FLORIDA EXTINCTIONS LAGGED ENVIRONMENTAL AND ECOLOGICAL PERTURBATIONS BY 1 MILLION YEARS

Although our planet is possibly in the midst of a 6^th great mass extinction, biologists are generally uncertain about the timescale of future species loss due to ongoing environmental and ecological changes. Some have speculated that extinction debts may last centuries or longer, well beyond the timescale of human observation. Here, we provide a paleontological perspective on the potential scale of extinction debt by reconstructing the timing of environmental decline, abundance shifts, and diversity loss in Florida’s Plio-Pleistocene marine molluscan assemblages, which experienced one of the most severe extinction events in the last 65 Ma.

Although upwelling cessation has been proposed as the cause of this event, no studies have directly quantified limiting nutrient concentrations for the time interval of interest. Our analyses of coral skeleton P/Ca, a proxy for seawater phosphate, reveal that availability of this essential nutrient in Florida’s coastal environments decreased sharply around 2.5 Ma, confirming previous inferences based on fossil assemblages. However, the biotic response to nutrient decline was complex, with timing and severity of response varying by phylogeny and ecological guild. Suspension-feeding bivalves, indirectly dependent upon these nutrients for food, experienced a pulse of extinction at 2.5 Ma, indicating a direct and rapid response to the environment. Sample standardized rarefaction shows that in older units deposited prior to nutrient decline, there are roughly 80 species of bivalves for every 550 individuals collected, whereas for younger deposits, the same effort produces less than 20 species. Simultaneously, bivalve survivors that prefer oligo- or mesotrophic conditions exploded in relative abundance, while gastropod relative abundances declined from 1.7 to <0.01 individuals per bivalve, i.e., to near absence. Gastropod extinctions at both the species and genus levels, however, did not peak until 1.8 Ma, nearly 1 Ma following the onset of environmental perturbations and dramatic population decline. These results highlight the complexities of deterministic extinction and warn of the potential impact of current anthropogenic change on future biotas.