Ocean acidification boosts reproduction in sea urchins to buffer against population collapse

Ocean acidification is forecast to drive a decline in populations of calcifying species, including sea urchins, particularly through its negative effects on reproduction and recruitment. Evidence for these predictions rests predominantly on laboratory experiments, which cannot fully incorporate the influence of ecological complexity in the natural environment. Therefore, we used natural volcanic CO2 seeps on a temperate reef, to test the prediction that ocean acidification drives a decline in urchin reproductive potential across their naturally occurring densities. We observed the opposite of this prediction, with urchins showing an increase in per capita reproductive potential under elevated CO2, with this effect enhanced at the decreased urchin densities observed at the seeps. This influence on reproductive potential appears to be an indirect effect of CO2 enrichment boosting the nutritional value and abundance of their algal food. Hence, the commonly observed direct negative effects of elevated CO2 in the laboratory may be countered by such positive effects that can only be observed in the field. So, whilst ocean acidification might put pressure on vulnerable early life stages to drive population decline, as observed in the laboratory, our field observations suggest that increasing reproductive potential may buffer the magnitude of these declines to maintain population persistence. Therefore, this study highlights the duality of ocean acidification to potentially suppress population sizes, whilst facilitating population persistence.