Kelp

Urchins

Blue-rayed limpet

Predators

The available literature indicates that the keystone species in kelp biotopes are likely (hypothesis only) to be the dominant algae themselves, and the grazers - especially urchins of various species. A great deal of observational work (and some experimental work) exploring the expected interactions between sea urchins and kelp plants has been undertaken in various parts of the world (for example, in Northern Europe, Echinus, Paracentrotus and Strongylocentrotus (Jones & Kain, 1967; Hiscock, 1983; Sivertsen, 1991; Rinde et al., 1992). Predators of urchins, such as lobsters (Homarus) and wolffish (Anarhicas), have also been studied in detail in several areas. In Alaskan kelp forests, the sea otter has been shown to be the controlling factor for the local populations of urchins (Dayton, 1975). Urchins also graze the understorey algae in kelp biotopes, and some species preferentially graze red algae. There has been little research published on the interactions between kelps and urchin species in the UK and none on interactions between urchins and other algae or urchin predators.

The population dynamics and biology of several of the kelp species in the UK are relatively well known. L. digitata and L. hyperborea are species that are harvested commercially and as such considerable research effort has been undertaken in the past into the recovery of the age structure of the population and the growth rates of the plants in areas that have been harvested. Without kelp plants, not only would there be no kelp biotopes but it is probable that much of the area of the coastal zone which is fuelled by the primary production of the kelp forests would be ecologically damaged.

A very obvious change that has been noted in kelp forests throughout the world is that, either at a certain depth (Jones & Kain, 1967) or in an area of kelp bed at a certain time, the kelp plants are lost and the bedrock becomes covered with encrusting coralline algae. The populations of the local species of sea urchin increase at the same time. These kelp free areas within or adjacent to kelp forests are frequently referred to as "urchin barrens" and may remain free of kelps for many years. In various parts of the world, the local species of sea urchin are thought to be responsible for much of the grazing of juvenile kelp sporophytes. Grazing effects on the kelp gametophytes are not known. Where urchins have been removed from an area in which kelps have been replaced by an urchin barren, the area is re-colonised by kelps (Lawrence, 1975).

Reports and observations of Helcion pellucidum on kelp plants in the UK are widespread although the available literature is sparse. A careful examination of the blades, stipes and holdfasts of kelp plants suggests that at any particular site at least half the kelp plants may be hosting one or more of these small, tissue destroying limpets. Since the paper by Kain & Svendsen (1969), only one study on this species has been published (McGrath, 1992) and this is based on studies in the south-east of Ireland. H. pellucidum appears as newly settled spat on the encrusting coralline algae of the low shore between February and April, after which period the growing limpets migrate to the fronds of macroalgal species. There is a seasonal pattern to the species that form the habitat for this limpet. Until August, Mastocarpus stellatus is the major host species for the juveniles, after which the limpet is more commonly found on Himanthalia fronds and on kelps. Growth rates were most rapid for those individuals feeding on kelps. The largest (oldest) limpets are found within the holdfasts of kelps, protected by the whorls of haptera.

Lobsters, crabs and some fish species are known to consume molluscan and echinoderm herbivores, but which species, in what quantities and with what effect on the population structures of the prey species is not known. In some instances, the trophic level and the dietary habits of even the most common and obvious animals in the kelp biotope are not known.

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