SIMULATION AND ANALYSIS OF THE DYNAMICS OF A FUCUS-DISTICHUS (PHAEOPHYCEAE, FUCALES) POPULATION

A general 9 x 9 matrix model based on recruit stages and plant size is proposed for a population of the brown alga Fucus distichus L. emend. Powell in False Creek, Vancouver, Canada. Twenty-six matrices were constructed each representing a monthly time interval covering the period from July 1985 to November 1987. Yearly matrices were also constructed as a product of monthly matrices covering periods of 1 yr. The characteristics of these matrices were evaluated by comparing the dominant eigenvalue (i.e. the population growth rate, lambda), the stable distribution, and reproductive values for each matrix. The relative contribution of each matrix parameter to population growth was assessed using elasticity analysis. The survival and transition of the plants among size classes contribute at least 50 % to the population growth rate. However, because F. distichus plants do not exhibit vegetative regeneration, the population can only experience positive growth in the presence of recruitment. Absence of a germling bank has little effect on monthly projected population growth, but can reduce the population growth rate projected from the yearly matrix by as much as 83 %. Current population size structure is very different from the projected stable distribution. The stable distribution generally has a large proportion of the plants in the recruit stage. Reproductive values are positively related to plant size but are comparable among the largest 4 size classes. Random combinations of monthly, seasonal, or yearly matrices were used to simulate population growth under various fluctuating environmental conditions. A negative growth rate was obtained in > 60 % of the simulation runs. In reality within the sampling period covered, the population is on the decline. It is likely that this population may recover by occasional pulses of a large number of recruits, as was observed in 1986.