Range roiled

The destructive impact of human activities on other animal species’ survival and biodiversity across the globe is increasingly clear. 

Urbanisation and habitat modification are well known culprits. But sporadic events such as hunting, recreation, military activity and aircraft have an even bigger impact, according to a meta-analysis of more than 200 studies.

The analysis focused on movements, which are essential for finding mates, foraging and hunting for food and avoiding predators. More broadly, animal movements help shape and sustain ecosystems through services such as pollination, seed dispersal and soil turnover.

Just how humans are disrupting this has been unclear, according to Tim Doherty from the University of Sydney, Australia, lead author of a study published in the journal Nature Ecology and Evolution.

“Lots of studies have been done on the impacts of humans on animal movement,” Doherty says. “But there have been few attempts to synthesise this data to uncover general patterns.”

Understanding this is vital, he explains.

“The consequences of changed animal movement can be profound and lead to reduced animal fitness, lower changes of survival, reduced reproductive rates, genetic isolation and even local extinction.”

The comprehensive study covered research that had tracked movements of 167 land and water species from every continent across the world. The creatures included mammals, birds, reptiles, fish, amphibians and insects, ranging from the lightweight sleepy orange butterfly (Eurema nicippe) to the hefty great white shark (Carcharodon carcharias).

The study found that human actions are disrupting animal movements for a wide range of species from all animal groups by forcing them to travel further or restricting their range. 

More than a third of cases increased or decreased their movements by 50% or more. For animals that moved further in response to disturbance, the average increase was 70%, while animals that moved less had an average decrease of 37%.

While 12% of these changes were attributed to habitat modifications such as logging, agriculture and urbanisation, more than a third were caused by sporadic disturbances.

“This tells us that humans have widespread impacts on animal movement, but in many cases these are going undetected and unaddressed,” says Doherty. “In everyday life, we generally only see animals in the wild for short periods and don’t get a proper understanding of how they move around and use space.”

For instance, squirrel gliders (Petaurus norfolcensis) living near residential areas of Brisbane, Australia, had smaller home ranges than those in the bush. Flightless weka (Gallirallus australis) near campgrounds in New Zealand moved 35% to 41% shorter distances than those further away, with implications for their seed dispersal capacity.

In the US, elk (Cervus canadensis) increased their movements in response to hunting, while Texas tortoises (Gopherus berlandieri) had reduced home ranges due to livestock grazing. River otters (Lontra canadensis) expanded their home ranges in areas polluted by an oil spill.

Norwegian moose (Alces alces) increased their range by 84% in response to military activity; their Swedish cousins run 33 times faster after being disturbed by cross-country skiers. Badgers (Meles meles) in Britain moved further to escape a culling program.

That sporadic activities caused larger changes in movement than habitat modification was surprising but understandable.

“This isn’t a result that we anticipated but it makes sense because those human activities are episodic in nature, which means that animals may move larger distances to escape these unpredictable disturbances,” says Doherty.

“On the other hand, habitat modification generally causes persistent changes to a landscape, which some animals can adapt to over time.”

The team says the study highlights the “global restructuring of animal movement”, underscoring the importance of addressing human impacts on their populations and minimising further disruption to natural habitats.

The results have important policy implications – and on the bright side, they suggest sporadic disturbances may be easier to address, such as curtailing activities in wilderness areas and avoiding breeding season or important activity periods. 

Overall, Doherty says, “our work supports calls for avoiding further habitat destruction and degradation, creating and managing protected areas, restoring habitat, and better managing human activities such as hunting, tourism and recreation”.