Abstract
Poleward range shifts are a global-scale response to warming, but these vary greatly among taxa and are hard to predict for individual species, localized regions or over shorter (years to decadal) timescales. Moving poleward might be easier in the Arctic than in the Southern Ocean, where evidence for range shifts is sparse and contradictory. Here, we compiled a database of larval Antarctic krill, Euphausia superba and, together with an adult database, it showed how their range shift is out of step with the pace of warming. During a 70-year period of rapid warming (1920s-1990s), distribution centres of both larvae and adults in the SW Atlantic sector remained fixed, despite warming by 0.5-1.0°C and losing sea ice. This was followed by a hiatus in surface warming and ice loss, yet during this period the distributions of krill life stages shifted greatly, by ~1000km, to the south-west. Understanding the mechanism of such step changes is essential, since they herald system reorganizations that are hard to predict with current modelling approaches. We propose that the abrupt shift was driven by climatic controls acting on localized recruitment hotspots, superimposed on thermal niche conservatism. During the warming hiatus, the Southern Annular Mode index continued to become increasingly positive and, likely through reduced feeding success for larvae, this led to a precipitous decline in recruitment from the main reproduction hotspot along the southern Scotia Arc. This cut replenishment to the northern portion of the krill stock, as evidenced by declining density and swarm frequency. Concomitantly, a new, southern reproduction area developed after the 1990s, reinforcing the range shift despite the lack of surface warming. New spawning hotspots may provide the stepping stones needed for range shifts into polar regions, so planning of climate-ready marine protected areas should include these key areas of future habitat.
Highlights
Temperature is a major driver of marine species distribution at global scales (Beaugrand et al, 2019; Burrows et al, 2011, 2019; Pinsky et al, 2013) and, one of the ‘universal’ responses to climatic warming is a poleward shift in distribution (Chen et al, 2011; Parmesan & Yohe, 2003)
This accords with the general principle of thermal niche conservatism, a central concept of species distribution models, which project distributions under future climatic states based on the assumption that the relationship of a species to its environment remains unchanged (Brun et al, 2016)
Using Arc GIS version 10.2.2 in South Polar Stereographic Projection, we provide in Figure 2 a simple visualization of mean gridded distributions during the three above-mentioned sampling eras
Summary
Temperature is a major driver of marine species distribution at global scales (Beaugrand et al, 2019; Burrows et al, 2011, 2019; Pinsky et al, 2013) and, one of the ‘universal’ responses to climatic warming is a poleward shift in distribution (Chen et al, 2011; Parmesan & Yohe, 2003). Within the Southern Ocean, Antarctic krill (Euphausia superba) play a central role in the food web, and the spatio-temporal dynamics of this single species impact both higher and lower trophic levels (Reiss et al, 2020; Schmidt et al, 2016) For this reason, studies are increasingly projecting krill distribution (Mackey et al, 2012; Piñones & Fedorov, 2016) or growth potential (Hill et al, 2013; Murphy et al, 2017; Veytia et al, 2020) based on the untested assumption that the relationship of krill with its environment remains fixed. The larval and adult databases allow us to hypothesize a mechanism, based on spawning dynamics, to explain such abrupt shifts
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