When you stop to think about it, few things are weirder than a tree. Like us, they’re largish organisms made up of many cells, each with a central nucleus – but we have little else in common. Plants diverged from our early ancestors well before there was anything bigger than a single cell around. They split from the animal lineage even before fungi, which leads to a shocking conclusion. That spot of mould in the vegetable drawer? It’s more closely related to you than the plants upon which you both depend.
Small wonder, then, that plants don’t live and die by the same rules as animals – but this could have dire implications. That’s the message of a new study by Jonathan Davies of McGill University, published in PLoS Biology. Davies and his international collaborators have shown that the factors causing extinction in plants are entirely unexpected, and the upshot is that the current IUCN Red List criteria for listing endangered species – which are based on animal studies – might be useless when it comes to plants.
Davies and his team used the latest the comprehensive Red List data for all flowering plant species in two locations: the United Kingdom and the South African Cape. The Cape is a biodiversity hotspot with thousands of endemic species: plants that evolved there, and that can be found nowhere else. The UK flora, in contrast, is made up of species from other regions that moved in after the retreat of Pleistocene glaciers.
Previous work has shown that among mammals, we are most likely to lose species with large body sizes and long generation times – giant pandas and elephants are classic examples. But according to the new analysis, plants break the mold. Davies and coauthors found that the kinds of plants most at risk in the UK are different from those at risk of extinction in the Cape, indicating that basic traits like size have nothing to do with it. Using a detailed evolutionary history of the Cape species, the team also found evidence that extinction risk in plants is tightly linked to mode of speciation: the Cape species most at risk tend to be ones from the younger, rapidly-evolving lineages.
This implies that in plants, extinction is pruning the tips of the evolutionary tree. The authors suggest an explanation: unlike animals, new plant species tend to arise from small isolated populations that are at the extremes of a much larger ancestral range. Thus, a new plant starts off with a limited distribution, and because range size is an important criteria for Red List risk, it is also highly vulnerable.
The team’s analysis of anthropogenic factors turned up an additional surprise. For the Cape flora, human-induced habitat changes such as urbanization and agriculture cannot explain extinction risk of local plants. In other words, there is no simple geographic correspondence between human activity and plant decline. As the authors put it, places like the South African Cape might therefore be both “cradles and graveyards of diversity”, regardless of human activities.
This study suggests that a major strategy revision is in order if we want to conserve the world’s plants – a group that we all depend upon for oxygen and energy. More generally, risk criteria for one taxonomic group cannot necessarily be applied to another, since the pathways to rarity may be as foreign as the species themselves.