Current biodiversity, the result of 3500 billion years of evolution , is the richest in the history of life on Earth. However, scientists are warning of a critical event: the sixth mass extinction . An international team of experts, in a study published in the journal Science , has pointed to the drastic loss of fauna and the accelerated extinction of species, which appear to be the first signs of this event, which they call the " defaunation in the Anthropocene ."
Since 1500, more than 320 species of terrestrial vertebrates have gone extinct , and a quarter of the remaining species have seen their populations decline. The situation is equally worrying for invertebrates, such as butterflies, spiders, and worms, whose populations have fallen by up to 45% in the last 35 years. Unlike previous extinctions caused by natural events, this time scientists point directly to human activity as the main cause.
Causes of the Sixth Mass Extinction and the Anthropocene
Unlike previous mass extinctions, the sixth extinction is unfolding during the current era, known as the Anthropocene . The term, coined by Nobel laureate Paul Crutzen, describes the era in which human activity has left a significant mark on Earth's natural systems. The main causes of the sixth extinction include habitat destruction , overexploitation of species , pollution , and climate change.
Overexploitation of species often affects large animals , or megafauna, such as elephants, rhinoceroses, and polar bears, which require large areas to maintain viable populations and whose reproduction rates are typically slower than those of smaller species. Furthermore, their size makes them prime targets for hunting. Although these animals represent a small percentage of endangered species, their disappearance could have destabilizing side effects on ecosystems, including the proliferation of disease-carrying species.
A clear example of this was observed in experiments in Kenya, where the disappearance of megafauna led to an increase in the rodent population, which in turn increased the abundance of disease-carrying ectoparasites. Rodolfo Dirzo, one of the report's authors, explains that the loss of large animals "can trigger a vicious cycle that affects the health of other organisms, including humans."
Previous Great Extinctions
A mass extinction is defined as the disappearance of at least 75% of species in a relatively short period of time in geological terms. Five major extinctions have occurred in Earth's history, the most well-known being the extinction that wiped out the dinosaurs 66 million years ago , likely caused by a meteorite impact on the Yucatán Peninsula. However, this was not the most devastating.
The Permian-Triassic extinction , which occurred approximately 250 million years ago, is known as the "Great Dying" and was responsible for the disappearance of up to 96% of marine species and 70% of terrestrial species. By comparison, current rates of species loss are beginning to approach dangerously close to those observed in these events, suggesting that we are at the beginning of another biological cataclysm.
Impact of Human Activity on Biodiversity
Human intervention has profoundly changed ecosystems. Our species' ability to modify the environment began with the domestication of plants and animals some 11.000 years ago. However, with the Industrial Revolution and exponential population growth, the environmental impact has grown unprecedentedly.
Agricultural and livestock activities have led to the massive deforestation of large areas of forest, especially in tropical regions. For example, logging has reduced the tropical rainforests of Africa and Latin America, which are home to extremely rich biodiversity, by 20%. At the same time, industrial agriculture has increased greenhouse gas emissions, including methane and carbon dioxide, contributing to climate change.
Climate change directly impacts ecosystems , altering rainfall patterns, modifying seasons, and increasing the frequency and intensity of extreme weather events. This leads to habitat loss and puts millions of species at risk, especially those with limited habitats or that cannot adapt quickly to these changes.
The Role of Invertebrates in Ecosystems
Often, when extinction is discussed, the focus tends to be on large mammals or birds. However, invertebrates play a crucial role in maintaining ecosystems. Insects such as bees, butterflies, and other pollinators ensure the pollination of 75% of the world's crops , which is invaluable for agriculture and food security.
In addition to pollinators, invertebrates also play a vital role in nutrient cycling, the decomposition of organic matter, and pest control. In the United States, the value of pest control provided by natural predators is estimated at $4500 billion annually . If these species continue to disappear at current rates, the consequences for humans could be devastating.
Consequences for Human Health
Defaunation not only affects biodiversity but could also have serious implications for human health . As mentioned earlier, the disappearance of keystone species can facilitate the proliferation of rodents and other animals that act as disease vectors. The increase in rodents in densely populated areas has already proven to be a problem, increasing exposure to pathogens such as those that cause bubonic plague or hantavirus.
Scientists also suggest that declining biodiversity is linked to an increase in global pandemics . As humans encroach on natural ecosystems and have closer contact with wildlife, the risk of zoonotic transmission—diseases that jump from animals to humans, as is suspected to have happened with Ebola and even COVID-19—increases.
Solutions and Actions to Prevent Further Defaunation
Given this bleak outlook, what can we do? Experts agree that there are several ways to help curb this sixth mass extinction . First, reducing the overexploitation of resources and land-use change is key. Preventing habitat destruction and ensuring the conservation of protected areas can provide safe havens for many species.
Furthermore, it is essential that conservation efforts focus on both megafauna and smaller species. Measures should include reforestation, restoration of degraded ecosystems and reduction of carbon emissions. The development and implementation of sound environmental protection policies are also essential.
Finally, education and awareness-raising play a key role. Local communities can be the greatest defenders of biodiversity if they are provided with the knowledge and resources to manage and value their environment. Community-led conservation movements are essential to changing the course towards a more sustainable future.
Although the current outlook is worrying, there are still opportunities to reverse some of the damage done. Collective action by governments, scientists and citizens could be the key to stopping this crisis before it becomes irreversible.