Resource homogenisation drives niche convergence between generalists and specialists in a future ocean

When humans drive rapid environmental change, is it favourable to be a generalist or specialist? To address this question, we compare how specialist and generalist marine herbivores adjust their isotopic niches (used as proxy for trophic niche) in response to predicted resource alterations under the simulated effects of ocean warming and acidification (based on a 6-month mesocosm experiment). Here, we show that when exposed to multiple climate stressors, food resources homogenized towards dominance of turf algae and suspended organic matter, with generalists and specialists adjusting their trophic niches in opposing ways. Whilst the niche breath of most generalists narrowed under climate stressors, those of specialists generally broadened, causing increasing overlap between their niches. The magnitude of this change was such that some generalists turned into specialists, and vice versa. Under ocean acidification, there was a greater probability of generalists increasing and specialists maintaining their biomass, respectively, but under warming the biomass of both specialists and generalists had a greater probability of collapse. For specialists, this collapse occurred even though they had adequate thermal tolerance and the capacity to expand their trophic niche. Climate change constrains or liberates resources, but where they are homogenized, generalists and specialists are likely to converge their trophic niches so they can exploit transforming environments for their survival or adaptive advantage.