Distribution

Macrocystis pyrifera forms dense forests in many parts of the world, including the cooler waters of the northeast Pacific (Alaska to Baja, California), the southern shores of South America, many Southern Ocean islands, and isolated areas of South Africa, southern Australia, and New Zealand.

Its distribution is greatly influenced by physical factors and it usually grows in the following conditions:

clean and clear coastal waters
temperature less than 20ºC
nutrient concentration high
water motion moderate

Its geographical limits are usually governed by the highest summer sea surface temperature.

Well-developed kelp forests are threatened by herbivorous grazers such as sea urchins, which can switch from grazing on detritus and drift weed to destructively grazing Macrocystis, leading to deforestation.

Various factors are thought to contribute to the sea-urchins’ switch in diet including:

extensive recruitment of grazers
low amount of drift algae
over-fishing
loss of urchin predators

Several hypotheses concerning the interactions between this kelp, sea urchins and El Niño events have been proposed to explain the extreme disruption of kelp forest communities in the past.

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