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Environmental Skeptics and Critics, 2014, 3(1): 8-16
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Invasions, adaptive radiations, and the generation of biodiversity

John C. Briggs
Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97333, USA. Present address: 2320 Guerneville Rd., Santa Rosa, CA 95403, USA

Received 3 November 2013;Accepted 8 December 2013;Published online 1 March 2014
IAEES

Abstract
When the subject of global biodiversity and its loss through human action became a focal point of conservation biology, there developed an increasing argument about the effects of the invasions of exotic species into native ecosystems. In order to place the current contention in a historical context, evidence from the geologic record has been examined. The record clearly indicates that numerous species invasions, extending from the Paleozoic through the Cenozoic, have led to adaptive radiations significantly increasing global biodiversity. Three invasion induced categories are recognized: (1) global radiations, (2) local radiations, and (3) single species effects. Together, the three may account for much of the biodiversity gain that has accumulated in the intervals between mass extinctions. In contemporary time, it has become apparent that exotic species colonizing a native ecosystem rarely cause extinctions. Instead, the invaders are accommodated by the native species that occupy the appropriate niches or habitats. The accommodation process results in a gain in the species diversity of the invaded area. Over time, local diversity gains can result in global gains as speciation among the invaders takes place. The majority of successful invasions occur via migration of species from centers of high species diversity to places of lesser diversity. The result is a dynamic world characterized by constant movement whereby the high diversity centers increase the diversity of outlying regions.

Keywords adaptive radiation;biodiversity;evolution;historic invasions;species invasions.

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