Ecology

Wild descendants of the hippo population introduced in Colombia by Pablo Escobar.Credit: Raul Arboleda/AFP via Getty

Colombia’s invasive hippo population is even larger than researchers had thought, according to the most thorough census of the animals conducted yet. Scientists were already concerned about the hippos — considered the largest invasive animal in the world — threatening native plants and animals in the country, and had been calling for drastic measures to reduce the population. The census results have only heightened that fear.
Pablo Escobar’s ‘cocaine hippos’ spark conservation row
A few years ago, researchers estimated how fast the animals were reproducing, to project that about 98 hippos were living along the country’s Magdalena River and its tributaries in 20201. But the new study, for which a research team counted the animals in person, by drone and using other tracking methods, estimates that there are 181–215 of them residing in Colombia.“Before, one argument against dealing with the hippos was that our information was limited and our arguments theoretical,” says ecologist Rafael Moreno, who participated in the study while at the Alexander von Humboldt Biological Resources Research Institute in Bogotá. “But we have put that argument to bed now. This study shows that this is a real issue, and that the state must act urgently.”By drone and on footColombia’s ‘cocaine hippos’ are all descendants of three females and one male illegally imported by drug-cartel leader Pablo Escobar. After he died in the 1993, the hippos (Hippopotamus amphibius) escaped from his estate and established themselves in the Magdalena River. Without the natural predators or droughts of their native Africa to keep them in check, the giant herbivores have bred rapidly to form the largest population of the animals outside that continent.Colombian officials have struggled to manage the hippos. After they ordered an aggressive male to be killed in 2009, a photo of soldiers posing with the corpse sparked outrage and halted efforts to rein them in. Some communities now depend on the tourism the hippos bring, whereas others — particularly fishing communities — live in fear of the highly territorial animals, which can weigh 3 tonnes and can tear off a person’s limbs or trample them.
Landmark Colombian bird study repeated to right colonial-era wrongs
Colombia’s environment ministry commissioned the census to get a better picture of the problem and how to manage it. Completing it was challenging: despite the hippos’ large size, it is difficult to find and count them accurately. They are nocturnal, immerse themselves in water for 16 hours a day and roam large distances.Comprising researchers at the National University of Colombia in Bogotá, the Humboldt Institute and Cornare, an environmental body managing an area where the hippos live, the team made various trips in 2021 and 2022 by car, boat and on foot to count the hippos. In locations where the animals could not be reached safely, they used drones to count them or footprints to estimate the population.

One way of managing the hippo population is by contraception, administered using a dart.Credit: Cornare via Reuters

The researchers found that 37% are juveniles, indicating that the animals are breeding rapidly. One hypothesis for this is that the hippos are reaching sexual maturity earlier than they do in Africa owing to the lush conditions in Colombia. Another is that the animals are having greater reproductive success because there are fewer fights among them for territory and resources, Moreno says. But evidence is needed to confirm the actual cause.While counting the hippos, the team documented the myriad ways in which the animals are damaging Colombia’s ecosystems. As they waddle their huge bodies to and from rivers, the hippos are eroding riverbanks and carving out muddy paths that divide forests. Moreover, they are outcompeting other animals for habitat and resources, with the West Indian manatee (Trichechus manatus), neotropical otter (Lontra longicaudis) and capybara (Hydrochoerus hydrochaeris) most under threat.Exploring solutionsWith serious attacks on humans in 2020 and 2021, and a car crash leaving a hippo dead on the highway in April, solutions are needed, scientists say.One strategy currently being tried is to administer contraceptives to the animals by dart. That might eventually get rid of the hippos by preventing their reproduction, but it is slow, costly and hasn’t been tested on hippos on such a scale before. A modelling study published in April2 estimated that this method could eradicate the hippos in 45 years at a cost of at least US$850,000.
The world’s species are playing musical chairs: how will it end?
Another strategy — capturing, anaesthetizing and transporting the hippos by helicopter to a facility to be castrated — would cost at least $530,000 and take up to 52 years for eradication, the study found. Both calculations are underestimates, given that the models were fed earlier, lower estimates of the hippo population, before the census results had been released.Meanwhile, the regional agency responsible for dealing with the hippos is strapped for funding and relying on donations of contraceptives from the United States. Agency officials are negotiating with countries, including India and Mexico, to export some of the animals to sanctuaries abroad. But this would cost $3.5 million, according to those working on the strategy.There is no single best solution, says Olga Montenegro, a biologist at the National University who worked on the census. The hippos live in various groups, some of which might be easily relocated, but others are well established and are breeding quickly.Many researchers advocate culling the animals. They say it is the swiftest, most humane thing to do and that it would solve the problem before it becomes impossible to fix. The cost of killing the hippos must be weighed against that of losing native flora and fauna in Colombia — the second-most biodiverse country in the world — they add.Nature asked the environment ministry how it would manage the growing hippo population, given the new findings, but it did not comment.“There is a moral weight to the decision to cull a hippo. But the weight of the other decision — inaction — is far greater,” Moreno says. “I hope this is something the politicians will understand.” More

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Extreme weather, such as has occurred in Bangladesh, is having a disproportionate impact on vulnerable communities.Credit: Tanbir Miraj/AFP/Getty

How many biophysical boundaries does our planet have? What are the limits of, say, carbon dioxide emissions, ocean acidification, chemicals and air pollution beyond which existence becomes unsafe for Earth and its inhabitants?
A safe operating space for humanity
Back in 2009, a team of researchers led by environmental scientist Johan Rockström grappled with these questions in an article published in Nature (J. Rockström et al. Nature 461, 472–475; 2009). In the researchers’ view, planet-altering human activities could be assembled into nine groups. Thresholds were calculated for most of them, beyond which the result could spell danger for the planet and its people. The scientists concluded that humanity has crossed three of these nine ‘planetary boundaries’, and that the remaining six would also be crossed unless remedial action was taken.That article, called ‘A safe operating space for humanity’, has been extraordinarily influential in a relatively short time. Cities around the world have been experimenting with how to apply the findings, and researchers (including many in the original 2009 team) have continued to refine the planetary boundaries in response to feedback and new data.A gap in the original concept was that it lacked environmental justice and equity — it needed to take into account the fact that everyone, especially the most vulnerable, has an absolute right to water, food, energy and health, alongside the right to a clean environment.This week, Rockström, together with sustainability scientist Steven Lade and a team of researchers, have modified their original concept to incorporate justice alongside the biophysical boundaries. The resulting findings, which build on a study published in March in Nature Sustainability (J. Gupta et al. Nature Sustain. https://doi.org/grwfbk; 2023), show that seven out of eight thresholds have been crossed (see ‘Planetary boundaries reboot’): the eight are climate, natural ecosystem area, ecosystem functional integrity, surface water, groundwater, nitrogen, phosphorus and aerosols.

Planetary boundaries reboot. The concept of ‘planetary boundaries’ has been updated to take into account the fact that everyone, particularly the most vulnerable, has an absolute right to water, food, energy and health, alongside the right to a clean environment. The red lines indicate a limit to what is ‘safe’ for the planet. The green space represents the threshold that is both safe for the planet and protects the world’s most vulnerable populations (‘safe and just’). The Earth-shaped icons show how, in seven of eight cases, thresholds for a safe and just world have already been crossed.Source: J. Rockström et al.

Stark warningThe findings are an even starker warning than were those reported in 2009. Arguably, the most striking change since 2009 is that the authors advocate that global warming should be limited to 1 °C above pre-industrial levels. This is lower than the 1.5 °C target agreed at the 2015 Paris climate conference. It is also a stricter constraint than the recommendation in the 2009 study to keep carbon emissions to 350 parts per million by volume (the pre-industrial value was 280 parts per million by volume).
Read the paper: Safe and just Earth system boundaries
The authors reason that keeping to 1.5 °C might well enable the world’s more-affluent people to protect themselves, but it would create significant harm for the most vulnerable. The researchers estimate that some 200 million people would be exposed to unprecedented temperature increases and that 500 million people would be exposed to long-term sea-level rise.In incorporating ideas of justice into their research, Rockström and colleagues build on a body of recent work. Not long after the 2009 paper was published, justice and equity were included in discussions that led to the United Nations Sustainable Development Goals (SDGs), announced in 2015. Environmental justice is also at the core of an innovative idea called doughnut economics. In 2012, economist Kate Raworth, then working for the aid agency Oxfam, and colleagues adapted the 2009 study’s findings to include what Raworth called a “safe and just space”. This space was represented by an area enclosed by a double circle, the ‘doughnut’, whose boundaries could not be crossed. Such a threshold would be measured using the indicators and methodology underpinning the SDGs.
How to identify unjust planetary change
The latest study shows how members of the 2009 team, working with a new generation of scientists and a more multinational team, have risen to the challenge of triangulating their original work with doughnut economics and the SDGs. This has not been easy, and the new work is very much an initial step. In an accompanying News & Views article, Stephen Humphreys, who studies law and social justice at the London School of Economics, acknowledges the difficulty of setting numerical values when integrating ideas from the natural and social sciences. Readers can see how this process worked: alongside the paper, we are publishing the full discussion between authors and reviewers (J. Rockström et al. Nature https://doi.org/10.1038/s41586-023-06083-8; 2023). It underscores where there is agreement and disagreement, and where more data and further refinement will be needed.This is a wonderful example of how well science functions when different teams study and refine each others’ work. But there is a troubling aspect to the new findings, too. If seven of the eight thresholds have been crossed, what does that mean for our still-feeble efforts to move to a more sustainable path?Researchers vary widely in their views on how this question should be addressed — from those who advocate working within the current economic system (known as green growth) to those arguing that the present economic system was itself a factor (if not the defining factor) in bringing about the present situation and requires transformation (known as post-growth or degrowth). Some months ago, we urged scientists representing these different approaches to forge more channels of communication between them.The paper we are publishing represents one such opportunity. If the findings are anything to go by, there is no time to lose. More

Definitions of marine heatwaves currently depend on baseline temperatures (see D. Amaya et al. Nature 616, 29–32; 2023). But use of these baselines fails to capture the impact of marine heatwaves on marine organisms (A. Sen Gupta et al. Nature 617, 465; 2023). To better inform adaptation measures to ocean warming, we suggest using a definition that derives from the effects on marine ecosystems. Lessons could be drawn from human responses to heat.
Competing Interests
The authors declare no competing interests. More

Golden snub-nosed monkeys (Rhinopithecus roxellana) are an endangered species, only found in China.Credit: Thomas Marent/Nature Picture Library

China’s government is the first to use satellites to monitor land set aside for conservation to ensure its protection from illegal development. Scientists hope that the move will safeguard ecologically important habitats and provide a model of remote-sensing use for conservation that other countries could follow. But they also have questions about how the nation has decided which areas to protect and where the boundaries, known as the ecological redlines, lie.“The decision makers have made a really bold step forward,” says Chi-Yeung Choi, an applied ecologist at Duke Kunshan University in Suzhou, China. He says that having a national system to protect ecologically important areas will stop provincial governments from prioritizing development over conservation. The policy “has really huge potential to conserve biodiversity hotspots”, he says.As a vast nation, China spans diverse habitats such as grasslands, forests, deserts and mountains. It is home to iconic species including the giant panda (Ailuropoda melanoleuca), south China tiger (Panthera tigris tigris) and snub-nose monkeys (Rhinopithecus spp.), as well as tens of thousands of lesser-known species. The International Union for Conservation of Nature (IUCN) counts it as the third most-biodiverse country in the world.On 22 April, China’s Ministry of Natural Resources announced that its ecological-redline map was finalized. Conservation areas in each mainland province are now organized into a central system managed by the ministry. Previously, China’s conservation zones included 2,750 nature reserves and thousands of other areas protected by different levels of government.According to the announcement, the protected zones cover 3 million square kilometres of land — approximately 30% of mainland China — and 150,000 square kilometres of sea. That’s in line with the 2022 Kunming–Montreal Global Biodiversity Framework target to have at least 30% of terrestrial and inland water areas conserved by 2030. Zhijun Ma, a conservation biologist at Fudan University in Shanghai, China, says that the redline map doubles the area that is legally protected from development.Lack of transparencyFour days after the redline map was finished, the Ministry for Ecology and Environment announced the launch of a monitoring platform to police the network of zones. It said that a fleet of 30 satellites has been launched to capture high-resolution images, with an algorithm designed to automatically detect changes to forest cover and land use within the redlines.The satellites are the space component of a three-layered monitoring network, referred to as ‘space, sky and land’, says Fangyuan Hua, a conservation biologist at Peking University in Beijing. Sky and land refer to drones and on-ground personnel that investigate human activity in the protected zones. The aim is to enable authorities to act swiftly if they detect illegal activity, such as land clearance for new mines or real-estate developments.But environmental scientists keen to research how effective the zoning will be are frustrated by a lack of transparency from the government. “Even though the map has been finished across the country, we actually don’t know what the map looks like,” says Hua. Without a public record of the conservation boundaries “there’s a possibility that the local governments might be able to shift their redlines to accommodate future [development] needs”, making it impossible for the public to monitor what’s going on, she says.Last year, Choi and his colleagues used a draft of the map to show that three times as many coastal sites that were important for waterbird conservation — around 75% of 172 sites — would be protected by the new plan than under its predecessor, the national nature reserve system. “Assuming that things will go as planned, then there’s actually a huge increase in the number of sites and the amount of areas that can be protected,” he says. But Hua says that without access to final versions of the map, few studies like Choi’s will be able to assess redline protections.Alice Hughes, a conservation biologist at the University of Hong Kong, says that other nations could follow China’s example, but “China also should take steps to make the data it is collecting more available.”China’s Ministry of Natural Resources did not respond to Nature’s request for comment on when the maps will be publicly available.Ecosystem servicesChina’s approach to defining its redlines differs from how many countries set aside protected land. Generally, decisions are based on an area’s value to wildlife or plants. The IUCN divides protected areas into seven categories that differ in size and level of protection. China’s redline map includes habitats that are identified as being important for protecting vulnerable species and ecosystems, but also draws on assessments of ecosystem services, a measure of an area’s benefits to humans. For example, the metric attributes value to areas with ecosystems that sequester carbon (such as trees), store or purify water, prevent soil erosion or desertification, and protect biodiversity. Including ecosystem services as a criterion for protection “is a very uniquely Chinese thing”, says Hua.Using these measures to justify land protection might be appealing to local governments, says Hughes, because preventing events such as landslides or sedimentation in waterways brings financial benefits. Overlapping conservation and economic priorities can lead to “mutual benefits”, she says.But areas worth protecting for their human benefit might not overlap with areas worth protecting for their biodiversity, says Hua. And whereas ecosystem services can be readily assessed using remote-sensing data — to evaluate canopy cover for carbon sequestration, for example — the same isn’t true for mapping biodiversity, she says. That requires greater on-the-ground assessments, which are more expensive to conduct.Hughes says that more ecological surveys are needed anyway, because detailed data on species vulnerability and distribution are lacking in China. As a result, conservation areas tend to favour large, charismatic animals such as the panda and snow leopard (Panthera uncia), and overlook less-celebrated species, especially in the southern parts of the country, for which the data are limited.Unlike most of the IUCN categories of protected areas, which strictly limit human activity, the redline zones often allow some human activity within the borders, says Hua. For example, people might be permitted to live or cultivate crops in certain areas. “The network of redlines is mostly intended to prevent large-scale developments, like mining and construction,” she says.The satellite monitoring system will be “a very powerful and effective system, if it can be done properly,” Choi says. And if the information on it is made open, researchers will also be able to interrogate land-use changes across the country. More

Researchers have discovered a treasure trove of arthropods such as these on the sea floor in the Clarion–Clipperton Zone, located in the central and eastern Pacific Ocean.Credit: SMARTEX Project, Natural Environment Research Council, UK (smartexccz.org)

A company is expected to request authorization in July, for the first time ever, to mine the ocean floor for metals such as cobalt and nickel. At the same time, researchers warn that a crucial database that maps deep-sea biodiversity and that could factor into the decision to approve such a licence contains errors and data gaps.
Seabed mining is coming — bringing mineral riches and fears of epic extinctions
The International Seabed Authority (ISA), a body associated with the United Nations that oversees deep-sea mining in international waters, currently allows only mining exploration. According to its website, it has approved 17 companies and government entities to study the mining potential of the Clarion–Clipperton Zone (CCZ), a region of the sea floor that spans up to 6 million square kilometres of the central and eastern Pacific Ocean and that holds metal-rich clumps of sediment. Nauru Ocean Resources, a subsidiary of The Metals Company, based in Vancouver, Canada, has been exploring the sea bed, with an eye towards gathering metals needed for electric-vehicle batteries and other electronics. It plans to apply for a commercial mining licence in a month or so. If approved, operations could begin in 2024.Scientists worry about allowing companies to start mining the sea bed because little is known about deep-sea habitats and biodiversity, so its environmental effects are unpredictable.The ISA runs a database called DeepData, which is meant to tackle some of these concerns, as well as to enable research projects. The database contains information that the ISA requires contractors to collect during their deep-sea exploration missions. These biological, geochemical and physical data include, for example, the species that they encounter, and the chemicals present in the water.But the analysis of DeepData, published in the journal Database on 30 March1, revealed flaws that worry the researchers who conducted the study.

Contractors would like to mine the sea bed for metal-rich clumps of sediment called polymetallic nodules.Credit: Courtesy of the NOAA Office of Ocean Exploration and Research, 2019 Southeastern U.S. Deep-sea Exploration.

“It strikes me as irresponsible to be relying on the database in its current form” to assess the impact of mining on the sea-floor environment, says Muriel Rabone, a data scientist at the Natural History Museum in London, who led the analysis. Rabone told Nature that the analysis was performed independently of the ISA, but that the agency cooperated to enable data access. It was also consulted on the scope of the study and an early draft of the manuscript.The ISA protests some of the findings, however, saying that the report is out of date. The researchers downloaded data collected in the CCZ on 12 July 2021 to run their analysis. Since then, the ISA has made “significant improvements” to address quality assurance and control issues with DeepData, it says.Responding to this criticism, Rabone maintains that the database still contains flaws. Even with its faults, it’s helping to point to thousands of species on the sea floor that had never been seen before — results published just this week. “There is work to do yet,” she says.Data gapsOf the 40,518 records that the researchers analysed for the Database study, about one-quarter were duplicates, which could lead to an underestimation of species richness in the deep sea, they say. The scientists think duplicates can arise partially because the database lacks unique codes to identify individual records.The ISA says that, like any other database, DeepData’s “features and the quality of its data are improving with the years due to technological advances”. It adds that it has identified and corrected duplicate records. Also, it is collaborating with the World Register of Marine Species, which catalogues and classifies marine organisms, and is sharing data with the Ocean Biodiversity Information System — a data hub that has helped to clean up the data and make it more widely available.

Brisingid sea stars like this one also live on the sea floor in areas rich with metals.Credit: Courtesy of the NOAA Office of Ocean Exploration and Research, 2019 Southeastern U.S. Deep-sea Exploration

Looking at the database today, however, Rabone says that some duplicate data still exist, and that many records still do not have a unique identifier.The team also found that DeepData contained inconsistent information — for instance, records that catalogued two species under the same name. And a lot of environmental data were missing. When contractors submit their data, they use a form with fields such as species name and fauna class size. The researchers found that 90% of the total data in various fields were missing.The ISA says it has already updated its forms to address some of these issues and is designing workshops and training for contractors to ensure that data quality and control are improved.
Scientists track damage from controversial deep-sea mining method
Rabone would like the workshops to be open to the scientific community, which she says can provide feedback on the database. Stefanie Kaiser, a deep-sea ecologist at Senckenberg Research Institute in Frankfurt, Germany, who was not involved with the study, agrees, and says that if the database were improved, it could be useful for researchers, giving them access to all the information collected by the contractors.But the ISA says workshops are for contractors only, because they provide the data, although it acknowledges that the academic community has assisted contractors with presentations and preparing annual reports.Despite the disagreements over DeepData, researchers are already learning from the database. Rabone formed an official partnership with the ISA to lead the first census of metazoan biodiversity on the CCZ’s sea floor. The endeavour found more than 5,500 species in the region, of which 92% are new to science, including many worms and arthropods. The findings were published on 25 May in the journal Current Biology2. More

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In this episode:00:45 Tree islands bring biodiversity benefits for oil-palm plantationGlobal demand for palm oil has resulted in huge expansion of the palm plantations needed to produce it, causing widespread tropical deforestation and species loss. To address this, researchers planted islands of native trees among the palms in a large plantation, and showed that this approach increases ecosystem health, without affecting crop yields. The team say that while protecting existing tropical rainforests should remain a priority, tree islands represent a promising way to restore ecosystems.Research article: Zemp et al.09:42 Research HighlightsThe oldest identified ‘blueprints’ depict vast hunting traps with extraordinary precision, and fossil evidence that pliosaurs swimming the Jurassic seas may have been as big as whales.Research Highlight: Oldest known ‘blueprints’ aided human hunters 9,000 years agoResearch Highlight: This gigantic toothy reptile terrorized the Jurassic oceans12:08 Briefing ChatWe discuss some highlights from the Nature Briefing. This time, how shredded nappies could partially replace sand in construction, and how CRISPR helped crack the mystery of the death cap mushrooms’s deadly toxin.Nature News: World’s first house made with nappy-blended concreteNature News: Deadly mushroom poison might now have an antidote — with help from CRISPRSubscribe to Nature Briefing, an unmissable daily round-up of science news, opinion and analysis free in your inbox every weekday.Never miss an episode. Subscribe to the Nature Podcast on Apple Podcasts, Google Podcasts, Spotify or your favourite podcast app. An RSS feed for the Nature Podcast is available too. More

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