Abstract
When conditions are stressful, reproduction and population growth are reduced, but when favourable, reproduction and population size can boom. Theory suggests climate change is an increasingly stressful environment, predicting extinctions or decreased abundances. However, if favourable conditions align, such as an increase in resources or release from competition and predation, future climate can fuel population growth. Tests of such population growth models and the mechanisms by which they are enabled are rare. We tested whether intergenerational increases in population size might be facilitated by adjustments in reproductive success to favourable environmental conditions in a large-scale mesocosm experiment. Herbivorous amphipod populations responded to future climate by increasing 20 fold, suggesting that future climate might relax environmental constraints on fecundity. We then assessed whether future climate reduces variation in mating success, boosting population fecundity and size. The proportion of gravid females doubled, and variance in phenotypic variation of male secondary sexual characters (i.e. gnathopods) was significantly reduced. While future climate can enhance individual growth and survival, it may also reduce constraints on mechanisms of reproduction such that enhanced intra-generational productivity and reproductive success transfers to subsequent generations. Where both intra and intergenerational production is enhanced, population sizes might boom.
Highlights
Population sizes under future climate increased by at least 2500% relative to contemporary climate (Fig. 1a; Table S1; Χ23,7 = 8.22, P = 0.042)
Male amphipods were larger under future climate relative to current conditions (Fig. 2a), driven by the effect of elevated temperature (Fig. S2, Table S2; F3,643 = 23.24, P < 0.0001)
Female gnathopod size decreased under elevated temperatures (Fig. S2; Table S3; F7,407 = 25.29, P < 0.0001)
Summary
Taxa (e.g. territorial birds[21], fish[22], and crustaceans[23]). Females can increase reproductive output when mating with high quality males[24] or reduce the intensity of male-male competition to increase overall reproductive success[16]. As a test of this enhanced reproduction mechanism, we propose that the relaxation of abiotic constraints imposes strong directional selection on male ornaments which increase access to mates as male sexually selected traits become more exaggerated and homogenized[25,26]. That as environmental constraints are relaxed, a higher proportion of males can maintain costly traits, predicting reduced variation in sexually selected characters[32] and an increase reproductive success[33] that together results in population growth[18,34]. We tested whether elevated temperature and carbon dioxide could increase population growth of a herbivorous amphipod (Cymadusa pemptos), and to account for this population increase we observed reproductive potential by testing the prediction that phenotypic variation in male secondary sexual characters is reduced while females fecundity is enhanced
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