A Predator‐Prey System in the Marine Intertidal Region. I. Balanus glandula and Several Predatory Species of Thais

On San Juan Island, Washington, three species of intertidal barnacles (Balanus glandula, Balanus cariosus and Chthamalus fissus) are the main prey of several species of predators, the commonest being three species of snails, Thais emarginata, Thais canaliculata and Thais lamellosa. Larvae of B. glandula settle throughout the intertidal zone but, except in quiet bays, survive to maturity only in a narrow zone at the top of the shore. In quiet bays, where predators of barnacles were scarce, and in areas of turbulent water where predators were excluded by other circumstances, adult B. glandula occurred over the whole intertidal zone. From the lower part of the intertidal zone. Predators were evidently eliminating all B. glandula from the lower part of the intertidal zone. The rates of feeding of Thais lamellosa and Thais emarginata were measured in cages on the shore. The existing population of Thais could account for all of the mortality of B. glandula which occurred at low shore levels in mid and late summer. By late autumn these predators could eat all the remaining B. glandula of that year's settlement at low levels, but at upper shore levels the Thais population could not account for all the mortality of B. glandula in summer. The Thais populations shift upwards from the lower shore levels in the autumn and it is very probable that this increased predation is enough to eliminate the barnacles at the middle and upper shore within the next year. The recruitment of B. glandula every year was quite regular. The breeding population in the narrow "refuge" zone at the top of the intertidal zone was capable of replacing itself as well as colonizing the middle and lower shore. The settlement below the refuge zone can be regarded simply as a regular food supply for the predator population. Because of the dependability of this food supply at upper shore levels, a predator, T. emarginata, has evolved to specialize at these upper levels. In Scotland, where the recruitment of barnacles is irregular, no such specialization is possible. Thus at least two species of predators are supported at different shore levels at San Juan Island, whereas only one species occupies the whole intertidal shore in Scotland. In Scotland, much of the mortality of young barnacles was caused by intraspecific crowding in dense populations, and these deaths provide food for scavengers, rather than predators. In contrast, at San Juan Island of the mortality of young barnacles is caused by predators. Thus the predators at San Juan Island are more efficient than those in Scotland, since they eat a much higher proportion of the available prey. Whereas in Scotland the distribution and abundance of barnacles is largely determined by competition for space, at San Juan Island the barnacles is largely determined by competition for space, at San Juan Island the barnacles are limited by predation.