The role of the thermal niche in past warming worlds (ThermNiche)

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Understanding the dynamics of past and present ecosystems and predicting directional changes in response to climate change are long-standing challenges in (paleo-)ecology. We propose to apply the concept of the thermal niche to study the effects of climate warming on shifts in the latitudinal distribution of species and in the thermal structure of faunal assemblages. Using occurrence data of marine benthic macroinvertebrates and maps of seawater temperatures derived from oxygen isotopes and climate modelling, we address in detail the effects of climatic ups and downs during the Pliensbachian-Toarcian (Early Jurassic) time interval, and subsequently extend our analyses to the Phanerozoic eon. First, we test whether warming is narrowing the latitudinal ranges of species (hypothesis 1) alongside the hypothesis that warming preferentially results in poleward shifts of species. Second, we argue that the thermal structure of an assemblage determines its response to warming (hypothesis 2). Particularly we hypothesize that immigrants have warmer realized thermal niches than the pre-existing assemblage average; that extirpation prevails among species for which raised temperatures exceed their upper thermal limits; and that increase or decrease in the abundance of species is predicted by their respective pre-existing thermal niche. Third, we explore whether changes in the composition and thermal structure of faunal assemblages are most distinct at the warm edge accumulations of tropical and temperate species respectively (hypothesis 3). In this context, we predict that local population losses occurred preferentially in assemblages with high proportions of the warmest-affinity species, whereas immigrations are negligible; that a second maximum of population losses is located at the equatorward distribution edge of temperate species; and that this warm temperate zone also receives significant immigrations of tropical species resulting in peaks of faunal turnover within assemblages. Understanding the role of the thermal niche, we expect to contribute knowledge about the extent to which marine ecosystems remain cohesive or are transformed, involving replacements of incumbent species by immigrants or newly evolved species, in the face of climate change.