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individuals differ

Among the simplifications in our ecological fact of life is the implicit assumption that all individuals are alike (since all we need do is count them), which is patently false on a number of counts. First, almost all species pass through a number of stages in their life cycle: eggs, larvae, pupae and adults in many insects; seeds, seedlings and photosynthesising adults in plants; and so on. The different stages are likely to be influenced by different factors and to have different rates of migration, death and, of course, reproduction. Second, even within a stage, individuals can differ in ‘quality’ or ‘condition’. The most obvious aspect of this is size, but it is also common, for example, for individuals to differ in the amount of stored reserves they possess.

unitary and modular organisms

Uniformity amongst individuals is especially unlikely when organisms are modular rather than unitary. In unitary organisms – mammals, birds, insects and so on – form and the succession of phases in the life cycle are predictable and ‘determinate’. That is, all dogs have four legs and each squid has two eyes, and dogs and squid that lived longer would not develop more of them. Similarly, we humans pass through an embryonic stage of nine months, a growth phase of around 18 years incorporating a prereproductive phase of 12 or so years, a reproductive phase lasting perhaps 30 years in females and rather longer in males, followed finally by a phase of senescence. Death can intervene at any time, but for surviving individuals the succession of phases, and even mostly the timing of phases, is, like form, entirely predictable.

But none of this is so simple for modular organisms such as trees, shrubs and herbs, chain‐forming bacteria and algae, corals, sponges, and very many other marine invertebrates (Figure 4.1). These grow by the repeated production of ‘ modules’ (leaves, coral polyps, etc.) and almost always form a branching structure. Most, following a juvenile phase, are rooted or fixed, not motile, and both their structure and their precise programme of development are not predictable but ‘indeterminate’. After several years’ growth, depending on circumstances, the same germinating tree seed could either give rise to a stunted sapling with a handful of leaves or a thriving young tree with many branches and thousands of leaves. It is modularity and the differing birth and death rates of modules that give rise to this plasticity. Reviews of the growth, form, ecology and evolution of a wide range of modular organisms may be found in Harper et al. (1986), Hughes (1989) and Collado‐Vides (2001).

Figure 4.1 Modular plants (left) and animals (right) show the underlying parallels in the various ways they may be constructed. (a) Modular organisms that fall to pieces as they grow: duckweed (Lemna sp.) and Hydra sp. (b) Freely branching organisms in which the modules are displayed as individuals on ‘stalks’: a vegetative shoot of a higher plant (Lonicera japonica) with leaves (feeding modules) and a flowering shoot, and a hydrozoa (Extopleura larynx) bearing both feeding and reproductive modules. (c) Stoloniferous organisms in which colonies spread laterally and remain joined by ‘stolons’ or rhizomes: strawberry plants (Fragaria) reproducing by means of runners, and a colony of the hydroid Tubularia crocea. (d) Tightly packed colonies of modules: a tussock of yellow marsh saxifrage (Saxifraga hirculus), and a segment of the sea fan Acanthogorgia. (e) Modules accumulated on a long persistent, largely dead support: an oak tree (Quercus robur) in which the support is mainly the dead woody tissues derived from previous modules, and a gorgonian coral in which the support is mainly heavily calcified tissues from earlier modules.

what is the size of a modular population?

It follows from this that in modular organisms, the number of surviving zygotes (individuals in a genetic sense) can give only a partial and misleading impression of the ‘size’ of the population. Kays and Harper (1974) coined the word genet to describe this ‘genetic individual’ – the product of a zygote – and we can see that in modular organisms, the distribution and abundance of genets is important, but it is often more useful to study the distribution and abundance of modules (ramets, shoots, tillers, zooids, polyps or whatever). The amount of grass in a field available to cattle is not determined by the number of genets but by the number of leaves (modules).