Life Histories of Modular Versus Unitary Organisms

Growth of the Individual and Size of Populations.For modular organisms there are two levels of population structure in a community (Chap. 1 in Harper 1977; Harper and Bell 1979): The genets, formally equivalent to the original zygotes (called N in animal population dynamics) and the quantity of modules (called ⴄ in plant population dynamics), a variable number of which occurs per genet. In theory both N and N · ⴄ (i.e., the number of modules in a given population) can be quantified. For aclonal modular organisms such as annual plants and most tree species, N can be counted easily. The difficulty occurs with clonally spreading perennials or among microorganisms and invertebrates where intermingling, sporulating, budding, or fragmenting clones necessitate either a logistically feasible marker or a mapping system to separate genets from ramets (discussed later under Longevity). Individuals tallied where possible as genets are meaningful to the geneticist or evolutionist interested in the genetic variation of a population. The production agriculturalist or population biologist or microbiologist is more interested in the number of ramets, that is, functional individuals, which gives a rough idea of biomass. The important point is that the basic demographic equation

N_t+1 = N_t + births — deaths + immigrants — emigrants
applies at both the genet and module levels of the population.

The most obvious difference, then, between a modular organism and a unitary organism with respect to growth is that growth of the former is a population event: There is an increase in number of modules, whether these are multicellular units of construction (e.g., leaves) or units of clonal growth capable of separate existence (i.e., ramets such as fronds of bracken fern or bacterial cells; see again Fig. 5.2). Growth (enlargement) of the module is formally equivalent to growth (enlargement) of the entire soma in unitary organisms (a single deer, a single bear, etc.), and this is distinct from population growth, which is the aggregate expression of each unit of construction. Hence, while each cell of the bacterial or yeast clone increases in mass more-or-less arithmetically prior to fission or budding, the population of such cells increases logarithmically, at least initially. Similarly, for the fungi, increase in germ tube length is initially exponential, then linear; however, because new branches are formed continuously the germling as a whole continues to expand exponentially, both in terms of total length of mycelium and number of branches. As summarized by Harper and Bell (1979, p. 31) “the parts of modular organisms have their own birth and death rates; a genet has its own internal population dynamics”.

Where modular organisms are also clonal, growth of the genet can be especially rapid. Thus, under favorable conditions, the number of ramets may increase (more poplar trees), or the number of modules per ramet (more leaves per poplar stem), or both. Conversely, ‘degrowth’ or shrinking can occur at either or both levels (Hughes and Jackson 1980; Hughes and Cancino 1985; Sebens 1987). For colonial clones, such as bryozoans and bacteria, there may be larger colonies, a larger number of colonies, or both. The prodigious, in some cases exponential, multiplication rates of bacteria are the extreme example.

Some implications of the population biology of modular organisms are nicely illustrated by the growth of Lolium grass (Harper and Bell 1979). The seed sown represents the new genets (N) and the young plants expand by producing tillers (branches from basal nodes) that represent new modules (ⴄ). The number of original zygotes will decline due to density- dependent controls, while the number of tillers will increase initially and then decline. In other words, the early effects of density will be reflected in the death rate of genets and the birth rate of modules. When this grass sward flowers and produces seed, the progeny will not reflect the balance of genes in the original population because some of the original genets will have died.

There will also have been differential growth and reproduction of the tillers, hence flowers and seeds, on the surviving genets (Harper and Bell 1979). This thinking can be extended to modular organisms in general, including microorganisms. For example, the number of clones of a bacterial or fungal species in an area would be represented by N, while ⴄ would represent the number of cells (or, alternatively, colonies) per clone. As the population grows the genetic structure will change as some clones outcompete and displace others, while yet others continue to arrive (Reeves 1992; Andrews 1998). This, of course, is a well- known phenomenon in plant pathogen and medical epidemiology (recall the worldwide ebb and flow of pathogenic bacterial clones carrying antibiotic resistance genes described in Sidebar in Chap. 2; e.g., Hawkey and Jones 2009).