? Does resource-based theory fail to describe adequately distribution and abundance in grasslands? Does the program of assembly processes predicted by threephase succession best describe distribution and abundance in grasslands while other successional theories fail? Like most important ecological studies, it is the challenge that such a study raises that is its principle message, not its particular findings. RADsare notoriously slippery measures of communities, and the processes behind their origin, such as how species divide resources, are barely known. Wilson et d’s study encourages us to apply similar, pluralistic methods to other systems. The mechanisms of resource-based ecology have been widely studied in laboratory and field experiments, the challenge now is to apply them more broadly to patterns in distribution and abundance. Results from such studies may contribute to our understanding of how communities may respond to changing levels of nutrients in a world where CO, and N fertilization is increasing worldwidel’j. For prop@ nents of other assembly rules that govern successional patterns, the challenge is to demonstrate that competing models can similarly generate the temporal patterns
8i COMMENT
Wilson et al. found. Further analyses of patterns in distribution and abundance will provide new momentum for our theoretical and experimental research. Shahid Naeem Dept of Ecolosy Evolution and Behavior, and Center for Community Genetics, Universily of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, St Paul, MN 55108, USA ([email protected])
References 1 Andrewartha, H.G. (1961) Introductionto the
2
3
4
5
6
7
8 9
10 11
12
13
StudyofAnimal Populations, University of Chicago Press Preston, F.W. (1962) The canonical distrfbution of commonness and rarity, Ecology43,185-215; 410-432 Wilson, J.B. etal. (1996) Are there assembly rules for plant species abundance? An investigation in relation to soil resources and successional trends, 1 Ecol. 84, 527-538 Lawler, S.P. and Morin, P.J. (1993) Food web architecture and population dynamics in laboratory microcosms of protists, Am. Nat. 141,675-686 Kelt, D.A.,Taper, M.L. and Meserve, P.L. (1995) Assessing the impact of competition on community assembly: a case study using small mammals, Ecology76,1283-1296 de Ruiter, P.C., Neutel, A. and Moore, J.C. (1995) Energetics, patterns of interaction strengths, and stability in real ecosystems, Science 269, 1257-1260 Colwell, R.K.and Winkler, D.W. (1984) A null model for null models in biogeography, in Ecological Communities: Conceptual Issues and the Evidence (Strong, D.R.,Jr et al., eds), pp. 344-359, Princeton University Press Pielou, E.C. (1975) Ecological Diversity,Wiley Bulla, L. (1994) An index of evenness and its associated diversity measure, Oikos 70, 167-171 Magurran, A.E. (1988) Ecological Diversityand its Measurement, Princeton University Press Camargo, J.A. (1993) Must dominance increase with the number of subordinate species in competitive interactions? J. Theor. Biol. 161,537-542 Wilson, J.B. (1993) Would we recognize a Broken-Stickcommunity if we found one? Oikos 67, 181-183 Watkins, A.J. and Wilson, J.B. (1994) Plant community structure and its relation to the vertical complexity of communities: dominance/diversity, spatial rank consistency and species richness, Oikos 70,91-98
14 Gitay, H. and Wilson, J.B. (1995) Post&~ changes in community structure of tall tussock grasslands: a test of alternative models of succession, and a new ‘three-phase?model, J. Ecol. 83,775-782 15 May, R.M. (1975) Patterns of species abundance and diversity, in Ecology and Euolution of Communities (Cody, M.L. and Diamond, J.M., eds), pp. 81-120, Belknap Press 16 Vitousek, P.M. (1994) Beyond global warming: ecology and global change, Ecology 75,1903-1910
401
&
COMMENT
Oceanic foraging flights The Crozet wandering albatrosses forage in all directions from the breeding island, irrespective of prevailing wind direction235Jl. The shape of the route can vary considerably, and the albatrosses do not use a search mechanism when returning to the island, but instead they usually approach the breeding island along a straight course even in crosswinds. Based on satellite trackings reported in the literature, Papi and Luschi concluded that the green turtles and the albatrosses probably rely on a mechanism of true navigation to find their way on these transoceanic travels5. With such a mechanism, the sea turtles and albatrosses can, based on local cues, calculate a specific direction that is then located based on compass information. True navigation can be based on a mosaic map or two physical and/or chemical gradients8J3X14.The relative location of the individual features contributing to a mosaic map, for example, topographical landmarks or local deviations of the geomagnetic field parameters, has to be learned by individual exploration and such a map is therefore limited in its extension. Because the sea turtles are myopic out of water and so, for example, are unable to see stars, and because they experience changing directions of ocean currents that are comparable with the changing wind conditions that birds experience during longdistance travels, visual and chemical cues might not prove to be as reliable on ocean migrations as a position-fixing mechanism. Instead an oceanwide position-finding mechanism based on the Earth’s magnetic field seems more plausible. The results reported by Lohmann and Lohmann on magnetic orientation by
hatchling loggerhead sea turtles is the first evidence that an animal can perceive two parameters that potentially contribute to a geomagnetic bicoordinate map. These data and the satellite trackings of adult sea turtles and foraging albatrosses, however, provide only circumstantial evidence for a geographic position-system based on the Earth’s magnetic field. Furthermore, one cannot rule out that the animals use a combination of different cues, for example, visual, magnetic and cheniical, to guide them. Such cues might be important in different phases of their journey. That animals performing longdistance travels in the ocean can use geomagnetic information for orientation and navigation is now quite clear (for reviews, see Refs 5,9; also see Ref. 15), but how they perceive the magnetic field remains a mystery. To solve the physiological function of the magnetic sense must be one of the greatest challenges that lies ahead. Acknowledgements Thanks to T. Ale&am, A. Hedenstram and R. Wehner for helpful comments on an earlier draft of this manuscript. Susanne Akesson Dept of Zoology, Ziirich University Winterthurerstrasse 190, CH-8057 Ziirich, Switzerland
References Lohmann, K.J. and Lohmann, C.M.F.(1996) Detection of magnetic field intensity by sea turtles, Nature 380,59-61 Jouventin, P. and Wiemerskirch, H. (1990) Satellite tracking of wandering albatrosses, Nature 343, 746-748
Patterns in the distributionand abundance of grasslandspecies
A
ndrewartha’sl definition of ecology as the study of the ‘distribution and abundance of organisms’ also serves well to define one of the central missions of ecology: to understand spatial and temporal patterns in the relative densities of species. Preston2 referred to such patterns as the ‘commonness and rarity of species’ and his early studies were instrumental in motivating ecologists to quantify such patterns rigorously and to find the mechanisms and processes that account for them. The search for mechanism and process in ecology, however, has 400
0
1996, Elsevier
Science
Ltd
often led to advances in theoretical and experimental knowledge, while our understanding of what governs patterns in relative abundance distributions (RADs) has lagged behind. That is, there is a tendency to get the theoretical and experimental ‘cart’in front of the RAD‘horse’that should be driving community ecology. The recent study by Wilson et a13 is an attempt to get the horse back in front of the cart. As Wilson et al. note at the outset of their paper, it is remarkable what little empirical information exists on how well patterns in RADs fit the predictions of
3 Luschi, P. et al. (1996) Long-distance migration and homing after displacement in the green turtle (Chelonia my&s): a satellite tracking study, J. Camp.Physiol.A 178, 447-452
4 Papi, P., Liew, H. and Luschi, P. (1995) Long-range migratory travel of a green turtle tracked by satellite: evidence for navigation ability in the open sea, Mar. Biol. 122, 171-175 5 Papi, F. and Luschi, P. (1996) Pinpointing ‘Isla Meta’:the case of sea turtles and albatrosses, J. @p. Biol. 199,65-71 6 Lohmann, K.J. and Lohmann, C.M.F.(1994) Detection of magnetic inclination angle by sea turtles: a possible mechanism for detection of latitude, J Exp. Biol. 194, 23-32 7 Lohmann, K.J. and Lohmann, C.M.F.(1996) Orientation and open-sea navigation in sea turtles, J Exp. Biol. 199, 73-81 8 Wallralf, H.G. (1991) Conceptual approaches to avian navigation systems, in Orientation in Birds (Berthold, P., ed.), pp. 38-51, Birkhauser 9 Wiltschko, R. and Wiltschko, W. (1995) Magnetic Orientationin Animals, Springer-Verlag 10 Alerstam, T. (1996) The geographical scale factor in orientation of migrating birds, J. Exp. BioL 199,9-19 11 Wiemerskirch, H. et al (1993) Foraging strategy of wandering albatrosses through the breeding season: a study using satellite telemetry, Auk 110,325-342 12 Prince, P. et al. (1992) Satellite tracking of wandering albatrosses (Diomedea exulans) in the South Atlantic, Antarct. Sci. 4, 31-36
13 Walcott, C. (1991) Magnetic maps in pigeons, in Orientation in Birds (Berthold, P., ed.), pp. 38-51, Birkhluser 14 Walcott, C. (1996) Pigeon homing: observations, experiments and confusions, J. Exp. Biol. 199,21-27 15 Quinn, T.P. (1994) How do sharks orient at sea? Trends Ecol. Evol. 9,277-278
major theories in community ecology. These authors focus on resource-based competition theory and successional theory, two important areas in ecology that predict how RADsshould vary in response to changing nutrient levels or aging of a community. Other areas in ecology, however, such as food-web theory4, competition-based theory5, Lotka-Volterrabased community theory6, and islandbiogeographic theory7, also exhibit the same shortcomings. Wilson et a/. examine plant species abundances in three, well-studied grassland communities in the UK. These communities are the experimental plots at Monk’s Wood in Huntingdonshire, the Park Grass experimental plots in Hertfordshire, and the Compton grassland plots at Wolverhampton. They examine how grassland RADs vary in response to resource
PII: SO169-5347(96)30039-s
TREE
vol.
I I, no.
I0 October
1996
NEWS manipulations at Park Grass and at Compton as a way of testing the predictions of resource-based community theory. They also examine how RADsvary over time at Monks Wood and at Compton in order to test the predictions of successional theory. The authors focus on three aspects of RADs:(1) evenness, (2) the fit of observed distributions to biologically-based RADs, and (3) rank-abundance consistency (C’J among temporal or spatial replicates. Evenness is a measure of uniformity in a proportional representation of species (either by biomass or by density) within a community. There are many indices, each with its pros and con+10, but Wilson et al. settle for one used by Camargo because it is insensitive to variation in species richness (the number of species in a community). The authors also examine the fit of observed RADs to four well-known, biologically-based, theoretical distributions: the Broken Stick, Geometric, General Lognormal, and Zipf-Mandelbrot distributionslz. The theoretical distributions that best fit the observed RADs are compared with patterns predicted from theory. Finally, C, is quantified by measuring variation in the pattern of rankings of densities of species from one place to another, or one time to another, using a method devised by Watkins and Wilsonl3. High spatial or temporal consistency of rank abundances supports intrinsic, biotic processes as being important processes regulating RADs.Alternatively, low consistency supports extrinsic environmental factors, like weather or variation in local abiotic conditions, as being important factors regulating RADs. The authors essentially describe a three-axis space describing (1) patterns in distribution and abundance comprising evenness, (2) the fit to biologically-based theoretical RADs, and (3) C,. If communities reside near the origin, where evenness is low, the observed RADfits no biologically-based theoretical RAD,and there is no spatial or temporal consistency in rank abundance, then the communities are likely to be regulated by extrinsic, abiotic processes. At the other extreme, if evenness is high, the observed RADclosely fits biologically-based theoretical RADs, and rank-abundances are highly consistent irrespective of space or time, then the community is more likely to be structured by intrinsic or biotic processes. Wilson et al. found that the observed response of RAD to resource manipulations did not fit predictions of resourcebased competition theory but did fit their model for succession. RAD responses to phosphorus treatments in Park Grass showed some changes in agreement with resource-based theory, however, few consistent responses to other manipulations TREE
uol. I I, no. IO October
1996
were observed. The Compton plots showed no constant responses at all. For successional theory, however, temporal trends in evenness, in the fit of observed RADs to the Broken Stick and Geometric RADs, and in C,.,were largely consistent with those predicted by Gitay and Wilson’s three-phase model of successionl4. How does one interpret these findings of Wilson et al.? Although the fit of observed RADs to theoretical RADs cannot tell us about biological processesaJ5, if communities shift within Wilson et d’s three-axis space then such a response qualitatively provides evidence to reject a biology-free, random, or null explanation for variation in RAD.If, for example, nutrient-based competition were regulating these systems, then manipulations of nutrients should have yielded declining evenness (increasing dominance of common species), predictable changes in bestfit theoretical RAD,and low C,among plots. It seems clear (with the possible exception of phosphorus in the Park Grass plots) that nutrient-based competition is not sup ported in this study as the factor responsible for grassland community composition and structure because there is very little support for patterns produced by biotic or intrinsic factors. Conversely, if succession was largely an abiotic process, then the predicted patterns in changing evenness, fit to the Broken Stick and Geometric RADs, and patterns in C,.,should not have been observed. Should we accept their conclusions? Does resource-based theory fail to describe adequately distribution and abundance in grasslands? Does the program of assembly processes predicted by threephase succession best describe distribution and abundance in grasslands while other successional theories fail? Like most important ecological studies, it is the challenge that such a study raises that is its principle message, not its particular findings. RADsare notoriously slippery measures of communities, and the processes behind their origin, such as how species divide resources, are barely known. Wilson et d’s study encourages us to apply similar, pluralistic methods to other systems. The mechanisms of resource-based ecology have been widely studied in laboratory and field experiments, the challenge now is to apply them more broadly to patterns in distribution and abundance. Results from such studies may contribute to our understanding of how communities may respond to changing levels of nutrients in a world where CO, and N fertilization is increasing worldwidel’j. For prop@ nents of other assembly rules that govern successional patterns, the challenge is to demonstrate that competing models can similarly generate the temporal patterns
8i COMMENT
Wilson et al. found. Further analyses of patterns in distribution and abundance will provide new momentum for our theoretical and experimental research. Shahid Naeem Dept of Ecolosy Evolution and Behavior, and Center for Community Genetics, Universily of Minnesota, 100 Ecology Building, 1987 Upper Buford Circle, St Paul, MN 55108, USA ([email protected])
References 1 Andrewartha, H.G. (1961) Introductionto the
2
3
4
5
6
7
8 9
10 11
12
13
StudyofAnimal Populations, University of Chicago Press Preston, F.W. (1962) The canonical distrfbution of commonness and rarity, Ecology43,185-215; 410-432 Wilson, J.B. etal. (1996) Are there assembly rules for plant species abundance? An investigation in relation to soil resources and successional trends, 1 Ecol. 84, 527-538 Lawler, S.P. and Morin, P.J. (1993) Food web architecture and population dynamics in laboratory microcosms of protists, Am. Nat. 141,675-686 Kelt, D.A.,Taper, M.L. and Meserve, P.L. (1995) Assessing the impact of competition on community assembly: a case study using small mammals, Ecology76,1283-1296 de Ruiter, P.C., Neutel, A. and Moore, J.C. (1995) Energetics, patterns of interaction strengths, and stability in real ecosystems, Science 269, 1257-1260 Colwell, R.K.and Winkler, D.W. (1984) A null model for null models in biogeography, in Ecological Communities: Conceptual Issues and the Evidence (Strong, D.R.,Jr et al., eds), pp. 344-359, Princeton University Press Pielou, E.C. (1975) Ecological Diversity,Wiley Bulla, L. (1994) An index of evenness and its associated diversity measure, Oikos 70, 167-171 Magurran, A.E. (1988) Ecological Diversityand its Measurement, Princeton University Press Camargo, J.A. (1993) Must dominance increase with the number of subordinate species in competitive interactions? J. Theor. Biol. 161,537-542 Wilson, J.B. (1993) Would we recognize a Broken-Stickcommunity if we found one? Oikos 67, 181-183 Watkins, A.J. and Wilson, J.B. (1994) Plant community structure and its relation to the vertical complexity of communities: dominance/diversity, spatial rank consistency and species richness, Oikos 70,91-98
14 Gitay, H. and Wilson, J.B. (1995) Post&~ changes in community structure of tall tussock grasslands: a test of alternative models of succession, and a new ‘three-phase?model, J. Ecol. 83,775-782 15 May, R.M. (1975) Patterns of species abundance and diversity, in Ecology and Euolution of Communities (Cody, M.L. and Diamond, J.M., eds), pp. 81-120, Belknap Press 16 Vitousek, P.M. (1994) Beyond global warming: ecology and global change, Ecology 75,1903-1910
401