Californians value oak woodlands for their aesthetic, natural resourceand economic significance. Over the past several decades, residents haveexpressed increasing concern, however, over the loss of oak woodland wildlifehabitat. The leading cause contributing to loss and degradation of habitatvalues is residential development.
Oak acorns, leaves, sap, and wood provide food for a multitude of mammals,birds, reptiles, amphibians, insects, and other wildlife. Along with food,oak communities contain the nooks, crannies, perches, and passages whereanimals eat, drink, reproduce, and find shelter. Oak woodland habitat providesvariety and a wealth of micro-habitats for wildlife to thrive. As we gaina better understanding of how residential development impacts wildlife habitat,we increase the possibilities for conserving California's oak woodlandsfor future generations.
Landscape Ecology Concepts
Over time, development activities may break up formerly large portionsof oak woodland into progressively smaller pieces, diminishing its naturalresource values, including its suitability as wildlife habitat (figure 1).Landscape ecologists term this process "habitat fragmentation."Some wildlife, especially the more mobile mammals and birds, can use habitatfragments. Fewer than 25% of the terrestrial vertebrate wildlife found inoak woodlands, however, can live in a fragmented environment. The remaining75%, which are unable to adapt, may experience declines in population size.
Figure 1. Oak Woodland showing continued habitat fragmentationover time (1890-1990) due to urbanization
Landscape Ecology Definitions
Habitat Fragmentation - The process by which contiguouslarge blocks of habitat are broken or sliced into progressively smallerpieces by housing and industrial development, intensive agriculture, roads,and other development activities.
Patch - A surface on the landscape differing in appearancefrom its surroundings. Patches may be due to natural (e.g., soil type) oranthropogenic (human-caused; e.g., housing development) factors. Areas ofoak woodland, chaparral, or residential development are examples of patcheswithin a landscape.
Edge Habitat - The outermost band surrounding a patch thathas an environment significantly different from the interior of a patch.Edges can be a few to several hundred feet wide depending on environmentalfactors.
Interior Habitat - Habitat within the interior of a patchthat is removed from edge habitat. Interior habitat is necessary for certainoak woodland species, providing insulation from edge effects such as noise,wind, solar radiation, and increased predation.
Corridor - A strip of land that differs from the surroundingarea on both sides. The strip of habitat along a stream or road, a windbreak,a road, or railroad right-of-way are examples of natural and human-designedcorridors that may function as habitat connections between patches.
Landscape - A heterogeneous land area composed of a clusterof patches. Landscapes can contain patches of human development and wildlifehabitat and vary considerably in size, shape, and structure.
Today, local planners frequently ask the following complex ecological questions:
- What is the smallest habitat patch required by wildlife?
- How does patch size and shape affect wildlife?
- What are edge species and interior species?
- What is a corridor and what is its value to wildlife?
- What minimum buffer dimensions are needed around patches and corridors to minimize the impact of residential development?
Although complete and easily applied answers to these questions are currentlyavailable, much has already been learned. Consensus is emerging among landscapeecologists on ways to minimize and mitigate the adverse impacts of residentialdevelopment on wildlife habitat.
Consequences of Habitat Fragmentation
Many animals require a variety of habitat patches, in close proximity,to meet their daily and seasonal needs. A deer may bed down beneath thecanopy of an oak, seek cover from predators within the thickets of riparianwillows, drink from a nearby stream, and leave the shelter of the treesto forage in adjacent grassland. This variety of vegetation types formsa patch-work quilt across the landscape. The size, vegetation diversity,and interconnectedness of patches that make up the landscape, determinethe population size and kinds of animals found within it.
For example, landscapes less than 250 acres (2.5 acres = 1 hectare) maysupport only a certain suite of species. The area may not be large enoughto include a diversity of habitat patches or meet the home-range requirementsof certain types of wildlife. Small, less mobile animals may be able tosurvive because their home-range requirements are small, however, survivalof medium-sized animals is compromised over time and large animals can berare or transient. Furthermore, when development activities isolate smallareas of woodland habitat, resident wildlife populations also become isolated,eventually reaching a point where loss of a species from an area becomesa real possibility due to inbreeding and restricted gene flow.
A landscape of interconnected patches between 250-12,000 acres beginsto approach sufficient size to support populations of medium-size animalssuch as coyotes, bobcats, and hawks. At this size, the region may encompassthe variety of habitats these animals need to live and reproduce.
Landscapes greater than 12,000 acres, begin to protect ecosystem integrityand function. These large areas not only supply a multiplicity of diversehabitat types for large mammals and birds; in addition, a full range ofsmall- and medium-size animals are provided quality habitat.
These landscape size categories (< 250 ac, 250-12,000 ac, > 12,000ac) are based on information from interviews with experts specializing inresearch on wildlife habitat size requirements and landscape ecology concepts.The catagories are based on habitat requirements of wildlife from the EasternUnited States and Southern California. To date, information on patch-sizerequirements for oak woodland wildlife is limited, and it is not known howwell these designations apply.
Edge and Interior Areas are Affected by Patch Size and Shape
Figure 2. (From Forman, T.T., 1981; Interactions of landscapeelements: a core of landscape ecology)
Patch Size and Shape.
The size of a habitat patch measurably affects the kinds and numbersof animals within the patch. Therefore, the species profile within a patchis at least partly a function of how we reduce and enlarge habitat as aconsequence of designing and constructing on the landscape. Small patcheshave been likened to "islands" floating within a sea of developmentand are typically too small or too isolated to provide resident wildlifewith the basic necessities of life: food, water, and cover.
The shape of a habitat patch causes similar effects to those of patchsize, since shape also influences the relative amounts of edge and interiorhabitat. As a patch is reshaped from circular to linear (figure 2), thedistance from the interior to the edge decreases, as it does when a patchbecomes smaller. For example, a long, thin patch caters almost exclusivelyto edge species because it provides little or no interior habitat. In contrast,a round patch of equal area may provide interior habitat.
Edge and Interior Habitat
Small habitat pieces favor edge species at the expense of interior wildlifespecies. The smaller patches provide relatively more edge habitat. For example,imagine a woodland patch of 16 acres divided into 4 smaller patches, andthen divided further into 16, one-acre patches (figure 4). The amount ofedge increases dramatically while the amount of interior habitat decreasesuntil finally the patch is all edge.
Edge habitat benefits only certain kinds of wildlife such as opossums,raccoons, deer, skunks, cowbirds, and red-tailed hawks-often at the expenseof interior habitat species, such as coopers hawks, bobcats, vireos, andthrushes-which mostly avoid edges. Highly mobile predators, such as mountainlions, will move through edge habitat in search of food, however, they dependon large patches for the cover and reproductive habitat the interior provides.
Corridors and Connectedness.
The path that a river or stream follows is an example of a natural habitatcorridor. Roads, both rural and urban, windbreaks, and railroad right-of-wayscan also function as corridors. The strips of vegetation along streams areespecially important to seasonally migrating wildlife. From a habitat perspective,these examples of natural and human-designed corridors may function as connectorsof two or more patches of habitat. A habitat connection may make an otherwiseisolated and unused patch into usable habitat. For example, recent researchhas shown that a mountain lion population of 15-20 adults requires, on average,625 square miles (1 km2 = 0.39 mi2) of unfragmented habitat. If a corridoris available, however, to allow movement of mountain lions between patches,an area of about 430 square miles (about 30% smaller) can support a mountainlion population, lessening the risk of local extinction.
Corridors-both natural and human designed-provide linkage between patchescreating a mosaic of habitats or vegetation types. Maintaining or creatingcorridors to link patches can increase usable wildlife habitat, providedispersal and migration avenues, facilitate gene flow between populations,and increase the likelihood the patches will be inhabited. To be effective,however, a corridor must be of adequate size to provide food, water, andshelter to meet an animal's needs as it moves within the corridor.
A GIS and the CDF Hardwood Maps
The CDF Maps
The Hardwood Maps were developed under contract for the Strategic PlanningProgram, California Department of Forestry and Fire Protection (CDF). Themaps are based on 1990 LANDSAT imagery. Two maps are available in both hardcopy and digital form (GIS) to California counties with oak woodlands.
The first, a polygon map, displays oak woodland polygons over 40 acresin size and below 5,000 feet in elevation along with their correspondingcanopy densities. Using GIS jargon, a polygon is an area characterized bya similar set of characteristics. The second is a grid map and displays12 land cover types (6 hardwood and 6 non-hardwood) mapped at a pixel resolutionof 164 by 164 square feet or about 0.5 acres. A pixel is the smallest unitof homogeneous vegetation.
The polygon map is most useful for determining the overall distributionof hardwoods in the county. The grid map can be useful in determining moresite specific information about vegetation. For example, if the polygonmap displays a 100-acre polygon as valley oak woodland with a canopy densityof <10%, the grid map will show how much of the polygon is grassland,shrubland, urban area, and water.
The Polygon Map
Displays polygons 40 acres or greater in size by the following hardwoodtypes:
- Blue Oak Woodland
- Valley Oak Woodland
- Blue Oak/Grey Pine Woodland
- Montane Hardwood Mix
- Coastal Oak Woodland
- Potential Hardwoods
and canopy densities (also termed crown closure or canopy cover):
<10% 10-33%
34-75% >75%
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