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Puget Trough Ecoregion: Biodiversity

Map: WA Department of Natural Resources

A great inland arm of the sea—Puget Sound—flanked by forested foothills and freshened by many rivers. The Puget Trough ecoregion is home to over 75% of Washington’s citizens.

Location

The Puget Trough ecoregion runs the length of Washington, rising to about 1000 feet elevation between the Cascade Mountains on the east and the Olympic Mountains and Willapa Hills on the west.

Interstate Highway 5 connects most of the Puget Trough’s urban centers: Vancouver, Centralia, Olympia, Tacoma, Seattle, Everett, Mt. Vernon, Bellingham. Rapidly growing communities such as Puyallup, Kent, Renton, Bellevue, and Redmond lie east of I-5.

Encompassing about 8% of Washington State’s area, this ecoregion is densely populated. It is part of the larger Willamette Valley-Puget Trough-Georgia Basin ecoregion that extends south into Oregon and north into British Columbia.

Outstanding Biodiversity Features

• Puget Sound—a globally important estuary. Home to orcas, porpoises, and harbor seals, with rich nearshore and deepwater habitats. Puget Sound’s distinctive underwater topography makes it vulnerable to activities onshore and upstream.

• Salmon, linking freshwater and saltwater habitats. Salmon are icons of the region, but several salmon species are at-risk due to habitat degradation.

• Grasslands and oak woodlands that support rare species. Many rare grassland species are declining because this habitat type has become highly fragmented. Fire suppression and invasive species are significant problems.

• Accessibility, rich natural resources, and economic potential. These factors have encouraged over 75% of Washington’s people to live here. The result is a mosaic of land uses that fragment high quality native habitats.

Landforms

The Puget Lobe of the continental ice sheet depressed the region under ice nearly a mile thick. Puget Sound itself was scoured out by glacial action. Its deep north-south channels and basins are the glacier’s legacy.

The hills of Seattle and Tacoma are north-south trending drumlins, or hills of glacial debris. Elevation in the Puget Trough ecoregion averages 450 feet, but ranges from sea level to about 2000 feet.

Several large rivers drain into Puget Sound, among them the Skagit, Stillaguamish, Snohomish, and Nisqually. Rivers and streams formed valleys with rich bottomland and productive deltas. They bring fresh water into the marine environment. The Skagit River is the greatest single source of freshwater to the Sound.

South of Puget Sound, near present-day Fort Lewis and Olympia, outwash from the glacier produced fast draining and gravelly soils. These soils gave rise to Mima Mounds and prairie vegetation.

Further south, near Kalama, Kelso, and Vancouver, river deposits formed the soils. Ice age floods of the Columbia River left behind deep silts.

Climate

The Puget Trough’s climate is notorious for high rainfall and gray skies. It’s a temperate maritime climate, with mild, wet winters. Summers are warm and dry—if not particularly sunny. Mean January temperature is 39° F and mean July temperature is 65° F.

Precipitation, mostly falling as rain, averages 40 inches per year. The local average can vary greatly in the ecoregion, due to the prevailing moisture-laden winds from the southwest.

Rain shadow areas, affected by the high Olympic Mountains, include the northeast corner of the Olympic Peninsula, Whidbey Island, and the San Juan Islands. Average annual precipitation is 15 to 30 inches.

In Seattle or Tacoma, near sea level, average annual precipitation is 35 to 50 inches.

Where clouds slam into the Cascade Mountains, in the foothills, average annual precipitation is 60 to 80 inches.

Marine Environment and Species

Puget Sound’s marine environment is rich and complex. The Sound features a wide variety of deepwater and nearshore habitats. These include coastal lagoons, kelp and seagrass beds, rocky shores, sandy beaches and spits, and salt marsh wetlands.

Steep underwater slopes in many parts of the Sound mean that the narrow strip of shallow water near the shore is all the more valuable. Sunlight allows eelgrass, seaweeds, and plankton to grow—providing food and shelter for myriad creatures.

Marine species include:

Some of these species are migratory and others reside year-round. Marine mammals include harbor seals, orcas, porpoises, and California sea lions.

Marine invertebrates include sea urchins and both native and introduced species of shellfish. Some of the largest octopus and barnacle species in the world live here.

Vegetation

Historically, coniferous forest dominated the vegetation in the Puget Trough ecoregion. Many of the planet’s most impressive stands of trees grew here. Also present were a mix of riparian habitats, oak woodlands, and prairies.

The vegetation in most of the ecoregion’s landscapes has now been altered. Cities, suburbs, and industrial lands are common. Managed forests and agricultural lands changed the vegetation, and themselves face pressure from sprawling development.

The native forest here is primarily of Douglas fir, western red cedar, and western hemlock. Red alder and big leaf maple grow in riparian areas. Red alder also colonizes areas disturbed by fire or logging.

Understory plants include sword fern and shrubs such as snowberry, Oregon grape, salmonberry, and many others. In places, the forests now struggle with invasive non-native plants, such as English ivy.

Other key trees are Pacific madrone, a frequent feature on dry bluffs, and Oregon ash, common in riparian areas south toward the Columbia River.

Washington has one species of oak, the Garry oak. It is often associated with grasslands. These grasslands are one of the most threatened ecosystems in Washington. They support spectacular floral displays, including blue camas, yellow buttercup, pink seablush, and the endangered golden paintbrush.

Terrestrial Animals

Terrestrial animals living in the Puget Trough ecoregion are diverse:

Some animal species are adaptable to cities and suburbs, and their populations are holding steady—or even increasing to sometimes alarming numbers. Typical urban wildlife includes raccoons, crows, and coyotes, and introduced species such as opossum, European starlings, and rock pigeons.

Other species in the ecoregion have declined significantly over the past 100 years. Their habitats have been altered and fragmented by development and use.

Notable population declines have occurred in the Taylor’s checkerspot butterfly, the Oregon spotted frog, the western pond turtle, the northern spotted owl, the marbled murrelet, and the western gray squirrel (not to be confused with its ubiquitous invasive cousin, the eastern gray squirrel).

People in the Puget Trough ecoregion

The earliest archaeological evidence of people in the Puget Trough ecoregion dates to about 8,000 years before present, at the mouth of the Fraser River in British Columbia and along the lower Columbia River. The ancestors of the Salish or Salishan-speaking peoples flourished in the region and developed into eighteen or more linguistic traditions.

These peoples created prosperous maritime cultures that employed the rich biodiversity of the region.

Key resources included:

• Salmon and shellfish for food and tools
• Cedar for housing, clothing, and canoes
• Plants such as nettle, bracken fern, berries, rice root, and camas for food and fiber.

Europeans, and later Americans, also made use of the marine and forest resources of the Puget Trough ecoregion. Land use patterns were established early.

As of 1991, more than 50% of the Puget Trough had been converted to urban and agricultural uses. Rural areas in the Puget Trough are managed largely for intensive industrial and private forestry. Pasture and cropland are also dispersed throughout the ecoregion.

In 1999, the ecoregion’s population was nearly 3.9 million—double that of the 1960s. By 2020 the population in the Puget Trough is expected to grow to 5 million. This increasing population is putting pressure on the remaining natural areas and on working lands.

Around Vancouver, Clark County’s population is rapidly growing. From 1990 to 1997 it increased 33%—tops for the state during that period.

Puget Sound itself suffers from pollution and other ills. The Endangered Species Act listing of Puget Sound’s wild Chinook salmon was the first to affect such a major urban area. The Puget Sound southern resident orca population has also been listed as endangered.

Although altered and under stress, both the terrestrial and marine environments of the Puget Trough ecoregion are still extremely productive.

Human Impact

More than three-fourths of Washington’s human population is concentrated in the Puget Trough ecoregion. A number of human activities and land uses adversely affect its biodiversity.

In marine and nearshore habitats, some of the most significant problems include:

• Shoreline modifications. Bulkheading, dredging, diking, and filling lead to loss of native habitat. Changes in sediment and wave energy alter beaches and subtidal areas.

• Environmental contaminants. In sediments and in water, these threaten the health of wildlife and people alike.

• Marine invasive non-native species. They threaten ecological stability. Once established, they are difficult and expensive to control and virtually impossible to eradicate.

• Habitat loss and fragmentation. This affects native plants and animals in forests, in grasslands and oak woodlands, and in riparian and wetland habitats.

• Management practices. Customary management practices have frequently degraded the wildlife habitat value of working forests and farms. New management methods and incentives to “grow habitat” help conserve biodiversity of these important non-urbanized areas.

• Terrestrial invasive non-native species. These pose threats to both the diversity and abundance of native plant and animal species.

The political will and creativity of the people who inhabit the Puget Trough ecoregion will be key to finding solutions to these issues.