SPATIAL DYNAMICS OF A HIMANTHALIA ELONGATA (FUCALES, PHAEOPHYTA) POPULATION

Abstract

In dense monospecific stands of plants intraspecific competition usually results in self‐thinning, the concurrent increase in biomass and decrease in density over time. Self‐thinning may also result in a change in the spatial pattern of individuals, but so far the spatial dynamics of marine plants has not been investigated. The brown alga Himanthalia elongata (L.) S. F. Gray forms dense monospecific stands on many northern temperate rocky shores, and various attributes (including its simple form) facilitated the study of the spatial dynamics of this species. The spatial pattern of settling zygotes was examined in the laboratory. In the absence of water movement, substratum heterogeneity, and a point source, zygotes usually settled in clumps rather than randomly. Within the clumps zygotes appeared to be regularly distributed at a scale similar to the size of the zygotes themselves. Furthermore, the clumps themselves seemed to be regularly distributed. On the shore, well‐established stands of “button‐stage” Himanthalia populations were examined during a period of extensive growth and self‐thinning. Individual plants were initially highly regular in spatial pattern but became less so over time. The pattern of plants dying during self‐thinning was also highly regular and probably reflected existing spatial regularity. However, using a hypothesis of mortality as a random event, I found that smaller plants had a less than average survival potential, while larger plants had a greater than average chance. A consideration of the spatial pattern of plants alive at the end of the study revealed regularity at a scale of 2–7 mm but a random spatial pattern at larger scales, which might indicate a small sphere of influence of competing individuals. The best predictor of mean nearest neighbor distances in the populations was mean plant diameter.

The development of size structure in a youngFucus serratuspopulation

“…Furthermore, as the populations of germlings developed, the sizes of plants became more variable, and a very strong positive skew had developed in population plant lengths by the end of the experiment. The development of young F. serratus is different from that of established Himanthalia elongata populations, in which inequality of sizes decreased over time as a result of self-thinning removing small plants (Gini coefficient day 0 --0"21, day 187 = 0"13; Creed, 1995). Martinez & Santelices (1992) reported Gini coefficients for the seaweeds Chondrus crispus (min.…”

Phenology of Sargassum spp. (Sargassaceae, Phaeophyta) from Reunion Rocks, KwaZulu-Natal, South Africa

“…It has been suggested that positive density dependence in algal species arises from benefits associated with the closer proximity of neighbours in a manner that outweighs competitive disadvantage (Creed, 1995;Scrosati, 1996). Such positive density dependence could be due to increased protection from desiccation (Hruby & Norton, 1979), protection from physical battering (Schiel & Choat, 1980), grazing (Lubchenco, 1983) or both (Ang & De Wreede, 1992).…”

SEASONAL CHANGES IN SIZE STRUCTURE OFSARGASSUMANDTURBINARIAPOPULATIONS (PHAEOPHYCEAE) ON TROPICAL REEF FLATS IN THE SOUTHERN RED SEA1

“…As in terrestrial plants (Weiner and Thomas 1986, Weiner et al. 2001), seasonal changes in population structure of large canopy algae appear to be associated with seasonal patterns of growth and strong changes in density due to seasonal recruitment and density dependent mortality (Ang and De Wreede 1992, Kendrick and Walker 1994, Creed 1995, Creed et al. 1998, Arenas and Fernández 2000, Arenas et al.…”