Herbivory

Predation

Shading

Spatial competition and patchiness

Kelp forests are a very species-rich environment (see Appendices 4 & 5) with more than 1,800 species having been recorded from kelp biotopes in UK waters. It is possible that many of these species are common within a given kelp bed but that due to the patchy nature of species distribution and the difficulties encountered in the detailed identification of species, the true diversity is inadequately reported. The high productivity of kelp forests (mg C m^-2 yr^-1) in comparison to other marine biotopes (Mann, 1972b) suggests that the surrounding coastal areas are dependent on the kelp biotopes as a major source of food energy. 90% of kelp production is estimated to enter the detrital food webs as POM or DOM, being exported from the immediate area of the kelp bed (C. Hurd, pers.comm.).

In terms of direct and possible indirect ecological interactions between species in the kelp beds:

• relatively little research effort, world-wide, has been directed at these topics, although data are available for some interactions of commercial significance
• for the majority of the 1,800+ species listed for UK kelp biotopes, we can only make hypotheses about their ecological interactions

Research conducted in terrestrial habitats has shown that the interactions between species within a habitat are sensitive to relatively small shifts in the populations of interacting species and the natural fluctuations in the physical environment.

It has been suggested that the grazing activities of the several species of sea urchin found in UK kelp beds are a critical factor in the maintenance of the heterogenicity of the understorey flora. Mixed grazing of terrestrial grassland (sheep, rabbits, horses, cattle) is known to maintain a high diversity of plants in the sward, but some conflicting evidence has been obtained from experiments in kelp beds. Vost (1983) examined the effect of urchin grazing on the understorey algae on the rock surface of a L. hyperborea forest in the Isle of Man. The understorey algae showed a patchy distribution prior to the removal of the urchins, and after 6-10 months this patchiness decreased. Algae with single attachment points became more frequent in the urchin free area and the total biomass and species richness of epilithic species increased.

The red algae form the bulk of the species in the undercanopy and understorey of the UK kelp beds. Aquaculture feeding preference experiments have been conducted with several urchin species around the world, and urchins often show a preference for red algae as food. In the west of Ireland, Paracentrotus lividus has been successfully reared using Palmaria palmata (P. Heath, pers. comm.). It is possible that the upper stipes of L. hyperborea are relatively heavily epiphytised as a result of the lower portions of the stipes being more readily accessible to urchins for grazing. The upper part of the stipe is moved about more by waves and currents and this may dislodge the grazers, or this pattern of biomass distribution on the stipes may be irradiance related.

Overall, within kelp beds there are relatively few species that are directly grazing either the kelp or the understorey algae as the enzymes required to directly utilise algae as food are not common. Those species able to graze directly on the kelp include the gastropods: Gibbula spp., Littorina spp., Acmaea spp., Haliotis tuberculata, Helcion pellucidum, Lacuna spp., Aplysia spp. and the Rissoidae, together with some amphipods and isopods that have the necessary laminarinases and alginate-lyases required for this diet (Dauvin, 1997).

Identification of predatory species within the kelp bed biotopes is incomplete. The complex food webs that occur within these ecosystems have not been explored in any detail. Some species are known to prey on others (starfish on mussels for example) but, other than observational records of interactions between a few groups of species, very little else is known of the predator prey interactions for the diverse assembly of species present in kelp beds. Predator-prey interactions have been studied in depth for numerous groups of species in terrestrial habitats and it has become apparent that the relationships between predator and prey are sensitive indicators of the conservation status of the habitat. More detailed studies in kelp biotopes may therefore have considerable conservation value.

The blades of mature kelps in the kelp forest form a canopy layer which, under certain conditions, may cut off as much as 90% of the incident irradiance. Any young sporophytes that are able to establish within the kelp forest are thus able to grow only slowly due to the dense shading. As a result there can be a bimodal size-frequency distribution of kelps in a stable forest, plants of the same age either becoming part of the canopy-forming community or part of the understorey flora (Kain, 1963b; 1977). The bimodal (or trimodal, Rinde et al., 1992) population structure does not apply in the deeper kelp biotopes (parkland) where the plants are so well spaced that a dense canopy does not develop.

The deep shade cast by the kelp forest restricts the development of species with high light demands. Furthermore, those species of algae which would be restricted to the lower infralittoral in kelp-free are able to compete more effectively in the reduced light levels of the forest and so are found at lesser depths than. The removal of the canopy results in major changes to the species composition of the understorey algae in the area (Rinde et al. 1992) and to the relative biomass of different understorey species and epiphytic species (Harkin, 1981).

In most kelp biotopes there is evidence of strong competition for space, especially for space on a favourable substratum. Competition may occur at a number of levels within the community, for example:

• among kelp species and individual kelp plants of a single species
• between kelps and substratum-colonising species of animals and plants
• between colonial animals and encrusting algae

If a sufficiently large area (hectares) of a pristine kelp biotope is completely surveyed (rather than sub-sampled) at yearly intervals, the species diversity is unlikely to fluctuate greatly over time. At most, the number and variety of species present in the area will vary around a "typical" community structure. However, if the area were sampled on a smaller scale (m²) the spatial interactions between different life forms and different species would be observed, with sample sites showing shifts in species composition and diversity over time. The interactions

between species which are competing for substratum result in the complex patchwork of species that is a characteristic of so many kelp biotopes. Furthermore, this competition for space between individuals and between species is dynamic, resulting in a constantly changing patchwork of species covering any suitable substrata within the biotope. The forces driving the development of these dynamic patterns and the time periods over which the various species and population changes take place are not known.

Natural or human mediated changes in the local conditions within a kelp biotope are thought to result in (often) slow but sweeping changes in the populations and species present, shifting between several extreme patterns of species domination. An understanding of these patterns and the forces driving them would be a very useful tool for the monitoring and management of these complex biotopes.

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