Effects of the removal of predator species.
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.
Case studies UK
Case studies elsewhere
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 Niņo, 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 Niņo of
1957-59 resulted from reductions in kelp standing stock and productivity below levels
necessary to satisfy the existing grazing demand.