California Sea Grant College Program

The purpose of this research was to gain an understanding of how physical and biological factors interact to transform local reef communities to and from a kelp forest and a barren state.

Data on plant, sea urchin and fish densities indicate that two unusually sever winter storms in 1980 (Storm I) and 1983 (Storm II), had different effects on a Southern California kelp forest. Storm Iremoved all canopies of giant kelp. Denied their preferred diet of drift kelp, urchins emerged from shelters and in the lack of predators, they consumed most of the remaining algal turf. This caused a decline in fish (Embiotocidae) that eat small animals living in turf. Storm II reversed the process by eliminating exposed urchins and clearing rock surfaces for kelp settlement and growth, thus extensive canopies redeveloped by summer 1984.

A study of the microhabitat use and food habits of five surfperch (Embiotocidae) showed them to exploit small animal prey in benthic turf. Turf contained inorganic debris and "low value" food items which fish mostly rejected, and "high value" items (amphipods etc.) which they selected. Two species took careful bites and swallowed all items. The three others selected food by winnowing bites of turf in their mouth and spitting out the cast. All species preferred microhabitats with highest densities of valued food items. Due to the ability of urchins to overgraze kelp and turf to a barren state of bare rock and crustose coralline, laboratory and field experimental investigations of physical obstacles to urchin movement was undertaken. Both red (Stronglyocentrotus franciscanus) and purple (S. purpuratus) urchins can negotiate sand using their oral spines, although purple urchins are more hesitant to do so. Starved red urchins transplanted to rock plots retreated to nearby crevices from where they ate attached kelp. Red urchins transported to sand plots with kelp present soon disappeared because individuals have difficulty holding and eating attached kelps on unconsolidated surfaces. In another experiment, red and purple urchins reached kelp on a rock ledge only by mounting an artificial ramp. In the absence of predator control, urchins can surmount most rock or sand barriers during periods of low water motion.

Observations of sea otter invasion of two rock reefs and predation on urchins showed contrasting outcomes. At one urchin-dominated low relief and barren reef, otter predation eliminated urchins as turf, kelp and sessile animals filled the space once dominated by urchins and crustose coralline algae. Similar changes occurred in urchin exclusion plots in an urchin barren ground at Naples reef in southern California where there are no otters. At a central California high relief reef not dominated by urchins and barrens, otter invasion and predation produced little change because urchins had not overgrazed the pre-otter reef. The organization of this reef was probably dominated by winter storm disturbance.

Results of caging treatments over several years in a barrens area as well as in kelp and turfed areas at Naples reef showed that urchin grazing is far more destructive to turf than is fish grazing. In the barrens area where urchins were exposed and actively grazing, differences in turf stands between urchin exclusion/total exclusion and fish exclusion/control plots were dramatic.In the kelp and turfed area urchin overgrazing was abated by the presence of drift kelp. Differences between grazing treatments and controls here were not significant and were overwhelmed by effects of season and year. Fish grazing was subtle and appeared only in the barrens area where they attacked and decimated any kelp sporophytes that recruited there.