Density-dependence in a population of Fucus distichus

Abstract

Regulation of a Fucus djstichus L. emend. Powell population is probably mediated through a density-dependent mortality of the recruits. Blocks seeded with different densities of germlings of E distichusand cleared plots with naturally occurring recruits of different densities and with artificially thinned densities were used to monitor the effect of density on mortality and growth among recruits in a populat~on of F: distjchus in False Creek. Vancouver, British Columbia. Canada. In the first 2 mo of development, germlings on the high density blocks experienced a lower mortality than those on the low density blocks. At later stages (> 2 mo), the effect of density on mortality was reversed. Plants recruited into the cleared plots were > 1 mm in length and they also experienced a positive effect of density on mortality. In both the density blocks and cleared plots, plant mortality eventually became independent of plant density. Plant growth rate was generally not related to density but was related to plant size (length). There was a large variation in the sizes of the plants. All 3 measures of si7c inequality: zero-centered skewness, coefficient of variation, and Gini coefficient are significantly correlated. Plants in both the density blocks and cleared plots showed a slight tendency towards a more normal or positively skewed size distribution from fall 1986 to spring 1987, probably due to an increased mortality of the smaller plants during this time. The mechanism of dominance-suppression and difference in the intrinsic growth rate or timing of settlement probably all contributed to the inequality in size distribution.

The development of size structure in a youngFucus serratuspopulation

“…Oospores released from the single F. serratus plant studied were more equal in size than any of these species. Ang & De Wreede (1992) demonstrated skewness of -1-5 to 2-5 in 8-to 16-month-old Fucus distichus populations.…”

“…It therefore seems likely that, in F. serratus populations on some shores, individuals are retarded in a "seed bank" year round. "Seed banks" have recently been found in other seaweed species (Ang, 1991;Hoffmann & Santelices, 1991;Ang & De Wreede, 1992;Kendrick & Walker, I994, but contrast Creed, 1995) and the seed bank contingent may buffer stochastic events which remove macroscopic plants from the population. The implication is that macroalgal population studies which ignore the microscopic phase do so at their peril.…”

“…Often the three measures are correlated (e.g. Ang & De Wreede, 1992), but apart from having slightly different properties (Bendel et al, 1989) all three measures were calculated so that our results could be compared with others which used only one or other of these statistics.…”

Effects of density on the vital rates of a modular seaweed

“…Considerable progress has recently been made with such experiments (Reed 1990, Ang & De Wreede 1992, Kendrik 1994, Creed et al 1996, Creed et al 1998, Lazo & Chapman 1998. However, observational studies are also valuable especially as a first attempt to investigate processes correlated with density.…”

“…Self-thinning or density-dependent mortality is common in macroalgal stands (Schiel & Choat 1980, Dean et al 1989, Kendrick 1994, Creed et al 1996 as it is in terrestrial systems (Harper 1977, Westoby & Howell 1986, Schmitt et al 1987. Algal individuals often have lower growth rates in crowded stands, and the effects are more dramatic in small plants (Reed 1990, Ang & De Wreede 1992, Creed et al 1998. Changes in the morphology of algae have likewise been reported, with plants at high density being more elongated or less ramified than at low density (Reed 1990, Holbrook et al 1991, Sjøtun & Frediksen 1995, Andrew & Viejo 1998, a response similar to that observed in terrestrial plants (Harper 1977, Silvertown & Lovett Doust 1993, Hutchings & de Kroon 1994.…”