An urchin barren is an area of the shallow part of the ocean where the population growth of

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

s has gone unchecked, causing destructive

grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to roam around and consume wild vegetations in order to convert the otherwise indigestible (by human gut) cellulose within grass and other ...

kelp forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...

Process

Sea urchins eat kelp holdfasts. This can be caused by a lack of

sea otter The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of the weasel family, but among the small ...

s or other predators in the area, which makes it extremely important to protect the ecological balance in a kelp forest.

Keystone species A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaini ...

such as the sea otter help maintain healthy kelp communities; however, likely because of increased killer whale predation, their numbers are in decline in areas of Western Alaska. Off the California coast, storm runoff, erosion and polluted water allow less light to penetrate, weakening the kelp. Sea urchins then can move in and settle. Despite their name, urchin barrens are usually abundant with

marine invertebrate Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evo ...

life,

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

in particular. Species such as the sunflower starfishes,

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomo ...

s, and the

purple sea urchin ''Strongylocentrotus purpuratus'', the purple sea urchin, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico, to British Columbia, Canada. This sea urchin species is deep purple in color, and lives in lower int ...

are common. Although macrofauna such as these are aplenty, there is little primary productivity among microorganisms. This makes it difficult for newly settled sea urchins (juveniles) to survive, making barrens more dangerous for juveniles than for adults. Once having wiped out a kelp forest, the environment becomes unsupportive of new sea urchin settlement and adults are forced to find a new resource.

Shift theories

An area of the

subtidal The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

where the population growth of sea urchins has gone unchecked causes destructive grazing of

kelp bed Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Ea ...

s or kelp forests (specifically the giant brown bladder kelp, ''

Macrocystis ''Macrocystis'' is a monospecific genus of kelp (large brown algae). This genus contains the largest of all the phaeophyceae or brown algae. ''Macrocystis'' has pneumatocysts at the base of its blades. Sporophytes are perennial and the indiv ...

''). The transition from kelp forest to barren is defined by phase shifts in which one stable community state is shifted to another. The continuous phase shift is widely accepted. This describes a transition from one ecosystem state to another where the threshold for the forward shift is at the same level as the threshold for the reverse shift back to the previous state. In other words, a kelp bed can re-establish itself when urchin grazing intensity decreases to the threshold density triggering the initial shift. Alternatively, another theory posits that both sea urchin barrens and kelp-beds represent

alternative stable state In ecology, the theory of alternative stable states (sometimes termed alternate stable states or alternative stable equilibria) predicts that ecosystems can exist under multiple "states" (sets of unique biotic and abiotic conditions). These alterna ...

s, meaning that an ecosystem can exist under multiple states, each with a set of unique biotic and abiotic conditions (i.e. barren except for urchins or flourishing with kelp). Those who argue for this theory propose several criteria: that different self-replacing communities dominate the site; each state exists longer than one complete turnover of the dominant community or species; and that following a disturbance (e.g. a storm), the system returns to the previous state.

Impacted areas

Over the past four decades, barrens have been reported along coastlines around the world, everywhere from Nova Scotia to Chile. They can either span over a thousand kilometers of coastline or occur in small patches.

References

Aquatic ecology Echinoidea {{Ocean-stub