Case studies: UK

Case studies: Norway

Case studies: California

The relationships between species within kelp beds have rarely formed the basis for scientific research. Other than some work on the interactions between kelps and sea-urchins, presumed interactions are based on field observations rather than statistically valid data.

Sea urchin barrens are found in kelp beds in most parts of the world but the factors influencing their formation and demise are not well understood. It has been postulated that the sweeping action of sub-canopy sporophytes in a kelp bed subject to wave action may dislodge grazing urchins (P. Zoutendyke, pers.comm.) and that mature plants are not directly affected as their longer stipes act as an urchin access barrier to the blade tissue. A change in the population structure of an area of the kelp bed would therefore give urchins the opportunity to develop and maintain a "grazing field". The interrelationship of urchins with kelps has been studied extensively in California, eastern Canada, Australia and Norway, but some of the earliest work took place in the British Isles (Kain, 1967), and heavily urchin-grazed biotopes are especially common in Scotland. Large areas of barren sea floor are found interspersed with the kelp beds in northern Norway but these barrens are apparently limited to the inner coast - the outer coast is unaffected and supports intact kelp forests. The kelp forests are an important source of zoospores for the potential reforestation of barren areas, and their dispersal range is of vital importance (Fredriksen et al., 1995). In New South Wales, Australia, the formation of urchin barrens is more predictable (and simpler) than in other temperate regions, because the sea urchins (Centrostephanus rodgersii) are found in shelters during the day, from which they emerge to forage at night and maintain patches of barrens habitat (Andrew, 1993). Artificial provision of shelter (boulders) led to the creation of barrens. Further discussion of urchin barrens in relation to harvesting of kelp is given in section V.B.3.

Other grazers may also reach plague proportions, such as the case of a North Pacific kelp-boring amphipod discussed below, but research on the effects of natural population fluctuations for other kelp bed grazers or their predators is largely lacking.

In the Isle of Man, Great Britain, Laminaria hyperborea and other algae were absent from the deepest 3 m of the seaward face of Port Erin breakwater (Kain, 1967). Over a 3-year period all sea urchins, Echinus esculentus, were removed by hand from a 10-m wide strip. Successful recruitment of young sporophytes of L. hyperborea occurred only in the urchin-cleared strip, young recruits elsewhere being destroyed by grazing. Kain therefore concluded that the lower limit of L. hyperborea at this site was determined by urchin grazing pressure.

The factors governing the urchin barrens in the Scottish kelp beds are unknown. It is possible that barrens result when urchins take advantage of kelp plants having been removed as the result of a combination of other biological and abiotic factors - such as the stipes having been weakened due to tissue removal by Helcion pellucidum and the plants subsequently removed by storms.

Leinaas & Christie (1996) examined the stability of the barren state of a kelp forest-sea urchin system in northern Norway. The ability of the sea urchin Strongylocentrotus droebachiensis to maintain high population densities and recover from perturbations, and the succession of kelp forest re-vegetation, were studied experimentally by reducing the urchin density on a barren skerry. Additional information was obtained from community changes following a natural, but patchy, sea urchin mortality that varied in degree between sites.

On the barren grounds, high urchin densities (30-50 m^-2) were maintained by annual recruitment. Severe reductions of urchin densities resulted in the initiation of luxuriant kelp growth, while more moderate population reductions allowed establishment of opportunistic algae (during spring and early summer), but not kelps. After a severe decline in sea urchin densities the succession of algal growth followed a predictable pattern. The substratum was colonised initially by filamentous algae, but within a few weeks these were outcompeted by the fast-growing kelp Laminaria saccharina. The slower-growing, long-lived kelp L. hyperborea became increasingly dominant 3-4 years after the urchin removal experiment. Increased availability of food after a reduction in urchin density led to increases in the growth of the remaining sea urchin individuals. However, the urchin population density did not increase, either by recruitment or by immigration from adjacent areas with higher sea urchin densities. Leinaas & Christie (1996) concluded that the early phases of the establishment of a dense kelp stand may represent a breakpoint in the ability of sea urchins to maintain a barren state.

The ability of L. saccharina to invade and monopolise an area quickly may have both positive and negative effects on the succession towards the climax L. hyperborea kelp forest. Competitive interactions between the kelp species may slow the process, but development of a dense stand of L. saccharina will also reduce the grazing risk on scattered recruits of the more slowly growing L. hyperborea.

Conlan & Chess (1992) reported a new species of ampithoid amphipod, Peramphithoe stypotrupetes, which bores into and occupies the interior of abraded stipes of kelps on the Pacific coast of North America. Adult bisexual pairs cohabit the stipes with their offspring of several generations. This amphipod was partly responsible for the creation of an urchin barren following the 1987 El Niño. Infestation of the kelp forest by this species reduced the kelp biomass possibly contributing to the loss of kelp plants which may have triggered damaging urchin grazing.

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