CONCLUSIONS

By analyzing the geographic ranges of Cincinnatian rhynchonelliform brachiopod species in a temporal framework, a clear picture of the biogeographic impact of the Richmondian invaders begins to emerge. Native Cincinnatian species with small geographic ranges in the Maysvillian do not cross into the Richmondian, whereas species that occupy large geographic ranges do. In fact, native taxa that carryover into the Richmondian have larger geographic ranges than the invader taxa until the C5 sequence—at least one million years after the invasion. Evidently narrowly adapted, ecological specialists were more significantly impacted by ecosystem change associated with the invasive regime than ecological generalists. Conversely, native generalist taxa continue to occupy large geographic ranges with no discernable contraction of range size following the influx of the invaders. The invader taxa exhibit statistically smaller range sizes than native carryover taxa initially, suggesting that native communities were somewhat resilient to invader domination. Speciation rate is low in the C4 sequence during the establishment of the invasive regime, but increases in the C5 sequence. New species that evolve in the Richmondian from Cincinnati natives occupy smaller geographic ranges than either the carryover or invader taxa in both the C4 and C5 sequences. The range size of these new taxa overlaps with the range size vacated by the ecological specialists of the Maysvillian Stage. In the C6 sequence, the differentiation between invader and native taxa appears to break down.

The long-term effects of invasive species of the Richmondian Invasion can be summarized as follows. Geographically restricted stenotopic species are the most susceptible to extinction during the invasive regime. Inferred competition between invader taxa and native eurytopic taxa does not result in either reduction in geographic range size or realized niches of native taxa. Speciation may be reduced during invader establishment, and new species that evolve following invader establishment are ecological specialists. Results also indicate that overall, the ecological effects of the Richmondian Invasion were long lasting—through both the C4 and C5 sequences. The protracted biogeographic response to invasion agrees with results of biofacies analyses by Holland and Patzkowsky (2007) in general, although this analysis suggests that a stable ecosystem may not have re-established until the C6 sequence.

Invasive species pose a significant problem for modern ecosystems. Invasion biologists have documented clear patterns of increased competition, predation, and ecosystem restructuring in the years and decades following invasion events (Lockwood et al. 2007) but lack the temporal data to determine how these short-term patterns may scale up over long time spans. Analyses, such as this one, can provide insight into the ultimate result of these short term impacts. The biogeographic response of native Cincinnatian taxa to the Richmondian invasion indicates that ecological specialists are most vulnerable to extirpation during invasive regimes, but competition between native generalists and invasive species does not result in extinction of either native or invasive species. Rather, total ecosystem diversity is enhanced. Although stenotopic species become extinct at the onset of the ecosystem reorganization, they are later replaced by newly evolved species characterized by similar niche breadth. New speciation, however, appears to be retarded during the initial waves of the invasion. Moreover, the results of this analysis indicate that invasion effects are likely to be long lasting. These results indicate that early intervention of new exotic introductions and protection of native ecological specialists in environments that are already impacted by invasion should be conservation priorities in modern ecosystems.