Landscape structure indices for assessing urban ecological networks

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270 E.A. Cook / Landscape and Urban Planning 58 (2002) 269–280Fig. 1. Patches and corridors and landscape scale.network. The plan is designed to enhance the overallecological viability of the study area by reducingfragmentation, maintaining naturalness, and improvingcompatibility of the landscape matrix with theecological network. The analysis demonstrates thatthe ecological network plan provides notable improvementover the current situation and managementschemes.2. Criteria for assessing viabilityAssessment of ecological network viability can beundertaken by analyzing the inherent characteristicsof the landscape elements, the interrelationshipsbetween landscape elements and external factorsaffecting the functioning of the ecological network.A range of landscape element attributes are assessedthrough patch and corridor content analysis. Inherentcharacteristics include size and type, vegetativestructure and diversity, and naturalness. Interrelationshipsbetween individual landscape elements and thelandscape matrix are assessed through a number ofindicators that are described as context. Networkstructure analysis considers the overall effect of theinterrelationship of patches and corridors within thecontext of the urban matrix. The network structureanalysis includes mesh density, circuitry and connectivity.This process is undertaken using plans describedin previous studies for the Phoenix urbanarea, principally at landscape scale. Both the existingsituation and an optimal situation are assessed.2.1. Patch content analysisThe patch content analysis incorporates four principlevariables, (1) patch size and type, (2) vegetativestructure and diversity, (3) patch context, and (4) patchnaturalness. Each variable has several sub-variables orattributes that are used to describe and assess patch
E.A. Cook / Landscape and Urban Planning 58 (2002) 269–280 271Table 1Summary of patch content analysisPatch size and typeOrigin 17 Natural 18 CulturalType 17 Remnant 18 IntroducedArea (mean)4650 haPerimeter length (mean) 22051 mPerimeter/area ratio (mean) 1.48Patch vegetative structure and diversityVegetative cover (mean) 0.33Native cover (mean) 0.17Structural diversity (mean) 0.62Total vegetation value (mean) 1.17Patch contextMatrix utility (mean) 0.59Isolation index (mean) 1.51T-links (mean)1.23 (43 Total)Patch naturalnessNative vegetation (mean) 0.17Soils (mean) 0.42Flows (mean) 0.35Total naturalness0.94value (mean)characteristics. Each of the patches were analyzedusing these variables. A summary of the results ofthe patch content analysis is included in Table 1.2.1.1. Patch size and typePatch size and type have been shown to affectinteractions of wildlife and affect ecological value.Forman (1995) uses five terms to classify patches,primarily based on origin and function. A remnantpatch is one that is left in its original characterfollowing widespread disturbance that changes thesurrounding matrix. A regenerated patch has beenpreviously disturbed or changed and has since naturallyreestablished vegetative cover. An introducedpatch is similar to a regenerated patch in that it waspreviously disturbed, but the establishment of newvegetation is of human origin. An environmental patchis one that exists within a predominantly naturalcontext and has significant natural values, but differsfrom the surrounding matrix. A disturbance patchis established by local disturbance of an area thatchanges the character, resulting in a patch that differsfrom its surroundings.The size of patches also affects their viability andecological value. Large patches are more valuablebecause they support large, persistent populations.Thus, there is a relationship between patch area andwildlifeabundance,persistenceanddiversity(Duerksonet al., 1997). Numerous researchers have demonstratedthis species area relationship. Large patchespreserve greater areas of interior habitat which isbeneficial to many indigenous species that are nottolerant of edge habitats. Smaller patches have moreedge area. Forman (1995, p. 47) lists ecological valuesof large and small patches.Large patches:1. Water quality protection for aquifer and lake.2. Connectivity of a low-order stream network forfish and overland movement.3. Habitat to sustain populations of patch interiorspecies.4. Core habitat and escape cover for large-homerangevertebrates.5. Source of species dispersing through the matrix.6. Microhabitat proximities for multihabitat species.7. Near-natural disturbance regimes at where manyspecies evolved with and require disturbance.8. Buffer against extinction during environmentalchange.Small patches:1. Habitat and stepping stones for species dispersal,and for recolonization after local extinction ofinterior species.2. High species densities and high population sizesof edge species.3. Matrix heterogeneity that decreases fetch (run)and erosion, and provides escape cover frompredators.4. Habitat for small-patch-restricted species. Occasionalexamples are known of species that do notpersist in larger patches.5. Protect scattered small habitats and rare species.The bottom line: large patches, large benefits, andsmall patches, small supplemental benefits.Three measures of patch size and type are used toevaluate the various attributes of patches within thissystem; patch area, perimeter length and perimeter/area ratio. Patch area is a simple measurement ofthe patch area in hectares and, with other factors,may be a good measure of long term viability of apatch. Perimeter length is used in combination with
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