Global Warming and Genomic Diversity Loss Alter the Biomass and the Size Distribution of Experimental Fish Populations

Warming and the loss of genetic diversity are two major components of global change for which the combined effects on the productivity and the size distribution of ectotherms have rarely been investigated. However, because genetic diversity should make populations more resilient to environmental changes, the loss of genetic diversity within populations could amplify the impacts of warming on ectotherm populations. Here, we fill this gap by using freshwater mesocosms in which we manipulated for 1 year both the genomic diversity of experimental fish populations (minnows, Phoxinus dragarum) and climatic conditions (ambient and warmed climates). We estimated across conditions the productivity (total fish biomass) and the size distribution (CV and size spectrum) of fish populations, as well as the individual growth rate and the survival rate of juvenile and adult fish. The productivity of minnow populations was not altered by climate warming, but decreased with the loss of genomic diversity (estimated using thousands of SNPs) within populations. However, populations were more homogeneous in body mass (lower CV and lower size spectrum exponent) under warm climate and when their genomic diversity was low. These impacts at the population level were underlined by contrasted effects of warming and genomic diversity on juveniles and adults. Specifically, adult survival was lower in warmer conditions, whereas juvenile individual growth rate was higher in the warmer treatments. Our study demonstrates that warming and the loss of genetic diversity have combined effects on the productivity and size distribution of fish populations. Although these combined effects are difficult to predict, we show that genetic diversity could play a crucial role in organism responses to climate warming, emphasizing the importance of intraspecific diversity for ecosystem resilience and adaptation.