Charles Darwin’s On the Origin of Species, the most important biology book ever written, contains only one figure. It’s the "tree of life", which shows how species are related through evolutionary history – from bacteria and molluscs to apes and humans. Sadly, human activity is now threatening this stunning diversity at a dramatic rate.
But even this could be an underestimate as we don’t know how many species there are on our planet and what losing any one of them could truly cost us.
"What really makes me weep, genuinely, is that there are species living in primary, natural rainforest that haven't been disturbed for hundreds of years that we never see," says Dr Will Pearse, a senior lecturer in applied ecology at Imperial College London. "We’ve never identified them – we’ve never even taken a picture of them."
Scientists have so far recorded about 1.7 million species of animals, plants and fungi, but there are likely to be ten to 100 times more. When you expand this count to include bacteria and other microorganisms, there could be up to a trillion different species. Despite this, the International Union for Conservation of Nature (IUCN), which produces the IUCN Red List, a list of species that are threatened with extinction, has only assessed 128,500 species to date. Of those, it estimates that 28% are threatened with extinction – including 40% of amphibians, 26% of mammals and 33% of coral reefs.
The diversity of life on Earth is the result of billions of years of evolution – and countless species have also been lost during this vast timespan. But the current rate of biodiversity loss is a thousand times higher than the historical background rate, which many scientists believe is a sign that we are undergoing a sixth mass extinction event.
Worse still, this tsunami of extinctions is being caused by human activity, such as the destruction of wild habitats to create farmland, pollution, climate change and unsustainable hunting and fishing practices.
"The human race inherited the planet at the most diverse it's ever been," explains Dr Bonnie Waring, a senior lecturer of ecology at the Grantham Institute – Climate Change and the Environment, Imperial College London. "And I think there is a moral and ethical perspective from which we should respect the rights of those species, with which we share the planet, to exist."
But biodiversity loss isn’t just a moral, ethical or aesthetic issue. When you lose species and entire ecosystems you also lose the benefits they naturally provide. The planet’s human population is growing, and to survive we must have water to drink, food to eat and clean air to breathe. We must also have healthy immune systems and medicines to protect us from devastating pandemics such as COVID-19. But the current rate of biodiversity loss threatens many of these things that we ultimately rely on to survive – whether we live in cities or in the rainforest. Biodiversity loss is also inextricably linked with climate change, with each compounding and making worse the other.
Oceans, trees and plants all store carbon and other pollution, helping to mitigate the effects of climate change and provide clean air. So the more forests we cut down and the fewer phytoplankton there are in the sea, the more carbon dioxide will hover in the atmosphere, warming our planet. Meanwhile, a decline in bees which pollinate crops threatens our supply of key foods such as fruits and nuts. Even the oceans are vulnerable, with overfishing, pollution and climate change putting at risk food sources of billions of people around the planet.
Biodiversity loss could even be making us ill.
By 2050, seven out of every ten people are expected to live in cities. While there is biodiversity in cities, it tends to be reduced and homogenous – home to the same invasive species, such as Japanese knotweed, raccoons and grey squirrels, which outcompete others and create numerous problems. And when we replace microbe-rich soils with concrete, we diminish our exposure to organisms that are beneficial to our health by boosting our immune systems. Our expansion into nature also makes pandemics that are passed from animals to humans more likely.
"There is research suggesting that more diverse natural communities essentially buffer disease transmission both within themselves and to humans," says Dr Waring. "So there’s a disease dilution effect of biodiversity."
We know that disease outbreaks such as SARS and COVID-19 have increased in the past few decades. One recent study suggested that while many animals are dying out, species like rats and bats survive and thrive – and they are more likely to pass dangerous viruses to humans. Biodiversity also threatens our supply of important medicines and antibiotics, many of which are sourced from nature.
Luckily, there are things we can do – both as individuals and nations – to reverse the damage before we cause widespread and irreversible ecosystem collapse, as we shall explore in the final part of this article.
Carnivores and herbivores
Mammals are the most studied species in the tree of life. From beavers and foxes to zebras and lions, there may seem to be a dazzling number of such species. But consider this: humans and our livestock make up an astonishing 96% of the mass of all mammals currently roaming the planet, while wild mammals account for just 4%. Similarly, 70% of all birds on Earth are poultry, such as chickens.
The situation is clearly concerning. A recent global review found that large mammals, such as primates, Indian elephants and rhinos, in the world’s poorest countries face the greatest threat, mainly from illegal hunting. Leopards, wild dogs and wolves are also declining rapidly, as are tiger populations, which, despite small increases over the past decade, have diminished by a staggering 96% over the last 100 years.
You may think that carnivores, which have to eat large amounts and hunt in vast areas, are at particular risk from biodiversity loss. But a surprising study from Imperial College London showed that this is wrong – herbivores are actually even more threatened. The findings were drawn from a database of the extinction risk and diets of more than 24,500 species of mammals, birds and reptiles.
This showed that while 15% of carnivores were at risk of extinction, 25% of herbivores were, too. 17% of omnivores, which eat both meat and plants, were also vulnerable. And just like previous studies, it found that larger animals were generally more at risk. Exceptions to this included plant-eating reptiles, such as tortoises, which are vulnerable to invasive species, such as rats, moving in and hunting their eggs.
"We are at the beginning of a mass extinction event," says Dr Pearse, one of the authors of the herbivore study. "What we're in danger of losing is not just the species themselves, but also the services that they provide us."
Herbivores that eat fruits and nuts, such as spider monkeys and sloths, for example, spread the seeds of trees that can store large amounts of carbon, helping to mitigate the effects of climate change. Elephants stomp forests and grasslands so that smaller species can live there. And rhinos are known to graze certain species of grass, making room for others to move in – providing food for yet other grazing animals.
This provides important lessons on how to best protect biodiversity. According to Dr Pearse, saving tigers isn’t everything – we need to preserve entire ecosystems. For example, while conservation programmes have managed to nearly double panda numbers since the 1970s, populations of other animals, such as leopards, wolves and dholes, have fallen in panda reserves. Similarly, overall tiger numbers have risen over the past decade. But genetic studies have shown that there are actually six distinct tiger subspecies – three have already gone extinct – with the South China tiger and the Sumatran tiger being particularly vulnerable. This suggests conservation work should take a more holistic approach, protecting different populations rather than just focusing on boosting the absolute number of an overall species.
And although habitat loss is the major threat to biodiversity among herbivores and carnivores at the moment, the impacts of invasive species and climate change are also on the rise. Many species across the world are struggling to survive as it becomes too hot, wet or dry, with events such as wildfires, droughts and flooding increasing. Many species, such as wolves, require snow to thrive, for example.
"We are seeing the impacts of climate change only now beginning to start – we’re beginning to have losses of species," Dr Pearse explains. "It's only going to get worse."
Insects and pollinators
It’s easy to get people to care about majestic creatures such as tigers, pandas and elephants. Many of us shudder, however, at the thought of cockroaches or spiders. But creepy crawlies are actually incredibly important for biodiversity as a whole – not least because many larger animals rely on them for food. They also prey on nasty pests and help decompose waste.
We know that insects are declining globally, although the pace of the loss is still being debated. One recent study estimated that land-based insects, which are the most common, have been declining at nearly 1% per year since 1925. Another suggested that 75% of flying insects were lost in the last 25 years in Germany, while bumblebee numbers have plunged by 30% in Europe and North America since 1970. This is a major cause for concern. More than 75% of the world’s crops benefit from insect pollination, including important food sources such as fruits and nuts.
"About 30% of the food we eat is thought to be pollinated by insects," says Dr Tilly Collins, a senior fellow in the Centre for Environmental Policy at Imperial with expertise in entomology, urban ecology and sustainability. "So without these pollinators, our diets would have to shift enormously towards wind-pollinated grains."
There are many different reasons for the decline in insects, ranging from light pollution and disease to the loss and fragmentation of habitats. For pollinators such as wasps, ants, beetles and butterflies, the main driver, however, is agriculture, which has led to the destruction of approximately 97% of wildflower meadows since the 1970s.
Modern farming also uses a number of pesticides and herbicides that are harmful to pollinators. Neonicotinoids, the most commonly used pesticides in the world, for example, have been linked to poor health in bees and have since been banned in the EU. Climate change is also an increasing problem.
"We understand very clearly why insect biodiversity has changed in our temperate agricultural zones," explains Dr Collins. "But we’re also seeing a great deal of species lost in some of the tropical insect fauna, which is scary."
This is strange, as there is relatively little agriculture and human disturbance in some tropical areas. The decline may be largely due to the subtle effects of climate change. Insects are highly sensitive to extreme and rapid weather changes, particularly species that evolved in the tropics, where the weather traditionally hasn’t changed much. Rapid changes in the seasons can also leave pollinators in more temperate latitudes out of sync with the flowering patterns of plants.
Soil and trees
Soils are home to an astonishing number of organisms, accounting for about 25% of all biodiversity on our planet, including nematodes, earthworms, ants, insects and microorganisms.
"If you went to your backyard right now and scooped up a teaspoon of soil, it would contain on average a couple of hundred species of fungi and a couple of thousand species of bacteria," explains Dr Waring.
Microorganisms in soils are important as they interact to break down organic material, such as dead plants or animals. They also cycle nutrients and filter water – ultimately helping plants to grow.
Exactly how much biodiversity is being lost in soils is difficult to estimate. It is only recently that genetic methods have enabled us to measure the diversity of soil organisms, meaning trends are hard to pick up.
"Starting about 30 years ago, we realised just how spectacularly diverse soils are because we couldn't do that until recently," says Dr Waring. "It also means that we don't know what baseline soil diversity there was 100,000 years in the past and that makes it difficult to assess."
But while we lack long-term measurements of soil microbial composition, there are signs that soil biodiversity is decreasing. Large mushrooms, for example, are declining due to human activity, so it is likely that microscopic fungi are also disappearing, argues Dr Waring. Another indication is that soils in natural woodlands differ in species composition from those in urban parks. We know that land loses biodiversity when it is degraded. And according to the IUCN, about a third of the world's land has already been degraded.
This degradation is mainly due to intensive agriculture and fertiliser use. Soil particles are often compressed due to heavy machinery, for example. This can happen to the extent that it reduces air and water infiltration. But extreme weather events such as heatwaves and droughts are also a problem, and could result in a 25% food production shortfall by 2050. Soil also plays a key role in the water cycle - it stores two thirds of Earth’s freshwater.
On a positive note, it is becoming increasingly clear that soils actually store huge amounts of carbon and play a key role in stabilising the climate. This realisation has triggered a real "paradigm shift" in Dr Waring’s research field recently. Estimates vary, but one initiative among scientists suggests that if we could globally increase soil carbon concentrations by 0.4% every year, we could offset the annual increase in anthropogenic emissions.
Trees also soak up a lot of carbon, and planting forests has therefore been a popular and cheap strategy to help limit the amount of carbon dioxide in the atmosphere. Scientists recently calculated that if we planted billions of trees, covering about 0.9 billion hectares, we may be able to store an equivalent of 25% of the current atmospheric carbon. Currently, there are 1.7 billion hectares of treeless land on Earth that we could use, although some of this area represents habitats like grassland or savannah, where planting trees could cause ecological harm.
Dr Waring agrees this could make a difference but warns we must also protect existing forests, which are home to 80% of the planet’s animals. Rainforests, for example, are among Earth’s most biodiverse places, while ancient forests are the most biodiverse land habitats in the UK. While planting young, fast-growing trees may be able to remove atmospheric carbon quickly, such trees can only store a relatively small amount of it. Old forests, on the other hand, capture carbon increasingly slowly over time, but can store large quantities of it.
"Cutting down existing forest to plant a bunch of trees that grow very fast is not effective for climate mitigation, and it's not effective from the biodiversity perspective," Dr Waring says. "We need to ensure that all of the solutions that we devise are confronting both crises at once."
Roll on, thou deep and dark blue Ocean,—roll!
Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin,—his control
Stops with the shore;—upon the watery plain
As Lord Byron noted in his poem, The Sea, some two centuries ago, the ocean is so enormous that humans were long thought to be incapable of altering it. Approximately 95% of the ocean is still completely unexplored – and the surfaces of Mars and the Moon have been mapped in higher resolution than the seafloor. But human activities – and their impacts – now spread far beyond the shoreline, explains Professor Guy Woodward, a professor of ecology at Imperial:
"We see that many global fisheries, for instance, are undergoing quite dramatic declines, even among the species that were previously common."
The ocean is threatened by human activity in many ways, ranging from overfishing and pollution to oil spills and climate change. The United Nations Food and Agriculture Organisation estimates that 31.4% of fish stocks are either fished to capacity or overfished. And overfishing is particularly affecting large species, such as tuna, sharks and rays. IUCN currently lists six tuna species as vulnerable, near threatened, endangered or critically endangered – including Bluefin tuna, Yellowfin tuna and Albacore tuna. Meanwhile, a recent report estimates that oceanic sharks and rays have declined by 71% since 1970.
The biggest creatures in the ocean are also dependent on the smallest ones. These tiny lifeforms are the base of the food chain and include microbes and plant-like cells called phytoplankton. And while these organisms aren’t threatened directly by human pressures such as overfishing, they may struggle as a result of climate change and chemical change including ocean acidification and chemical pollution. According to one study, phytoplankton have declined by 40% since 1950, and changes of this magnitude could potentially have consequences for feedbacks with climate change, as phytoplankton store about half of the Earth’s carbon, being a major source of oxygen as well as sink for carbon dioxide.
Climate change affects the oceans in many other ways, too. Rising waters are a threat to many species that live in shallow coastal areas, such as mangroves and salt marshes, for example. And when carbon dioxide from the atmosphere is absorbed by the seawater, it becomes more acidic – affecting the ability of certain invertebrates and the fish that feed on them to thrive. Warm waters also damage the ocean’s coral reefs, making them eject the algae living in their tissues. This makes them turn white or "bleach" – becoming skeletons of their former selves. And dead coral is also a threat to the ocean’s biodiversity.
"Coral reefs are really the aquatic world’s rainforests," says Professor Woodward, adding they are home to a wide range of species.
They also protect shores from hurricanes, mitigating the worse effects of extreme weather events that are expected to increase in intensity under a warming climate. So how dangerous is the situation?
"These are still vast systems and in many cases, they can bounce back as well," says Professor Woodward. "We do see recovery in fisheries if harvesting rates are reduced to more sustainable levels."
Consequently, we are not looking at an immediate global collapse of marine ecosystems, he argues – even bleached corals can recover. But that doesn’t mean we should be complacent. Because the ocean is so vast, the impact of climate change will take a long time to ripple through – with effects seen over centuries or even millennia to come.
"Even if we stopped excessive carbon emissions today, you would still see responses happening for a long time into the future," explains Professor Woodward.
We don’t know exactly at what point the biodiversity loss we are seeing now could prompt widespread and irreversible ecological collapse – the likely timeframes are something researchers are still trying to work out. But clearly, now is the time to act.
"We're conducting this massive planetary experiment where we know that we're losing biodiversity," says Dr Waring. "But we're driving blind in terms of what threshold can we push that biodiversity loss to before we start seeing major human impacts."
The good news is that we know what is required to tackle these problems - reducing emissions, protecting land and developing more sustainable systems of food production. The new Biden administration in the US has recently rejoined the Paris climate agreement. And a 600-page review of the economic importance of nature, commissioned by the UK Treasury, has just recommended that world governments should rethink their national accounting, currently based on GDP, and instead create a system which takes into account the destruction of natural resources.
Ahead of a UN summit on biodiversity later this year, Dr Pearse would also like to see more protected areas. At present, only 15% of the terrestrial surface and almost 7.5% of marine regions are covered by protected areas, he says. We should also investigate whether such regions are, in fact, protected enough. "A big and ongoing debate in the UK is that many of our protected areas have agricultural grazing in them, which can reduce that native biodiversity," he argues.
Dr Waring adds that it is also important that local communities are involved when creating protected areas and other policies. "Having sort of a top down approach doesn't really work," she argues. "You need buy-in from whoever is local to that place." Meanwhile, Dr Collins wants to see more efficient agriculture. "We have to focus on biotechnology," she says, adding that this could involve using solar power to desalinate sea water, so that you can grow plants year round.
Limiting climate change is another crucial task. According to Professor Woodward, this is tricker because it requires a much more global approach.
"We've seen a lot of policy interventions help put the brakes on some of these more damaging consequences, or even reverse them," he argues, referring to efforts like conserving species or introducing fishing quotas. "But other systems might be exposed to much more diffuse pressures which become more difficult to regulate, particularly when they span international boundaries," he argues. "So climate change is a particular challenge because of the huge scale of the problem."
Most scientists agree that the best approach is a mix of individual and governmental action. We can all reduce our carbon footprint by flying or driving less, insulating our homes and turning off electrical goods when we don't use them. Dr Collins argues we can also directly create more biodiversity by embracing "scruffiness" and, for example, rewilding our gardens and communal areas.
Shifting our diets to consume less meat and more locally sourced food could also have a big impact. After all, 50% of the habitable land on Earth is devoted to agriculture, of which 80% is pasture. Diet changes can also benefit ocean biodiversity. Professor Woodward advises to eat smaller fish that are lower in the food chain. "If you have got a choice between tuna and sardines, sardines are the more ecologically efficient choice." But ultimately the problems we are seeing are largely a result of government policies and industry’s response to them, so perhaps the most important thing we can all do on an individual level is to lobby our elected representatives for change.
Many scientists hope we will learn important lessons from the COVID-19 pandemic. Dr Pearse says he has been motivated and inspired by how people have come together during the pandemic and how epidemiologists have communicated with policymakers. But Professor Woodward argues that COVID-19 has also shown how difficult it can be to get this right – you need to be persuasive, but if you exaggerate, the argument can backfire and you may lose public trust.
"In many cases, the evidence is inconclusive," he points out. "I'm very wary of making statements about things such as a mass extinction event being literally just around the corner, but we are certainly already seeing very worrying declines in the abundance of many species - a precursor to extinction - and many previously dominant species are in decline around the world."
But the public may already be committed. According to Dr Collins, people are now beginning to understand that human encroachment into forests risks bringing about new pandemics. The hope is that businesses, keen to avoid financial losses associated with future pandemics, as well as flooding and other climate-related disasters, will recognise the risks and go greener as a result. But perhaps most importantly, consecutive lockdowns have been an opportunity for many of us to truly enjoy nature, bringing about an awareness of the positive impact that green spaces can have on our mental health and emotional wellbeing.
In his autobiography, Darwin wrote that his mind had become "a machine for grinding general laws", which he worried may have harmed his moral and emotional character. He’d be pleased to know that centuries later, his tree of life would fill scientists with the emotion and moral conviction needed to make our planet better.
Title image and moray eel: Dzulkeflee Khairuddin via dzulkeflee.com
Fallow deer and fox cub: Professor Richard Templar, Director of Innovation at the Grantham Institute
Lioness: Abigail Croker, studying the impacts of colonialism on fire regimes in East and Southern Africa at Imperial College London
Daphnia, juvenile Antilles pinktoe tarantula (Caribena versicolor) and ghost mantis: Samantha Ibbott, Digital Communications Officer at the Grantham Institute
Whale shark and bees flying into hive: Imperial College London asset library
Preserving biodiversity, conservation action: ZSL