Evolving to stay the same: life history evolution and trade-offs in response to high and low total food

Food drives ecology and evolution, but few studies have directly investigated the impacts of the total amount of food on life history evolution within-species. Among the limited number of available case studies that do directly test total food effects on life history evolution we still lack consensus, partially owing to incompletely described life histories. We explored life history trade-offs across the whole life cycle, and the consequences for trait and population dynamics, in a marine copepod evolved under high and low total food using an integral projection model (IPM). Populations were subjected to high- and low-food regimes and a common garden experiment after 30 generations of evolution. We then sampled and measured individual vital rates (growth, reproduction, and survival) from hatching until death, which were used to parameterise IPMs. Food regime had a significant but slight effect on life histories, which appeared 'slow' and 'fast' in low-food and high-food lineages, respectively. Low-food lineages grew bigger and produced larger offspring to genetically compensate for their environment, but this compensation came with costs; notably shorter lifespans and less chance of producing clutches of eggs. Despite these differences, population ecology and fitness were similar in high- and low-food lineages as anticipated by per-capita rather than total food effects. Consequently, though natural planktonic populations may genetically mitigate the effects of climate-induced food scarcity, there are limits to this compensation and likely unforeseen impacts effects for wider food webs.