We are in the early stages of the Earth’s sixth mass extinction, according to a study from Stanford University. And while previous extinctions have been caused by natural planetary transformations or asteroid strikes, it seems that humans may be responsible for this one.
Biologists have drawn a chilling connection between the decline of animal populations and the knock-on effects it could have on human health – with risks including plague epidemics in densely populated areas.
Up to 33% of all vertebrates species are estimated to be threatened or endangered globally. Now a team of scientists, led by Stanford biology Professor Rodolfo Dirzo, has revealed how, through a complex chain of cascading effects, human lives in large numbers could be at stake if we don’t ensure the survival of these animals.
“We tend to think that extinction is a phenomenon that will affect a particular population,” says Dirzo, who coined the term ‘defaunation’ describing the decline of animals as a consequence of human impact. But if one animal population is driven to local extinction, the effects on the ecosystem could scale up all the way to a global level, the scientist warns.
Experiments conducted by Dirzo and his colleagues in Kenya have studied how the absence of large animals such as zebras, giraffes and elephants impacts on the ecosystem. They observed that rather quickly, affected areas will be overwhelmed with rodents as seeds and shelter from grass and shrubs become more easily available and the risk of predation drops.
Consequently, the number of rodents doubles – as does the number of disease-carrying ectoparasites they harbour. Many of the pathogens the researchers found on the rodents in Kenya pose a threat to human health, including the bacteria that cause plague.
This could cause a disastrous chain of effects, particularly in densely populated areas, says Dirzo. “Where human density is high, you get high rates of defaunation, high incidence of rodents, and thus high levels of pathogens, which increases the risks of disease transmission.”
So what can we do to prevent an apocalyptic scenario of human populations being eradicated by rodents carrying the plague?
Defaunation is driven directly by hunting, poaching and illegal trade of animals and indirectly by changes in land use, which can reduce or isolate natural habitats, preventing native species from maintaining healthy populations. In an earlier study Dirzo and colleagues estimated that 50% of all mammal species could be placed under serious risk of extinction in the next 200 years.
Finding a solution is tricky, the scientist admits. Immediately reducing rates of habitat change and overexploitation would help. But these approaches need to be tailored to individual regions and also need do address rural poverty which often drives hunting, poaching and illegal trade.
While reforestation projects are already working to reverse the catastrophic effects of declining rainforests, Dirzo says we need to create a similar process of ‘refaunation’ – the restoration of endangered animal species and their habitats.
Clearly, such a process will take time and significant changes in human habits and activities. But maybe the awareness that the ongoing mass extinction will not only affect large, charismatic animals but could also wipe out human populations will provide the incentive to spur change.