Human-driven land clearing and climate change are sending plants extinct at a rapid rate, risking a devastating biodiversity crash.Earth is seeing an unprecedented loss of species, which some ecologists are calling a sixty mass extinction. In May, a United Nations report warned that 1 million species are 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, has the rate of extinction increased? I found some plants have been going extinct up to 350 times faster than the historical average – with devastating consequences for unique species.
We are currently witnessing the start of a mass extinction event the likes of which have not been seen on Earth for at least 65 million years. We noticed how human actions over the past 500 years have affected the extinction rates of vertebrates: mammals, fish, birds, reptiles, and amphibians. It found a clear signal of elevated species loss which has markedly accelerated over the past couple of hundred years, such that life on Earth is embarking on its sixth greatest extinction event in its 3.5 billion year history.
However, there is always a risk to overestimates modern extinction rates because they need to make a number of assumptions given the very limited data available. To establish extinction rates for species that were very conservative, with the understanding that whatever the rate of species loss has actually been, it could not be any lower. This makes our assumptions even more significant because even with such conservative estimates we find extinction rates are much, much higher than the background rate of extinction – the rate of species loss in the absence of any human impacts.

Here again, we err on the side of caution. A number of studies have attempted to estimate the background rate of extinction. These have produced upper values of about one out of every million species being lost each year. Two out of every million species will disappear through natural causes each year. This should mean that differences between the background and human-driven extinction rates will be smaller. But the magnitude of more recent extinctions is so great as to effectively swamp any natural processes.

Cumulative vertebrate species recorded as extinct or extinct in the wild by the IUCN (2012). The dashed black line represents the background rate. This is the ‘highly conservative estimate’. The “very conservative estimate” of species loss uses International Union of Conservation of Nature data. This contains documented examples of species becoming extinct. They use the same data source to produce the “conservative estimate” which includes known extinct species and those species believed to be extinct or extinct in the wild.The paper has been published in an open-access journal and I would recommend reading it and the accompanying Supplementary Materials. This includes the list of vertebrate species known to have disappeared since the year 1500. The Latin names for these species would be familiar only to specialists, but even the common names are exotic and strange. These particular outer branches of the great tree of life now stop. Some of their remains will be preserved, either as fossils in layers of rocks or glass eyed exhibits in museum cabinets. But the Earth will no longer see them scurry or soar, hear them croak or chirp.

We may wonder to what extent does this matter? Why should we worry if the natural process of extinction is amplified by humans and our expanding industrializedcivilization?One response to this question essentially points out what the natural world does for us. Whether it’s pollinating our crops, purifying our water, providing fish to eat or fibers to weave, we are dependent on biodiversity. Ecosystems can only continue to provide things for us if they continue to function in approximately the same way.

The relationship between species diversity and ecosystem function is very complex and not well understood. There may be gradual and reversible decreases in function with decreased biodiversity. There may be effectively no change until a tipping point occurs. The analogy here is popping out rivets from a plane’s wing. The aircraft will fly unimpaired if a few rivets are removed here or there, but to continue to remove rivets is to move the system closer to catastrophic failure.

This latest research tells us what we already knew. Humans have in the space of a few centuries swung a wrecking ball through the Earth’s biosphere. Liquidating biodiversity to produce products and services has an endpoint. Science is starting to sketch out what that endpoint could look like but it cannot tell us why to stop before we reach it.

If we regard the Earth as nothing more than a source of resources and a sink for our pollution, if we value other species only in terms of what they can provide to us, then we will continue to unpick the fabric of life. Remove further rivets from spaceship earth. This not only increases the risk that it will cease to function in the ways that we and future generations will depend on but can only reduce the complexity and beauty of our home in the cosmos.

Measuring the rate of extinction:  “How many species are going extinct” is not an easy question to answer? To start, accurate data on contemporary extinctions are lacking in most parts of the world. But that doesn’t mean coldspots aren’t worth conserving – they tend to contain completely unique plants.

I recently examined 291 modern plant extinction between biodiversity hot- and coldspots. I looked at the underlying causes of extinction, when they happened, and how unique the species were. Armed with this information, we asked how extinctions differ between biodiversity hot- and coldspots.

Unsurprisingly, I found hotspots to lose more species, faster, than coldspots. Agriculture and urbanization were important drivers of plant extinctions in both hot- and coldspots, confirming the general belief that habitat destruction is the primary cause of almost extinction. Overall, herbaceous perennials such as grasses are particularly vulnerable to extinction.

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

I also found that recent extinction rates, at their peak, were 350 times higher than historical background extinction rates. Scientists have previously speculated that modern plant extinctions will surpass background rates by several thousand times over the next 80 years.  So why are our estimates of plant extinction so low? First, a lack of comprehensive data restricts inferences that can be made about modern extinctions. Second, plants are unique in – some of them live for an extraordinarily long time, and many can persist in low densities due to unique adaptations, such as being able to reproduce in the absence of partners.

Let’s consider a hypothetical situation where we only have five living individuals of Grandidier’s baobab (Adansonia grandidieri) left in the wild. These iconic trees of Madagascar are one of only nine living species of their genus and can live for hundreds of years. Therefore, a few individual trees may be able to “hang in there” (a situation commonly referred to as “extinction debt”) but will inevitably become extinct in the future.

Finally, declaring a plant extinct is challenging, simply because they’re often very difficult to spot, and we can’t be sure we’ve found the last living individuals. Indeed, a recent report found 431 plant species previously thought to be extinct have been rediscovered. So, real plant extinction rates and future extinctions are likely to far exceed current estimates.

There is no doubt that 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 particularly severe on plant biodiversity. Current estimates of plant extinctions are, without a doubt, gross underestimates.

In the near future, utilization > urbanization > hydrological disturbance and habitat destruction will remain the primary drivers of plant extinctions in hotspots, while the main drivers in coldspots are largely unknown but with certain impacts from habitat destruction.

 Concluding Remarks: Along with previous estimates suggesting 20% of the world’s plants to be threatened by extinction, the recently released IPBES report paints a depressing picture of global biodiversity. Yet, in our study, we found surprisingly few documented plant extinctions and that these accrued at rates much lower than previously thought. Plants may represent a particularly challenging taxon to unambiguously claim extinction, given their often-inconspicuous presence in the environment and the difficulties associated with locating the last-living individuals or propagules.

Our glimpse into the causes and trends in plant extinctions over the last three centuries left us with more questions than answers. The only way to better understand the magnitude of, and processes driving, the extinction crisis faced by plants, and biodiversity in general, is to urgently initiate regional- (preferably based upon a biogeographical framework) or at least country-level biodiversity assessments and to update existing ones.

This needs to be done using standardized methodologies documenting not only specific reasons for extinction but also key functional and life-history traits (e.g., breeding system, pollination ecology, seed crop sizes, and seedbank persistence). Such assessments will be vital to more accurately infer the main drivers and trends of plant extinctions during the Anthropocene and to make informed forecasts about the future and preservation of Earth’s flora.

The writer is Former Head, Department of Medical Sociology, Institute of Epidemiology, Disease Control & Research (IEDCR),  Dhaka, Bangladesh