THE RICHMONDIAN INVASION: A DETAILED VIEW OF INCUMBENT AND INVADER DISTRIBUTIONS

Species invasions and their causes are of major interest to ecologists because of potentially detrimental effects on native communities. The upper Ordovician of the Cincinnati Arch region (OH, IN and KY) geologic record permits study of these events over scales of 10³ to 10⁵ yrs. During the Late Ordovician Richmondian Age (~447.5 to 445 Ma) a suite of more than 70 genera, including brachiopods, bryozoans, and corals, immigrated into the area. Previous studies have examined the “Richmondian invasion” at a coarse scale, but questions remain as to first appearance, persistence, and dominance of invading taxa and effects on incumbent taxa. This study utilized a two-pronged approach to probe these issues. First, it employed an extensive presence/absence dataset (187 genera, 128 samples through 7 intervals) spanning the Richmondian Stage, to study aspects of the diversity of incumbents (111 genera total) and invaders (76 genera) and patterns of appearance and persistence. Second, a series of 125 bulk samples through a critical ~20 m interval, were counted to quantify patterns of relative abundance of incumbents and invaders. A majority of incumbents span the entire 2.5-to-3-million-year Richmondian interval, with only a single extinction, despite the influx of invaders. Only about half of the invading genera were successfully established; a majority (67%) of these genera invaded in two brief events (Clarksville and Liberty phases) and then persisted, without changing to the end of the Richmondian. The Clarksville invaders did not expand during the subsequent interval but persisted in extremely low abundance for several hundred thousand years. Subsequently, in the overlying Liberty Formation, invaders once more became abundant with a lesser number of newly established invaders. However, even here, invaders dominated only in limestone rich packages associated with transgressions, while incumbents remained strongly dominant in shale rich highstands. This pattern indicates that invaders remained at a disadvantage throughout their history, likely had little impact on incumbent communities, and were more sensitive to environmental changes. From our results, it appears that immigration events and relative success of invaders were controlled mainly by environmental changes rather than ecological interactions.