As the world population continues to grow, there will be increasing human-animal overlap on more than half of the Earth’s land area by 2070, according to a study from the University of Michigan.

Greater human-wildlife overlap could lead to more human-wildlife conflict, the UM researchers say. But understanding where that overlap is likely to occur—and which animals are likely to interact with humans in certain areas—will be critical for urban planners, conservationists, and countries committed to international conservation. Their findings were published in Scientific advances.

“We found that the overlap between human and wild populations will increase to about 57 percent of the world’s land area, but will only decrease to about 12 percent of the world’s land area. We also found that there will be a substantial increase in the overlap between agricultural and forest areas in the future,” said Deqiang Ma, lead author of the study and a postdoctoral fellow at the UM Institute for Global Change Biology in the School for Environment and Sustainability.

The study showed that the overlap between humans and animals is caused by population growth rather than climate change. That is, the increasing settlement of humans in previously undeveloped areas is the cause of the overlap, rather than climate change, which will cause animals to shift their habitat.

“In many places around the world, more people will come into contact with wildlife in the coming decades, and these wildlife communities will often consist of different species than those that live there today,” said Neil Carter, the study’s lead researcher and associate professor of environment and sustainability.

“This means that in the near future all sorts of new interactions – good and bad – will occur between humans and wildlife.”

A man-made problem

To calculate future human-wildlife overlap, the researchers created an index that combined estimates of where humans are likely to settle on land and the spatial distribution of 22,374 species of terrestrial amphibians, birds, mammals and reptiles.

They obtained information about the spatial distribution of vertebrates from previously published data that provide predictions about where species will live based on their climatic niches. Their estimates of where humans are likely to live were based on projections of economic development, global society, and demographics.

“The index we created found that most countries around the world will see an increase in human-wildlife overlap. This increased overlap is due far more to the expansion of human populations than to changes in species distribution caused by climate change,” Ma said.

In particular, the researchers found that the areas where there is high human-wildlife overlap now and is expected to continue to be so in 2015 and 2070 are concentrated in regions where population densities are already high, including China and India.

In addition to areas where there is already high overlap, “forests are another area of ​​great concern, particularly forests in Africa and South America, where we see a large increase in overlap in the future,” Carter said. “The reason for this is that these areas have very high biodiversity that will be under even greater pressure in the future.”

The researchers also found that mean species richness – the diversity of species in a given area – is expected to decline in most forests in Africa and South America. In South America, mammal richness is expected to decline by 33%, amphibian richness by 45%, reptile richness by 40% and bird richness by 37%. In Africa, mammal richness is expected to decline by 21% and bird richness by 26%.

The need for biodiversity

Maintaining biodiversity in these overlap zones has real benefits, says Carter.

“There are cases of human-wildlife interactions that are both good and bad, but we expect them to increase. COVID19, for example, was the result of human contact with wildlife, and there is concern that new diseases will emerge from increased encounters between humans and certain wildlife species,” he said. “But there are also species that provide important benefits to humans, such as reducing pest populations.”

For example, part of Ma’s data analysis looked at birds that eat insects in agricultural areas and where those birds will migrate as climate change changes. He found that more than two-thirds of the cultivated areas that are likely to see increased human-wildlife interactions by 2070 will see a decline in bird species that can help reduce pests.

“We look at these areas and ask ourselves if there is cropland or pasture there and whether there will be species that will move there or move away from those areas,” Carter said.

“Will these new croplands or pastures pose an additional threat to species or could the ecosystem services be used for free?”

Scavengers such as vultures and hyenas also play an important role by removing waste from urban areas and other landscapes, Carter said. By removing waste, scavengers can reduce the spread of some human diseases such as rabies, anthrax and bovine tuberculosis.

“Hyenas and other species that are demonized or persecuted because they are scavengers offer many benefits in disease control,” Carter said. “On the one hand, they are seen as a threat, but on the other hand, they provide free health benefits.”

Equal nature conservation

Future conservation strategies will need to evolve, particularly in regions that have previously been largely uninhabited by humans, the researchers say. In the past, a key conservation strategy has been to establish protected areas where access to humans is restricted, but this is becoming increasingly difficult to implement as there are fewer such places.

“There is also an important environmental justice argument about the legality of telling communities that may have lived in a particular area for generations that they have to move,” Carter said.

“Our study suggests that conservation planning needs to become more creative and inclusive as more and more areas of the world are expected to be shared by both humans and wildlife.”

Conservationists need to engage local communities to gain their interest in improving the conservation process. This process may include establishing habitat corridors to connect existing protected areas with potentially new areas or creating temporary protected areas during critical periods for wildlife, such as breeding seasons, and other conservation innovations.

“We care deeply about which areas can support populations of endangered species like tigers and how human communities interact with these species,” Carter said.

“In some places it will be really difficult to do everything at once: grow crops, create urban areas and protect these species and their habitats. But if we start planning now, we have many tools to help us promote sustainable coexistence.”

Study co-authors include Briana Abrahms of the University of Washington, UM ecologist Jacob Allgeier, Tim Newbold of University College London and UM evolutionary ecologist Brian Weeks.