Intraspecific variation in response to climate drives population patterns and dynamics
Intraspecific variation is ubiquitous, yet ecological theory has historically tended to treat all individuals of a species as ecologically equivalent. When and to what degree the inclusion of intraspecific variation alters ecological dynamics are still outstanding questions with important consequences for our understanding of species interactions and distributions. We constructed models to explore the effects of ecological differentiation between the sexes of a dioecious plant on population dynamics. First, we quantified sex-specific demographic vital rates – annual growth, survival, and offspring production – over complementary axes of climatic variation spanning a gradient of 1800 m elevation (space) and 35 years of climate change (time). We parameterized sex- and size- structured integral projection models (IPMs) using data from 7 contemporary populations over space and 3 populations over time (paired historic and contemporary data). By comparing these models over both axes of climatic variation, we tested for population-level consequences of intraspecific variation in response to climate. We hypothesized that sex-specific responses to climatic variation could alter population dynamics by two non-mutually exclusive mechanisms: (A) linear effects of sex differences, where vital rate changes in either sex affect population growth by altering the mean vital rate and (B) non-linear effects of sex differences, where vital rate differences change the sex ratio of the population and in turn alter mate availability, thus rendering reproductive rates sensitive to population composition.
The sexes differed significantly in demographic vital rates in all populations, rejecting the hypothesis of ecological equivalence among conspecifics. Moreover, demographic differences between the sexes had a strong impact on population dynamics. IPM projections with and without sex-based demographic variation (hypothesis A) differed in equilibrium population growth rate and sex ratio, supporting the inclusion of intraspecific variation. Specifically, our models projected an increase in female frequency with elevation and an increase in male frequency over time. Field surveys of population sex ratios corroborated these projections over space (+8.8% female frequency per 1000 m) and time (+5.5% male frequency per decade). Comparisons of models with and without a feedback of population sex ratio on offspring production (hypothesis B) indicated that population composition per se can affect population growth rate through mate limitation. In summary our work demonstrates that intraspecific variation can fundamentally alter population dynamics. We suggest that explicitly including intraspecific variation in theory and models may reveal otherwise cryptic mechanisms that enable or constrain responses to ongoing environmental change.