1 Samuel M. Scheiner and Michael R. Willig, eds. 1. A General ...

trend were based on the ratio between body volume and surface area. The square-versus-cube
relationship makes the area of a solid proportional to the two-third power of its mass, so
metabolic rate should also be proportional to mass. But a thorough study by Kleiber (1932)
found that, for mammals and birds, metabolic rate was mass to the power of 0.73 (approximately
three-quarters) and not 0.67 (two-thirds as postulated based on surface to volume ratios). Other
research supported this new three-quarter-power law, although consensus is still elusive (Grant
2007). It was, however, much harder to find a theoretical reason why metabolic rate should be
proportional to mass raised to power of 3/4. Furthermore, it was not clear why quarter-power
scaling laws should be so prevalent in biology. Nevertheless, the generalization and the
difficulties with the putative causes of it led West et al. (1997) to develop a new explanation of
why metabolic rate should equal the three-quarter power of body mass.
Indeed, the generalization that larger organisms have lower metabolism is such a noisy
one that some question its relevance. What is important is that an early strong empirical
generalization inspired development of sophisticated theoretical explanations. A considerable
value of these explanations to ecology arises not from whether they are correct or accepted but
rather whether they have been formulated and tested. The very process of doing so identifies
further problems, questions, and directs new empirical and theoretical pursuits; which advances
understanding.
Another theory formulated in response to an empirical generalization is the neutral theory
of species diversity (Hubbell 1997). Most communities, irrespective of the scale of data
collection, harshness of the environment, or productivity, show a general qualitative regularity of
species abundances: few species are abundant while many more are rare, with species of
intermediate abundance filling the range in between according to one of several abundance
partitioning models. Numerous theoretical efforts have been made in the past to account for this
general pattern (e.g., May 1975, Ugland and Gray 1982, Tokeshi 1993). The most familiar of
those is the broken stick model of MacArthur’s (1957). None have gained full acceptance.
Hubbell’s proposition is a culmination of these efforts and a stimulus for further tests.
Admittedly, sometimes a good generalization does not lead to the development of a theory (but
see Fox et al., Chapter 14, on gradients regarding a possible solution). Holdridge’s schematic,
illustrating vegetation types as a function of evapotranspiration and annual precipitation (Figure
2.3), represents a useful generalization that does not need or inspire a complex theory. In case of
34
34