Imagine living in a world where your only possible sexual partners are your relatives. For mountain goats in Washington's Cascade range, that isn't such a stretch.
Interstate-90 splits mountain goat habitat, halving the number of potential mates and increasing their chances of inbreeding, which lowers the population's ability to survive and reproduce. The constant high-speed traffic on the freeway, combined with human-related development near the road, makes the I-90 corridor an impassible barrier for the small population of mountain goats in Washington's Cascade Mountains.
"Any high alpine animal won't be too thrilled about all the roads, clear-cuts and cabins [in the I-90 corridor]," said Kelly McAllister, a habitat connectivity biologist with the Washington State Department of Transportation. "There are a number of things going on that make that area a barrier for some species."
Mountain goat habitat in the Cascades extends from Mount Baker in the north to Mount Adams in the south. While much of the high-alpine habitat is intact, roads, logging and other human-related development prohibit mountain goats from roaming freely through the terrain.
I-90 isn't the only thing blocking mountain goat movement and gene flow, but it is the biggest factor, said Andrew Shirk, an environmental science graduate student at Western Washington University.
Shirk has been researching mountain goats for his master's thesis, which is focused on identifying areas in the Cascades where mountain goat movement and gene flow is blocked. He spent the last two summers collecting samples of mountain goat genetic material throughout the Cascades.
"There are other barriers like Mount Baker Highway, Baker Lake and North Cascades Highway, but they are insignificant compared to I-90," Shirk said.
Small towns and highways can't be bulldozed and reforested to help the plight of the isolated mountain goats, but I-90 could be made crossable for the horned, high-alpine inhabitants.
The Department of Transportation is planning to upgrade a 15-mile section of I-90 in order to improve habitat connectivity, meet projected traffic demands and increase driver safety.
The plan to upgrade the freeway from Hyak to Easton, Wash., includes a combination of bridges and wildlife overpasses that will enable mountain goats and other wildlife to cross under and over the freeway, McAllister said.
Freeway bridges will be built much longer than the width of the creeks they cross, so animals can pass under the freeway. For example, Gold Creek is 40 to 60 feet wide. The planned bridge over Gold Creek will be 1200 feet long, allowing ample room for terrestrial animals to cross underneath the freeway, along the creek, McAllister said.
In addition, the Department of Transportation plans to build two wildlife overpasses that will allow wildlife to cross over the freeway. The fenced overpasses will be covered with soil and native plants and will be wide enough that animals won't see the road while they're crossing over it, McAllister said.
The Department of Transportation's engineers are working closely with engineers from Banff National Park in Canada, who successfully implemented a similar project on the Trans-Canada Highway. Wildlife crossing structures over the Trans-Canada Highway get significant animal use, and traffic-related animal mortality has decreased by more than 80 percent since construction, according to the journal "Wildlife Biology."
Animals are probably leery of crossing over a freeway at first, but every year the number of animals that use the bridges in Banff increases, said McAllister.
Several small-scale wildlife underpasses in Arizona and Montana have also been successful, said Larry Mattson, Assistant Environmental Manager of the I-90 Snoqualmie Pass East Project.
Studies conducted by the Department of Transportation identified areas along I-90 where animals crossed or attempted to cross the freeway. The two wildlife overpasses will be built in those high-traffic areas.
Construction on the first five miles of the project will begin summer 2009 and is expected to take six years to complete.
However, only the first section, five miles from Hyak to Keechelus Dam, has been funded. That section, which doesn't include either of the wildlife overpasses, will cost $545 million, according to the Washington State Department of Transportation Web site.
"We don't know where funding will come from for the rest of the project," Mattson said.
If the proposed wildlife corridors don't get funded, the small population of mountain goats in the Cascades might not be around much longer.
For mountain goats to persist in the Cascades, they need a population of 500 or more, Shirk said. Currently there are close to 150 goats on each side of the freeway. Unless connectivity between the North and South Cascades improves and the mountain goat population increases, they will have trouble surviving in the future.
"If animals aren't free to move through the landscape uninhibited, the landscape can fragment a population into smaller groups," Shirk said.
When an isolated group of animals gets too small, it can experience inbreeding depression. Inbreeding depression occurs when a population doesn't have enough genetic diversity to reproduce or adapt to changes in their environment, Shirk said. If all the mountain goats in one herd have similar genes, they can be wiped out by a single disease.
Shirk used a genetic approach to locate barriers hindering mountain goat movement. If two animals are close relatives but far apart in distance, the habitat is easy to move through, he explained. Conversely, if animals are genetically distinct but geographically near each other, there is probably some kind of landscape barrier preventing them from mating.
Shirk and members of The Washington Department of Fish and Wildlife collected fur samples with a biopsy rifle. The biopsy rifle shoots air darts, which grab a patch of fur before bouncing off the goat and onto the ground. This method may be uncomfortable for mountain goats, but it produced great genetic samples, Shirk said.
"The goats are so tough," Shirk said. "They would be a little shocked that something happened, but within seconds they would go back to eating."
Shirk analyzed 149 genetic samples, some of which he collected on foot and some that were collected by the Washington Department of Fish and Wildlife. Shirk hiked to typical mountain goat terrain-high-alpine meadows near cliffs-and darted the animals when they came out to graze in the mornings.
Analysis of the genetic samples showed that goats on the south side of I-90 were genetically different from goats on the north side, revealing that mountain goats from either side of the interstate weren't breeding with each other, he said.
The already low numbers of goats in the Cascades adds to the risk of inbreeding. The Northwest's harsh climate keeps the mountain goat population small. Mountain goats repopulate slowly because less than half of their offspring survive their first winter, Shirk said. Mountain goats survive the cold winter temperatures of their alpine environment by growing thicker coats and descending from snowy mountaintops into the forest.
Hunting between the 1950s and 1980s also played a large role in the decline of the mountain goat population. During that time, their population shrank from 10,000 to 3,000, according to David Wallin, a professor of environmental science at Western.
"Hunting pressure was dramatically reduced from the late 80s to the early 90s, so you would imagine that populations should recover quickly, but that's not the case," Wallin said.
Currently, fewer than 20 hunting permits are issued every year, a dramatic reduction compared to the 300 to 400 hunting permits that were issued every season between the 1950s and 1980s.
According to the Banff Wildlife Crossings Report, the Banff wildlife crossing bridges are frequently used by elk, deer, wolves, sheep and coyotes, but mountain goat don't cross as frequently. Mountain goats live near cliffs and steep-terrain, which is usually found at higher elevations then the relatively low I-90 valley. They are agile climbers and they use steep-terrain to outmaneuver and escape predators.
Although mountain goats are less likely to use a bridge than other animals, they would probably learn to cross I-90 using the overpasses, Shirk said. The I-90 valley is part of their historic habitat. In fact, Wallin and Shirk have identified a couple of goats that managed to cross the freeway and breed on the other side.
Funding has not yet been secured for the wildlife crossing structures planned for I-90, but if they are built, they will be the first of their size in the United States, Mattson said. They will set a precedent, as the Department of Transportation plans to incorporate wildlife crossing structures into future plans.
"The I-90 wildlife overpass will be one of the best examples of a wildlife corridor in the world," Shirk said.
For drivers, wildlife crossing structures will mean less road kill. For mountain goats, the ability to cross the freeway would minimize inbreeding and offer a gateway to a reproductive haven.
Oliver Lazenby studies environmental journalism. He has been published in The Western Front.