Landscape and Habitat Connectivity in the Urban Environment
Connecting Forest Park and Tryon Creek State Natural Area
The Impact of Urbanization
In the US, less than 20% of urban area has remained as natural habitat. Cities have focused on improving connectivity and habitat for humans which has resulted in the loss of area for other species.
In the area around Portland, Oregon, urban growth has removed or fragmented natural habitat that native species depend on. The Coast Range of Oregon has maintained its role as an important habitat for many species and remains connected to the Portland Metro area via Forest Park. Vital corridors like Forest Park along, with a patchwork of habitats connect the Willamette Valley to Oregon's Coast Range.
Maintaining healthy ecosystems and biodiversity in the Portland area means improving the connectivity within the city and to the city through parks, woodlands and natural areas.
-Explore the interactive map to the right to see how the Coast Range connects to Forest Park.
Forest Park
More than 5,200 acres make Forest Park one of the largest urban parks in the United States. The park contains 11 separate watersheds, sections of rare old-growth forest and provides excellent habitat for a great number of species. Despite it's size, connecting the park with other patches in the Portland area can be a challenge because of urbanization.
The Park as a wildlife corridor acts to funnel species in from more wild areas outside of the metro area and helps to explain the diversity that we see in Portland today.
Tryon Creek State Natural Area
Tryon Creek State Natural Area (SNA) comprises of 659 acres of land located south of Forest Park. The mostly contiguous park is a large patch of habitat for many species. The forest around Tryon Creek provides a refuge in the urban landscape and helps to mitigate negative impacts to the Tryon Creek watershed as a result of increased devleopment. However, the ability for species and resources to move between Tryon Creek and places like Forest Park is limited by barriers between the two locations and distances between patches.
The West Willamette Corridor
The West Willamette Corridor is the region that extends from Forest Park to Tryon Creek State Natural Area (SNA) and the focus of this study.
The Corridor between the parks is a matrix of urban land-uses which includes parks and natural areas that serve as habitats as well as roads and development that act as barriers to species. Increasing the connectivity between these areas can greatly benefit species diversity and overall urban ecosystem health.
Habitat Connectivity
Source: www.yardmap.com
Patches are islands of suitable habitat that can provide shelter and resources for species. The species-area relationship theory suggests that the larger an area that is suitable habitat for species, the more individuals and diversity that they can support. Corridors connect patches and facilitate migration, support genetic diversity and provide access to resources and new habitats.
Habitat connectivity is species specific where as landscape connectivity is the degree to which the urban matrix impedes or facilitates the movement between patches. Understanding both is important for improving the resilience and diversity of natural ecosystems.
Improving the West Willamette Corridor
What is limiting connectivity between Forest Park and Tryon Creek SNA?
Click on the numbered links in the interactive map to learn more about the impacts of urbanization on habitat connectivity.
Increasing Habitat
There are many ways to enhance habitat in cities: replacing demolished buildings with parks, removing parking lots and planting native vegetation or planting trees throughout the city.
While removing buildings and de-paving parking lots can make space for restoration, these options are often not economically or socially feasible. The Backyard Habitat Certification Program provides a real solution for increasing patch density by creating functional habitats in residential yards of less than 1 acre.
By identifying residential properties in the West Willamette Corridor that can be enhanced through the Backyard Habitats program, managers and stakeholders can project how connectivity between Forest Park and Tryon Creek can be improved. Neighborhoods can be identified based on their location relative to connectivity paths and priority areas can be chosen.
Backyards that were once a mono-culture of grass or sparse vegetation, can be converted to viable habitat that attracts a variety of native species. The transformation of backyards to vital habitat softens the urban matrix, increasing the amount of habitat and landscape connectivity of the Portland Metro area.
Habitat and Landscape Connectivity Analysis
Current habitat connectivity
The Cost Connectivity tool in ArcGIS was used to evaluate habitat connectivity for 3 surrogate species native to the Pacific Northwest: Swainson's Thrush, red-legged frog and Douglas squirrel. Species-specific habitat layers provide an accurate way to evaluate habitat connectivity that can then be used to make informed decisions for similar species.
The baseline least-cost paths from Forest Park to Tryon Creek SNA were calculated for each species. While it is not likely that an individual species (other than avian species) would travel the entire distance, the paths provide a baseline cost and highlight how parks, green spaces and other areas act as stepping stones.
Landscape connectivity (Legend icon in lower left of map) was calculated using reclassified National Land Cover Database (NLCD) data; the least cost path (light blue path) follows a similar route as that of the thrush and Douglas squirrel.
Natural Areas like Marquam Nature Park, Keller Woods, Hoyt Arboretum and even Riverview Cemetery serve as patches of habitat.
-Explore the interactive map to take a closer look at the areas that species may use along the connectivity paths.
Streams and Natural Areas
Red-legged frog
The least cost path calculated for the red-legged frog highlights important habitats for the species. The red line shows that habitats and permeable regions found near streams like Tryon Creek are important for this species.
Red-legged frog (Rana aurora). Image credit: www.californiaherps.com
The area from I-5 to Marshall park con be considered important for the red-legged frog, but I-5 is a significant barrier that keeps this area isolated from habitats to the Northwest.
Parks & Green Spaces
Swainson's thrush & Douglas squirrel
Locations like George Himes Park appear to act as functional patches that serve as 'stepping stones' for species like the Douglas squirrel and Swainson's thrush. While preserving these areas is important, softening the urban matrix through habitat enhancement efforts like the Backyard Habitat program can strengthen connectivity.
Swainson's thrush (Catharus ustulatus). Image credit: www.allaboutbirds.com
Seeing that the least cost paths for two surrogate species and landscape connectivity follow a path parallel to 99-W suggests that this is an important portion of the West Willamette Corridor.
Douglas squirrel (Tamiasciurus douglasii). Image credit: www.coniferousforest.com
Some large and small parks are isolated from the current connectivity routes either as a function of distance or their land-use may not serve as habitat for the surrogate species. These areas most likely help support meta-populations despite not being being connected in this scenario.
Fanno Creek Natural Area and Albert Kelly Park are examples of patches that can help connect Tryon Creek SNA to Forest Park. Additional studies that include smaller patches like these can provide more information on connectivity.
Pan or zoom to see more parks or green spaces that can be connected through increased habitat.
Backyard Habitats
In order to explore the scenario of increasing the number of backyard habitats, 5-meter buffers were selected around every residential building in the study area in order to simulate the enhancement of those areas in the urban matrix.
The resistance layers for each surrogate species were reclassified to reflect the change in habitat and to determine what areas, if any showed improved connectivity.
The neighborhoods that intersect with the enhanced connectivity paths can be areas of focus for the Backyard Habitat Program.
Backyard Habitat Scenario Results
Increasing Connectivity
Examine the interactive map to the right; the legend is in the lower left corner.
New least-cost connectivity paths were calculated for each of the 3 surrogate species. While the connectivity paths for the surrogate species are similar, there were noticeable changes following the addition of backyard habitat.
The calculated cost for each species declined which reflects an increase in suitable habitat and/or permeable areas. Although a path may be similar to the baseline connectivity, a decrease in cost may indicate that a species may be more likely to utilize those areas in addition to established habitats.
Changes in least-cost path totals for each surrogate species.
Habitat connectivity for Swainson's thrush improved with lower cost paths through neighborhoods and could increase the available habitat in many backyards that were previously categorized as barriers or only permeable.
An increase in the amount of backyard habitats would benefit other avian species that would be expected to use similar areas to that of the thrush.
The analysis for the Douglas squirrel also saw new paths that utilize residential neighborhoods away from traditional green spaces like parks.
Because the red-legged frog relies on habitat that is primarily upland woods and riparian areas, backyard habitat enhancements would most likely have little effect on this or similar species. Although both least-cost path analyses 'connect' Forest Park with Tryon Creek SNA, amphibian species face many more barriers compared with other terrestrial species.
For amphibian species, focusing habitat enhancement to areas near streams and riparian zones around least-cost paths could be beneficial.
Conclusion
Increased Connectivity
- The reduced costs for the surrogate species show that the Backyard Habitat Certification Program can increase connectivity in the West Willamette Corridor
- Based on where paths intersect geographic areas, individual neighborhoods or streets can become focus areas for enhancement projects
- Improving the West Willamette Corridor will better connect the large habitats of Tryon Creek SNA and Forest Park
- Significant barriers like Highway 26 and I-5 will continue to act as significant barriers, fragmenting habitats for most species, so restoration and enhancement should focus on the area between these barriers.
Moving Forward
- Small scale connectivity analyses between smaller, closer patches should be done
- Additional connectivity tools are available that can be used to further investigate the West Willamette Corridor such as Circuitscape and Fragstats
Methods & Data
Habitat Connectivity
Tool: ArcMap (v 10.7.1) Cost Connectivity (Spatial Analyst)
Use: Model least-cost paths between Forest Park and Tryon Creek SNA
Data: Reclassified land-cover to surrogate species data used to create cost raster layer (5 meter resolution), ORCA layers (Portland RLIS) used to identify the polygons for Forest Park and Tryon Creek SNA. Raster layers for each species were categorized as either habitat, permeable or barrier. Each category was then assigned cost value: habitat = 1, permeable = 25, and barrier = 100
Backyard Habitat
Tools: ArcMap (v10.7.1) Buffer (Analysis), Select by attribute, Polygon to Raster
Use: Create 5-meter buffer around all residential buildings in study area and use raster calculator to create new cost raster reflecting habitat change
Data: Building footprints (Portland RLIS)
References
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- Gilbert, L. E., & Singer, M. C. (1973). Dispersal and gene flow in a butterfly species. The American Naturalist, 107(953), 58–72.
- Gurrutxaga, M., Lozano, P. J., & del Barrio, G. (2010). GIS-based approach for incorporating the connectivity of ecological networks into regional planning. Journal for Nature Conservation, 18(4), 318–326.
- Li, H., Chen, W., & He, W. (2015). Planning of green space ecological network in urban areas: An example of Nanchang, China. International Journal of Environmental Research and Public Health, 12(10), 12889–12904.
- Lin JP. (2009). The Functional Linkage Index: A Metric for Measuring Connectivity among Habitat Patches Using Least-Cost Distances. Journal of Conservation Planning, 5(NA), 28-37.
