Climate change may influence kelp forests through the direct effects of temperature, moving the boundaries of the areas where they have the potential to survive. In addition, nutrient inputs necessary for the kelp to thrive may be altered due to changes in the geographical or seasonal distributions of rainfall and upwelling. In kelp forests off southern California, the proportion of cold water fish such as the greenspotted rockfish, have fallen and warm water species like the Garibaldi have increased since the 1970s. Since the 1940s in Tasmania an increase of 1.5 – 2.0°C in minimum water temperature associated with a southward shift in the boundary between the warm East Australian current and the cold, nutrient-rich sub-Antarctic waters has contributed to a decline in the extent of what was once the largest kelp forest in Australia. An increase in the numbers of sea urchins has also contributed to the decline. These are normally passive grazers of algae but graze the kelp when the waters become warmer, as in El Niño events. In the Northern Hemisphere former kelp forests have become 'sea urchin barrens' due to overfishing, removing the predators that would normally control the urchins. Disruption of ecosystems due to climate change may re-inforce the imbalances that have resulted in the overgrazing of the kelps.