Selective Pressures and Coevolution in a Kelp Canopy Community in Southern California

Abstract

The community of epiphytes on the giant kelp Macrocystis pyrifera, is simple, consisting primarily of the bryozoans Membranipora membranacea, Hippothoa hyalina, and Lichenopora buskiana; the serpulid polychaete Spirorbis spirillum; and the hydroids Obelia sp. and Campanulaia sp. Abundances of the organisms vary rapidly, by an order of magnitude or more in 2 wk. In spite of this short—term unpredictability, broad patterns of distribution are predictable. Membranipora, the competitive dominant, is abundant during winter and spring and absent during summer and fall when surface waters are warm and thermally stratified. When present, it occurs primarily in the canopy along the outer edge of the large (1 X 11 km) Point Loma, San Diego, kelp bed. The distribution of the subordinate epiphytes complements that of Membranipora. They occur in the interior portions of bed, and on the lower, older blades of Macrocystis plants. These patterns are maintained by a sophisticated suite of larval behaviors and settlement preferences. The subordinate epiphytes settle preferentially on the older blades of Macrocystis, and avoid blades from plants on the outer edge of the bed where competitive overgrowth by Membranipora is more likely. In addition the extremely short larval life—spans of the subordinate epiphytes (a few hours) mean that offspring will settle near where they were released, in the interior of the bed. Membranipora, in contrast, has a planktonic lifetime of 2 to 4 wk. Nearshore current measurements indicate that as a result its larvae are flushed from the bed, and that the source of larvae ready to metamorphose and settle must be external to the bed, from other kelp beds in the area. This helps restrict Membranipora to the outer edge of the bed, since larvae are removed from the water as it passes through the bed both by settlement on Macrocystis blades and by plankivorous fish that feed on them. The labrid fish Oxyjulis californica is an important predator in this system. It is a curious, actively searching generalist that excludes large, erect epiphytes such as the barnacle Lepas pacifica and the bivalve Leptopecten latiauritus from the nearshore kelp bed. Field and laboratory experiments imply that Oxyjulis forms short—term search images and switches among prey species. One of the species it regulates is the herbivorous isopod Pentidothea resecta, which, when released from regulation, multiples rapidly and destroys the kelp canopy. Oxyjulis, though it buffers the system from extinction due to Pentidothea, is also potentially destructive. Small kelp beds, too small for the filtering action to take effect, become heavily encrusted with Membranipora and can be destroyed by Oxyjulis predation on the encrusted blades. The set of relationships among species in this community can fruitfully be viewed as having coevolved to contribute to persistence in this unstable environment. Indeed, some factors that contribute to instability are the very ones necessary for persistence. Competition, predation, habitat selection, and life history strategies interact to ensure that the number of species in the community and their interrelationships remain constant. There appear to be no mechanisms acting to regulate epiphyte species to equilibrium levels of abundance. This system is thus best described as one that fluctuates unpredictably within well—defined boundaries.