Thermal niches of planktonic foraminifera are static throughout glacial–interglacial climate change

Abstract

Significance We examined the degree to which temperature tolerances changed on 8,000-y timescales across 700,000 y of glacial–interglacial climate change. We coupled a new fossil occurrence database of planktonic foraminifera, an abundant type of zooplankton, with Atmosphere–Ocean Global Circulation Model reconstructions of past climates. Our suite of analyses demonstrated that foraminiferal species have not shifted their temperature tolerances in response to glacial cycles; species occupied the same temperature conditions regardless of the magnitude of global temperature change. The limited tendency of planktonic foraminifera to change their tolerances suggests that ongoing global change could hasten local or global extinctions of plankton and other widely dispersing marine species. Abiotic niche lability reduces extinction risk by allowing species to adapt to changing environmental conditions in situ. In contrast, species with static niches must keep pace with the velocity of climate change as they track suitable habitat. The rate and frequency of niche lability have been studied on human timescales (months to decades) and geological timescales (millions of years), but lability on intermediate timescales (millennia) remains largely uninvestigated. Here, we quantified abiotic niche lability at 8-ka resolution across the last 700 ka of glacial–interglacial climate fluctuations, using the exceptionally well-known fossil record of planktonic foraminifera coupled with Atmosphere–Ocean Global Climate Model reconstructions of paleoclimate. We tracked foraminiferal niches through time along the univariate axis of mean annual temperature, measured both at the sea surface and at species’ depth habitats. Species’ temperature preferences were uncoupled from the global temperature regime, undermining a hypothesis of local adaptation to changing environmental conditions. Furthermore, intraspecific niches were equally similar through time, regardless of climate change magnitude on short timescales (8 ka) and across contrasts of glacial and interglacial extremes. Evolutionary trait models fitted to time series of occupied temperature values supported widespread niche stasis above randomly wandering or directional change. Ecotype explained little variation in species-level differences in niche lability after accounting for evolutionary relatedness. Together, these results suggest that warming and ocean acidification over the next hundreds to thousands of years could redistribute and reduce populations of foraminifera and other calcifying plankton, which are primary components of marine food webs and biogeochemical cycles.