COS 218-5 - CANCELLED - Demographic lability instead of buffering in a long-lived tropical tree

Background/Question/Methods

Environmental fluctuations and anthropogenic activities are two fundamental drivers of population dynamics. Understanding species response to environmental stochasticity is critical for predicting potential effects of global changes and developing conservation strategies. The demographic buffering hypothesis suggests that natural selection will favor low variability of highly influential demographic processes. Demographic buffering is expected due to the nonlinearity in population dynamics response to perturbations of demographic processes. Because some organisms are more likely to shrink instead of dying when faced with harsh conditions, thereby limiting population size variation, shrinkage is expected to be a driver of demographic buffering. Previous tests of the demographic buffering hypothesis yield mixed results, but long-lived species are expected to buffer against environmental variability. In addition, our understanding of the role of chronic anthropogenic disturbance in influencing demographic buffering or lability is limited. In this study, we tested the demographic buffering hypothesis and examined the effects of disturbance, shrinkage, nonlinearity in species response over the short and long terms. We used four years of demographic data of a long-lived tropical tree species in two contrasting climatic environments to develop matrix projection models and estimate lower-level vital rates elasticities and coefficient of variation.

Results/Conclusions

We found no support for the demographic buffering hypothesis but rather a positive relationship between vital rates coefficient of variation and elasticities indicating demographic lability. This trend was consistent over the short-term and long-term and irrespective of climatic conditions. The strength of demographic lability increased with harsh environmental conditions and chronic anthropogenic disturbance. Contrary to expectation, shrinkage, seedling, and adult survival have no effects on the strength of demographic lability. However, we found a significant nonlinear positive relationship between the slope of the transfer function, a proxy for the degree of non-linearity between viral rates and population growth rate, and the strength of demographic lability for both long and short terms. Our study demonstrates that contrary to predictions that longevity buffers against demographic variation, demographic lability can be selected for in a long-lived species with a limited role of shrinkage but a stronger role of nonlinear population response to perturbation.