IN FIGHT AGAINST EXTINCTION, PLANTS DIG IN

EARTH is seeing an unprecedented loss of species, which some ecologists are calling a sixth mass extinction. In May, a UN report warned that a million species were threatened by extinction. More recently, 571 plant species were declared extinct.

But extinctions have occurred for as long as life has existed on Earth. The important question is whether the rate of extinction has increased.

Our research, published recently in Current Biology, found some plants have been going extinct up to 350 times faster than the historical average – with devastating consequences for unique species.

How many species are going extinct? This is not an easy question to answer. To start, accurate data on contemporary extinctions are lacking from most parts of the world. And species are not evenly distributed – for example, Madagascar is home to around 12 000 plant species, of which 80% are endemic (found nowhere else). England, meanwhile, is home to only 1 859 species, of which 75 (4%) are endemic.

Areas like Madagascar, which have exceptional rates of biodiversity at risk from human destruction, are called “hot spots”. Based on numbers, biodiversity hot spots are expected to lose more species to extinction than cold spots such as England. But that doesn’t mean cold spots aren’t worth conserving – they tend to contain unique plants.

We are part of an international team that examined 291 modern plant extinctions between biodiversity hot and cold spots. We looked at the underlying causes, when they happened and how unique the species were. Armed with this information, we asked how extinctions differed between hot and cold spots.

Unsurprisingly, we found hot spots lost more species, faster, than cold spots. Agriculture and urbanisation were important drivers of plant extinctions in both, confirming the belief that habitat destruction was the primary cause. Overall, herbaceous perennials such as grasses are particularly vulnerable to extinction.

However, cold spots stand to lose more uniqueness than hot spots. For example, seven cold spot extinctions led to the disappearance of seven genera, and in one instance, a whole plant family. Clearly, cold spots also represent important reservoirs of biodiversity that need conservation.

We also showed that recent extinction rates, at their peak, were 350 times higher than historical background extinction rates. Scientists have speculated that modern plant extinctions would surpass background rates by several thousand times over the next 80 years.

Why are our estimates so low? First, a lack of comprehensive data restricts inferences that can be made. Second, plants are unique – some of them live for a long time, and many can persist in low densities due to adaptations, such as being able to reproduce in the absence of partners.

Let’s consider a hypothetical situation where we have only five living individuals of Grandidier’s baobab

(Adansonia grandidieri) left in the wild. The trees of Madagascar are of only nine living species of their genus and can live for hundreds of years. Therefore, a few individual trees might be able to “hang in there” (referred to as “extinction debt”) but will inevitably become extinct.

Finally, declaring a plant extinct is challenging because they’re often difficult to spot, and we can’t be sure we’ve found the last living individuals. A recent report found 431 plant species thought to be extinct have been rediscovered. Real plant extinction rates and future extinctions will probably far exceed estimates.

Biodiversity loss, together with climate change, are some of the biggest challenges faced by humanity. Along with human-driven habitat destruction, the effects of climate change are expected to be severe on plant biodiversity. Estimates of plant extinctions are gross underestimates.

However, the signs are crystal clear. If we were to condense the Earth’s 4.5-billion-year-old history into one calendar year, then life evolved somewhere in June, dinosaurs appeared somewhere around Christmas, and the Anthropocene starts within the last millisecond of New Year’s Eve. Modern plant extinction rates that exceed historical rates by hundreds of times over such a brief period will spell disaster for our planet’s future.