Indicator: Habitat Fragmentation
Data and Data Discussion provided by
Sustainable Seattle
Sustainability Snapshot:
While direct habitat loss is the largest cause of species loss worldwide, habitat fragmentation is considered the principal threat to temperate zone species. Over the past century, clear-cut logging, road building and urbanization have significantly degraded our Central Puget Sound forests. A huge road network now cuts across these lands, home to a wide range of wildlife species. Several of the species, which require large intact core areas to survive, are on federal threatened and endangered species lists.
Sustainability Trend:
The pattern of intense clear-cutting, road construction and urbanization threatens to separate the Central Cascade forests into two ecological regions.
-
Degree of forest habitat fragmentation as measured by edge density, mean patch size, and core area.
Edge density is a measure of the length of habitat patch per unit area. Calculating the density of the edges across a landscape provides a measurement of the degree of habitat fragmentation.
Core areas are the habitat patches that are distant enough from habitat edges or other disturbances to ensure that the species within them remain relatively unaffected by the edge impacts. To identify the remaining core forest habitat areas, "edge-effect" zones of varying widths are subtracted.
-
Forest habitat fragmentation stems from clear cutting and road construction. Fragmentation occurs when a habitat area is separated into smaller and smaller patches, increasing the number of patches and the amount of abrupt edges in forest habitat. When created by roads edges can lead to increased erosion,
sedimentation in streams, noise, dust, hunting pressure and
introductions of non-native invasive plant and animal species.
Two landscape scenarios were developed from 1999 Landsat (satellite) data: pre-settlement (or natural) and human-disturbance.
Landscape fragmentation metrics were used to evaluate and compare the
two scenarios. The study area was 3.5 million acres of Central Cascades forest habitat in King and Pierce counties and the eastern flank of the Cascades.
The average edge density increased by a factor of 4 to 5 when roads and clear-cuts were added to the natural scenario. (Figure 1) Most of the windows in the pre-settlement scenarios have edge densities between 0 and o.5 miles per mile squared. Overall road density in the human-disturbance scenario was 1.9 miles per mile squared.
The largest mean forest patch sizes shift from low elevations to protected areas and decrease substantially in size when roads and clear-cuts are added to the pre-settlement scenario. In the human-disturbance scenario, patch size does not exceed 5 square miles and size drops from the pre-settlement scenario by factors of 21 to 69. The study's overall average patch size is less than 0.2 square miles. These results confirm that as the amount of disturbance on the landscape increase, the mean patch size decreased.
The amount of forested core area dropped substantially with the addition of roads and clear-cuts. The overall number of core areas increased from 255 in the pre-settlement scenario to 7706 in the human-disturbance scenario, while the average core size decreased from 8.06 to 0.1 square miles.
-
The data and data discussion are based on the habitat fragmentation analysis reported in Cascade Crest Forests, Forest Loss, Habitat Fragmentation, and WIldness.
The analysis is limited to forest habitat and does not include urban
and agricultural lands or non-forested alpine lands which are for the
most part already protected as national park area and congressionally
designated wilderness.
- collapse all
Additional Resources
- Cascade Crest Forests: Forest Loss, Habitat Fragmentation, and Wildness
This Wilderness Society report presents the results of three analyses of landscape conditions conducted for 3.5 million acres in Washington State’s King, Pierce, and Kittitas counties. The analyses examined the rate and distribution of forest loss, the degree of forest habitat fragmentation, and the degree of “wildness” across the landscape.
