Philip Grime's fourth corner: are there plant species adapted to high disturbance and low productivity?
Abstract
Grime's CSR species life-strategy theory (competition-stress-ruderality) provides a conceptual framework to classify species into competitive (growing under high productivity, low disturbance), stress-tolerant (low productivity, low disturbance) and ruderal (high productivity, high disturbance). Importantly, this classification is based on the assumption that the niche space of disturbance and productivity is filled unevenly: while in productive habitats species can adapt to different disturbance regimes, species of low-productivity and disturbed habitats do not exist, resulting in a triangular distribution of species optima along axes of disturbance and productivity.
This assumption has often been criticised, but it has not yet been put under a rigorous test. Here we use existing data on niche positions of central European plant species to test this hypothesis, namely its prediction that species adapted to jointly stressed (low-productive) and disturbed habitats do not exist.
We use Ellenberg indicator values and newly developed indicator values for disturbance as proxies of species positions in the space of productivity and disturbance. We found that positions of species optima along the gradients of productivity and disturbance severity are not independent of each other, with very few species adapted to low-productive and severely disturbed habitats.
In contrast, there is no relationship between productivity and disturbance frequency; a number of species occur in low-productive and frequently disturbed habitats. The relationship between productivity and disturbance severity can be either due to tradeoffs between life history traits responsible for response to disturbance and productivity (as originally assumed by Grime) or due to historical rarity of severely disturbed habitats in unproductive conditions and consequent absence of evolution of species adapted to them.
Our data are based on one specific flora, shaped by glaciations and early introduction of agriculture, but the question of what causes this pattern can be resolved by future analyses of floras with different evolutionary and ecological histories.