The harvesting of one or more species from a biotope may result in an ecological imbalance. If this is not ameliorated by the influx of replacement individuals of the harvested species, then long-term shifts in the composition of the biotope may occur. The Californian giant kelp forests provide the best-known examples of the effect of predator removal on kelp biotopes. With the near-extinction of the sea otters for their fur by the mid-nineteenth century, sea urchins were no longer controlled and urchin barrens were created, as already discussed. The sea otter is now making a comeback under protection, but does not appear to be having any effect on urchins as yet. In eastern Canada, the lobster fishery was similarly implicated in the explosion of urchin barrens in the 1970s but definitive evidence is still lacking.

None known

A fishery for red urchins, Strongylocentrotus franciscanus, which became extensive in the Californian kelp forests in the late 1970s, has resulted in a decline in their density since the mid 1970s. A study by Dayton et al. (1992) found that the Point Loma (San Diego) kelp forest showed significant recovery during the year after the 1982-84 El Niño, although a similar degree of recovery had taken more than 5 years in the 1960s. The reduction in the urchin population was cited as an important contributor to the faster recovery of the kelp forest. Dayton et al. considered that, in the absence of evidence for increased recruitment or temperature effects on urchin grazing demand, the destructive grazing observed during and after the El Niño of 1957-59 resulted from reductions in kelp standing stock and productivity below levels necessary to satisfy the existing grazing demand.

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