Bouncy, big-eared icons of the American West, deer are declining rapidly across Colorado and other states — forcing difficult decisions.

The causes vary from energy development to hard winters and aren’t always clear.

But dwindling numbers already have driven cutbacks on deer hunting, reducing potential funds for land conservation.

State wildlife biologists are scrambling to reverse the declines. This is spurring scrutiny of intensifying oil and gas drilling on federally managed deer habitat.

The nation’s largest mule deer herd, located in northwestern Colorado, has decreased over the past decade by more than two-thirds — to 32,000 at latest estimate, down from 105,900 in 2005.

DATA: Deer population in Colorado

Colorado Parks and Wildlife data also show a statewide deer population decrease since 2005 by about 36 percent. Across the West, deer decreased by at least 10 percent.

“Mule deer are an indicator species. If mule deer herds are in poor health, it probably means the land itself is in poor condition and that a lot of other species are at risk,” National Wildlife Federation public lands policy director Kate Zimmerman said. “For example, sage grouse and mule deer occupy the same habitat for part of the year, and both are in trouble.

“There are a lot of things we can do. We can get better in the types of activities we allow in deer habitat — like oil and gas development. We can get better in the ways we do oil and gas development in order to reduce the impact on deer habitat.”

Colorado-based wildlife biologists have pinpointed multiple factors driving deer declines:

• a one-two punch of hard winters followed by drought;

• commercial and residential development in the mountains;

• chronic wasting disease;

• aggressive fire suppression that leads to overly thick forests;

• coyote and mountain lion predation;

• more than 2,000 vehicle collisions a year in western Colorado;

• energy development that disrupts deer habitat and migration.

In response to the multi-year decline, Colorado wildlife managers have reduced the number of deer hunting licenses they offer from 130,106 in 2007 to about 80,000 for this year.

Hunters in Colorado kill 35,000 to 40,000 deer a year, said Chad Bishop, CPW assistant director for wildlife natural resources.

DATA: Demand for deer-hunting licenses

Next month, CPW leaders are staging a summit in Glenwood Springs to work on strategies to try to boost deer.

“We’ve had a significant decline that is not satisfactory to us and to the public,” Bishop said. “We’re going to work collaboratively with all of our constituents to increase mule deer numbers as best as we can. … If our deer continue to decline, it will hurt us financially.”

The extent of oil and gas development on public land “is a consideration,” he said. “We work closely with energy companies, like we would any entity — work with them on achieving mitigation as best we can when energy impacts occur.”

In Colorado, the latest CPW population estimates, provided in response to Denver Post queries, show a statewide decline in mule deer — the main deer in the West — down to 390,600 in 2013 from 614,100 in 2005.

Some of the decline may be because of changes in methods for estimating deer populations.

Across western states, deer decreased by about 10 percent overall between 2003 and 2009, said Arizona-based wildlife biologist Jim Heffelfinger, who chairs the Western Association of Fish and Wildlife Agencies’ Mule Deer Working Group, which draws expertise from 23 states and Canadian provinces.

Deer aren’t likely to join polar bears facing extinction, but the sharp downward trend requires concerted human action, Heffelfinger said.

“We certainly cannot have it all. We need to be smart about our wildlife habitat, especially our mule deer habitat and how we manage the population,” Heffelfinger said. “There are so many different things that are stressing mule deer around the West. Fire suppression has closed the forest canopies, and that has reduced the amount of shrubs and weeds that deer rely on.

“You don’t really like big, catastrophic fires — certainly where human structures are damaged. But we really need to open up the canopies for deer.”

Conservationists pointed to plans for increased oil and gas development on habitat of the already-declining White River herd in northwestern Colorado. Bureau of Land Management officials said they anticipate 15,000 wells will be drilled over the next 20 years on these 1.5 million acres in northwestern Colorado, located between Meeker and Rangely, south of the Yampa River and north of the Colorado River.

That land serves as habitat for other big game, too — and simultaneously holds significant energy resources extractable using the latest technology.

Conservationists urge better spacing of oil and gas facilities, consolidation of facilities, limited construction of roads and adjustments to make sure roads and pipelines don’t cut across migration routes.

BLM officials point out that the rights to develop oil and gas in that area already are leased. The federal land managers are developing a plan to manage environmental impact, BLM spokesman Dave Boyd said.

“We’re looking at what can be done to mitigate oil and gas impacts,” he said. “Should it turn out that it would be more than 15,000 wells, then we would need to do another environmental impact statement.

“We are limited in what we can do once lands are leased.”

Bruce Finley: 303-954-1700, bfinley@denverpost.com or twitter.com/finleybruce