Gradual Warming Drives Life-History Shifts and Reveals Reproductive Ceilings in Daphnia magna

Abstract

Global warming often pushes species toward their physiological limits, yet the pace of change may be as important as the absolute temperature reached. Most experiments rely on abrupt shifts, leaving the demographic and evolutionary consequences of gradual warming poorly understood. We conducted a 30-generation experiment with clonal populations of Daphnia magna maintained either under constant temperature (26 {degrees}C) or gradual warming (+1 {degrees}C every 4-6 generations, up to 32 {degrees}C). We measured fecundity, growth rate, age at maturation, body size, and lifespan across generations, and used a reciprocal transplant assay to disentangle genetic and plastic responses. Warming populations matured earlier, grew faster, and initially reproduced more than controls, but suffered shorter lifespans and lost their reproductive advantage near 30 {degrees}C, a sub-lethal ceiling that coincided with extinction events. This rate-dependent trajectory contrasts with one-step warming studies that typically report uniformly reduced reproduction. Reciprocal transplants revealed a persistent reduction in adult body size of warming-line offspring, regardless of rearing environment, consistent with heritable or trans-generational change, while growth rate remained largely plastic. Together, these results show that gradual warming can simultaneously elicit plastic and heritable trait shifts, and that reproductive ceilings well below lethal limits act as tipping points for population persistence.