Again and again throughout evolutionary history, one clade of plants has arisen and had its ‘moment in the sun’ while another has gone into permanent decline. This is referred to as clade replacement. Because the factors that influence the success or failure of a group are many and complex, it is difficult to say conclusively whether competition between the two is what led to the replacement, even if the two occupied the same habitat or niche.
A very broad-scale example of clade replacement has been the decline of gymnosperms alongside the meteoric rise of the angiosperms. This shift was set against the backdrop of a cooling climate that may also have been a key factor, given that gymnosperms were a major component of tropical ecosystems at the time. Furthermore, several mass extinction events took place during the age of gymnosperm dominance and may have negatively impacted them. Whether a changing climate, mass extinction, competition with flowering plants, or a combination of the three was responsible for gymnosperm decline has proven difficult to demonstrate definitively.
In a new article published in the Proceedings of the National Academy of Sciences, lead author Fabien Condamine and colleagues used fossil and molecular phylogenetic evidence to weigh the influences of the possible drivers of gymnosperm decline, using conifers as a proxy for gymnosperms. Models for different scenarios were evaluated for best fit against data on conifer speciation and extinction rates.
The researchers found that conifers had low rates of diversification even prior to the rise of the angiosperms. Condamine, a research scientist at the French National Centre for Scientific Research, speculates that this may be due to their slow growth and long maturation times, which result in a lower potential for adaptation. Beginning in the mid-Cretaceous (100 to 110mya), the conifer extinction rate increased significantly and remained high thereafter, overtaking the speciation rate during the Cenozoic and leading to net decline.
Though global cooling – specifically, the Eocene-Oligocene cooling event – does appear to have significantly influenced conifer extinction rates, the factor most influential in their decline appears to have been angiosperm speciation. Mass extinction events do not appear to have been a major factor. “[W]e found remarkably consistent signals from the fossil and phylogenetic data, both providing strong support for the angiosperm-driven extinction model. Our results thus suggest that the Cretaceous rise of angiosperms, extended through the Cenozoic, indeed had a large effect on conifer diversification,” write the authors.
Is this clade replacement still ongoing today, when more than a third of remaining conifer species are threatened with extinction? “Based on temporal surveys of contemporary forests, it seems that flowering plants are gaining ground on conifers, especially towards the northern high latitudes,” says Condamine.
But in the Anthropocene, humans are a bigger threat than flowers. “Living conifers are generally adapted to cold climates, however the recent human-mediated warming is too fast for conifers to respond,” he says. “Perhaps more than ever, conifers are endangered and I am quite concerned about their future in the current global context.”