Tracing the effects of environmental drivers and phenotypic plasticity with hierarchical population models

Background/Question/Methods

The life history of species can vary markedly over their geographical distribution, but also over smaller, local scales. This is partly because individuals respond to different environments by developing different phenotypes, which is generally seen as a mechanism through which individuals can buffer adverse environmental conditions and increase their fitness. Understanding how life history variation is affected by nutrients and climate is crucial for predicting how such a species will respond to climate change in a heterogeneous landscape. To disentangle the effects of e.g. climate, substrate, nutrients and herbivory on the life history of plant species, we developed hierarchical population models in which the impact of environmental drivers, through plant traits and vital rates, on population growth can be explicitly traced with variance decomposition techniques. We illustrate the use of these hierarchical population models with 2 case studies: one on the impact of eutrophication on nutrient-poor grassland herbs like Succisa pratensis and Cirsium dissectum, and one on the impact of various environmental drivers on Arabidopsis lyrata petraea across its distribution range.

Results/Conclusions

The hierarchical population models showed clear patterns in the population dynamics. In the case of Succisa pratensis positive effects of eutrophication on population growth through increased plant size and reproductive allocation were largely buffered by a decreased seedling establishment rate. In the clonal Cirsium dissectum the nutrient enrichment was strongly negative through reduced survival and clonal propagation rates. For Arabidopsis lyrata petraea, climate (minimum summer night temperature and total degree days below 0°C), nutrients (soil ammonium concentrations) and herbivory had differential effects on the vital rates, and these effects differed considerably per substrate type (organic, sand, and rock). The hierarchical population models enabled us to trace the integrated effect of these environmental drivers on the life history variation of A. l. petraea through the effects on each separate vital rate. Ammonium, for instance, affected population dynamics on rocky substrates mostly through winter survival and growth. Thus hierarchical population models are useful tools for unraveling the different environmental effects on life history variation. In combination with common garden experiments such field-data based hierarchical population models can also be used to reveal the role of certain plant traits and of phenotypic plasticity.