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communities Table 8.4 The 3 most influential taxa contributing to the dissimilarity . 220 between patch and non-patch communities xiv
BACKGROUND CHAPTER 1 INTRODUCTION Aquatic and terrestrial environments may be viewed as being primarily structured by large-scale physical processes (currents and winds in aquatic systems) that cause gradients on the one hand and patchy structures separated by discontinuities on the other (Platt and Sathyendranath, 1992). These large-scale patches and gradients induce the formation of similar responses in biological systems, both spatially and temporally (Legendre and Fortin, 1989; Raffaelli et al., 1993). For example, within the relatively homogeneous zones within large-scale patches, smaller-scale biological processes such as reproduction, predator-prey interactions and parasitism take place resulting in more spatial structuring. Legendre (1990) stated that several theories and models, such as those for predator-prey interactions, implicitly or explicitly assume that elements of an ecosystem that are close to one another in space are more likely to be influenced by the same process. Spatial heterogeneity is therefore functional in ecosystems and not the result of some random, noise-generating process (Legendre, 1993). The realisation that almost every ecological variable, biotic or abiotic, has a non-random spatial distribution (Valiela, 1984; Addicott et al., 1987; Caswell and Cohen, 1991) has resulted in ecologists not treating spatial heterogeneity as a 'nuisance' in ecological studies (Legendre, 1993) which hinders the estimation of population densities (Reise, 1979; Valiela, 1984; McArdle et al., 1990), but as a useful tool in assessing the processes operating within a particular system (Hall et al., 1993). Observing patterns of variation in space helps with the understanding of population processes which occur over time scales too long to be amenable to studies (Sokal and Wartenberg, 1981). More fundamentally, assessment of spatial patterns also helps to determine suitable study areas (Livingston, 1987), appropriate sampling- scales (Downing, 1979; Eckman, 1979; Taylor, 1984) and the optimisation of mathematical models (Hanski, 1994; Petersen and DeAngelis, 1996). 1
- Page 1 and 2: AN INVESTIGATION INTO THE PROCESSES
- Page 3 and 4: ABSTRACT The spionid polychaete Pyg
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communities<br />
Table 8.4 The 3 most influential taxa contributing to the dissimilarity . 220<br />
between patch and non-patch communities<br />
xiv
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