The Mercury


- JACO LE ROUX | The Conversati­on Jaco le Roux Associate Professor, Macquarie University, Sydney

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.

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