Given the vast advances in knowledge over the last 158 years, it is amazing that some of Charles Darwin's ideas are only just being tested now. One of Darwin's lesser known theories that predicts what would limit the range of species in different conditions has finally been tested. Although the great man's expectations have largely been confirmed, the truth turns out to be a little more complex than he anticipated.

All living things have a certain range over which they can thrive, at least without human intervention. We take it for granted that reindeer don't populate the tropics (other than unconfirmed reports of an annual visit) and crocodiles abhor the poles that we seldom think that hard about the details.

Darwin, of course, spent a lifetime contemplating the questions others avoided. In doing so, he proposed a solution: In stressful environments, it is climate factors (such as cold or lack of water) that limit a species' range. In more benign conditions, ranges are restricted not by the climate but by competition or predation from other species.

Dr Allison Louthan, then at Colorado University Boulder, tested Darwin's theory at three sites in Kenya over a period of four years for her PhD. She focused on the plant Hibiscus meyeri rather than on animals, since it is easier to study something that doesn't move around. In Proceedings of the National Academy of Sciences, she reports something similar to what Darwin anticipated. However, the nature of the interactions with other species were different.

"Darwin and others have said that what drives this pattern is gradients in density or diversity of interacting species, but instead it seems to be effects of stress on growth, survival, reproduction and germination of the plant species," Louthan said in a statement. 

At dryer locations, H. meyeri expanded as far as there was enough water to support it, and then stopped. Other species appeared to have little influence. Where water was abundant, however, the little flowering plant lived or died depending on the animals that might eat it, competition from other plants, and whether pollinators preferred it to alternatives.

The work could be important for protecting threatened species and predicting the fate of endangered ecosystems. "Understanding the different forces that set limits to a species range and allow populations to expand or contract is crucial for understanding both ecological and evolutionary dynamics," Louthan said. This will be particularly important as climate change causes some species to move from their existing ranges.

Louthan and her co-authors plan to replicate the research in a wider range of environments.