Review

Biodiversity losses and conservation responses in the Anthropocene

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Science  21 Apr 2017:
Vol. 356, Issue 6335, pp. 270-275
DOI: 10.1126/science.aam9317

Abstract

Biodiversity is essential to human well-being, but people have been reducing biodiversity throughout human history. Loss of species and degradation of ecosystems are likely to further accelerate in the coming years. Our understanding of this crisis is now clear, and world leaders have pledged to avert it. Nonetheless, global goals to reduce the rate of biodiversity loss have mostly not been achieved. However, many examples of conservation success show that losses can be halted and even reversed. Building on these lessons to turn the tide of biodiversity loss will require bold and innovative action to transform historical relationships between human populations and nature.

Extinction has always been a feature of life on Earth, but the domination of global ecosystems by people has caused a sharp rise in the rate of extinctions to far above pre-human levels. Loss of biodiversity affects the functioning of natural ecosystems and threatens human well-being. In this Review, we place the current extinction crisis in the context of long-term impacts of humanity and assess current trends in biodiversity loss. We identify successes as well as failures in our response to this crisis and draw lessons on what is needed to turn the tide of biodiversity loss.

A brief history of human-caused extinction

The imprint of humanity on biodiversity reaches back 2 million years, when our ancestors in the genus Homo began to use the large-carnivore niche in Africa. This was associated with a two-thirds decline in other large carnivores, as species such as sabretooth cats and long-legged hyenas disappeared (1). Diversity of large herbivores also declined. For example, the 12 species of elephants and their relatives living in Africa around 3 million years ago were reduced to two (2). Similar disappearances began elsewhere as species of Homo spread beyond Africa (3), then accelerated in step with the global expansion of H. sapiens through the past 60,000 years (Fig. 1A).

Even before the dawn of the modern era of extinctions in 1500 CE, the wave of extinctions that followed our species around the world had large impacts on biodiversity. At least 140 genera of mammals, more than 10% of the global total, were lost over the 100,000 years to 1500 CE (table S1), a pace of extinction that far exceeds background rates estimated from the fossil record (4). Similarly, 23% of the world’s turtle and tortoise species have disappeared over approximately the past 300,000 years (5). Prehistoric occupation of Pacific islands alone was associated with extinction of at least 1000 bird species, which is around 10% of all birds (6). In New Zealand, 36% (44 of the original 117) of land bird species have gone extinct since human settlement began 750 years ago, most of them in the prehistoric period between Polynesian and European arrival (Fig. 1B). Most prehistoric extinctions were of terrestrial animal species, but marine biodiversity also declined because of local extirpations and loss of abundance as human use of the oceans expanded (7).

Prehistoric extinctions are predominantly associated with human arrival rather than climate change (8) and are best explained by the impacts of hunting (9). Habitat modification and predation by alien species were additional factors in some places, especially islands. Throughout the Pacific and Indian Oceans, human-lit fires transformed island ecosystems with unprecedented speed (10); in New Zealand, anthropogenic fire saw the loss of over 40% of forest cover in the drier lowland regions within 10 to 70 years of human arrival (11). The main causes of recent extinctions and declines continue to be overexploitation and conversion of habitat, along with invasive species, disease, and urban development (12). Global climate change is already causing large disruptions to ecosystems (13) and is likely to grow in importance as a cause of extinction.

Estimates of the recent rate of extinction are limited by poor knowledge of most species. Our best information is for vertebrates: At least 363 vertebrate species have gone extinct since 1500 CE, according to the International Union for the Conservation of Nature (IUCN) Red List (14). The rate of extinction of vertebrates rose through the past two centuries as human populations industrialized and grew (Fig. 1C and fig. S1). Levels of current threat of extinction in those groups of plants and invertebrates that have been systematically assessed cover a similar range to vertebrates (Fig. 1D), suggesting that vertebrates provide a useful yardstick for patterns of species decline and extinction among less well-studied organisms. Systematic monitoring of birds and mammals shows that global levels of threat are increasing by 1 to 2% per decade (Fig. 1D). This masks rates of population decline that are often far greater. For example, coastal wetlands are fast disappearing because of encroachment by people (15). Among 155 species of coastal waterbirds from east Asia, populations are declining at rates of 5 to 9% per year and as much as 26% for some species (16).

Comparison of trends in recent extinctions with current levels of threat suggests that the rate of extinction may be about to increase. Extrapolation of recent trends suggests that there will between 269 and 350 further extinctions of birds and mammals by 2100 (fig. S1). However, 1341 birds and mammals are currently classed as Critically Endangered or Endangered and are therefore likely to be extinct by 2100 if the processes causing their endangerment continue to operate; the fact that most (85%) of these species are currently decreasing suggests that is the case (details are provided in the supplementary materials). That is, our knowledge of current threats suggests that the rate of extinction could soon rise to at least five times higher than it has been in the recent past (Fig. 1C).

Species loss and ecosystem change

All species are connected to others through ecological interactions. Extinctions therefore reverberate through ecosystems, as do extirpations of local populations and declines in abundance, which are widespread even in species not close to extinction (17). Past extinctions and population declines were apparently concentrated on large-bodied vertebrates (9). Disappearance of these animals removed powerful consumers with strong effects on ecosystem composition and function, both on land and in the oceans (9, 18). More generally, species declines disrupt many interactions, with far-reaching consequences for ecosystems (19). For example, many woody plants produce large fruits and rely on large vertebrates for seed dispersal (20). Large seed size is positively correlated with wood density and hence high carbon-storage capacity. Loss of mutualistic partners can therefore lead to tropical forests dominated by fast-growing, small-seeded plants with lower carbon stores (20). Growth of reef-building corals depends partly on abundance of large herbivorous fish. In the Caribbean, overfishing of these keystone herbivores over the past thousand years caused shifts from coral to algal dominance of reef habitat (21). The stability and productivity of commercial fisheries are enhanced by diversity of both fished and unfished species. Large declines in diversity over the past two centuries correlate with lower catches, lack of resilience to exploitation, and higher incidence of collapse of stocks, along with degradation of other values such as quality of coastal and estuarine waters (22).

“[O]ur knowledge of current threats suggests that the rate of extinction could soon rise to at least five times higher than it has been in the recent past.”

Global responses

Several international initiatives have attempted to coordinate action to halt or reverse biodiversity loss. The most important is the Convention on Biological Diversity (CBD; www.cbd.int), to which 196 nations are party. In 2002, world leaders pledged through the CBD “to achieve by 2010 a significant reduction of the current rate of biodiversity loss.” The 2010 target was succeeded by the “Aichi Biodiversity targets” for 2011–2020, a more complex plan to reduce loss of species and natural habitats and safeguard ecosystem services, while also improving planning, financing, knowledge, and benefits from sustainable management of the natural world.

The 2010 target was not reached (23), and thus far there has been too little progress on most of the Aichi targets for them to be met by 2020 (2426). Most indicators of the global state of species and ecosystems show continuing deterioration, with little or no evidence of recent slowdown in rates of change (24). Indicators of the capacity of ecosystems to provide ecological services also show declines, despite increases in the benefits that human populations derive from them, suggesting that the natural capital on which human populations depend is being rapidly run down (25).

The continuing decline of global biodiversity is clear despite weaknesses in our ability to monitor progress against targets (27, 28). We lack good indicators for some of the Aichi targets, and available indicators often give somewhat inconsistent signals. For example, The Biodiversity Intactness Index, based on models of effects of land use on species abundances (29), suggests that all human land use to date has reduced the abundance of species by a global average of ~15%. In contrast, the Living Planet Index estimates an average 58% population decline in monitored vertebrates worldwide since 1970 (30). In response to such problems, work is under way on a consistent set of Essential Biodiversity Variables (EBVs; http://geobon.org) to capture spatial and temporal change at the levels of genetics, species abundances, species traits, community composition, and ecosystem structure and function (28). There is also much potential for improved use of existing ecological records, such as the International Long-Term Ecological Research Sites network (ILTERS; www.ilternet.edu). These networks have broad coverage (Fig. 2) and use shared protocols to record ecological changes over long periods (31) but are currently underused in assessments of global change.

Causes of failure

Why have we failed to stem the tide of biodiversity loss? We suggest four interrelated reasons. First, responses to biodiversity decline are being more than offset by rising pressures, related ultimately to increasing human population size and per capita consumption. Between 1993 and 2009, the Human Footprint index, a measure of cumulative human impacts on land, increased by 9%, mostly because of conversion of habitat for agriculture (Fig. 2) (32), whereas the total area of forest landscapes unmodified by human use fell by 7.2% between 2000 and 2013 (33). The area of ocean fished at high intensity (that is, removing >30% of available primary productivity) has increased since 1950 (34). A recent leveling off in that trend may represent the limit of productivity of wild fisheries as well as improvements in fishery management (34, 35), but cumulative human impacts continue to increase across two-thirds of the world’s oceans, mainly owing to intensifying effects of climate change (35).

Second, interactions and synergies among threatening processes often amplify their effects, producing large and accelerating combined impacts (36). Policy responses and actions tend to tackle threatening processes separately and are therefore often not appropriately scaled to pressures. A related problem is that some changes have large effects in catalyzing disparate threats (37). For example, road development in tropical forests has direct impacts by fragmenting habitat and causing mortality to wildlife but also triggers rapid escalation of a complex of threats, including overexploitation, conversion of marginal habitats for farming, fire, and invasive species (3739). Climate change is likely to amplify impacts of other drivers of species decline; for example, there is evidence that rising temperature variability increases the susceptibility of amphibians to disease (40).

Conclusion

Although conservation efforts have produced some encouraging results, these have done little more than forestall some losses by tackling symptoms of unsustainable use of environments. Our successes have been valuable in buying time that could allow recovery of species and ecosystems in the future and providing lessons on how conservation actions can be made effective. However, the problem of transforming the fundamental drivers of unsustainable use of nature remains largely unaddressed.

Correction (2 May 2017): Credits for the photos in Fig. 3, A and B, were added to the figure caption.

Supplementary Materials

www.sciencemag.org/content/356/6335/270/suppl/DC1

Supplementary Text

Figs. S1 to S3

Tables S1 to S3

References

References and Notes

1. Acknowledgments: For information, we thank M. Brooke, S. Butchart, R. Green, S. Jennings, A. Latawiec, W. Lau, P. Martin, T. Martin, A. Purvis, F. Saltré, S. Turvey, W. Xu, and E. zu Ermgassen.
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