Diversity differs among three variations of the stress gradients hypothesis in two representations of niche space

Simulations reveal climate and legacy effects underlying regional beta diversity in alpine vegetation

Introduction

Whether the distribution and assembly of plant species are adapted to current climates or legacy effects poses a problem for their conservation during ongoing climate change. The alpine regions of southern and central Europe are compared to those of the western United States and Canada because they differ in their geographies and histories.

Methods

Individual-based simulation experiments disentangled the role of geography in species adaptations and legacy effects in four combinations: approximations of observed alpine geographies vs. regular lattices with the same number of regions (realistic and null representations), and virtual species with responses to either climatic or simple spatial gradients (adaptations or legacy effects). Additionally, dispersal distances were varied using five Gaussian kernels. Because the similarity of pairs of regional species pools indicated the processes of assembly at extensive spatiotemporal scales and is a measure of beta diversity, this output of the simulations was correlated to observed similarity for Europe and North America.

Results

In North America, correlations were highest for simulations with approximated geography and location-adapted species; those in Europe had their highest correlation with the lattice pattern and climate-adapted species. Only SACEU correlations were sensitive to dispersal limitation.

Discussion

The southern and central European alpine areas are more isolated and with more distinct climates to which species are adapted. In the western United States and Canada, less isolation and more mixing of species from refugia has caused location to mask climate adaptation. Among continents, the balance of explanatory factors for the assembly of regional species pools will vary with their unique historical biogeographies, with isolation lessening disequilibria.

Interactions and constraints in model species response to environmental heteroscedasticity

Increasing environmental variability could exacerbate the effects of climate change on ecological processes such as population dynamics, or positive and negative effects (favorable or unfavorable weather) could balance. Such a balance could depend on constraints of the processes. Biological and spatial constraints are represented in a spatially explicit individual based simulation of an ecotone reduced to two species on a single environmental gradient. The effects of climate amelioration are simulated from a plant's-eye-view by increasing the establishment and decreasing the mortality rates. Variability is introduced as a random multiplier of these rates, and the strength of the variation is increased through the period of climate change. The biological constraints limit change in the rates, and the extent of the simulation grid represents a spatial constraint. A small increase in environmental variation, multiplied through time with climate change, increases extinction rates. The biological and spatial constraints have little effect on the response of populations. Instead, competition, based on the form of the species response functions to the environmental gradient at the point where they intersect, determines differences in population responses. Positive and negative variations in the environment do not balance because the responses are hierarchical and asymmetric. Differences persist because extinction during a negative anomaly cannot be reversed by a later positive one.