The Effects of Variable Settlement and Early Competition on Patterns of Kelp Recruitment

Abstract

Recruitment from newly settled propagules is an important process in the population dynamics of many species especially those living in highly disturbed areas and subjected to frequent local extinctions. For many primitive plants such as kelps (large brown algae in the order Laminariales), the process of recruitment (i.e., the first appearance of macroscopic individuals) is quite complex and requires the successful completion of an alternate microscopic free—living sexual stage. To experimentally investigate the consequences of variable spore settlement and early (i.e., prerecruit) competition on the recruitment patterns of the sympatric sublittoral kelps Macrocystis pyrifera and Pterygophora californica, I seeded bare rock substrates with known densities of spores of the two algae in both single— and mixed—species assemblages. Results from 12 field experiments done over 3 yr indicated that for both kelps, a minimum density of at least 1 spore/mm2 was needed for successful recruitment. This minimum density was probably determined in part by the maximum distance that could separate male and female gametophytes while still allowing fertilization to occur. Although greater spore density often resulted in higher numbers of sporophyte recruits, strong density—dependent mortality still occurred whenever there was recruitment. The requirement for spores to settle at relatively high densities (i.e., >1 spore/mm2), coupled with the drastic difference in size between spores (0.01 mm) and early recruits (20 mm), insures that density—dependent mortality will occur at early stages in these algae.“Safe sites” for the recruitment of kelp were highly variable in time; recruitment of both kelps varied significantly among experiments and was negatively correlated with the natural recruitment of ephemeral species of filamentous brown algae. Differences in postsettlement mortality between Macrocystis and Pterygophora often caused patterns of sporophyte recruitment to vary from those of initial spore settlement. In monospecific treatments, the recruitment rates of the two kelps usually differed in the same experiment, indicating they responded differently to the same environmental conditions. In mixed—species treatments, interspecific competition between microscopic prerecruit stages also caused species—specific differences in early mortality. Pterygophora spore settlement generally inhibited the recruitment of Macrocystis at both high (750—1500 spores/mm2) and low (10—20 spores/mm2) densities. Competition between the two kelps was asymmetrical; Macrocystis settlement had no effect on Pterygophora recruitment. Thus, changing environmental conditions and competition among early microscopic stages modified the ways in which spore settlement influenced sporophyte recruitment. The effects of these factors on kelp recruitment were interactive, and their relative importance will undoubtedly vary with species, location, and time.