The importance of modularity

The sedge Carex bigelowii, growing in a lichen heath in Norway, illustrates the difficulties of constructing any sort of life table for organisms that are not only iteroparous with overlapping generations but are also modular (Figure 4.13). Carex bigelowii has an extensive underground rhizome system that produces tillers (aerial shoots) at intervals along its length as it grows. It grows by producing a lateral meristem in the axil of a leaf belonging to a 'parent' tiller. This lateral is completely dependent on the parent tiller at first, but is potentially capable of developing into a vegetative parent tiller itself, and also of flowering, which it does static life tables: flawed but sometimes useful, none the less

Table 4.4 Mean clutch size and age of great tits in Wytham Wood, near Oxford, UK. (After Perrins, 1965.)

1961 1962 1963

Age (years) Number of birds Mean clutch size Number of birds Mean clutch size Number of birds Mean clutch size

Yearlings 128 7.7 54 8.5 54 9.4

Figure 4.12 Age-dependent reproduction (average litter size) in a population of moose (Alces alces) in Sweden (means with standard errors). (After Ericsson et al., 2001.)

when it has produced a total of 16 or more leaves. Flowering, however, is always followed by tiller death, i.e. the tillers are semel-parous although the genets are iteroparous.

Callaghan (1976) took a number of well-separated young tillers, and excavated their rhizome systems through progressively older generations of parent tillers. This was made possible by the persistence of dead tillers. He excavated 23 such systems containing a total of 360 tillers, and was able to construct a type of static life table (and fecundity schedule) based on the growth stages (Figure 4.13). There were, for example, 1.04 dead vegetative tillers (per m2) with 31-35 leaves. Thus, since there were also 0.26 tillers in the next (36-40 leaves) stage, it can be assumed that a total of 1.30 (i.e. 1.04 + 0.26) living vegetative tillers entered the 31-35 leaf stage. As there were 1.30 vegetative tillers and 1.56 flowering tillers in the 31-35 leaf stage, 2.86 tillers must have survived from the 26-30 stage. It is in this way that the life table - applicable not to individual genets but to tillers (i.e. modules) - was constructed.

There appeared to be no new establishment from seed in this particular population (no new genets); tiller numbers were being maintained by modular growth alone. However, a 'modular growth schedule' (laterals), analogous to a fecundity schedule, has been constructed.

Note finally that stages rather than age classes have been used here - something that is almost always necessary when dealing with modular iteroparous organisms, because variability stemming from modular growth accumulates year upon year, making age a particularly poor measure of an individual's chances of death, reproduction or further modular growth.

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