Christopher N. Johnson
School of Natural Sciences and Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage, University of Tasmania, Hobart Tasmania 7000, Australia.
One of the largest effects of humans on the natural world has been to raise the rate of extinction of species far above natural levels. This began many thousands of years ago, and as a result the human-caused loss of global biodiversity was already significant before the modern era. Now, the extinction rate is accelerating, biodiversity is in rapid decline, and many ecosystem processes are being degraded or lost.
1. History of human-caused extinctions
The effect of humans on global biodiversity first became significant as modern Homo sapiens migrated from Africa to occupy the other continents. Between about 60,000 and 10,000 years ago, a wave of extinctions of giant animals – mammoths, ground sloths, giant kangaroos, and many others – followed the arrival of people in Eurasia, Australia and the Americas. Probably, these megafauna disappeared because of hunting by humans1 and 2.
Then, between about 5,000 and 500 years ago people discovered and settled oceanic islands3. This resulted in extinction of whatever megafauna lived on those islands, such as New Zealand’s moa and Madagascar’s giant lemurs and elephant birds. As well, many smaller vertebrates succumbed to the combined pressures of hunting, forest removal, and impacts of alien species transported by voyaging people; presumably there were extinctions of other components of biodiversity as well, but these are not as well known. Because of the remarkable distinctiveness of biodiversity on islands this second wave of extinctions accounted for a great many species. For example, more than 100 endemic mammal species disappeared from the Caribbean islands alone3, and human occupation of Pacific Islands resulted in extinction of at least 1,000 bird species, around 10% of all the world’s birds4.
Since 1500 CE a third and still greater wave of extinction has been growing. This third wave is being driven ultimately by growth of the global human population, increased consumption of natural resources, and globalization. It is affecting a wider range of animals and plants than the preceding two extinction waves, in the oceans as well as on land5. Our knowledge of which species have gone extinct since 1500 is collated in the IUCN Red List6 and is most complete for vertebrates, especially birds, mammals and amphibians: 711 vertebrates are known or presumed extinct since 1500, including 181 birds, 113 mammals and 171 amphibians6. We know of almost 600 extinctions each of invertebrates6 and plants7 since 1500, but given limited basic knowledge, survey, and assessment of conservation status, the true magnitude of losses in these groups is certain to be far higher.
2. Accelerating extinction rates
The list of known recent extinctions is still only a small fraction of all species on the planet. For example, the tally of bird extinctions since 1500 amounts to 1.6% of all bird species that were living in 1500; the figures for mammals and amphibians are 1.9% and 2.1% respectively. What is more concerning than the raw numbers of extinctions is that they represent a rate of extinction far above pre-human levels. The extinction rate for any group of organisms is expressed as the number of extinctions that would occur each year among a million species8 (or equivalently, the number that would occur in a century among 10,000 species). Standardizing rates in this way allows comparison of extinction rates in different groups of organisms and time periods. Our best estimates suggest that extinction rates in the recent past have been running 100 or more times faster than in pre-human times8 and 9, and that the pace of extinction has accelerated over the last few centuries (Figure 110 and 11). If this continues, the loss of species will soon amount to a large fraction of all species on the planet.
Figure 1. Estimated extinction rates in various animal groups through time, expressed as extinctions per million species per year. The height of each bar represents the range of estimates. Pre-human extinction rates are inferred from the fossil record, recent values from documented extinctions in selected groups, and near-future extinctions are projected from the current rates at which species are transitioning between IUCN categories (data from refs 2, 9, 10, 12 and 13).
There are two reasons to think that the extinction rate is about to rise still further. The first is that current levels of threat of extinction signal a steep increase in the number of extinctions over the next few decades.
In those groups of plants and animals that have been systematically assessed under IUCN Red List criteria, the proportions classified as threatened with extinction (that is, Critically Endangered, Endangered, or Vulnerable) are typically high, about 25% on average6, 11 and 14. This figure implies that a total of approximately one million of the world’s species are currently threatened with extinction14. Five groups (mammals, birds, amphibians, corals, and cycads) have been comprehensively assessed two or more times since 1980. In all cases the reassessments show an increasing trend in the proportion of species that are threatened15.
The most severe category of extinction risk is Critically Endangered. To qualify for this, a species must have some combination of very small total population (250 adults or fewer), extremely restricted distribution (10 km2 or less), and continuing population decline at rates high enough to guarantee extinction within decades6. Currently 6,811 species are listed as Critically Endangered (of a total of 120,372 that have been formally assessed, a number still far short of the estimated two million-plus species so far described).
In short, the threat of extinction is now so widespread, and so many species stand on the brink of extinction, it is clear we could be about to lose many more. Even if the future brought nothing worse than the extinction of a significant proportion of all species now listed as Critically Endangered, that would amount to a very large increase in the total number of extinctions since 1500 CE. Because population size is still decreasing in most Critically Endangered species11 we must consider it likely that many of them will soon be gone.
The near-future rate of extinction depends not just on current levels of threat, but on the speed with which now-threatened species decline all the way to extinction; it depends also on the rate at which species not currently threatened become so, and how quickly they then travel the full path to extinction. These dynamics of extinction risk are unknown for most groups, but they can be described for a few. The best-studied case is the worlds’ birds, illustrated in Figure 2.
Between 1988 and 2016 a large cohort of bird species travelled all the way from being Near Threatened (that is, secure, but within sight of one of the markers of Vulnerable) to Critically Endangered (Figure 2). At the same time more than twice as many species joined the ranks of Near Threatened from Least Concern (that is, at minimal risk) and could soon follow the others on the path towards extinction. Another large cohort moved from Least Concern to Vulnerable or Endangered, having crossed the broad territory of Near Threatened in just a few years (Figure 2). The speed of these recent movements from low to high risk suggests that the number of bird extinctions is about to increase much more dramatically than might be suggested by the rather small rise in overall percentage of species listed as threatened (from 12.6% to 13.5% between 1988 and 2016).
Figure 2. Changes of Red List categories for bird species from 1988 to 2016. Numbers of taxa making each change are shown in the circles. Total birds in each category in 2016 are shown in parentheses; the total for extinctions is the number confirmed since 1500 CE. Data from the IUCN Red List, as compiled by Monroe et al 201912.
Recent studies of birds12 and 13 and mammals2 have used information like that shown in Figure 2 to estimate transition probabilities between IUCN Red List categories (in both directions, for the better or worse) and forecast rates of extinction over the coming decades. These studies suggest that extinction rates for birds and mammals are about to increase by more than tenfold (Figure 1). Similar accelerations in extinction are likely for other groups that are not as well-known; if anything, the increases could be even greater in many groups of organisms that are given less attention than birds and mammals and so are less likely to be helped by specific conservation actions.
3. Intensifying pressure on biodiversity
The second reason to anticipate a steep rise in extinction is that the forces that caused recent extinctions are as strong as ever. The main direct causes of extinction are loss and degradation of habitats due to human use of land and sea; overexploitation of wild populations; and the impacts on populations and ecological communities of invasive alien species, pollution, and climate change14, 16 and 17. These direct causes are driven ultimately by demographic, economic and societal factors that increase the pressures that human populations place on biodiversity. Most indicators of the direct and ultimate causes of biodiversity decline show that they are continuing to grow stronger18 and 19. There have been some improvements—most notably, the expansion of protected areas since 2000 (from 10% to 15% of the land surface of the globe, and 3% to 7% of the oceans) and a recent fall in the (still substantial) global rate of deforestation18 —but they are too small to offset the general increase in pressure.
There are several other reasons to think that the pressures that have caused extinction in the recent past will have worse effects in the future. Populations of many species are becoming smaller and more geographically restricted, whether or not those species yet qualify as threatened15. This makes them more susceptible to threats they might have resisted when abundant and widespread, because for each population affected by some insult such as habitat loss or overexploitation there are fewer others to offset local declines and supply immigrants to replenish losses. Also, the general increase in human impact on nature makes it more likely that remaining natural areas are subject to several different threats at the same time, leading to compounding or synergistic effects with greater total impact20.
The future will also bring an increasingly important overlay of global climate change to the long-standing forces of habitat loss, overexploitation, and so on. The most significant effect of climate change may well be to increase the frequency or magnitude of extreme events. These include many that recent experience shows have great potential to damage biodiversity, such as intense tropical cyclones, marine heat waves, and El Niño and La Niña events21. Extreme events that affect large areas can force large, abrupt and unexpected declines of many species at one time.
The climate-driven fires that recently burned much of southern Australia supply an illustration of such an extreme event22. During the 2019/20 fire season, 97 000 km2 of southern Australian woodlands, forests and associated habitats were burnt by fires of exceptional intensity. The fires caused extreme damage to habitats that typically experience recurrent fires of lower intensity and extent, while consuming other habitats that normally do not burn at all. In the broad region affected by the fires, 243 vertebrate species or subspecies are listed as threatened. Fires overlapped significant portions (>10%) of the ranges of 46 of these threatened vertebrates. Some had most of their habitat burnt, for example 82% for the long-footed potoroo Potorous longipes and 98% for the Kangaroo Island dunnart Sminthopsis griseoventer aitkeni. Another 49 vertebrates not currently listed as threatened had 30% or more of their habitat burned, 100% in the case of Kate’s leaf-tailed gecko Saltuarius kateae. It is possible that many currently listed vertebrates will move into more severe threat categories, while re-assessment of previously secure species could see the total number of threatened vertebrates in Australia increase by 14%, as a result of this single event22. At this stage there is less complete knowledge of impacts on other species, but rapid appraisals have identified 191 invertebrate and 486 plant species as potentially severely affected23.
4. Loss of abundance
The figure of 25% of all species threatened with extinction is one measure of a more general decline of populations of wild species. Analysis of aggregated data on population trends of vertebrates from around the world indicates a general decline in abundance of 68% between 1970 and 2016, due both to extirpation of local populations and reduced numbers in those that remain. Roughly similar trends have occurred in most major regions of the world, in freshwater and dryland ecosystems, and in the oceans as well as on land5, 15 and 24. There is also growing evidence of widespread decline in the abundance of invertebrates, especially insects15 and 25. This is best studied in Europe, where it is becoming clear that the abundance and diversity of arthropods is declining even in relatively undisturbed habitats, evidently because of spill-over effects from agricultural land use26.
So far, conservation action has had little success in reversing the general decline in abundance of wild species. We have prevented some extinctions; for example, interventions between 1993 and 2020 prevented 21-32 bird and 7-16 mammal extinctions, such that extinction rates in both groups would otherwise have been 2.9-4.2 times higher27. This is encouraging, but most improvements have been in moving species out of the Critically Endangered category into Endangered (Figure 2), that is, holding the line against extinction for some of the most severely threatened taxa. Few threatened species have recovered their original distribution and abundance against the much stronger tide running in the other direction (Figure 2).
Image caption: the Critically Endangered orange-bellied parrot Neophema chrysogaster by Tiana Pirtle.
Not all species are being forced into decline; some are becoming more abundant as a result of human disturbance. In general, however, large-bodied and ecologically specialised species are more likely to decline28, being replaced by less diverse sets of species that either tolerate disturbance or benefit from it and are capable of invading new or altered habitats29 and 30. The result of this process is that a great part of the original diversity of nature is being lost from much of the planet. As this happens, ecological communities are being made simpler and some important ecosystem functions are degrading.
5. Species extinction and ecosystem decline
In the places that still have them, very large herbivores such as elephants and rhinos control the structure and pattern of vegetation, promote habitat heterogeneity, limit the extent of wildfire, transport nutrients, and disperse seeds. These various effects combine to promote diversity among smaller animal and plant species; megaherbivores could even influence the climate through alterations to land-surface albedo31. The wholesale extinction of mega-herbivores many thousands of years ago damaged ecosystems and diminished biodiversity in ways that are only beginning to be understood31, 32 and 33. Big predators also sustain biodiversity and stabilize ecosystems by regulating populations of smaller predators and intermediate-sized herbivores34.
Among living mammals, amphibians, birds, reptiles and fish, the very largest species continue to be at highest risk of extinction: 59% of living megafauna are threatened, and 70% are decreasing in numbers35. The threat of extinction is also exceptionally high among the smallest vertebrates36. That is, human impact is deleting the smallest and largest vertebrates, and thereby confining survivors to a narrower size-range than was produced by evolution.
This is one instance of a more widespread phenomenon, in which human impact reduces the diversity and range of traits of organisms in natural assemblages28. The general result is a reduction of functional diversity as well as total numbers of species, simplification of ecosystems, and in consequence loss or destabilisation of important ecosystem functions. So, for example in forest and woodland ecosystems the loss of mega-herbivores can result in higher incidence of destructive wildfire37; in the oceans the loss of large species is reducing connectivity among ecosystems and thereby making them unstable5; and declining diversity of insects diminishes many essential ecosystem functions such as pollination and nutrient re-cycling15.
Our knowledge of past and future extinction rates makes it clear that the problem of extinction is urgent. The problem has two main components. First, the extinction of species is reducing the total diversity of life on the planet. Although the extinction tally does not yet represent a large proportion of all species, it is substantial and ecologically important. The rate of extinction is rising fast, and on current trends a large fraction of all the world’s species could soon be gone. Second, the combination of extinction of some species and declining abundance of many others is causing a general loss of abundance and diversity of wild species and compromising the functioning of ecosystems.
The highest priority for action should be the prevention of extinction, because the extinction of any species is an irredeemable loss9. Science-based interventions have a good record in saving species from extinction and recent losses would have been significantly worse without them27 and 38. The actions that have been most consistently successful are establishment of protected areas, habitat restoration, and intensive management of small populations including reintroduction27. The reason that we have not seen more success is primarily that investment in and resourcing of species conservation is in general far too low39.
The problem of preventing broader decline of wild species requires more complex solutions, based on retention of large areas of intact habitat together with rewilding of degraded areas, improvements in the sustainability of exploitation of wild populations, development of strategies for reducing the impacts of invasive species at landscape scales, and mitigation of climate change. Accomplishing these changes will require transformations of the relationship of human communities to nature that will depend on the application of science, development of new socio-economic and governance models, and restoration of indigenous knowledge and practices of environmental management13, 15 and 26.