On Being a Macroorganism or a Microorganism

The major frame of reference for this book, as the title implies, is size differences across major groupings of taxa, that is, microorganisms versus macroorganisms. As noted in Chap. 1, however, there is obviously a continuum between the two categories for several reasons; in other words, the demarcation is one of convenience and is not a simplistic binary choice. Indeed, the impetus for this book is the arbitrary distinction, reinforced by disciplinary isolation, between microbial ecology and plant/animal ecology. Instead it is possible to compare all cellular life essentially along six axes as we have done here with respect to: genetics, nutrition, size, growth form, life cycle, and environment.

Be that as it may, size is clearly an ecologically important attribute of an organism and influences profoundly its interactions with the biotic and abiotic environment. Microbes are affected primarily by intermolecular forces while large creatures live in a world governed by gravity. The direct consequences of this were discussed in Chap. 4 and were developed, along with many indirect consequences, in each of the other chapters. Most if not all attributes of an organism are influenced by its size, and much of the variation in life histories is correlated with size. One cannot compare organisms strictly on the basis of size, however, because this factor is not independent of others.

First, organisms are locked into developmental and ecological channels established by phylogenetic differences, the most obvious of which are design constraints (Chaps. 1 and 4). A wolf and a fish of equivalent weight have very different life histories and evolutionary potentials. Second, even among geometrically similar organisms of a common phylogeny (such as different species of lizards), shape within limits tends to change with increasing volume leading to proportionate rather than progressively diminishing changes in surface area of many structures. Efficiency in biological (exchange) processes dictates an approximately constant ratio of surface:volume. The same relationship means that supporting elements (bones, stems) must be proportionately thicker as size increases if the organism is not to fall under its own weight.

Within the scale from microorganism to macroorganism, increasing size also means increasing complexity (more cell types) and concomitantly increasing division of labor and centralized control; increasing independence of the external environment (homeostatic ability); and increasing chronological time to maturity and between generations (although ratios of life history features to physiological time seem to be constant at least in unitary organisms; Chap. 4). Population densities (and intrinsic growth rates) of microorganisms are consequently much higher than those of macroorganisms.

Since for a given nucleotide sequence the rates of base substitution are approximately constant per unit time across lineages (molecular evolutionary clock; Chap. 2), favorable mutations will spread more rapidly in a bacterial than in an elephant population. Put differently, this means that microorganisms, though more exposed to environmental variation (Chap. 7), can evolve more rapidly in response to it.