and abundance, other factors contributing toaare: (i) the fraction of the
metabolism at level 0 that is allocated to biomass production, or production
efficiency, which appears to be approximately 50% across a wide variety of
ectothermic organisms; (ii) the fraction of the individuals or biomass produced
by trophic level 0 that is consumed by trophic level 1, i.e. the consumption
efficiency; and (iii) the fraction of the energy content of this consumed resource
that is actually absorbed and transformed in the metabolism of organisms at
level 1, i.e. the assimilation efficiency, which has been measured for the diets of
many kinds of consumers.
The above framework can be applied to understand quantitatively how
Lindeman efficiencies, rates of energy and material flow, biomasses and abun-
dances vary within and between ecosystems as a consequence of the body sizes
of the organisms at different trophic levels. For example, Brownet al.(2004)
derive explicitly the conditions required to have the inverted ecological pyra-
mids (i.e. ratios>1) of biomass or abundance that are sometimes observed
empirically (see alsoJones & Jeppesen, this volume). Additionally, ifa is
known, Eqs. (1.7) and (1.8) can be applied, and the body sizes of the organisms
occupying adjacent trophic levels can be used to predict the ratios of biomass
and abundance. Conversely, if the body-size ratios are known, the above equa-
tions can be used to explore the contributions of body size and the production,
consumption and assimilation efficiencies to the Lindeman efficiency.
More generally, the above framework represents a start at synthesizing the
different approaches to food webs that have traditionally been taken by ecosys-
tem and community ecologists. The former have useddE/dtcurrencies to quan-
tify rates of energy and material flow, whereas the latter have useddN/dt
currencies to focus on the dynamics of consumer populations and their re-
sources. By considering explicitly the allometry of resource use and abundance,
metabolic theory shows how these currencies are inextricably related (see also
Yodzis & Innes,1992; Kerr & Dickie,2001; Brown & Gillooly,2003; Brownet al.,
2004 ; Gilloolyet al., 2006).Concluding remarks
Much of ecology is concerned with the exchanges of energy and materials (i.e.
elements) between organisms and their environments. These exchanges deter-
mine the life histories of individual organisms, the abundances and turnover of
populations, the allocation of resources among coexisting species, and the
fluxes and pools of energy and materials in ecosystems. These exchanges are
direct consequences of metabolism as organisms take up energy and nutrients
from their environments, transform them within their bodies, and allocate
them to maintenance, growth and reproduction. The metabolic rates of organ-
isms vary predictably, or scale quantitatively, with body size and temperature.
The metabolic theory of ecology uses these scaling relations to make and test12 J. H. BROWNETAL.