The mechanisms that buffer small populations in the face of environmental variation are crucial to their long-term persistence. Building from diversity-stability concepts in community ecology, within-population diversity is gaining attention as an important component of population stability. Genetic and microhabitat variation within populations can generate diverse responses to common environmental fluctuations, dampening temporal variability across the population as a whole through portfolio effects. Yet, the potential for portfolio effects to operate at small scales within populations or to change with systematic environmental shifts, such as climate change, remain largely unexplored. We tracked the abundance of a rare alpine perennial plant, Saussurea weberi, in 49 0.8 m2plots within a single population over 20 years. We estimated pairwise correlations between plots in annual abundance and population growth rates. We tested for population-level synchrony and quantified portfolio effects across the 20-year study period and also in 5-year subsets based on June temperature quartiles. Finally, we used multi-site population models to explore the importance of correlations among plots for population persistence.
We found evidence that small-scale diversity buffers this rare alpine plant population. We found little synchrony among plots, due in part to different plot-level responses to June temperature. Asynchrony among plots reduced overall fluctuations in abundance and the probability of decline in population models. However, plots became more synchronous and portfolio effects decreased during the warmest years of the study, suggesting that future warming may erode buffering mechanisms in this rare plant. Further, June temperature has steadily increased at this site over the 20-year study period. Taken together, this study suggests that 1) portfolio effects can operate at small scales to stabilize populations even in the absence of obvious habitat or life history variation, and 2) such buffering mechanisms can weaken with systematic environmental change. The loss of buffering mechanisms could magnify and accelerate the negative impacts of climate change in small populations.