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18 <br />wildlife community but have also been shown to facilitate regional wildlife movement. Riparian <br />areas can vary from tributaries winding through scrubland to densely vegetated riparian forests. <br />A riparian zone can be defined as an area that has a source of fresh water (e.g., rill, stream, <br />river), a defined bank, and upland areas consisting of moist soils (e.g., wetter than would be <br />expected simply due to seasonal precipitation). These areas support a characteristic suite of <br />vegetative species, many of which are woody, that are adapted to moister soils. Such vegetation <br />in the project region may include California buckeye (Aesculus californica), dogwood (Cornus <br />sp.), California hazelnut (Corylus cornuta var. californica), elderberry (Sambucus sp.), Oregon <br />ash (Fraxinus latifolia), walnut (Juglans sp.), California laurel (Umbellularia californica), toyon <br />(Heteromeles arbutifolia), oaks (Quercus sp.), and willow (Salix sp.). <br />Beier and Loe (1992) noted five functions of corridors (rather than physical traits) that are <br />relevant when conducting an analysis regarding the value of linkages. The following five <br />functions should be used to evaluate the suitability of a given tract of land for use as a habitat <br />corridor: <br />1.) Wide ranging mammals can migrate and find mates; <br />2.) Plants can propagate within the corridor and beyond; <br />3.) Genetic integrity can be maintained; <br />4.) Animals can use the corridor in response to environmental changes or a catastrophic <br />event; <br />5.) Individuals can recolonize areas where local extinctions have occurred. <br />A corridor is “wide enough” when it meets these functions for the suite of animals in the area. It <br />is important to note that landscape linkages are used differently by different species. For <br />instance, medium to large mammals (or some bird species) may traverse a corridor in a matter of <br />minutes or hours, while smaller mammals or other species may take a longer period of time to <br />move through the same corridor (e.g., measured in days, weeks and even years). For example, <br />an individual cougar may traverse the entire length of a long narrow corridor in an hour while <br />travel of smaller species (such as rodent or rabbit species) may best be measured as gene flow <br />within regional populations. These examples demonstrate that landscape linkages are not simply <br />highways that animals use to move back and forth. While linkages may serve this purpose, they <br />also allow for slower or more infrequent movement. Width and length must be considered in