Habitat fragmentation is among the most significant drivers of urban biodiversity loss. When green spaces exist as isolated islands within a matrix of pavement and buildings, species populations become genetically isolated, dispersal is blocked, and local extinctions cannot be reversed by recolonization from adjacent populations. Research published in Conservation Biology demonstrates that isolated urban habitat patches support 40 to 60 percent fewer species than connected patches of equivalent size.
Portland has significant green space assets, approximately 11,600 acres of parks, natural areas, and greenways. But connectivity between these spaces is uneven. TerraFuture's Urban Biodiversity Corridor project, launched in January 2024, aims to create a 12-mile network of habitat connections across Southeast Portland, linking Forest Park's 5,200 acres to the Springwater Corridor and the Oaks Bottom Wildlife Refuge through a series of enhanced street trees, pocket habitats, green roofs, and restored riparian buffers.
The Science of Connectivity
Our corridor design is based on circuit theory modeling, the same mathematical framework used to model electrical current flow through networks of resistors. In ecological applications, the landscape is treated as a conductive surface where natural habitats have low resistance to animal movement and roads, buildings, and parking lots have high resistance.
Using data from our urban tree canopy mapping, land cover classification from Sentinel-2 satellite imagery, and species occurrence records from the Portland Audubon Society and Oregon Department of Fish and Wildlife, we modeled connectivity pathways for five focal species: Anna's hummingbird, western painted turtle, Pacific tree frog, red-legged frog, and band-tailed pigeon.
The modeling identified seven critical pinch points where connectivity between major habitat areas narrows to corridors less than 50 meters wide. These pinch points, primarily located where streams pass under major roads and where residential areas lack tree canopy, represent the highest-priority locations for habitat enhancement.
Implementation Strategy
The corridor is being built through a combination of five intervention types, each tailored to the specific constraints of the location.
Riparian buffer widening along Johnson Creek and Crystal Springs Creek will add native plantings to 3.8 miles of stream bank, expanding the average vegetated buffer from 15 feet to 35 feet. These enhanced buffers serve as movement corridors for amphibians and provide habitat for nesting birds.
Street tree densification along six key arterials will add 840 native trees, primarily Oregon white oak, Pacific madrone, and bigleaf maple, to create aerial corridors for birds and arboreal mammals. Street tree spacing will be reduced from the current average of 45 feet to 25 feet in corridor segments.
Pocket habitat installations at 18 locations will create small refugia ranging from 500 to 5,000 square feet, featuring native plantings, log structures, rock piles, and seasonal water features. These function as stepping stones for species moving between larger habitat patches.
Biodiversity does not recognize property lines or zoning designations. Creating functional habitat networks in cities requires thinking at the landscape scale and working at the parcel scale.
Community Partnerships
The corridor passes through 14 neighborhoods, and its success depends on community engagement. We have recruited 42 residential properties as Habitat Steward sites, where homeowners have committed to maintaining native plantings and wildlife-friendly features. Each Steward site receives a free landscape consultation, native plants, and ongoing support from TerraFuture staff.
Four schools along the corridor route have incorporated habitat installations into their grounds, creating both ecological function and outdoor learning opportunities. The combined area of school habitat sites is 1.8 acres, a meaningful addition to the overall corridor footprint.
Measuring Success
We are monitoring corridor effectiveness through annual bird point counts at 48 stations, amphibian call surveys at 12 wetland sites, camera trap deployments at all seven pinch points, and vegetation cover assessments along the entire 12-mile route.
Baseline surveys completed in spring 2024 documented 87 bird species, 6 amphibian species, and 14 mammal species within 200 meters of the planned corridor. Our five-year target is a 25 percent increase in species richness and a measurable increase in wildlife movement through the seven identified pinch points, as documented by camera trap data.
The total project cost is estimated at 3.4 million dollars over five years, funded through a combination of Metro regional bond funds, Oregon Watershed Enhancement Board grants, and TerraFuture's capital campaign. On a per-mile basis, this represents an investment of approximately 283,000 dollars, comparable to the cost of 0.3 miles of urban roadway construction.
Residents along the corridor can participate by joining the Habitat Steward program, volunteering at restoration events, or contributing to the capital campaign. Visit our programs page for details on how to get involved.
