American Southwest Drought Shrinks Wildlife Habitat Across the Food Chain

The American Southwest is enduring a historic drought. Communities are dealing with water limits, failed crops and reservoirs that are steadily running low.

Yet these same conditions are also affecting wildlife in ways that have attracted much less notice.

Drought is reducing the habitats available to animals throughout the food chain.

That loss curbs reproductive success and raises urgent concerns about the future of these species if droughts continue to intensify.

A new University of Michigan-led study analysed 12 years of GPS records for mule deer, black bears and cougars across Nevada and Utah, covering almost 518,000 square kilometres.

Its findings showed that drought had a substantial effect on all three species, irrespective of their position in the food web.

“The take-home message is that the effects of drought are huge and widespread. These results aren’t just from one small study system,” said study lead author Kirby Mills from the Institute for Wildlife Studies in California.

What the data shows

Researchers used data from more than 3,000 individual animals monitored from 2010 to 2022.

They intentionally selected three species with different diets: mule deer are herbivores, black bears are omnivores and cougars are carnivores.

Collectively, they represent the food web, enabling the team to identify patterns in the way drought influences animals with fundamentally different ecologies.

In periods of severe drought, the area of their most preferred habitat fell by at least 10% for every species.

The decline was roughly 10% for mule deer, 14% for black bears and 18% for cougars.

Why cougars were affected most

At first, the researchers were surprised by the pattern. It might seem logical that herbivores would be hit hardest by drought because of their direct reliance on vegetation.

“Cougars can’t just go and chomp on whatever they find that’s green like deer can,” Mills said.

“That means cougars have to work harder for their food and they’re more limited in their opportunities to find food, so their populations can be more sensitive to perturbations.”

When drought limits vegetation, deer have several possible responses. They may travel over a wider area, change their food sources or alter their diet.

Cougars, however, are dependent on hunting prey. When drought puts prey populations under strain and scatters them, cougars are affected disproportionately. Drought’s impact becomes greater further up the food chain.

Beyond individual behaviour

Predators face another source of vulnerability that is not simply about individual behaviour.

Population density naturally falls at higher levels of the food web: the study tracked more than 2,800 mule deer, compared with only 105 cougars.

As a result, impacts on individual cougars are more strongly felt across the wider cougar population.

A pressure that can be spread among thousands of animals in one species becomes far more acute where there are only hundreds.

Cascading effects of reproductive stress

The research found that drought influences reproductive success as well as habitat use.

During extreme drought, the number of new mule deer fawns born per doe may fall by more than 30%.

“What we’re seeing is that drought is having a major impact not just on habitat suitability, but also on fitness, on the survival of wildlife,” said Martin Leclerc, who co-led the study and is now an assistant professor at the University of Quebec at Chicoutimi.

A 30% fall in fawn survival, if maintained across several drought years, can place substantial pressure on overall population numbers.

The consequences then move upwards through the food chain: reproductive stress in deer reduces prey availability for the bears and cougars that rely on them.

A management problem

The study brings into focus an issue that wildlife managers are only starting to confront.

Climate, habitat, vegetation, predator-prey relationships and wildlife behaviour have traditionally been regarded as separate areas, overseen by different agencies using different frameworks. The growing body of evidence indicates that they cannot be treated in isolation.

Study senior author Neil Carter is an associate professor at the University of Michigan School for Environment and Sustainability.

“The study highlights the growing intersection of climate patterns, including drought and wildfire, with landscape planning and management, natural resource management, vegetation dynamics, wildlife behavior and management – all of these things that are often looked at separately,” said Carter.

“Now we’re finding that they’re enmeshed so tightly and that demands different management strategies moving forward.”

Implications of the study

One immediate implication is that deer management and cougar management can no longer be viewed as wholly separate issues.

If drought-related stress in deer populations is passed on to cougars in an amplified way, choices about managing deer also affect cougars - and the reverse is true as well.

“People are typically managing deer populations, not deer and cougar simultaneously, so I think there will start to be more conversation and communication around that,” Carter said.

More broadly, he maintains that planning to shield people from climate vulnerability is not being matched by equivalent efforts for wildlife.

The necessary tools and frameworks are available, while the data is becoming increasingly accessible. What is required is the commitment to combine them.

“There’s pretty robust planning going on for mitigating human vulnerability to climate change, but we don’t have the same level of planning for mitigating wildlife vulnerability. I certainly think there are opportunities to bring those together,” Carter concluded.