DO INVASIVE SPECIES CAUSE EXTINCTION?: COMPARING INVASION EVENTS DURING INTERVALS OF LATE ORDOVICIAN AND LATE DEVONIAN BIOTIC TURNOVER

Intervals during which species from one biogeographic region invade into another region occur throughout the geologic record and can be studied as analogs to the modern problem of human-introduced invasive species. In modern ecosystems, competition from invading species rarely results in global extinction of a native species, but instead may result in local extirpation and ecosystem reorganization. Modern invasive species, however, can only be studied on timescales of years to decades. The longer term effects (tens of thousands of years or more) of interbasinal species invasions on diversity overturn and ecosystem structuring can be best examined by studying similar events in the fossil record.

The biogeographic and biodiversity changes associated with species invasions are assessed using Geographic Information Systems (GIS) to map the geographic range of shallow marine brachiopod species over short temporal intervals (0.5 to 1 Ma) during two intervals of known invasive activity during the Paleozoic: the Richmondian Invasion (Late Ordovician, Cincinnati Arch) and the Frasnian Biodiversity Crisis (Late Devonian, Appalachian Basin). Reconstructed species ranges are compared with species-level phylogenies to infer relationships between invasive history, speciation, and extinction events. In general, endemic species with larger geographic ranges preferentially persist in the presence of invading species, although species with smaller geographic ranges may not. This relationship suggests that broadly distributed generalist species should be able to better survive during episodes of biotic invasion. Extinction rates, however, are not statistically elevated relative to background rates. Interestingly, speciation appears to be more strongly related to invasion than extinction. When invasion intensity is greatest, speciation rates decline and vicariant speciation, in particular, is retarded. The combination of generalist survival and reduction in vicariant speciation suggests that one consequence of the modern biodiversity crisis will be the establishment of a species-poor but biogeographically widespread fauna—not unlike the observed biotas of the latest Ordovician and Devonian.