ECOLOGY ON GEOLOGIC TIMESCALES: THRESHOLD DYNAMICS IN RESPONSE TO CLIMATE CHANGE

Environmental management is built upon a model of gradual ecosystem changes, but there is an increasing recognition of abrupt changes and threshold dynamics in modern systems. Fossil records of past environmental change and biological response can provide tests of the generality of these dynamics in the absence of anthropogenic effects, a critical step in building predictive models.

I examine the drivers and dynamics of ecological changes in continental shelf benthic foraminifera and molluscs from the Early Miocene Newport Member of the Astoria Formation in Oregon (20.3–16.3 mya). During deposition, the region warmed by 2-4^OC, productivity and organic carbon flux increased, and oxygen levels declined. On the Oregon shelf in the last decade, low-oxygen conditions have increased in occurrence due to intensified wind-driven upwelling tied to modern warming. Thus, faunal patterns from the Newport Member might inform predictions of future ecological dynamics on the Oregon coast with global warming.

Observed changes in community composition and structure of both the foraminifera and the molluscs are abrupt relative to the duration of community states, but respond differently under climate change. The foraminiferal community composition changes gradually (~linearly) with δ¹³C and Δδ¹³C suggesting it closely tracks changes in productivity and carbon flux. In contrast, molluscan community change in the same strata is non-monotonic and non-linear with respect to δ¹³C , undergoing a step-change despite gradual environmental change. This difference in response may be explained by the shorter life cycles of benthic foraminifera and their ability to respond to seasonal changes in upwelling or oxygen stress. Temperature itself had little direct influence on community composition.

This study shows that (a) high-resolution time series of paleoecological change and environmental variables can reveal the tempo and drivers of ecological changes and (b) different taxonomic groups respond differently to the same environmental pressures, both of which are critical to predicting future ecological change. Finding abrupt patterns in a pre-anthropogenic record also suggests that human influences are not necessary to push systems over a threshold: climate changes within natural ranges are sufficient.