A competitionbased model of algal zonation

The zonation of the fucoid algae on sheltered shores of the British Isles is accurately reproduced by an individual-based computer simulation model (see IndividualBased Models). The relative growth rates differ among three competing species according to tradeoffs between net photosynthetic performance and tolerance to desiccation. Parameters for these relationships were drawn directly from field and laboratory measures. Thus, F. serratus, which naturally dominates the low shore, has the fastest growth per unit of photosynthetically active radiation (PAR), but is slow to recover photosynthetic performance following moderate water loss during emersion. F. spiralis, which naturally dominates middle shore levels, has intermediate values for PAR-based growth and for recovery from desiccation. Pelvetia canaliculata, which naturally dominates upper shore levels, has relatively low values of PAR-based growth, but relatively quick recovery from extreme levels of desiccation. Incident PAR depends on (1) shore level, because light attenuates with increasing depth and duration of submergence; and (2) shading of the photosynthetic tissues as an individual's fronds overlap or are overgrown by other plants. Very low incident PAR incurs the complete suppression of growth as photosynthesis declines to match rates of respiration. Therefore, success in competition is determined when individuals of one species grow taller than adjacent individuals of another species, suppressing that species as their own growth increases. Over the course of simulations, an initially uniform distribution of algal sporlings gives arise to the distinct zones of adults, because the advantage in size-dependent competition shifts among the three species over the compound gradient of desiccation and PAR.

The central premises - that competition structures the zonation of Fucus spp., and success in competition depends on a tradeoff between growth rate and tolerance to desiccation - was proposed a century ago based on some of the first laboratory studies of the adaptations of intertidal algae. The model provides a framework to develop this idea by incorporating detailed estimates of the underlying physiology in the context of the compound environmental gradient.

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