THERMAL NICHES OF PLANKTONIC FORAMINIFERA ARE STATIC THROUGHOUT GLACIAL-INTERGLACIAL CLIMATE CHANGE

Abiotic niche lability reduces extinction risk by allowing species to adapt to changing environmental conditions. Species with static niches, in contrast, must keep pace with the velocity of climate change as they migrate to track suitable habitat. The rate and frequency of niche lability have been studied on human scales (months to decades) and geological scales (millions of years), but lability on intermediate (millennial) timescales remains largely uninvestigated. Here, we quantified abiotic niche lability at 8-ka resolution across the last 700 ka of glacial-interglacial fluctuations, using the exceptionally well-known fossil record of planktonic foraminifera coupled with atmosphere-ocean global climate model reconstructions of paleoclimate. We developed a new R package ("kerneval") to characterize niches as continuous probability distribution functions while accounting for sampling bias, and to compare kernel densities in a quantitative way. We tracked foraminiferal niches through time along the univariate axis of mean annual temperature, measured both at the sea surface and from species’ depth habitats. Evolutionary trait models fit to time series of occupied temperature values supported widespread niche stasis above randomly wandering or directional change. Intraspecific niches were equally similar through time, regardless of the magnitude of global temperature change on short timescales (8ka) and between extreme contrasts of glacial and interglacial intervals. Ecotype explained little variation in species-level differences in niche lability after accounting for evolutionary relatedness. The findings of abiotic niche stasis suggest that warming and ocean acidification over the next hundreds to thousands of years could hasten local and global extinctions in foraminifera and other calcifying plankton, which are primary components of marine food-webs and biogeochemical cycles.