Structure and dynamics of pollination networks: The past, present, and future

The Systematics Association Special Volume 81: Evolution of Plant-Pollinator Relationships

Volumn , Issue , 2011, Pages 374-391

  Olesen, Jens M ^a   Dupont, Yoko L ^a   Hagen, Melanie ^a   Rasmussen, Claus ^a   Trøjelsgaard, Kristian ^a  

^a AARHUS UNIVERSITY   (Denmark)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 84931840968     PISSN: None     EISSN: None     Source Type: Book    
DOI: 10.1017/CBO9781139014113.014     Document Type: Chapter

References (125)

1. Aizen, M. A., Morales, C. L. and Morales, J. M. (2008). Invasive mutualists erode native pollination webs. PLoS Biology, 6, 396-403.
2. Alarc ó n, R. (2010). Congruence between visitation and pollen-transport networks in a California plant-pollinator community. Oikos, 119, 35-44.
3. Albert, R., Jeong, H. and Barab á si, A.-L. (1999). Diameter of the world-wide web. Nature, 401, 130-1.
4. Ara ú jo, M. S., Guimar ã es, P. R. J., Svanbäck, R., Pinheiro, A., Guimar ã es, P., Dos Reis, S. F. and Bolnick, D. I. (2008). Network analysis reveals contrasting ef ects of intraspecii c competition on individuals versus population diets. Ecology, 89, 1981-93.
5. Arroyo, M. T. K., Primack, R. and Armesto, J. (1982). Community studies in pollination ecology in the high temperate Andes of Central Chile. I. Pollination mechanisms and altitudinal variation. American Journal of Botany, 69, 82-97.
6. Bartomeus, I., Vilà, M. and Santamaría, L. (2008). Contrasting ef ects of invasive plants in plant-pollinator networks. Oecologia, 155, 761-770.
7. Bascompte, J. (2009a). Disentangling the web of life. Science, 325, 416-419.
8. Bascompte, J. (2009b). Mutualistic networks. Frontiers in Ecology and the Environment, 7, 429-36.
9. Bascompte, J. and Jordano, P. (2006). The structure of plant-animal mutualistic networks. In Ecological Networks: Linking Structure to Dynamics in Food Webs, ed. M. Pascual and J. A. Dunne. Oxford, UK: Oxford University Press, pp. 143-59.
10. Bascompte, J. and Jordano, P. (2007). Plant-animal mutualistic networks: the architecture of biodiversity. Annual Review of Ecology, Evolution and Systematics, 38, 567-93.
11. Bascompte, J., Jordano, P., Meli á n, C. J. and Olesen, J. M. (2003). The nested assembly of plant-animal mutualistic networks. Proceedings of the National Academy of Sciences USA, 100, 9383-7.
12. Bascompte, J., Jordano, P. and Olesen, J. M. (2006). Asymmetric coevolutionary networks facilitate biodiversity maintenance. Science, 312, 431-3.
13. Basilio, A. M., Medan, D., Torretta, J. P. and Bartoloni, N. J. (2006). A year-long plant-pollinator network. Austral Ecology, 31, 975-83.
14. Batagelj, V. and Mrvar, A. (2009). Pajek- Program for Large Network Analysis. Home page: http://vlado.fmf.uni-lj.si/pub/networks/pajek/
15. Bauer, P. J. (1983). Bumblebee pollination relationships on the Beartooth plateau tundra of southern Montana. American Journal of Botany, 70, 134-44.
16. Bosch, J., Martin, A. G., Anselm, R. and Navarro, D. (2009). Plant-pollinator networks: adding the pollinator's perspective. Ecology Letters, 12, 409-19.
17. Bundgaard, M. (2003). Tidslig og rumlig variation i et plante-best ø vernetværk. MSc Thesis, Aarhus University, Denmark.
18. Burgos, E., Ceva, H., Perazzo, R. P. J., Devoto, M., Medan, D., Zimmermann, M. and Delbue, A. M. (2007). Why nestedness in mutualistic networks? Journal of Theortical Biology, 249, 307-13.
19. Butts, C. T. (2009). Revisiting the foundations of network analysis. Science, 325, 414-6.
20. Clements, R. E. and Long, F. L. (1923). Experimental pollination. An outline of the ecology of flowers and insects. Carnegie Institute of Washington, Washington DC.
21. Cowling, R. M. and Campbell, B. M. (1980). Convergence in the vegetation structure in the Mediterranean communities of California, Chile, and South Africa. Plant Ecology, 43, 191-7.
22. Dalsgaard, B., Martín, A. M. G., Olesen, J. M., Ollerton, J., Timmermann, A., Andersen, L. H. and Tossas, A. G. (2009). Plant-hummingbird interactions in the West Indies: floral specialisation gradients associated witThenvironment and hummingbird size. Oecologia, 159, 757-66.
23. Dalsgaard, B., Martín, A. M. G., Olesen, J. M., Timmermann, A., Andersen, L. H. and Ollerton, J. (2008). Pollination networks and functional specialization: a test using Lesser Antillean plant-hummingbird assemblages. Oikos, 117, 789-93.
24. Dicks, L. V., Corbet, S. A. and Pywell, R. F. (2002). Compartmentalization in plant-insect flower webs. Journal of Animal Ecology, 71, 32-43.
25. Dormann, C. F., Fründ, J., Blüthgen, N. and Gruber, B. (2009). Indices, graphs and null models: analyzing bipartite ecological networks. Open Ecology Journal, 2, 7-24.
26. Dormann, C. F., Gruber, B. and Fründ, J. (2008). Introducing the bipartite package: analysing ecological networks. R News, 8, 8-11.
27. Dupont, Y. L. and Olesen, J. M. (2009). Modules and roles of species in heathland pollination networks. Journal of Animal Ecology, 78, 346-53.
28. Dupont, Y. L., Tr ø jelsgaard, K. and Olesen, J. M. (2010, online) Scaling down from species to individuals: a flowervisitation network between individual honeybees and thistle plants. Oikos.
29. Elberling, H. and Olesen, J. M. (1999). The structure of a high latitude plant- pollinator system: the dominance of l ies. Ecography, 22, 314-23.
30. Fortuna, M. A., García, C., Guimar ã es, P. R. J. and Bascompte, J. (2008). Spatial mating networks in insect-pollinated plants. Ecology Letters, 11, 490-8.
31. Fortuna, M. A., Stouf er, D. B., Olesen, J. M., Jordano, P., Mouillot, D., Krasnov, B. R., Poulin, R. and Bascompte, J. (2010). Nestedness versus modularity in ecological networks: two sides of the same coin? Journal of Animal Ecology, 79, 811-7.
32. Gibson, R. H., Nelson, I. L., Hopkins, G. W., Hamlett, B. J. and Memmott, J. (2006). Pollinator webs, plant communities and the conservation of rare plants: arable weeds as a case study. Journal of Applied Ecology, 43, 246-57.
33. González-Álvaro, V. (2004). Estimaci ó n de la riqueza y abundancia de polinizadores potenciales basada y par á metros morfológicos florales de plantas ruderales. PhD Thesis, University of Castilla-La Mancha, Toledo, Spain.
34. Guimarães, P. R. and Guimarães, P. (2006). Improving the analyses of nestedness for large sets of matrices. Environmental Modelling and Software, 21, 15112-13.
35. Guimarães, P. R., Machado, G., Aguiar, M. A. M. d., Jordano, P., Bascompte, J., Pinheiro, A. and Furtado dos Reis, S. (2007). Build-up mechanisms determining the topology of mutualistic networks. Journal of Theoretical Biology, 249, 181-9.
36. Guimerà, R. and Amaral, L. A. N. (2005). Functional cartography of complex metabolic networks. Nature, 433, 895-900.
37. Hegland, S. J., Nielsen, A., L á zaro, A., Bjerknes, A.-L. and Totland, Ø. (2009). How does climate warming af ect plant-pollinator interactions? Ecology Letters, 12, 184-95.
38. Heithaus, E. R. (1974). The role of plant- pollinator interactions in determining community structure. Annals Missouri Botanical Garden, 61, 675-91.
39. Herrera, C. M. (1996). Floral traits and plant adaptation to insect pollinators: a devil's advocate approach. In Floral Biology, ed. D. G. Lloyd and S. C. H. Barrett. London, UK: Chapman and Hall, pp. 65-87.
40. Herrera, J. (1988). Pollination relationships in Southern Spanish Mediterranean shrublands. Journal of Ecology, 76, 274-87.
41. Hocking, B. (1968). Insect-flower association in the high Arctic with special reference to nectar. Oikos, 19, 359-88.
42. Ings, T. C., Bascompte, J., Blüthgen, N., Brown, L., Dormann, C. F., Edwards, F., Figueroa, D., Jacob, U., Jones, J. I., Lauridsen, R. B., Ledger, M. E., Lewis, H. M., Montoya, J. M., Neutel, A.-M., Olesen, J. M., Veen, F. J. v., Warren, P. H. and Woodward, G. (2009). Ecological networks: food webs and beyond. Journal of Animal Ecology, 78, 253-69.
43. Inoue, T., Kato, M., Kakutani, T., Suka, T. and Itino, T. (1990). Insect-flower relationship in the temperate deciduous forest of Kibune, Kyoto: an overview of the flowering phenology and the seasonal pattern of insect visits. Contributions from the Biological Laboratory, Kyoto University, 27, 377-463.
44. Inouye, D. W. (1978). Resource partitioning in bumblebees: experimental studies of foraging behavior. Ecology, 59, 672-8.
45. Johnson, S. D. and Steiner, K. E. (2000). Generalization versus specialization in plant pollination systems. Trends in Ecology and Evolution, 15, 140-3.
46. Joppa, L. N., Bascompte, J., Montoya, J. M., Solé, R., Sanderson, J. and Pimm, S. L. (2009). Reciprocal specialization in ecological networks. Ecology Letters, 12, 961-9.
47. Jordano, P. (1987). Patterns of mutualistic interactions in pollination and seed dispersal: connectance, dependence, and coevolution. American Naturalist, 129, 657-77.
48. Jordano, P. (2010). Coevolution in multispecii c interactions among freeliving species. Evolution, Education, and Outreach, 3, 40-6.
49. Jordano, P., Bascompte, J. and Olesen, J. M. (2003). Invariant properties in coevolutionary networks of plant- animal interactions. Ecology Letters, 6, 69-81.
50. Jordano, P., Bascompte, J. and Olesen, J. M. (2006). The ecological consequences of complex topology and nested structure in pollination webs. In: Plant-Pollinator Interactions: From Specialization to Generalization, ed. N. M. Waser and J. Ollerton. Chicago, IL: Chicago University Press, pp. 173-99.
51. Jordano, P., V á zquez, D. P. and Bascompte, J. (2009). Redes complejas de interacciones mutualistas plantaanimal. In: Ecología y Evolución de Interacciones Planta-Animal: Conceptos y Applicaciones, ed. R. Medel, M. A. Aizen and R. Zamora. Santiago de Chile, Chile: Editorial Universitaria, pp. 17-41.
52. Kaiser-Bunbury, C. N., Memmott, J. and Müller, C. B. (2009). Community structure of pollination webs of Mauritian heathland habitats. Perspectives in Plant Ecology, Evolution and Systematics, 11, 241-54.
53. Kakutani, T., Inoue, T., Kato, M. and Ichihashi, H. (1990). Insect-flower relationship in the campus of Kyoto University, Kyoto: an overview of the flowering phenology and the seasonal pattern of insect visits. Contributions from the Biological Laboratory, Kyoto University, 27, 465-521.
54. Kanstrup, J. and Olesen, J. M. (2000). Plant-flower visitor interactions in a neotropical rain forest canopy: community structure and generalisation level. In The Scandinavian Association for Pollination Ecology honours Knut Fægri, ed. Ø. Totland, W. S. Armbruster, C. Fenster, U. Molau, L. A. Nilsson, J. M. Olesen, J. Ollerton, M. Philipp and J. Aagren. Oslo, Norway: The Norwegian Academy of Science and Letters, pp. 33-41.
55. Kato, M. (2000). Anthophilous insect community and plant-pollinator interactions on Amami Islands in the Ryukyu Archipelago, Japan. Contributions from the Biological Laboratory, Kyoto University, 29, 157-252.
56. Kato, M., Kakutani, T., Inoue, T. and Itino, T. (1990). Insect-flower relationship in the primary beech forest of Ashu, Kyoto: an overview of the flowering phenology and the seasonal pattern of insect visits. Contributions from the Biological laboratory, Kyoto University, 27, 309-75.
57. Kearns, C. A., Inouye, D. W. and Waser, N. M. (1998). Endangered mutualisms: the conservation of plant-pollinator interactions. Annual Review of Ecology and Systematics, 29, 83-112.
58. Kevan, P. G. (1970). High Arctic Insect- Flower Relations: the Interrelationships of Arthropods and Flowers at Lake Hazen, Ellsmere Island, Northwest Territories, Canada. PhD Thesis, University of Alberta, Edmonton, Canada.
59. Levin, D. A. and Anderson, W. W. (1970). Competition for pollinators between simultaneously flowering species. American Naturalist, 104, 455-67.
60. Lewinsohn, T., Prado, P., Jordano, P., Bascompte, J. and Olesen, J. M. (2006). Structure in plant-animal interaction assemblages. Oikos, 113, 174-84.
61. Lundgren, R. and Olesen, J. M. (2005). The dense and highly connected world of Greenland's plants and their pollinators. Arctic, Antarctic, and Alpine Research, 37, 514-20.
62. MacArthur, R. H. (1972). Geographical Ecology. New York, NY: Harper and Row.
63. Macior, L. W. (1971). Coevolution of plants and animals. Systematic insights from plant-insect interactions. Taxon, 20, 17-28.
64. Martín, A. M. G., Dalsgaard, B., Ollerton, J., Timmermann, A., Olesen, J. M., Andersen, L. H. and Tossas, A. G. (2009). Ef ects of climate on pollination networks in the West Indies. Journal of Tropical Ecology, 25, 493-506.
65. May, R. M. (1973). Stability and Complexity in Model Ecosystems. Princeton, NJ: Princeton University Press.
66. McCann, K. (2007). Protecting biostructure. Nature, 446, 29.
67. Medan, D., Montaldo, N. H., Devoto, M., Mantese, A., Vasellati, V. and Bartoloni, N. H. (2002). Plant-pollinator relationships at two altitudes in the Andes of Mendoza, Argentina. Arctic, Antarctic, and Alpine Research, 34, 233-41.
68. Medan, D., Perazzo, R. P. J., Devoto, M., Burgos, E., Zimmermann, M. G., Ceva, H. and Delbue, A. M. (2007). Analysis and assembling of network structure in mutualistic systems. Journal of Theoretical Biology, 246, 510-21.
69. Meli á n, C. J., Bascompte, J., Jordano, P. and Krivan, V. (2008). Diversity in a complex ecological network with two interaction types. Oikos, 118, 122-30.
70. Memmott, J. (1999). The structure of a plant-pollinator food web. Ecology Letters, 2, 276-80.
71. Memmott, J. (2009). Food webs: a ladder for picking strawberries or a practical tool for practical problems. Philosophical Transactions from the Royal Society in London, B Biological Sciences, 364, 1693-9.
72. Memmott, J., Craze, P. G., Waser, N. W. and Price, M. V. (2007). Global warming and the disruption of plant-pollinator interactions. Ecology Letters, 10, 710-7.
73. Memmott, J., Godfray, H. C. J. and Gauld, I. D. (1994). The structure of a tropical host-parasitoid community. Journal of Animal Ecology, 63, 521-40.
74. Memmott, J. and Waser, N. M. (2002). Integration of alien plants into a native flower-pollinator visitation web. Proceedings of the Royal Society, London B, 269, 2395-9.
75. Memmott, J., Waser, N. M. and Price, M. V. (2004). Tolerance of pollination networks to species extinctions. Proceedings of the Royal Society, London B, 271, 2605-11.
76. Moldenke, A. R. (1979). Pollination ecology within the Sierra Nevada. Phytologia, 42, 223-82.
77. Moldenke, A. R. and Lincoln, P. G. (1979). Pollination ecology in montane Colorado: a community analysis. Phytologia, 42, 349-79.
78. Montero, A. C. (2005). Pollination Generalization Level and Habitat Width of Temperate Forest Insects. MSc Thesis, Aarhus University, Denmark.
79. Montoya, J. M., Pimm, S. L. and Solé, R. V. (2006). Ecological networks and their fragility. Nature, 442, 259-64.
80. Morales, C. L. and Aizen, M. A. (2006). Invasive mutualisms and the structure of plant-pollinator interactions in the temperate forests of north-west Patagonia, Argentina. Journal of Ecology, 94, 171-80.
81. Mosquin, T. (1971). Competition of pollinators as a stimulus for evolution of flowering time. Oikos, 22, 398-402.
82. Mosquin, T. and Martin, J. E. (1967). Observations on the pollination biology of plants on Melville Island, NWT, Canada. Canadian Field- Naturalist, 81, 201-5.
83. Müller, H. (1883). The Fertilization of Flowers. London, UK MacMillan.
84. Nielsen, A. and Bascompte, J. (2007). Ecological networks, nestedness and sampling ef ort. Journal of Ecology, 95, 1134-41.
85. Olesen, J. M. (2000). Exactly how generalised are pollination interactions? In: The Scandinavian Association for Pollination Ecology honours Knut Fægri, ed. Ø. Totland, W. S. Armbruster, C. Fenster, U. Molau, L. A. Nilsson, J. M. Olesen, J. Ollerton, M. Philipp and J. Aagren. Oslo, Norway: Norwegian Academy of Science and Letters, pp. 161-178.
86. Olesen, J. M. and Jordano, P. (2002). Geographic patterns in plant/pollinator mutualistic networks. Ecology, 83, 2416-24.
87. Olesen, J. M., Bascompte, J., Dupont, Y. L. and Jordano, P. (2006). The smallest of all worlds: pollination networks. Journal of Theoretical Biology, 240, 270-6.
88. Olesen, J. M., Bascompte, J., Dupont, Y. L. and Jordano, P. (2007). The modularity of pollination networks. Proceedings of the National Academy of Sciences USA, 104, 19891-6.
89. Olesen, J. M., Bascompte, J., Elberling, H. and Jordano, P. (2008). Temporal dynamics of a pollination network. Ecology, 89, 1573-82.
90. Olesen, J. M., Dupont, Y. L., Bascompte, J., Elberling, H. and Jordano, P. (2010a online). Missing and forbidden links in mutualistic networks. Proceedings of the Royal Society, London B.
91. Olesen, J. M., Dupont, Y. L., O ’ Gorman, E., Ings, T. C., Layer, K., Melían, C. J., Tr ø jelsgaard, K., Pichler, D. E., Rasmussen, C. and Woodward, G. (2010b). From Broadstone to Zackenberg: space, time and hierarchies in ecological networks. Advances in Ecological Research, 42, 1-69.
92. Olesen, J. M., Eskildsen, L. I. and Venkatasamy, S. (2002). Invasion of pollination networks on oceanic islands: importance of invader complexes and endemic super generalists. Diversity and Distribution, 8, 181-92.
93. Ollerton, J. and Cranmer, L. (2002). Latitudinal trends in plant-pollinator interactions: are tropical plants more specialized? Oikos, 98, 340-50.
94. Padrón, B., Traveset, A., Biedenweg, T., Di á z, D., Nogales, M. and Olesen, J. M. (2009). Impact of alien plant invaders on pollination networks from oceanic and continental islands. PLoS ONE, 4, e6275.
95. Paine, R. T. 1980. Food webs: linkage, interaction strength and community infrastructure. Journal of Animal Ecology, 49, 667-85.
96. Pauw, A. (2007). Collapse of a pollination web in small conservation areas. Ecology, 88, 1759-69.
97. Percival, M. (1974). Floral ecology of coastal scrub in southeast Jamaica. Biotropica, 6, 104-29.
98. Petanidou, T. (1991). Pollination Ecology in a Phryganic Ecosystem. PhD Thesis, Aristotelian University, Thessaloniki, Greece.
99. Petanidou, T., Kallimanis, A. S., Tzanopoulos, J., Sgardelis, S. P. and Pantis, J. D. (2008). Longterm observation of a pollination network: l uctuation in species and interactions, relative invariance of network structure and implications for estimates of specialization. Ecology Letters, 11, 564-75.
100. Philipp, M., Böcher, J., Siegismund, H. R. and Nielsen, L. R. (2006). Structure of a plant-pollinator network on a pahoehoe lava desert of the Gal á pagos Islands. Ecography, 29, 531-40.
101. Potts, S. G., Vulliamy, B., Dafni, A., Ne ’ eman, G. and Willmer, P. G. (2003). Linking bees and flowers: how do floral communities structure pollinator communities? Ecology, 84, 2628-2642.
102. Primack, R. B. (1983). Insect pollination in the New Zealand mountain l ora. New Zealand Journal of Botany, 21, 317-33.
103. Ramos-Jiliberto, R., Domínguez, D., Espinoza, C., López, G., Valdovinos, F. S., Bustamante, R. O. and Medel, R. (2010). Topological change of Andean plant-pollinator networks along an altitudinal gradient. Ecological Complexity, 7, 86-90.
104. Reader, R. J. (1975). Competitive relationships of some bog ericads for major insect pollinators. Canadian Journal of Botany, 53, 1300-5.
105. Rezende, E. L., Lavabre, J. E., Guimarães, P., Jordano, P. and Bascompte, J. (2007). Non-random coextinctions in phylogenetically structured mutualistic networks. Nature, 448, 925-9.
106. Robertson, C. (1929). Flowers and Insects: List of Visitors of Four Hundred and Fifty-h ree Flowers. Carlinville, IL: Science Press.
107. Saavedra, S., Reed-Tsochas, F. and Uzzi, B. (2008). A simple model of bipartite cooperation for ecological and organizational networks. Nature, 457, 463-6.
108. Schef er, M., Bascompte, J., Brock, W. A., Brovkin, V., Carpenter, S. R., Dakos, V., Held, H., Nes, E. H. v., Rietkerk, M. and Sugihara, G. (2009). Early-warming signals for critical transitions. Nature, 461, 53-9.
109. Schemske, D. (1983). Limits to specialization and coevolution in plantanimal mutualisms. In Coevolution, ed. M. Nitecki. Chicago, IL: Chicago University Press, pp. 67-110.
110. Schemske, D., Willson, M., Melampy, M., Miller, L., Verner, L., Schemske, K. and Best, L. (1978). Flowering ecology of some spring woodland herbs. Ecology, 59, 351-66.
111. Stang, M., Klinkhamer, P. G. L. and Meijden, E. v. d. (2006). Size constraints and flower abundance determine the number of interactions in a plant- flower visitor web. Oikos, 112, 111-21.
112. Stang, M., Klinkhamer, P. G. L. and Meijden, E. v. d. (2007). Asymmetric specialization and extinction risk in plant-flower visitor webs: a matter of morphology or abundance? Oecologia, 151, 442-53.
113. Stiles, F. G. (1975). Ecology, flowering phenology and hummingbird pollination of some Costa Rican Heliconia species. Ecology, 56, 285-301.
114. Straw, R. M. (1972). A Markov model for pollinator constancy and competition. American Naturalist, 106, 597-620.
115. Sugihara, G. and Ye, H. (2009). Cooperative network dynamics. Nature, 458, 979-80.
116. Thompson, J. N. (2005). The Geographic Mosaic of Coevolution. Chicago, IL: Chicago University Press.
117. Traveset, A. and Richardson, D. M. (2006). Biological Invasions as disruptors of plant reproductive mutualisms. Trends in Ecology and Evolution, 21, 208-16.
118. Tylianakis, J. M. (2008). Understanding the web of life: the birds, the bees, and sex with aliens. PLoS Biology, 6, 224-7.
119. V á zquez, D. P. and Aizen, M. A. (2003). Null model analyses of specialization in plant-pollinator interactions. Ecology, 84, 2493-501.
120. V á zquez, D. P. and Aizen, M. A. (2004). Asymmetric specialization: a pervasive feature of plant-pollinator interactions. Ecology, 85, 1251-7.
121. Vespignani, A. (2009). Predicting the behavior of techno-social systems. Science, 325, 425-8.
122. Waser, N. M., Chittka, L., Price, M. V., Williams, N. and Ollerton, J. (1996). Generalization in pollination systems, and why it matters. Ecology, 77, 1043-60.
123. Williams, R. J., Berlow, E. L., Dunne, J. A., Barab á si, A.-L. and Martinez, N. D. (2002). Two degrees of separation in complex food webs. Proceedings from the National Academy of Sciences USA, 99, 12913-6.
124. Woodward, G., Ebenman, B., Emmerson, M., Montoya, J. M., Olesen, J. M., Valido, A. and Warren, P. H. (2005). Body size in ecological networks. Trends in Ecology and Evolution, 20, 402-9.
125. Yodzis, P. (1980). The connectance of real ecosystems. Nature, 284, 544-5.