Landscape metrics and ecological assessment methods

Landscape Metrics

Today's ecology professional is faced with many challenges in terms of climate and landscape change. Part of this is natural processes that affect the resources and wildlife of an area over the long-term, while another part is constituted by human activity. Landscape metrics then offer a way for professionals to determine the historical extent of landscape change, the differences among adjacent areas, and the ways in which this affects both human, animal and plant life in these areas. To demonstrate this, the ecological regions in the Washington, Oregon, and Idaho states will be considered in terms of five metrics, including patch density, edge density, number of classes, Shannon's diversity index, and the interspersion and juxtaposition index.

Patch Density

According to Eiden, Kayadjanian, and Vidal (n.d.), patch density means that a single land cover class over a given area is measured, which represents one patch. To arrive at patch density, the number of patches within the area is determined by dividing the number of patches by the size of the area (per 100 ha). What this means is that patch density increases with the increase of the number of patches within the given area size. A higher number of land cover classes or patch types would then mean a higher patch density within the area.

In the Coast Range, current patch density is relatively low in terms of Douglas-fir plantations that dominate the coniferous forests. The landscape is intensely logged and managed, which means human habitation has significantly lowered patch density, where Sitka spruce forests originally dominated the area, with western red cedar and western hemlock previously accompanying the occurrence of Douglas fir. This demonstrates the way in which landscape metrics can help determine the impact of human habitation on an area, with the addition of possible measures that might be necessary to improve the ecological viability of the area.

Edge Density

And edge means the border between different classes of landcover (Eiden, Kayadjanian, and Vidal, n.d.). Edge density is also known as "Perimeter/Area Ration" and is used to calculate the length of all borders between different patch types. This is calculated by dividing the length of borders between patch types by the total area size. Edge density is then measured in m/ha. Edge density adds a further dimension to the landscape investigation by taking into account the complexity and shape of the patches in the area.

In the Puget Sound area, the many islands, peninsulas, and bays in the area lend themselves well to patch density investigations. The area is characterized by ground moraines, outwash plains, floodplains, and terraces. Forest species are specifically affected by the rainshadow from the Olympic Mountains. These effects over time can be calculated by the edge density metric.

Number of Classes

The number of classes in an area is a means of calculating biodiversity. A high number of classes would then mean a diversity rich area (Eiden, Kayadjanian, and Vidal, n.d.). One disadvantage of this metric is that it does not take into account the area that each class covers, or its importance. Nevertheless, in an ecologically sensitive area such as the Idaho Batholith, it is useful to monitor the biodiversity of the area to determine the potential damage of human activity, as well as determining ways to mitigate this.

Shannon's Diversity Index (SHDI)

The SHDI is a somewhat more complex method of determining the diversity of a certain area. To calculate this, the number of patch types as well as their proportional area distribution are taken into account (Eiden, Kayadjanian, and Vidal, n.d.). These components are also generally known as richness and evenness. The Index increases with the number of different patch types and also if the proportional distribution among patch types becomes more even. The calculation of this Index allows the ecologist to determine the differences in an area's biodiversity with time. This could be an important component of landscape calculations of an agriculturally important area such as the Willamette Valley. This area originally had a wide range of prairies, oak savannas, coniferous forests, wetlands, and deciduous riparian forests. Today, Oregon's population, industry, commerce, and cropland have caused significant changes in biodiversity. The sustainability of agricultural practices is therefore a significant concern.