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Natural dynamics and matrix models of a fucus distichus (phaeophyceae, fucales) population in Vancouver, British Columbia, Canada Ang, Put Ong


Patterns of reproduction, micro-recruitment, macro-recruitment, age- and size-dependent reproduction, growth and mortality in a population of the brown alga Fucus distichus in Vancouver, British Columbia, Canada, were examined from May 1985 to November 1987. Using log linear and association analyses, age and size are both found to be significant, but size more so than age, as descriptors of the demographic parameters. Reproductive plants were found throughout the sampling period, but peaked in fall and winter of each year. Estimated monthly egg production, calculated by the observed number of eggs in clusters extruded from the receptacle, is independent of plant size. Two types of recruits were monitored. Microrecruits (< 1 month-old of microscopic size) are germlings developed from fertilized eggs. Their numbers were assessed using settling blocks. Macrorecruits are detectable by the unaided eye and are plants appearing in the permanent quadrats for the first time. The recruitment pattern of microrecruits is significantly correlated with reproductive phenology and patterns of potential and estimated monthly egg production. However, peaks in micro-recruitment are not always followed by peaks in macro-recruitment. This apparent discrepancy is probably due to a differential survivorship of microrecruits over time or to the possible existence of a "germling bank". Patterns of survival and emergence of macrorecruits may be independent of those of microrecruits or may be unrelated to the prevailing reproductive phenology. This population of Fucus distichus showed seasonal variations in plant mean length and growth rates. Mean length was greater in winter (4.2-5.3 cm) and lower in summers of 1986 and 1987 (2.7 - 4.3 cm). Absolute growth rates showed a significantly opposite trend, being higher in spring and summer (0.24 - 1.17 cm/month) and lower in fall to winter (-0.5 - 0.4 cm/month). The relationship between reproduction, growth and mortality was also evaluated in terms of the cost of reproduction. There is no clear indication of cost of reproduction with respect to the longevity or mortality of fertile vs. non-fertile plants. Fertile plants, especially those > 17 cm in length, tend to exhibit negative or zero growth much more often than non-fertile plants, suggesting the cost of reproduction may be manifested in the form of reduced growth rather than in greater mortality or shorter longevity of the fertile plants. The failure to detect cost of reproduction may be due to the modular character of the plants, where cost occurs at the level of the modules (branches) rather than at the level of the whole plant. The effect of density on mortality and growth among recruits in this population was also monitored. In the first 2 months of development, germlings growing at high density experienced a lower mortality than those growing at lower densities. At later stages (> 2 months), the effect of density on mortality was reversed. Plant growth rate was generally not related to density but was related to plant length. The dynamics of the population was further evaluated with a 9 x 9 matrix model based on recruit stages and plant size. From the elasticity analysis, the survival and transition of the plants among size classes was found to be the most important parameter and contributed at least 50% to the population growth rate (lambda). Fucus does not grow by vegetative propagation, the population can only experience positive growth in the presence of recruitment. The current population size structure is unstable and is very different from the projected stable distribution. Overall, the population is on the decline. However, it is likely that the population may recover by occasional pulses of a large number of recruits, an example of which was observed in 1986.

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