Description
Adult giant kelp, Macrocystis pyrifera, release billions of zoospores into the surrounding kelp forest community during each reproductive season. The young sporophytes that ultimately develop from these zoospores undergo a process of self-thinning until their densities are reduced to levels that can be sustained by environmental conditions at that time. We examined the effects of recruitment density on the survival of juvenile M. pyrifera within naturally occurring patches in the Point Loma kelp forest, San Diego, CA and found that the total density of all macroalgae within the patch had no effect on M. pyrifera survival. Instead, we found that density-dependent mortality within each patch was driven almost entirely by the density of conspecifics. This pattern, and the direct effects of recruitment density on juvenile M. pyrifera survival was further tested experimentally by oputplanting two size classes of juvenile M. pyrifera (medium size class, 5-25cm in height; large size class, ~1m in height) at three densities (20, 60 and 120 individuals m-2, and 2, 4 and 8 individuals m-2 for medium and large size classes, respectively). While the large size class exhibited the overall greatest survival, declines in survival were consistent through time for all density treatments and for each size class, with the surviving sporophytes converging on similar densities (8 m- 2 ±4 for the medium size class, and 1 m-2 ±1 for the large size class). Together, our observational and our experimental studies demonstrate that, regardless of the initial density at which M. pyrifera recruits, the resulting patches of juvenile sporophytes will thin to similar densities that are presumably appropriate for their survival under the existing environmental conditions. This self-thinning process is an example of density-dependent mortality that operates in addition to other stressors such as grazing, sand scour, entanglement and dislodgement due to wave energy, each of which effectively reduces the population size of new M. pyrifera recruits to a level that is sustainable by the kelp forest community.
