Local and Regional Forces

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Section B. Is local diversity a product of regional forces, local forces, or both, and how might we know?

In ecology, the change in species diversity across space and time is one of the oldest studied subjects, however the underlying causes of many diversity patterns is under much debate (Field et al. 2001; MacArthur, 1972; Rosenzweig, 1995). How does diversity arise (Erwin 1991)? How is diversity distributed in space (Dean et al. 1997, Fonseca et al. 2000)? And how is diversity maintained (Lande, 1988)? Are three fundamental questions that ecologist look to answer. Early studies focused around the role of competition in determining diversity at local scales, today however it is emphasized that many factors on top of competition, such as dispersal, predation, variation in the physical environment, and historical factors, influence diversity patterns across a range of scales (Rosenzweig, 1995). According to MacArthur (1965) local dynamics happened quickly compared to regional dynamics and so due to the filling of ‘niche space’, communities would be ecologically saturated with species. This led to the conclusion that historical/biogeographic process like speciation and extinction rates could be dis-included when explaining local diversity. This resulted in local ecological processes being a default belief for terrestrial community ecologists. In contrast to this ‘Niche theory’, where diversity is limited to local factors, MacArthur & Wilsons (1967) equilibrium theory of Island Biogeography considers that local diversity is thought of as a chance of events of immigration and extinction, determined by regional processes. The conflict between the two theories simultaneously explaining community structure at different scales was referred to as ‘MacArthurs Paradox’ (Schnoener, 1983) and is still debated today.

Communities are assembled through a series of filters, representing historical and ecological constraints on arrival and survival of local organisms (Lawton, 1999; Zobel 1997). Local species richness, regional species richness and spatial turnover or differentiation diversity are three commonly recognized components in determining processes that may affect the diversity (Field et al, 2001) In marine ecology, there is a long tradition of measuring and modeling the influence of regional dispersal on the dynamics of local populations and communities. By analysing the roles of speciation, dispersal and extinction using a combination of modeling and data, marine ecologists have created large –scale gradients in species diversity and composition (Bellwood et al. 2003, 2008; Buzas et al 2009; Jablonski et al. 2006). On the other hand terrestrial ecologists have made some progress in this area (Allen et al. 2007; Latham and Ricklefs 1993), but community ecology remains predominantly centered on local species interactions, as evidenced by a recent review proposing a research agenda with little reference to regional processes (Ackerly et al. 2007). It is considered that dispersal from the regional species pool is more significant in marine than in terrestrial communities. However through the theory of island biogeography, the relationships between species numbers and area or isolation suggest ways to ‘manipulate’ (in a comparative observational sense) the regional species pool to test for its effects on local richness (MacArthur and Wilson 1967). This manipulation is exemplified through Cornells (1985) study of Cynipid gall wasps (Cynipidae) that are highly specialized to an oak species indigenous to the pacific slope of California. The distributions of the oaks vary widely, with some oaks being specific to one wasp species. The relationship between the more broadly distributed oaks and number of gall wasp species suggests larger regional species richness (RSR), which in turn predicts local species richness (LSR) of gall wasps on local populations of different oaks (Cornell, 1985). This means of increasing...
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