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

Species invasions and their causes are of major interest to ecologists because of their potentially detrimental effects on native communities. Studying impacts of invasions on geologic timescales (10³ to 10⁵ yrs) is possible in the Cincinnati Arch region (OH, IN and KY). During the Late Ordovician Richmondian Age (~447.5 to 445 Ma) a suite of over 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 regarding the first appearance, persistence, coordination, and dominance of invading taxa and effects on incumbent or native taxa. This study utilized a two-pronged approach to probe these issues. First, it employed an extensive presence/absence dataset (187 genera, 128 sampled intervals) spanning the Richmondian Stage, to study aspects of the diversity, appearance patterns, and persistence of a recorded 111 incumbent and 76 invader genera. 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. Despite the influx of invaders, many incumbents span the entire Richmondian interval, with only a single extinction. In contrast, only about half of the invading genera successfully established. Most invading genera (67%) established in two brief events (Clarksville and Liberty phases), though three minor phases of invasion were identified (Arnheim, Blanchester, and Whitewater phases). Of the major phases, earlier Clarksville invaders became rare in the overlying interval before becoming abundant once more in the Liberty alongside new arrivals. However, Liberty invaders only dominated 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 perturbations. It appears that immigration events and the relative success of Richmondian invaders were controlled mainly by environmental changes rather than ecological interactions.