Chapter 2: Coming Together
Animal groups vary in size from two magpies sitting on a branch to plagues of millions of locusts crossing the desert. Not only do the sizes of groups vary between species, but they can change dramatically within species. In some cases, the change in group size depends on changes in the environment. For example, locust outbreaks are thought to originate where resources are patchily distributed and locusts move towards these limited resources. In other cases, individuals in similar environments are found in very different sized groups. Fishermen are used to such intrinsic variation in fish school size. Some days a net contains three fish the next day in contains tens of thousands. Human settlements also show this remarkable variety in size, ranging from tiny villages to massive cities, with differences in size arising without large differences in the environments in which they were originally founded.
Key ideas covered in this chapter: Costs and benefits of group membership; optimal and stable group sizes; Sibly’s group size model; empirical examples of group sizes of birds, spiders, primates and other species; power law distributions of fish schools, mammalian herds and human city sizes; other empirical examples of power laws; mechanistic models of how power laws arise; accumulation models; Niwa’s merge and split model; preferential attachment; limitations of power law models; group size and population density; alternative explanations of grouping.
Links
Richard Sibly's homepage
Charles Brown's hompage
Hiro-Sato Niwa's Homepage
References
Brown, C. R. & Brown, M. B. 1996 Coloniality in the Cliff Swallow. Chicago: The University of Chicago Press.
Krause, J. & Ruxton, G. D. 2002 Living in groups. Oxford series in ecology and evolution. Oxford ; New York: Oxford University Press.
Newman, M. E. J. 2005 Power laws, Pareto distributions and Zipf's law. Contemporary Physics 46, 323-351.
Niwa, H. S. 2003 Power-law versus exponential distributions of animal group sizes. Journal Of Theoretical Biology 224, 451-457.
Sibly, R. M. 1983 Optimal group size is unstable. Animal Behaviour 31, 947-948.
Sornette, D. 2004 Critical Phenomena in Natural Sciences: Springer Verlag.
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