Who are the real pests?

Since Charles Darwin sailed around South America less than 200 years ago – a blip in the Earth’s history – the human population has mushroomed exponentially, from less than 1.2 to around 3.5 billion in 1968 and now approaching eight billion people.

This uncontrolled population growth is the inconvenient truth we can no longer skirt around, as David Attenborough recently warned. Now, two papers have highlighted its impact on wildlife through disrupted ecosystems.

People and livestock comprise a whopping 97% of the global mammal biomass, notes one group of scientists in the journal Frontiers in Ecology and Evolution, and our tentacles have infiltrated the natural balance on every continent.

“We have affected most life forms through climate modification, harvest, erasure and fragmentation of habitat, disease, and the casting of alien species,” the international team of field scientists writes.

Led by Joel Berger from Colorado State University, US, they each drew from their own work on different continents, merged with other research, to document changes among and between species that impact native ungulates.

These are hoofed mammals ranging from the tiny Javan mouse deer (Tragulus javanicus) to the seven tonne African elephant (Loxodonta Africana), that are embedded in complex predator-prey and broader ecological dynamics.

The impacts the team identified ricochet throughout the biosphere.

Changing climates have caused toxic algal blooms in the Patagonian Pacific, in turn decreasing fish available for harvest. In search of other food sources, fishermen use dogs to hunt huemul (Hippocamelus bisculcus), a beautiful and now rare species of deer that has dwindled to 1% of its former range.

Receding ice in once-pristine areas of the Himalayas has attracted human colonisation, along with stray and feral dogs that hunt rare, endangered ungulates including kiang (Equus kiang), chiru (Pantholops hodgsonii), saiga (Saiga tatarica) and takin (Budorcas taxicolor). They’ve also spread diseases and driven away snow leopards (Panthera uncia).

The multi-billion-dollar fashion industry’s appetite for cashmere has accelerated breeding of domestic goats – mostly in Mongolia, China and India – which compete with native ungulates for food and are in turn at risk from feral dogs.

Feral pigs have spread to every continent except Antarctica, and in 70% of US states, disrupting fish, reptiles, birds and other small mammals, plants and soils.

In the US, a century of intensive livestock grazing in the Great Basin Desert has disrupted plant communities, encouraging mule deer (Odocoileus hemionus) populations which attracted pumas (Panthera concolor) and have thus permanently changed predator-prey dynamics.

The impacts the team identified ricochet throughout the biosphere.

Colombia is now home to wild hippos, Australia has banteng, and New Mexico has gemsbok (Oryx gazelle) and Barbary sheep, while Burmese pythons contributed to the downfall of white-tailed deer and reconfigured the Everglades food web.

It’s a mess, they say, and future repercussions are unpredictable. What we do know is that things can never go back to the way they were.

“For many assemblages of animals, we are nearing a moment in time, when, like Humpty Dumpty, we will not be able to put things back together again,” says Berger.

In an effort to conserve biodiversity, protected areas have been implemented worldwide; however, alien species threaten to infiltrate them, according to the second paper published in the journal Nature Communications.

Along with habitat destruction, pollution and CO₂ emissions, alien species are one of the top five major threats to global biodiversity loss, says co-author Tim Blackburn from University College London, UK – and they are becoming more pervasive.

He and a team of Chinese researchers, including lead author Xuan Liu from the Chinese Academy of Sciences, investigated nearly 900 terrestrial animal species, including mammals, birds, reptiles and invertebrates, known to have become established in new environments around the world.

They then determined if these species live within or near the boundaries of nearly 200,000 protected areas, defined by the International Union for Conservation of Nature, including wilderness, national parks and natural monuments or features.

Encouragingly, less than 10% of the protected areas, which cover about 15% of the Earth’s land surface, currently contain alien species, suggesting they are doing their job. However, nearly all are at risk of invasion – 99% of protected areas had alien species within 100 kilometres of their boundaries, and 89% within a mere 10 kilometres.

This is a problem, because the team determined that more than 95% of protected areas have environments within which at least some of the alien species could flourish once they get in.

Tellingly, areas that already contained alien species were those with a larger human footprint due to transport links and high human population density nearby.

“At the moment, most protected areas are still free of most animal invaders, but this might not last,” says senior author Li Yiming. “Areas readily accessible to large numbers of people are the most vulnerable.”

Alien species that have infiltrated include the cane toad (Rhinella marina) in 265 protected areas including Australia’s Kakadu National Park, and the Harlequin ladybird (Harmonia axyridis) and wild rabbit (Oryctolagus cuniculus) in 2686 and 1673 protected areas, respectively.

The most invaded parks were found in Hawaii’s Volcanoes National Park, Hakalau Forest National Wildlife Refuge and Kipuka Ainahou.

“Human activities are putting lots of different pressures on the natural world,” Blackburn says. “The so-called developed nations are the worst offenders, as the richest 10% of humanity uses 50% of current resources.

“We urgently need to move to a way of living that is not destroying the life support systems that we depend on.”

Berger’s team says although food webs and ecosystems are changing, with concerted efforts all is not yet lost.

“Basically, the world is changing, it has changed, and it will continue to change,” he says. “From a more positive side, though, there are so many places worth protecting that even with change, they still reflect processes, species and hope.”