Local and Regional Forces

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

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