The island rule

Mark Lomolino has been one of the central figures in recent work on body size, energetics, and evolution -- especially with respect to the evolution of body size in island species. With several of his colleagues, he had an editorial in the Journal of Biogeography last year, calling for a renewed research agenda in ecogeographical rules -- that is, rules relating body size and other characters to biogeographic variation of different kinds. The island rule is one such rule, other examples are Bergmann's and Allen's rules (relating body size and limb lengths with temperature) and Rapoport's rule (relating north-south species range extents with actual latitude).

Most of the possibility for a substantial renewal has come from the increased exchange and computerized search capacity for basic observations on body size and shape in different species. The editorial mainly reviews the prospects for developing new theoretical understandings of these rules and their exceptions using new data, and points to the necessity of the comparative approach.

The editorial treats the island rule as a case study -- and it is probably the best one possible because of the substantial density of recent research on the topic. The authors provide a nice nutshell history of the development of the island rule as a scientific explanation:

Here we first summarize the epistemology of the island rule as an illustrative case study. This pattern was first described by Foster (1963, 1964), but it was not labelled as a rule (and one 'with fewer exceptions than any other ecotypic rule in animals') until Van Valen's papers in 1973 (Van Valen, 1973a,b: p. 35). Originally, Foster described the pattern as a tendency for different taxa (orders of mammals) to exhibit different evolutionary trends on islands, rodents tending to increase, and carnivores and ungulates tending to decrease in body size. Later, Heaney (1978), Lomolino (1985) and others reinterpreted the rule to be a graded trend from gigantism in the smaller species to dwarfism in the larger species of mammals. This recasting of the island rule reflected the heuristic tension between process and pattern, and theory and empiricism. The earliest articulation of the island rule suggested disparate evolutionary trends among mammalian orders, but this was soon found to be inconsistent with modern evolutionary theory. Rather than invoking some overriding importance of phylogenetic inertia that somehow differs among mammalian orders, the island rule (sensu nova) instead is inferred to reflect differences in selective pressures on islands and among species of different body size (therefore the graded trend should occur within, as well as among, mammalian orders; see explanations by Heaney, 1978; Lomolino, 1985, 2005; Adler & Levins, 1994; Adler, 1996; Marquet & Taper, 1998; McNab, 2001, 2002; Gould & MacFadden, 2004). Note, however, that the geographical context of the island rule's primary pattern is typically limited and binary: terrestrial populations occur in just two types of ecosystem, islands or mainland sites. Yet we know that islands vary in area, isolation, latitude and other characteristics that influence the abilities of organisms to colonize and maintain populations. Thus, in addition to the primary pattern, body size of insular populations of particular species should not be constant, but should vary in a non-random manner among islands and archipelagoes. These secondary or corollary patterns provide excellent opportunities to evaluate alternative explanations for the island rule, for example by testing for correlations between body size and the area, isolation and latitude of islands (see studies summarized in Table3 of Lomolino, 2005), or equivalently by testing for dynamics in body size following range expansion and vicariance, or tectonic events, climatic fluxes and associated changes in productivity, community structure and isolation of habitats and populations (Case, 1976, 1982, 2002).

The editorial quickly details some of the current range of hypotheses addressing body size evolution in island contexts, and has a great set of references. I like this paragraph a great deal, it makes many suggestions for research strategies:

Perhaps most insightful among these new, more integrative research initiatives are the opportunities afforded by the many thousands of introduction 'experiments' performed by human civilizations during their advances across the globe. Each of these episodes of invasion provides an opportunity to investigate how the dynamics in one of the most fundamental characteristics of an organism - its body size - is associated with the dynamics in one of the most fundamental characteristics of a species - its geographical range. A limited, but intriguing number of studies have demonstrated that ecogeographical patterns can evolve in surprisingly short periods of time as an invasive species expands its exotic range and, as a result, experiences repeated founder events and novel selection regimes (Johnston & Selander, 1964; Huey et al., 2000, 2005; Gilchrist et al., 2001; Sax, 2001; Campbell & Echternacht, 2003; Fridley et al., 2006; Patterson et al., 2006). The converse phenomenon - changes in body size and other characteristics of individuals as the species' geographical range contracts - may prove just as insightful. We know that geographical range collapse is far from a random process, with final populations typically persisting in the most isolated reaches of the species' historical range, either along the range periphery, in montane areas or on oceanic islands (Lomolino & Channell, 1995; Channell & Lomolino, 2000a,b; see also Safriel et al., 1994; Towns & Daugherty, 1994; Gaston, 2003; Laliberte & Ripple, 2004). Yet we know of no studies examining the consequences of this highly non-random pattern of range collapse on body-size variation in native or invasive species (Lomolino et al. 2006:1505).

An integrative perspective on population ecology, adaptations, and displacement in the face of demographic change would certainly improve our understanding of human evolution. In particular, the events of the Late Pleistocene may be explicable only in these terms. But also, taking a broader view of ecological change, ecogeographical changes in Holocene human populations may be examined with this perspective.

In their list of suggestions, Lomolino and colleagues include this:

We should capitalize on the thousands of unplanned but well chronicled introduction 'experiments' as opportunities to investigate simultaneously the dynamics of morphological traits and geographical range size. Among these, we include the many waves of invasions and subsequent ecological and evolutionary adaptations of Homo sapiens. Given the available global record on colonization by human civilizations and the substantial morphological variation among individuals and regional populations, ecogeographical studies of our own species may prove especially intriguing (Roberts, 1953, 1978; Ruff, 1994; Bindon & Baker, 1997; Brown et al., 2004; Morwood et al., 2004) (Lomolino et al. 2006:1507).

Lately, with the exception of Flores, research into hominid body mass has emphasized global patterns rather than regional ones. But the most interesting questions lately have arisen as a result of observing interregional and within-region diversity in body sizes (particularly at the Australopithecus-Homo transition).

My favorite factoid lately -- you can ask my classes -- is the location of the tallest contemporary human population. The Netherlands.

References:

Lomolino MV, Sax DF, Riddle BR, Brown JH. 2006. The island rule and a research agenda for studying ecogeographical patterns. Journal of Biogeography 33:1503-1510. doi:10.1111/j.1365-2699.2006.01593.x