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The Purple Plague

The Purple Plague

Author: Hugh Bollinger/Monday, October 29, 2018/Categories: natural history, video, marine life, sustainability, environment, climate change

                    Purple Sea Urchins, Coastal California (credit: UCSB Dept. Ecology, Evolution, & Marine Biology)

A purple plague has descended on coastal California and is causing massive death not unlike the "black plague" carried by rats in the Middle Ages. However, this plague doesn't impact people directly but causes havoc with marine environments, especially the underwater kelp forests. The plague consists of millions of destructive purple sea urchins.

Along the coastal zone, these marine forests consist of a giant brown algae (Macrocystis sp.) that forms dense tangles of underwater stems. Kelp are attached to the seafloor by a holdfast structure that grips the rocks like a clump of roots. As with land-covering forests, marine environments are greatly influenced by the presence of these undersea "trees". Kelp provides a nursery for young fish, habitat for seals, otters, and birds, as well as influencing the general ecological structure for the coastal ecosystem.


         Healthy Kelp Forest Ecosystem (credit: NOAA)              Denuded Sea Urchin Barrens (credit: Channel Island NP)

This is why the purple plague is so deadly. The urchins attack the holdfast base on the rocks letting the stem float away and die. The urchins then overgraze the seafloor rocks producing a desert inhabited largely by the urchins. According to the National Park Service that manages Channel Islands National Park off Southern California:

if the balance of the kelp forest community becomes upset, dramatic consequences ensue. Sea otters were once the top predators in the kelp forests off California's coastline feeding on sea urchins. However, by 1900 hunting for otter fur had driven the marine mammals close to extinction. Without the otters, purple sea urchin populations exploded and overgrazed the kelp beds, leaving behind large areas of 'urchin barrens'. 

Research is underway to determine the best methods for removal of the purple urchins from California's coastal zone. The work is time-consuming and labor intensive but where the urchins have been removed or sea otters have re-populated coastal bays, the urchins can be brought under control. Kelp has shown a remarkable ability to regrow and rapidly produce a new undersea forest that can be measured in months rather than years.

The sea urchins will remain a purple plague until the marine ecosystem if brought back into balance.



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