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The term 'biodiversity' is the simple contraction of 'biological diversity', and at first sight the concept is simple too: biodiversity is the total sum of all biotic variation from genes to ecosystems (Purvis & Hector, 2000). The challenge comes in measuring such a broad concept in ways that are useful. The measurement of diversity of one place, group or time is in itself more or less useless. But comparable measurements of diversity from multiple places, groups or times can help us to understand how diversity arose and how we can maintain it. Marine biodiversity is higher in benthic rather than in pelagic systems, and in coasts rather than in open ocean since there is a greater range of habitats near the coast. In addition, losses of marine biodiversity are highest in coastal areas as a result of conflicting uses of coastal habitats (Gray, 1997b). The spatial characteristics of inshore coastal systems are small enough for reserves to have the potential to play a useful role in conserving species and unique ecosystems (Angel, 1994). Seagrass beds are an example of coastal ecosystems that are under threat of human activities. Within the tropical zone, seagrass beds show a large number of seagrass species and several morphological forms, so-called 'growth forms' (Philips & Mefiez, 1988). The present study aimed to evaluate different spatial levels of marine biodiversity and their underlying factors. In order to so, meiofauna (organisms passing through a 1 mm sieve but retained on a 38 µm sieve) samples were collected following a standardized protocol in tropical seagrass beds in three sites. Harpacticoid copepods (Crustacea, Copepoda, Harpacticoida) were selected as key-taxon in view of their high habitat specificity and morphological adaptations to the seagrass habitat (Noodt, 1971, Hicks & Coull, 1983). In view of the characteristics of the selected organisms and the selected habitat, it was possible to analyse and discuss three different spatial levels of biodiversity (as defined by Whittaker, 1960, 1967, 1972, 1977): (1) alpha diversity or within-habitat diversity, (2) beta diversity or between-habitat diversity and (3) gamma diversity or landscape diversity .Apart from the theoretical definition, each level can reveal some interesting ecological aspects. The first spatial level is alpha diversity or within-habitat diversity, defined as diversity of a sample or a community regarded as homogeneous (Whittaker, 1960, 1967, 1972). Given the structure of the selected habitat (seagrass beds), alpha diversity can be interpreted as the diversity of the meiofauna/copepod communities associated with one seagrass species or one of its corresponding subhabitats (roots or leaves). A first question to address is whether separate 'benthic' (occurring in or near the bottom) and 'epiphytic' (living in close association with plants) meiofauna communities exist. This level was mainly discussed for the first sampling site, Gazi Bay (Kenya) (see Chapter 3). Meiofauna was sampled for five different seagrass species belonging to four genera with different functions in the colonisation process. An additional question was how meiofauna was structured by seagrass characteristics and relevant environmental variables. On the higher taxon level, three distinct 'benthic' and two 'epiphytic' meiofauna assemblages could be identified and these assemblages corresponded entirely with those identified for the seagrass species by Coppejans et al. (1992): a high intertidal pioneer association (<i>Halophila ovalis/Halodule wrightii</i>), an intertidal climax assemblage (<i>Thalassia hemprichii</i>) and a high subtidal pioneer association (<i>Halophila stipulacea/Syringodium isoetifolium</i>). These data support the hypothesis that meiofauna communities correspond to the characteristic zonation of the seagrass vegetation in Gazi Bay. Meiofauna densities were clearly higher in the high intertidal benthic communities and in the more subtidal epiphytic communities.
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