Biodiversity definition and application on rocky shores

Trends in biodiversity

Biodiversity is a general term covering all aspects of biological diversity, from genetic variability within species to species richness: a measure of number of species compared with the number of individuals in a community and ecosystem complexity. As described above, rocky shore ecosystems are often characterised by strong biological interactions, with the presence or absence of some species having indirect repercussions on others. An important contribution to biodiversity at the level of the physical habitat is the wide range of biotopes found on rocky shores resulting from environmental gradients and structural complexity of the shore. Each species reaches the peak of its competitive ability at a given intersection of the habitat gradients and yet its distribution is ultimately influenced by interactions with other species. Rocky shores encompass a vast range of habitat and community characteristics over a very narrow spatial scale.

The diversity of species on rocky shores increases towards the low shore where conditions are more benign. A limited number of species are able to survive on extremely exposed shores, particularly those consisting mainly of steep, smooth rock. As a result these shores have a low biodiversity. Protected microhabitats on exposed shores, such as algal turfs or deep crevices, can however support a surprising variety of species (Raffaelli and Hawkins, 1996). The presence of zonation patterns indicates that large proportions of available space are dominated by a few species. However, these species often provide a microhabitat for others. Older barnacle shells are pitted by boring blue-green Cyanobacteria and the pits shells provide shelter for large numbers of diatoms (Thompson et al., 1996). Algal canopies, and kelp holdfasts especially, support a variety of epiphytic species as described above and mussel beds provide a refuge and habitat for a great number of species including representatives from most of the main invertebrate phyla (Seed, 1996). At higher shore levels, tardigrades live beneath lichen crusts. These examples illustrate the importance of biological microhabitat provision on rocky shore diversity. Physical complexity is also important with crevices, rock pools and the underside of boulders all harbouring diverse species assemblages, including species which are restricted to these microhabitats.

In general, species diversity and numbers increase with habitat complexity (Kostylev et al., 1996). The abundance of any species is determined by the resources available and its ability to compete for them. Therefore, species diversity will be greatest in those shores which are rich in varied microhabitats. Mussel beds are recognised as an important source of biodiversity (Seed, 1996; Holt et al. 1998). The presence of biogenic microhabitats can have a negative effect of the number of individuals in a given area. For example, more species but fewer individuals are found between Fucus clumps than on barnacle encrusted rock on UK shores (Thompson et al., 1996). This is entirely due to the negative influence of Fucus on barnacle numbers.

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