The COVID-19 pandemic has led to global issues that were previously high on the list of urgent concerns, such as climate change and biodiversity loss, being moved to the backburner. However, these threats are no less pressing today than they were before the coronavirus hit. For the Conference of the Parties to the Convention on Biological Diversity, which in 2021 is due to set new guidelines for the coming decades, some experts have suggested setting a target of less than 20 extinct species per year. Interestingly, however, not all conservation efforts are focused on saving living beings; sometimes the purpose is the opposite: to exterminate them. Certain invasive species, or others that have proliferated excessively and threaten valuable ecosystems, are now the focus of eradication campaigns, like an ecological version of amputating a leg to save a life.
Possum, New Zealand
The possum is a friendly-looking mammal, especially recognizable by the female’s habit of carrying a clutch of young on her back. But while not many food sources fall outside its omnivorous diet, it is also a voracious predator. In New Zealand, the common brushtail possum (Trichosurus vulpecula) was introduced from its native Australia from 1837 in order to support a fur trade, a mistake that today is deeply regretted. These marsupials have now colonized the entire country, competing for food with native species, devouring eggs and chicks from previously widespread birds such as the kōkako or the kea, and spreading bovine tuberculosis, without their natural predators, wild cats, able to control their populations. Four official New Zealand agencies are involved in a genuine crusade to exterminate these animals, as part of a major strategy to rid the country of introduced predators by 2050.
Cane toad, Australia
Back in 1935 it seemed like a good idea. Sugar cane farmers in Australia were suffering huge losses to their crops due to the native beetles that were decimating their plantations. In tropical Central and South America, sugarcane pests were kept at bay by a voracious insect predator, the cane toad (Rhinella marina). What better than to import these creatures into the big island nation? The problem is that the behaviour of a species cannot be controlled once released into the wild. And while the toads did eat some cane beetles, the truth was that they preferred other insects.
With the exponential growth of toad populations, which have already occupied 20% of the country, other species have been harmed, including native frogs, which are losing their habitat to these American invaders. Even worse, the introduced amphibians came with an extra disadvantage—they are poisonous, so their predators often fare poorly. Species native to Australia, such as monitor lizards, snakes and marsupial cats, have seen their populations greatly reduced in some places due to toad toxins. So far, attempts to eradicate this pest in Australia have been unsuccessful. Meanwhile, some local artisans use these animals to make bags, key chains and other objects.
Crown-of-thorns starfish, Australia
Seen on the seabed, the crown-of-thorns starfish (Acanthaster planci) is a curious sight, a large disc studded with spines with arms radiating from the central body, and all this sometimes in bright warning colours. But this starfish, unusual among echinoderms for having lost its pentaradial symmetry (five arms) and with sophisticated eyes at the tip of its seven to 23 arms, is a ferocious scourge for the hard corals it feeds on. Since the 1960s, scientists have warned of an explosive increase in the population of these animals in Australia’s Great Barrier Reef, which has been attributed to increased nutrient runoff to the sea from agriculture and sewage, where it supports the growth of sea star larvae. It is estimated that 42% of the loss of the Great Barrier Reef is due to these spiny predators, so the Australian government and conservation organisations are fighting hard against them.
Monk parakeet, Spain
Many biological invasions have their origin in the trade in exotic species sold as pets, which end up making their way into nature. Some do not survive in environments very different from their own, but others end up invading the world. An example is the monk parakeet, also called the Quaker parrot (Myiopsitta monachus), a bird native to subtropical South America that today is invasive across large regions of the planet, including temperate or cold climate areas where it has adapted marvellously, and where its abundant reproduction and aggressive behaviour threaten native species, not to mention that its immense collective nests can sometimes bring down trees and kill anyone unfortunate enough to be standing underneath. In Madrid, which has been invaded by some 12,000 monk parakeets, the city council has launched a plan to eliminate these birds that endow parks and gardens with a noisy, tropical vitality, but are destroying the biodiversity of the urban environment.
House mice, Gough Island
Gough Island or Gonçalo Álvares is a place unknown to most, and not without reason; this British overseas territory is a desolate piece of volcanic rock with an area of 91 km2 located in the South Atlantic, uninhabited and with an inhospitable climate similar to that of Scotland. However, its limited attraction to humans has made it a paradise for eight million seabirds, which in 1995 led it to being designated a World Heritage Site by UNESCO. But this Manhattan of birds is threatened by a lethal pest: the simple house mouse (Mus musculus), unwittingly introduced in the 19th century from the ships of seal hunters. Since then, the rodents have become strong there, and in the most literal sense; due to a curious evolutionary phenomenon of insular gigantism, Gough’s mice are now the largest in the world, with a size that is twice as big as normal. In 2007, scientists warned that these brutes were driving species like the Tristan albatross or the Atlantic petrel to extinction. The mice devour not only the chicks, but also the adult birds. In 2018, an ambitious plan was conceived to launch poison from helicopters in order to make the island mice-free by 2022.
It is not one of the most worrying cases of invasive species for global ecology, but it is one of the most unique, and also one of the thorniest because of the animals involved: hippopotamuses. When Colombian drug lord Pablo Escobar was gunned down by police in 1993, his property was seized by the government, including his private zoo. Most of the animals were removed from the estate, but the four hippos were left to fend for themselves, and they eventually escaped. Today, there are more than 80 animals living around the old Escobar ranch and the Magdalena River, and Colombia has not yet been able to decide what to do with them. These are dangerous animals for humans, and they are profoundly modifying their ecosystems, but while some experts warn of the risks involved, others say their presence could help with ecological regulation, so the debate goes on.