Growth Form as the Integrator of Life Histories Across Size Categories

If size establishes fundamental differences at the level of the individual, then growth form is arguably the best unifier. Instead of segregating the biological world by size into micro- and macro organisms, it can be cut along the axis of growth form into modular or unitary organisms. Such a scheme constitutes a rational as opposed to an arbitrary demarcation and is more biologically justifiable than is one based on size.

The unifying feature of growth form is that microorganisms share with a whole set of macroorganisms a modular life style. Foremost among such characteristics are sessility, indefinite growth by iteration, and a developmental mode that is effectively based on totipotency or somatic embryogenesis (Chap. 5). As developed earlier, the main ecological and evolutionary implications include: (i) replacement of active mobility with growth by extension, passive dispersal mechanisms, and dormancy; (ii) exposure of the same genetic individual to different environments (hence to different selection pressures); (iii) close interaction among subsets of a population (neighbor effects); (iv) high phenotypic plasticity; (iv) the potentially significant role of somatic mutation in evolution; and (v) substantially attenuated or absence of senescence at the level of the genet.

By varying their growth form directly in response to environmental variation, modular organisms make trade-offs visible in a way that their unitary counterparts do not. Consider a fungus that can either forage for nutrients by a diffuse mycelial network, or extend rapidly away from resource depletion zones into new terrain by aggregating its hyphae into telephone cable-like strands (rhizomorphs), or divert biomass into wind disseminated spores and the specialized structures built to broadcast them. Dispersal in some form is evident in all organisms, but the costs are shown most dramatically within the modular category. As addressed in earlier chapters, mortality may exceed 90% of the cell mass in dictyostelid cellular slime molds, where the stalk of the fruiting body consists of dead cells ‘sacrificed for the cause’.

Among the fungi (e.g., Aphyllophorales of the Basidiomycetes), resources are diverted into massive bracket conks, structures that exist purely for dispersal and are ultimately destined to die. Moreover, the wood-rotting fungus may produce conks prolifically, thus expending its finite resource quickly ‘in brief but riotous living’ Alternatively, it may opt to stagger reproduction over time, thereby prolonging the food base but risking loss to competitors or by mishap. Among plants, the compromise is between the competing needs of photosynthetic and non-photosynthetic tissue. Among sessile marine invertebrates, it is between biomass allocated for support against turbulence, for food interception, or for production of progeny. While the modular life style makes the concept of trade-offs especially compelling, sorting out the forces involved in any situation and the relative roles of each in the evolution of form is challenging.

What limitations does growth form impose on the extent to which basic ecological concepts can be generalized? A good example is the logistic or sigmoid equation developed by Verhulst (1838) to describe growth of human populations. The sigmoid model and variations of it have been used almost universally as descriptors of population increase (Chap 4; for microbial analogues see Andrews and Harris 1986). In terms of a unifying ecological principle, logistic theory at the population level is as basic a tenet as is that of trade-offs at the level of the individual. It is the foundation for fundamental sequels such as the Lotka-Vol- terra model of interspecific competition, and r- and К-selection (Chap. 4).

Although density-dependent regulation of populations in some form is generalizable, as is r/К theory, the predicted outcome may vary depending on growth form. Sackville-Hamilton et al. (1987) propose that the effects of r- and К-selection may not be the same on unitary as on clonal (modular) organisms. For instance, whereas genets with a phalangeal growth form (Chap. 5) are expected to behave according to the theory, those with the guerrilla habit are not. In the latter case, r-selection favors nonreproductive (nonsexual), so-called immortal genets that spread clonally unless there is a lethal condition that kills all stages in the module-to- module cycle. К-selection acts similarly, provided the environment is reasonably stable over many generations (if it is not, then К-selection should favor reproduction).

These predictions are of course opposed to convention, which associates greater reproduction with r-selection and competitive growth with К-selection (7Chap. 5). The authors observe that the genets of some higher plants are ‘immortal’ and never reproduce. It is also interesting that numerous fungi appear to grow indefinitely as clones. The significance of the paper by Sackville Hamilton et al. (1987) in our context is that growth habit, not size per se, may greatly influence response to selection pressure. Hence, it would be informative to align evolutionary predictions with respect to growth form, rather than segregated by size to microorganism as opposed to macroorganism.