Determining the effects of selection on scaling exponents
The value of applying scaling laws to predict diverse ecological phenomena
depends on the reliability of the underlying laws, and whether selection can
alter the scaling exponent. Of particular recent interest is a theory that can
predict life history, population growth and carrying capacity, species richness
and rates of biomass production (reviewed by Brownet al., 2004; Brownet al.,this
volume). This theory incorporates a long-established, fundamental generalization
in biology that respiration (metabolic) rate (R)¼aM3/4(e.g. Westet al., 1997;Savage
et al., 2004). An important question is to what extent the scaling exponent,b,is
fixed at 3/4? If this exponent varies with environment (e.g. pelagic vs. benthic)
substantially across taxa with diverse body plans and of widely separated phylo-
genies, then a strong role for selection on this exponent is suggested, and a
possible role for life-history theory. This is exactly what was found in a recent
survey of 642 published regressions of scaling of laboratory-measured routine
aerobic metabolic rate during ontogeny of invertebrates (Glazier, 2005 ;Glazier,
2006). Fifty per cent were significantly different from 3/4, and this rose to 70% in
analyses with more statistical power (N50 or where body mass ranges2orders
of magnitude). Further, the exponent varied systematically with habitat: the mean
exponent (95% CL) of pelagic species was nearly 1 (b¼0.950.05,N¼58) and
was very highly significantly different (p<0.0000000001) from that shown by
benthic species (b¼0.740.02,N¼355). This distinction was robust, occurring
between pelagic and benthic members of each of four diverse phyla, and even
between pelagic larvae and benthic adults within species. This case of convergent
evolution suggests a strong role for natural selection. Research is needed to
discover whether near-isometric scaling of metabolic rate in pelagic species is
associated with, for instance, size-dependent costs of locomotion or maintaining
buoyancy, or size-dependent evolutionary responses to higher levels of predation.
Relationship between intraspecific and interspecific allometries
Typically, allometric relationships between some traits and body size across
species use data from adults only (e.g. Savageet al., 2004). If it is assumed that
natural selection has optimized body size through optimal allocation of re-
sources to growth and reproduction independently in each species, then inter-
specific allometries become by-products of many body-size optimizations across
the species included (Kozłowski & Weiner, 1997 ). This can produce different
slopes for intraspecific and interspecific allometries. For instance, if one species
(speciesA) is adapted to conditions that allow faster growth and reproduction
than a related speciesB, then all else being equal, speciesAwill have a higher
mass-specific metabolic rate because of its faster rate of growth, and is predicted
by simple evolutionary models to mature at a larger size (Kozłowski & Weiner,
1997 ; Daan & Tinbergen, 1997 ; Day & Rowe, 2002 ). Thus, in this case the inter-
specific allometric relationship will be steeper than the intraspecific ones.
LIFE HISTORIES AND BODY SIZE 45