Limits to Biodiversity (Species Packing) dubious There may be groups of species, in which each one is closely connected to a few others but only loosely connected to other groups The term ‘‘community’’ was once extremely useful and is still of pedagogic value if carefully used As reified objects for research, the concept of communities is now threatening to become what Simberloff and Dayan have referred to as a ‘‘panchreston,’’ an idea as likely to generate confusion as enlightenment The prospect of a general species packing theory has melted away Despite the unreality of the models, they did direct attention to what seemed to be a real phenomenon and encouraged experimentalists and field biologists to ask important questions Differences in species richness can be partially explained by a multiplicity of factors that not necessarily relate to each other so as to permit formation of a single coherent theory Each of the multiplicity of theories of diversity focuses on one or a few of the empirical factors that are known to enhance or diminish the possibility of species coexistence Many of these can be seen in laboratory experiments, which have the advantage of clarity but may be of questionable applicability For example, local geometry complexity influences species diversity in nature as well as in the laboratory Certainly in some cases the term ‘‘species packing’’ is used in the sense of species being squeezed into a space In several cases individuals of particular species in species-rich regions are believed to have a narrower range of activities than individuals of the same species in species-poor situations Diet or nest sites may be more restricted In these cases the individual organisms can be imagined to have been constricted by some packing process, like individual pillows in a crate but even this has two possible meanings depending on whether we are concerned with the population level or with individuals In comparing different locales, the range of variation among organisms within a unispecific natural population may be reduced when more other species are present In the case of comparisons of different islands or lakes, this might be tentatively attributed to species packing Returning to the initial analogy of packing actual objects, an island is clearly a container But if packing means filling the ecological space, either by pillows or tumblers, we would not expect islands or speciose lakes to easily admit invasive species In lakes there may be enormous species richness of fishes, as in the ancient African lake cichlids, but I don’t know of any comparisons showing that species-rich lakes show less within species, among individual variation, than species-poor lakes Is the attempt to crowd multispecific collections of fish together in the same lake equivalent to packing glass tumblers rather than pillows? There is a general impression that species rich systems seem at least as likely to be invaded by exotic species as species-poor systems, violating our sense of what packing might mean If the container walls are not apparent but among individuals variation is reduced in one or more populations of a species, is this a sign of species packing? Does the mere fact that among individuals variation is reduced when more species are present imply that there must exist a container wall which may not be obvious? 647 The overall conclusion is that the theory of species packing does not conveniently predict very much about natural systems but that the images of packing that it generates informally suggest interesting phenomena to look for See also: Competition, Interspecific Diversity, Community/Regional Level Habitat and Niche, Concept of Plant Invasions Species–Area Relationships Species Coexistence Stress, Environmental References Abrams P, Nyblade C, and Sheldon S (1986) Resource partitioning and competition for shells in a sub-tidal hermit-crab species assemblage Oecologia 69: 429–445 Adams DC, di Biteti MS, Janson CH, Slobodkin LB, and Valenzuala N (1997) An ‘‘audience effect’’ for ecological terminology: Use and misuse of jargon Oikos 80: 632–636 Anderson MG (1995) Interactions between Lythrum salicaria and native organisms: A critical review Environmental Management 19: 225–231 Ayala F (1971) Competition between species: Frequency dependence Science 171: 820–824 Barrett GW, Peles JD, and Odum EP (1997) Transcending processes and the levelsof-organization concept Bioscience 47: 531–535 Bazzaz F, Ceballos G, Davis M, et al (1998) Ecological science and the human predicament Science 282: 879 Berryman AA (1992) Intuition and the logistic equation Trends in Ecology and Evolution 7: 316 Boake CRB and Wade MJ (1984) Populations of the Red Flour Beetle Tribolium castaneum (Coleoptera, Tenebrionidae) Differ in Their Sensitivity to Aggregation Pheromones Environmental Entomology 13: 1182–1185 Burgman M and Lindenmayer D (1998) Conservation Biology for the Australian Environment Chipping Norton, NSW: Surrey Beatty and Sons Cornell H and Lawton J (1992) Species interactions, local and regional processes, and limits to the richness of ecological communities: A theoretical perspective Journal of Animal Ecology 61: 1–12 Crombie A (1945) On competition between different species of graminivorous insects Proceedings of the Royal Society of London Biological Sciences 132: 362–395 Crombie A (1946) Further experiments on insect competition Proceedings of the Royal Philosophical Society of London Series B 133: 76–109 Daily GC, Alexander S, Ehrlich P, et al (1997) Ecosystem Services: Benefits Supplied to Human Societies by Natural Ecosystems Washington, DC: Ecological Society of America D’Ancona U (1954) The Struggle for Existence Leiden: E J Brill Davis M (1986) Climatic instability, time lags, and community disequilibrium In: Diamond J and Case T (eds.) Community Ecology, pp 269–284 New York: Harper and Row Diamond J and Case T (1986) Community Ecology New York: Harper and Row Dykhuizen D (1978) Selection for tryptophan auxotrophs of Escherichia-coli in glucose-limited chemostats as a test of energy-conservation hypothesis of evolution Evolution 32: 125–150 Elton C (1958) The Ecology of Invasions by Animals and Plants London: Methuen and Co Feldman MW and Roughgarden J (1975) Populations stationary distribution and chance of extinction in a stochastic environment with remarks on theory of species packing Theoretical Population Biology 7: 197–207 Ferson S, Stewart S, Downey P, et al (1986) Competing reviews, or why Connell and Schoener disagree? American Naturalist 127: 571–576 Fryer G and Iles TD (1972) The Cichlid Fishes of the Great Lakes of Africa: Their Biology and Evolution New York: Crown Gause GF (1934) The Struggle for Existence Baltimore: Williams and Wilkins Glasser JW (1983) Variation in niche breadth with trophic positionFOn the disparity between expected and observed species packing American Naturalist 122: 542–548 Griffing TC (1965) Dynamics and Energetics of Populations of Brown Hydra Ph.D Thesis, University of Michigan Hairston N Sr (1991) Ecological Experiments: Purpose, Design and Execution Cambridge: Cambridge University Press