The Establishment and Development of a Marine Epifaunal Community

Because of the nature of their substratum, the sessile invertebrate species of the marine epifaunal community living on rocks occur in discrete patterns of distribution. The rocks are finite patches or habitat islands with a limited space for colonization and growth. Such a system is ideal for studying the parameters affecting the distribution of species within a community. Also, because of the small size and immobility of the adults, the system is also ideal for studying the pattern of change in species composition and diversity within a community. This study used multiple series of manipulated experimental plates, which both duplicated natural rock surfaces and could be compared with samples of the rocks, to investigate the developmental and distributional processes of this community. Five major factors were found to be important to both the development of the community and its distribution on the rocks: (1) the selectivity of the metamorphosing larvae as to site of attachment; (2) the seasonal fluctuation in larval abundances; (3) the biological interactions within and between species; (4) the size of rock substrata; and (5) the physical disturbance of the substrata (rock turnover). Initially, the developmental process can be uncoupled from the effects of the substrate size and disturbance. Predation is relatively unimportant as a biological interaction within this community, but the species can be ranked according to their ability to compete for the available space on a substratum. This ranking implies a type of successional sequence in the development of the community; however, the sequence is greatly affected by historical components. The colonization of a substratum is directly dependent upon the abundance of settling larvae, which in turn is a function of seasonality and selectivity. The eventual competitive outcome and development of the community will depend upon which species have immigrated onto the substratum and is thus dependent upon history. The process is, therefore, open ended: colonization will be highly variable and change seasonally and, although one species may eventually dominate the substratum, it may be one of nine different species depending upon the individual history of that area. The frequency with which a substratum is disturbed (with the resultant extinction of its fauna) is a function of wave force and is inversely proportional to both the size of the substratum and the depth at which it occurs. Disturbance will determine when a substratum is initially exposed for colonization and how long it will have for development. In the shallow subtidal (mean low water to -2.5 m), the frequent disturbance of small rocks will cause them to support less than their equilibrium number of species and their fauna will reflect immediate larval abundances. Large rocks will remain stable for long periods of time and will usually be dominated by a single species. Intermediate-sized rocks (1 to 10 dm3) will remain stable long enough to develop an equilibrium number of species but will be disturbed before dominance occurs. They will thus have the highest diversity because of their 'optimal' frequency of disturbance. In deeper water (10 m) smaller rocks will be more stable. More rocks will develop dominance and the smaller size of a rock at the optimum frequency will mean that substrata at this frequency will have a lower equilibrium number of species and thus a lower diversity. The increased stability with depth will mean a lower overall diversity for the community. Lastly, the increased disturbance and lower pool size for rocks in the intertidal will also cause diversity to be lower in this area. There appears, therefore, to be an optimal frequency of disturbance at which diversity is maximized. An increase or decrease in this level causes a reduction in diversity because of a decrease in the number of species present or an increase in dominance. This optimum will vary with the physical environment and the type of community.