Utilities of edge-based metrics for studying landscape fragmentation

Computers, Environment and Urban Systems

29 (2005) 159–178

www.elsevier.com/locate/compenvurbsys

Utilities of edge-based metrics forstudying landscape fragmentation

Hui Zeng a, X. Ben Wu b,*

a Department of Ecology, College of Environmental Science, Peking University,

Beijing 100871, PR Chinab Department of Rangeland Ecology and Management, Texas A&M University,

College Station, TX 77843-2126, USA

Abstract

Landscape fragmentation associated with human activities poses serious threats to con-

servation of the Earth’s natural ecosystems. A central issue in landscape fragmentation is the

ecological effect of habitat edges, but edge characteristics and associated metrics have not been

widely used in landscape fragmentation studies. This study explored the utility of edge-based

metrics in quantifying landscape fragmentation using simulated artificial landscapes and a

real-world example of landscape fragmentation in southwest China’s Wolong Nature Reserve.

Results from artificial landscape simulations demonstrate that edge-based landscape metrics

are effective measures of landscape fragmentation, capturing important aspects of landscape

fragmentation not detected by patch-based metrics. The landscape of the reserve has become

more fragmented due mostly to effects of past reforestation efforts and of sustained human

disturbances at lower elevations during the study period. This paper illustrates that the use of

edge-based metrics, in conjunction with patch-based metrics, can provide more comprehensive

information on the patterns and dynamics of landscape changes, and thus facilitate the as-

sessment and monitoring of landscape fragmentation and restoration.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Landscape fragmentation; Edge-based metrics; Wolong Nature Reserve

1. Introduction

Landscape fragmentation associated with human activities leads to loss of habi-

tat, reduced habitat patch size, and increased isolation among habitat patches. It is

* Corresponding author. Tel.: +1-979-845-7334.

E-mail address: [email protected] (X. Ben Wu).

0198-9715/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compenvurbsys.2003.09.002

mail to: [email protected]

160 H. Zeng, X. Ben Wu / Comput., Environ. and Urban Systems 29 (2005) 159–178

considered one of the most serious threats to the conservation of the Earth’s natural

ecosystems (Collinge, 1996; Knight, Smith, Romme, & Buskirk, 2000; Laurance

et al., 1997; Rochelle, Lehman, & Wisniewski, 1999; Turner, 1996) as well as a threatto the health of agricultural and urbanized ecosystems (Carsjens & van Lier, 2002).

Even in protected natural areas such as nature reserves, landscape fragmentation can

be substantial due to direct and indirect effects of human activities within and around

the reserves (Zheng, Wallin, & Hao, 1997).

A central issue in landscape fragmentation is the ecological effect of habitat edges,

boundaries between adjacent ecosystems, which have been long recognized by

ecologists and often referred to as the ‘‘edge effect’’ (Burgess & Sharpe, 1981; Foggo,

Ozanne, Speight, & Hambler, 2001; Leopold, 1933; Paton, 1994). After a period ofdeclining interest in the edge effect in ecological literature, interest in its study has

resurged in the 1990’s with the appearance of more mechanistic approaches based on

strong landscape perspectives (Lidicker, 1999).

An impressive body of recent literature, both empirical and theoretical, has shown

that habitat edges and their dynamics modify microclimate (Chen et al., 1999;

Gehlhausen, Schwartz, & Augspurger, 2000) and influence plant dispersal and

community structure (Euskirchen, Chen, & Bi, 2001; Herlin & Fry, 2000; Kupfer,

Malanson, & Runkle, 1997), animal assemblage structures (Manson, Ostfeld, &Canham, 1999), reproductive ecology (Flaspohler, Temple, & Rosenfield, 2001a,

2001b), abundance and distribution (Demaynadier & Malcolm, 1998; Golden &

Crist, 2000), predation and herbivory (Cadenasso & Pickett, 2000; Gardner, 1998;

Heske, Robinson, & Brawn, 1999), species interaction (Fagan, Cantrell, & Cosner,

1999), species diversity (Gascon & Lovejoy, 1998; Kollmann & Schneider, 1999;

Metzger, 2000), and the distribution and fluxes of invasive species (Cadenasso &

Pickett, 2001; Stiles & Jones, 1998). Furthermore, two processes have been recog-

nized as driving habitat fragmentation: within-patch effects linked to the creation ofhabitat patches and outside-patch influences of matrix habitat (Gascon & Lovejoy,

1998). It is important to examine the ecological mechanisms associated with these

two landscape processes (Collinge & Forman, 1998; Donovan, Jones, Annand, &

Thompson, 1997; Wiens, 1995).

Edge attributes include the amount, length, and distribution of edges between

specific patch types. They are directly related to and influence the ecological pro-

cesses of within-patch edge effects and outside-patch influences of neighboring pat-

ches. These edge attributes can provide critical information for quantifying andunderstanding landscape fragmentation, and yet, in fragmentation studies, compo-

sitional and patch-based landscape metrics are most often used for qualifying

landscape changes (Hargis, Bissonette, & David, 1998). When edge metrics are used,

they are usually limited to the total amount of edges or edge density (cumulative

edge length per unit area) for the entire landscape or for a given patch type

(McGarigal & Marks, 1995). These metrics can be considered as ‘‘patch type-based’’

landscape attributes. ‘‘Edge type-based’’ attributes, which are the attributes of edges

between specific patch types, however, are seldom used. Changes in these edge at-tributes will likely have a direct link to impacts on ecological processes and to

mechanisms of disturbances. These edge characteristics and associated metrics are

H. Zeng, X. Ben Wu / Comput., Environ. and Urban Systems 29 (2005) 159–178 161

likely to be effective measures of landscape fragmentation and can enrich landscape

pattern analysis by providing insights that cannot be gained using traditional ap-

proaches (Metzger & Muller, 1996).During recent years, increased attention has been paid to edge characteristics in

the analysis of landscape pattern and changes. Several studies have focused on the

contribution of edge characteristics to the understanding of changes in landscape

patterns and functions related to landscape fragmentation (Metzger, 2000; Metzger

& Muller, 1996) and on the spatial patterns and scaling of edge influences associated

with fragmentation (Rescia, Schmitz, deAger, dePablo, & Pineda, 1997; Zheng &

Chen, 2000). Furthermore, a number of quantitative approaches and metrics have

been developed for edge/boundary analysis in landscape studies (Bogaert, VanHecke, Moermans, & Impens, 1999; Fortin et al., 2000; Rescia et al., 1994; Zheng &

Chen, 2000). But so far there have not been systematic evaluations of the utilities of

edge-based metrics in the context of landscape fragmentation.

The objective of this study was to explore the forms and evaluate the utilities of

edge-based landscape metrics at both edge type and landscape levels, using simulated

artificial landscapes and a real-world example of landscape fragmentation in

southwest China’s Wolong Nature Reserve. Although there are different opinions

about the meaning of the terms ‘‘boundary’’ and ‘‘edge’’ (Forman, 1995; Kent, Gill,Weaver, & Armitage, 1997), these terms have been used more or less interchangeably

in fragmentation studies. In this paper, the term ‘‘edge’’ is used to represent a rel-

atively sharp boundary between two patches of different types.

2. Methods

2.1. Edge-based metrics

Patch-based landscape metrics, such as patch density, mean patch size, and mean

patch shape index, are frequently used in quantifying landscape fragmentation

(Frohn, 1998; McGarigal & Marks, 1995; Turner, Gardner, & O’Neill, 2001; Wu,Thurow, & Whisenant, 2000). Although edge density, the cumulative length of edges

per unit area (McGarigal & Marks, 1995), is frequently used, it has been most often

applied to patch types (e.g., edges between forest patches and patches of all other

land cover types) rather than edge types (e.g., edges between forest patches and

cropland patches).

At the edge type-level, edge attributes may include the cumulative length of edges

per unit area (‘‘edge density’’, m/km2), number of edge segments per unit area (‘‘edge

segment density’’, #/km2), and average length of edge segments (‘‘mean segmentlength’’, m) for specific edge types. An ‘‘edge segment’’ refers to a continuous edge of

a given edge type. These attributes can not be derived from a landscape analysis

based on individual patches using popular landscape analysis software packages

such as FRAGSTATS, but can be generated through GIS analysis or using other

software such as LANDSOFT (Sui & Zeng, 2001). Landscape level edge attributes,

such as the diversity and interspersion of edge types in a landscape, also have a


significant relationship to the degree of landscape fragmentation (Metzger & Muller,

1996; Rescia et al., 1997; Trani & Giles, 1999). Landscape-level metrics commonly

based on areal cover of patch types, such as diversity, evenness, and dominance(O’Neill et al., 1988), can also be defined based on the relative abundance of edge

types, both in terms of edge density (length) and edge segment density (number).

Shannon’s diversity index �P

iðPi ln PiÞ, Shannon’s evenness index �P

iðPi ln PiÞ=lnN , and the dominance index lnN þ

PiðPi ln PiÞ, where Pi is the relative abundance

of an edge type measured by edge density or edge segment density and N is the

number of edge types, can be used to quantify landscape fragmentation.

2.2. Parameters of edge frequency distribution as metrics of landscape fragmentation

In addition to edge density and edge segment density, the frequency distribution

of edge segments of different lengths is also important for quantifying landscape

fragmentation. It is suggested that not only does the number and total length of

edges tend to increase, but the relative abundance of short edges will also likely

increase with increased landscape fragmentation (Collinge & Forman, 1998). Fre-quency distribution of edge segments of different length can provide information on

pattern of fragmentation, which cannot be derived from edge density or edge seg-

ment density, and can yield insights on associated ecological processes.

Direct examination of frequency distributions of landscapes is not efficient for

quantification and comparison, especially when dealing with large numbers of

landscapes. Therefore, we developed a new approach to characterize frequency

distributions of edge segments by fitting a logarithmic model to the frequency dis-

tribution, and using the model parameters as a parsimonious quantification of edgefrequency distribution. A logarithmic model in the form of lnNe ¼ �a lnðLeÞ þ b,where Ne is the number of edges of a given length Le, is fitted to the frequency dis-

tribution of edge segments in a landscape. The parameters a and b in the logarithmic

model can be particularly useful for quantifying edge characteristics and landscape

fragmentation. The parameter a indicates the steepness of the curve––the larger the

value of a, the steeper is the curve, and hence a higher proportion of short edges.

This parameter, therefore, can be used as a metric of ‘‘edge segmentation’’, or rel-

ative abundance of short edges. The parameter b reflects the average differences inedge abundance between adjacent length intervals––for a given a, a higher value of bmeans a larger number of edges in all length intervals. Since b ultimately reflects the

total number of edges of landscapes, we should rather use edge segment density,

which has direct physical meaning, along with a to quantify edge characteristics for

assessing landscape fragmentation.

2.3. Behavior of edge metrics based on artificial landscapes

In order to evaluate the behavior of the edge-based metrics as measures of

landscape fragmentation, a set of artificial landscapes were generated using the

SIMMAP software (Saura & Mart�ınez-Mill�an, 2000). Four levels of fragmentation

were defined by setting the P parameter of SIMMAP to 0.49, 0.52, 0.55, and 0.58,


which represent a realistic range of landscape fragmentation. Four patch types (A, B,

C, and D) were used, where three levels of compositional evenness were considered:

high evenness (25% initial cover in every patch type), medium evenness (35%, 35%,20%, and 10% initial cover for A, B, C, and D, respectively), and low evenness (70%,

10%, 10%, and 10% initial cover for A, B, C, and D, respectively). The percent covers

of the patch types in the artificial landscapes generated can deviate from the initial

values due to stochasticity in the generating process. The dimension of the land-

scapes was 500 · 500 pixels and the minimum patch size was set to 4 pixels. Six

replicate landscapes were generated for each combination of levels of fragmentation

and evenness. Each artificial landscape was analyzed using LANDSOFT (Sui &

Zeng, 2001) to determine the values of edge-based metrics, at both edge type andlandscape levels, as well as patch-based metrics related to fragmentation.

2.4. Edge-based metrics and landscape fragmentation in the Wolong Nature Reserve

Landscape fragmentation in the Wolong Nature Reserve was used as a case ex-

ample to explore the utility of edge-based metrics for quantifying fragmentation of

actual landscapes. Located in southwestern China (102�520 to 103�240E, 30�450 to31�250N) and covering around 2000 km2 (Fig. 1), the Wolong Nature Reserve is a

flagship protected area established by the National Council of China in 1975, and

subsequently designated as a ‘‘Man and Biosphere’’ reserve by UNESCO in 1980

(Qin & Allen, 1993). Alpine ecosystems and rare species such as the giant panda

(Ailuropoda Melanolenca) (Schaller, Hu, Pan, & Zhu, 1985) are its main protectiontargets (Qin & Allen, 1993). This area was subjected to small-scale logging during the

first half of the 20th century. Large-scale forestry operations, logging and the

planting of coniferous tree seedlings, were conducted in part of the area from 1965 to

1975. Since the establishment of the reserve, large-scale human disturbances such as

commercial logging activities have been prohibited. Human impact has been largely

restricted to agricultural activities of local residents in limited areas, mostly around

residential areas.

Land cover data for the Wolong Nature Reserve was developed based on TMimages acquired in 1987 and 1997. The images were rectified and georeferenced using

12 ground control points (RMS were 0.0673 and 0.0741 for the 87 and 97 images,

respectively). A maximum likelihood supervised classification method in IDRISI was

used to classify the images based on raw bands and NDVI to generate land cover

maps. Six vegetation or land cover types were used in the classification: coniferous

forest, mixed coniferous–broadleaf forest, broadleaf forest, shrub, herbaceous, and

bare. A total of 43 sites (10 coniferous forest, 11 mixed forest, 12 broadleaf forest, 6

shrub, and 4 herbaceous sites) were sampled and used as training sites for classifi-cation, along with another 15 bare sites selected from the imagery. Each non-bare

site consisted of a 25 m · 25 m plot located in a homogeneous area with a single cover

type, and its location was recorded using a GPS with differential correction. Results

of plant community sampling and analysis at these sites suggested that no cover type

changes occurred between 1987 and 1997, therefore these training sites were used for

both the 1987 and 1997 classifications. Another set of 119 sites (20 coniferous forest,

Fig. 1. Study area––Wolong Nature Reserve, a flagship protected area established in 1975 for the pro-

tection of the endangered giant panda (A. Melanolenca), Sichuan Province, Southwestern China (102�520to 103�240E, 30�450 to 31�250N).


21 mixed forest, 20 broadleaf forest, 24 shrub, 19 herbaceous, and 15 bare sites) was

selected in the vicinity of the road network, and assessed in the field for evaluatingthe accuracy of the classification. It was verified that these sites, among a larger pool

of randomly selected sites, had kept the same vegetation cover types during the study

period, both using historical data and consultation with the reserve personnel and

other local experts.

The overall accuracy based on these evaluation sites was 90.8% for the 1987

classification and 93.3% for the 1997 classification. All producer’s accuracy (measure

of omission error) and user’s accuracy (measure of commission error) for individual

cover types in both 1987 and 1997 classifications were P82%. The number andspatial distribution of the verification sites were not the ideal due to logistical limi-

tations, and the selection of the stable (no change in cover type from 1987 to 1997)

sites might cause over-estimation of the classification accuracy. These limitations

should be kept in mind when considering ecological interpretations of the landscape

change, but they should not compromise the goal of this study to illustrate the

utilities of landscape metrics in landscape fragmentation studies. A 3 · 3 (pixel) four-


direction filter procedure was used to reduce small-scale noise in classification. The

TM images from both years had some areas (collectively about 20% of the reserve

area) that were covered with clouds, and thus were excluded from the final classi-fication maps. The 1987 and 1997 land cover maps were analyzed using LAND-

SOFT to determine the values of edge-based metrics at both edge type-level and

landscape-level, as well as patch-based metrics related to fragmentation.

3. Results

3.1. Edge-based metrics and their behavior based on artificial landscapes

Edge type-based edge density and edge segment density increased and mean edge

segment length decreased consistently with increased fragmentation, and patch-

based edge density also increased (Fig. 2). Comparisons between these metrics forlandscape with different levels of compositional evenness, however, differ consider-

ably among edge and patch types. Patterns in edge-based metrics are complex and

can be different from those found in patch-based edge density. For example, the edge

density for patch type A had the highest value for landscapes with medium com-

positional evenness, which represents, however, a mix of drastically different densi-

ties of edges between patch type A and the other patches types (B, C and D). These

landscapes had the lowest A–B edge densities and an exceptionally high A–C edge

density. Such complexities can provide critical information for understanding thepatterns, mechanisms, and ecological implications of landscape fragmentation. Al-

though the landscape metrics were calculated for the same set of artificial landscapes,

their values have different ranges. In order to facilitate comparisons, results for each

of these metrics were standardized by subtracting the minimum value and divided by

the difference between the maximum and minimum values of the metric (Fig. 3). The

values of the patch type-based diversity index decreased with decreasing composi-

tional evenness as expected. They showed small changes along levels of fragmenta-

tion within the evenness levels, which was due to the variations in percent cover ofthe patch types in the generated landscapes. Edge (type) density-based diversity

showed a considerably different trend. Landscapes with medium compositional

evenness had higher diversity, compared to those with low compositional evenness

(Fig. 3), which is due to greater variation in the density of edge types (Fig. 2).

Edge segment density-based diversity had a similar trend as patch-based diversity,

but greater separation among levels of compositional evenness. Dominance showed

an opposite pattern when compared to diversity. The apparent insensitivity of di-

versity and dominance to increasing levels of fragmentation in the artificial land-scapes quite likely happened because the algorithm induced fragmentation equally to

all patch types, resulting in relative stable evenness; actually fragmentation would

likely occur preferentially to selected landscape elements in real landscapes. Despite

the lack of changes in diversity and dominance with higher fragmentation, both the

number of patches and the number of edges increased with increasing fragmentation

(Fig. 3). Although landscapes with medium and high compositional evenness had a

25

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High evenness Medium evenness Low evenness

Fig. 2. Edge densities based on patch type and edge type, edge segment density, and mean edge segment

length for artificial landscapes with four patch types (A, B, C and D) and three levels of compositional

evenness, generated using SIMMAP. The x-axis is a combination of patch type and level of fragmentation

for the top plot and a combination of edge type and level of fragmentation for the bottom three plots.

Four levels of fragmentation were defined by the P parameter of SIMMAP (0.49, 0.52, 0.55, and 0.58), a

lower P represents greater fragmentation.


High evenness Medium evenness Low evenness

Initial P (increasing fragmentation with decreasing initial P)

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Fig. 3. Standardized diversity and dominance based on patch type, edge (type) density, and edge segment

density, number of patches (patch #), number of edges (edge #), and edge segmentation metrics for

artificial landscapes with four patch types and three levels of compositional evenness, generated using

SIMMAP. The x-axis is the level of fragmentation defined by the P parameter of SIMMAP; a lower Prepresents greater fragmentation.


similar number of patches, the number of edges increased along with increasing

compositional evenness. The edge segmentation metric, representing the relative

abundance of shorter edge segments, increased with both increasing fragmentation

and increasing compositional evenness.

3.2. Edge-based metrics and landscape fragmentation in Wolong Nature Reserve

Although the amount of forest coverage in the Wolong Nature Reserve remained

virtually unchanged from 1987 to 1997, the landscape became more fragmented withincreased patch density (16.5%), decreased mean patch size ()14.3%), and increased


edge density (10.9%) (Table 1), as a net result of dynamics in the coverage of different

forest types. Broadleaf forests decreased considerably ()13.1%), while mixed forests

decreased only slightly ()2.0%), but both became considerably more fragmented,with more and smaller patches. These decreases in forest coverage were compensated

for by a substantial increase in coniferous forests (39.8%), with a slightly increased

mean patch size but substantially increased patch density and edge density. Simi-

larly, the stable but more fragmented non-forested coverage resulted from the sub-

stantially decrease in shrub and herbaceous coverage being balanced by an increased

in bare grounds (Table 1). Both shrub and herbaceous coverages became more

fragmented over the period, whereas bare grounds became less fragmented.

There were apparently two major processes that contributed to the landscapechanges in Wolong from 1987 to 1997. One was past reforestation efforts that led to

the increase of coniferous forests. Another was the sustained human disturbance that

increased bare grounds, mostly at lower elevations. The effect of these processes on

landscape structure and dynamics can be better understood through examination of

edge attributes, especially edge density (total edge length per unit area) and edge

segment density (number of edge segments per unit area), which provide more

comprehensive quantifications of fragmentation. These attributes for individual edge

types (Table 2) can be used to further elucidate the specific dynamics of landscapestructure.

The significant increase of coniferous forests was due primarily to reforestation

efforts since the mid 1960’s. The former Red Flag Forestry Bureau planted about

7000 ha of spruce-fir forest following logging during the period between 1965 and

1975. The Management Branch of Wolong Nature Reserve has also organized the

planting of pine forests since 1980. In 1987, considerable amounts of these reforested

areas were occupied by young conifer stands with low canopy cover of conifer

saplings mixed with other plants; some of these young conifer stands could not bedistinguished from shrubs in the 1987 TM imagery. By 1997, however, most of these

reforested areas displayed the characteristics of coniferous forests, with a substantial

canopy cover of conifers, and were successfully classified. This increase in the

number of small conifer forest patches had a substantial influence on the landscape

structure. Both edge density and edge segment density increased in every type of

conifer edges (Table 2).

The substantial increases in conifer–mixed and conifer–broadleaf edges indicate

that considerable amounts of planting took place in disturbed areas in mixed andbroadleaf forest areas occurring at elevations lower than the typical zone of natural

occurrence of conifer forests. Conifer–shrub edges also increased substantially,

which may reflect adjacencies between planted areas and unplanted disturbed areas

undergoing slower recovery, or adjacencies between grown planted areas and re-

cently planted areas. Overall, when all vegetation types are considered, reforestation

with conifers may have increased landscape fragmentation. It may have, however,

offset to a certain degree the overall trend of forest fragmentation, when forest

coverage is considered as a whole.There appeared to be some sustained or perhaps slightly intensified human dis-

turbances generating or maintaining bare ground areas, especially at lower eleva-

Table 1

Dynamics in the landscape composition and spatial patterns in the Wolong Nature Reserve from 1987 to 1997

Cover type Areal cover (%) Patch density (#/km2) Mean patch size (ha) Edge density (m/km2)

1987 1997 Change 1987 1997 Change 1987 1997 Change 1987 1997 Change

Forest 62.4 62.5 0.07 11.90 13.89 1.97 8.40 7.20 )1.20 2677.2 2940.3 263.1

Conifer 9.9 13.8 3.9 1.95 2.61 0.66 5.06 5.29 0.23 1702.5 2560.1 857.7

Mix 27.0 26.5 )0.5 1.99 2.29 0.30 13.59 11.56 )2.03 3835.9 4202.6 366.7

Broadleaf 25.5 22.2 )3.3 1.75 1.94 0.18 14.54 11.44 )3.10 3254.0 2991.7 )262.3Shrub 15.9 13.4 )2.4 1.93 2.57 0.64 8.23 5.20 )3.03 2654.5 2640.5 )14.0Herbaceous 4.5 4.1 )0.5 0.51 1.13 0.61 8.84 3.62 )5.22 530.4 815.3 284.9

Bare 17.2 20.1 2.9 3.77 3.35 )0.41 4.58 6.00 1.42 2045.1 1888.2 )156.8

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169

Table 2

Edge density (length of edges per km2) and edge segment density (number of edge segments per km2) by edge type in the Wolong Nature Reserve in 1987 and

1997

Edge type Edge density (m/km2) Edge segment density (#/km2)

1987 1997 Change (% Change) 1987 1997 Change (% Change)

Conifer–mixed 923.0 1477.3 554.4 (60.1) 3.2 5.0 1.8 (55.0)

Conifer–broadleaf 38.2 100.8 62.6 (163.6) 0.6 1.3 0.7 (123.7)

Conifer–shrub 281.7 485.6 203.9 (72.4) 1.9 3.3 1.5 (78.7)

Conifer–herbaceous 9.8 10.0 0.2 (1.8) 0.1 0.2 0.1 (74.7)

Conifer–bare 449.8 486.4 36.6 (8.1) 3.9 4.0 0.2 (4.5)

Mixed–broadleaf 2096.3 1829.0 )267.4 ()12.8) 4.3 4.9 0.7 (16.1)

Mixed–shrub 371.1 413.7 42.7 (11.5) 2.9 3.3 0.4 (13.9)

Mixed–herbaceous 5.8 55.9 50.0 (858.6) 0.1 0.6 0.5 (562.5)

Mixed–bare 439.6 426.7 )12.9 ()2.9) 4.4 4.1 )0.3 ()7.1)Broadleaf–shrub 799.5 727.5 )72.0 ()9.0) 2.8 2.8 0.0 ()1.7)Broadleaf–herbaceous 3.6 149.7 146.1 (4051.5) 0.1 1.3 1.2 (1934.0)

Broadleaf–bare 316.3 184.8 )131.4 ()41.6) 2.9 1.9 )1.0 ()34.1)Shrub–herbaceous 437.0 411.6 )25.4 ()5.8) 1.4 2.3 1.0 (69.7)

Shrub–bare 765.2 602.0 )163.2 ()21.3) 4.9 3.8 )1.0 ()21.4)Herbaceous–bare 74.1 188.2 114.1 (153.9) 0.6 1.3 0.7 (111.6)

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tions where most human activities are concentrated. Bare ground coverage increased

substantially (16.8%) from 1987 to 1997, while shrub and herbaceous coverage de-

creased. Shrub and herbaceous coverage became more fragmented, with substan-tially increased patch density and decreased mean patch size. These changes were

likely due to a combination of uneven rates of vegetation recovery through succes-

sion related to site variability, and intensified human disturbances that expanded

bare grounds. In contrast, bare grounds had become more aggregated, with de-

creased patch density, increased mean patch size, and decreased edge density (Table

1). This trend suggests the apparent coalescence of bare areas, possibly due to in-

tensified disturbances. Dynamics in the attributes of individual types of bare area

edges show differential changes of bare edge types (Table 2). There were considerabledecreases, both in edge density and edge segment density, for bare–broadleaf and

bare–shrub edges, as well as a slight decrease for bare–mixed edges. Edge density and

edge segment density for bare–conifer and bare–herbaceous edges, however, in-

creased over the 10-year period.

Human disturbances generate bare grounds largely at lower elevations, where

mixed and broadleaf forests dominate. Decreased bare–broadleaf and bare–mixed

edges indicate reduced new disturbances (like logging) in these forests, while older

disturbed areas in the forests recovered. The small increase of bare–conifer edgesresulted from an increase in small conifer plantations. The increased bare–herba-

ceous edges may indicate aggregated areas, likely near residential areas, which were

subject to sustained or frequent disturbances. These changes in edge types suggest

spatial redistribution and, possibly, the stabilization and concentration of disturbed

areas near residential areas at lower elevations.

Not only edge density and edge segment density increased from 1987 to 1997 in

the Wolong Nature Reserve (Table 2), but ‘‘edge segmentation’’ also increased, in-

dicating a rise in the relative abundance of short edges resulting from higher land-scape fragmentation. A large proportion of the edge segments in the 1987 and 1997

landscapes of Wolong Nature Reserve were short, and the number of edges de-

creased exponentially with increasing edge lengths.

4. Discussion

4.1. Edge-based versus patch-based metrics for quantifying landscape fragmentation

Edge type-based metrics are able to capture complexities in landscape structure

and change that cannot be detected by patch type-based metrics. Quantification of

these complexities can be critical for understanding ecological changes in landscapepatterns and the associated functional consequences of landscape fragmentation.

Compared to patch type-based edge density, edge type-based edge density, edge

segment density, and mean edge segment length can reveal more information about

differential patterns of landscape fragmentation. They can help to quantify the dy-

namics in the abundance and attributes of specific types of edges (ecotones), and

infer the associated ecological effects.


In addition to metrics for individual edge or patch types, more-parsimonious

landscape-level metrics of landscape fragmentation are also necessary, especially for

assessing fragmentation at multiple points in time and for a large number of land-scapes. Diversity, evenness, and dominance are commonly used landscape-level

metrics for quantifying the heterogeneity of landscapes. Although these metrics were

originally defined based on areal proportions of patch types, they can be constructed

based on edge (type) density and edge segment density in order to quantify different

aspects of landscape heterogeneity related to landscape fragmentation (Fig. 3).

Landscape diversity metrics based on Shannon’s index measures both the richness

(number of types) and evenness (equality in abundance of types) of a landscape.

Diversity based on areal proportion measures compositional diversity withoutproviding information on the patterns of interaction among different patch types.

Diversity based on edge density or edge segment density, however, quantifies

heterogeneity based on the functional interfaces among patch types. When landscape

fragmentation changes the relative abundance of patch types, both patch diversity

and edge diversity will change. When the fragmentation process breaks patches into

smaller units, however, it can lead to considerable changes in edge length and edge

number without substantial changes in the relative abundance of patch types. An

edge type-based diversity measure, therefore, can potentially capture landscapechanges due to fragmentation better than patch type-based diversity. Similarly,

evenness and dominance based on edge types may likely be more sensitive than those

based on patch types. Data on landscape fragmentation collected for the Wolong

Nature Reserve in 1987 and 1997 corroborated this idea, and suggest that edge type-

based diversity, evenness and dominance measures may be slightly more sensitive to

landscape changes in the reserve than patch type-based ones (Table 3).

Evenness, which is independent from the number of types, is likely a more useful

measure than diversity for most landscape fragmentation studies, because thenumber of patch types in a given landscape usually changes little, and different

landscapes in comparative studies may have different numbers of patch types.

Evenness based on edge density is actually identical to the widely used interspersion

and juxtaposition index (IJI) proposed by McGarigal and Marks (1995). Edge

evenness or IJI measure the interspersion of different patch types, as reflected by

evenness in the relative abundance (in length) of different edge types, which is in-

fluenced by landscape fragmentation (Trani & Giles, 1999). Dominance (O’Neill

et al., 1988), a reciprocal measure of evenness but lacking a standardized range, isoften used to replace evenness, probably because it has a more straightforward

physical meaning in a landscape context.

Diversity, evenness, and dominance based on edge segment density (number)

appear to be more sensitive to landscape changes in the Wolong Nature Reserve than

the metrics based on edge density (length) (Table 3). Fragmentation can increase

considerably the number of edge segments without large changes in the cumulative

edge length, which may make edge number-based metrics more sensitive to land-

scape fragmentation. The opposite situation, with large changes in the edge lengthand small changes in the edge number, can also result from fragmentation but likely

are less common.

Table 3

Changes in overall landscape pattern based on areal proportion and edge attributes

Metric Areal proportion based Edge density based Edge segment density based

1987 1997 % Change 1987 1997 % Change 1987 1997 % Change

Diversity 1.67 1.68 1.0 2.16 2.29 5.8 2.38 2.53 6.3

Evenness 0.93 0.94 1.0 0.8 0.84 5.8 0.88 0.93 6.3

Dominance 0.13 0.11 )13.0 0.55 0.42 )23.1 0.33 0.18 )45.3

H.Zeng,X.Ben

Wu/Comput.,

Enviro

n.andUrbanSystem

s29(2005)159–178

173


4.2. Edge-based versus other metrics for quantifying landscape fragmentation

Several other non-edge-based landscape metrics have been used for quantifyinglandscape fragmentation. The patch-based fragmentation index (Turner, 1990), de-

fined as ðN � 1Þ=A, where N is the number of patches in the landscape and A is the

average patch size, is generally an effective tool for measuring landscape fragmen-

tation. It cannot, however, capture effects of different edge types, i.e., the different

spatial interfaces among patch types. For example, a landscape with four spatially

separated square patches embedded in a forest matrix, two recent clear-cuts and two

older clear-cuts halfway through succession, would yield the same fragmentation

index as a landscape having the same structure except where each pair of recent andolder clear-cut patches were budding against each other. On the other hand, edge-

based dominance or IJI would be able to reflect the difference between these two

landscapes.

Contagion, a pixel-based metric that measures the extent to which landscape

elements are aggregated within a landscape (the opposite of fragmentation) (Li &

Reynolds, 1993; O’Neill et al., 1988), can also be used to measure landscape frag-

mentation. It is, however, highly scale dependent (Turner, O’Neill, Gardner, &

Milne, 1989), making it difficult to use in comparative studies. In contrast, edgedensity for a landscape is less sensitive to scale and, since it has a strong negative

correlation with contagion (Hargis et al., 1998), it can capture similar information

on landscape fragmentation.

Lacunarity is another landscape metric that quantifies the spatial heterogeneity of

a landscape, or the deviation of a geometric object from translational invariance

(Plotnick, Gardner, & O’Neill, 1993). Multiple-scale lacunarity analysis has been

efficiently used to quantify landscape fragmentation in both binary and continuous

landscapes (Derner & Wu, 2001; Wu et al., 2000). Lacunarity analysis, however, isrestricted by the need to use a single binary or continuous variable (Plotnick,

Gardner, Hargrove, Prestegaard, & Perlmutter, 1996). Although it is possible to use

lacunarity analysis with multiple-element landscapes in a piecewise approach (Wu &

Sui, 2002), computation becomes intensive and the representation of the results can

become cumbersome, especially for comparative studies involving a large number of

landscapes.

4.3. Utilities of edge-based metrics for quantifying landscape fragmentation

Compositional and patch-based landscape metrics have been most often used in

studies of landscape fragmentation (Coppedge, Engle, Fuhlendorf, Masters, &

Gregory, 2001; Hargis et al., 1998; Zheng et al., 1997). Our study suggests that edge-based landscape metrics, such as edge density (reflecting the amount of edge and

spatial aggregation), edge-based dominance or the IJI (reflecting the relative abun-

dance of edge types and interspersion), and edge segment density and edge seg-

mentation (reflecting the edge segment length distribution), are effective measures to

quantify important aspects of landscape fragmentation either not or ineffectively

captured by other types of landscape metrics.


Although changes in edge attributes can affect interior habitat, edge attributes do

not provide direct information on changes in the amount and attributes of interior

habitat. Direct measures of interior habitat, such as core-area attributes (McGarigal& Marks, 1995), should be used in conjunction with edge-based metrics when spe-

cies-specific interior habitats are of interest. Use of both patch-based and edge-based

landscape metrics can be very useful by providing more comprehensive information

for the understanding of landscape patterns and dynamics.

5. Conclusions

Edge-based landscape metrics, especially edge density (length) and edge segment

density (number) for specific edge types, can be used in conjunction with patch-based

metrics for a more comprehensive characterization of fragmentation, and to eluci-

date specific dynamics of landscape structure and function. In addition to metricsbased on individual edge types, landscape-level edge-based metrics can provide

parsimonious measures for landscape fragmentation. Diversity, evenness, and

dominance measures based on edge types can capture different aspects of landscape

heterogeneity and may be more sensitive to some landscape changes related to

fragmentation than those based on patch types. These edge-based landscape metrics

capture information related to the functional interfaces among patch types, thus

enhancing our ability to quantify and understand landscape fragmentation.

When a landscape becomes more fragmented, not only its edge density and edgesegment density increase, but also the frequency distribution of the edge segments

tends to change, with an increasing proportion of shorter edge segments as an im-

portant signature of fragmentation. Using the slope parameter of a logarithmic

model fitted to the frequency distribution of edge segment lengths, we defined an

‘‘edge segmentation’’ metric that quantifies the relative abundance of shorter edges.

Edge segmentation was sensitive to both fragmentation and compositional evenness

in simulated landscapes.

A case example showed that the landscape in the Wolong Nature Reserve becamemore fragmented from 1987 to 1997, although the total amount of forest cover re-

mained unchanged. This fragmentation is reflected in the increasing patch density

and decreasing mean patch size for all the vegetation types, except conifer forests and

bare grounds. Both edge density and edge segment density increased from 1987 to

1997, as well as the relative proportion of short edges as measured by edge seg-

mentation metrics, all indicating increased landscape fragmentation. Two major

ecological processes apparently contributed to the landscape changes in Wolong

from 1987 to 1997: one is past reforestation efforts that led to an increase in coni-ferous forest coverage, and another is the sustained human disturbance that has

increased bare grounds, especially at lower elevations.

Although compositional and patch-based landscape metrics have been dominant

in studies of landscape fragmentation, our study shows that edge-based landscape

metrics can better quantify multiple aspects of landscape fragmentation, such as the

amount of edge and spatial aggregation, the relative abundance of edge types and


interspersion, and edge segment length distribution. Most of these attributes are not

captured by patch-based metrics. The combined use of both patch-based and edge-

based landscape metrics can provide more comprehensive information for under-standing landscape patterns and dynamics, and thus facilitate the assessment and

monitoring of landscape fragmentation and restoration.

Acknowledgements

This study is partially supported by the National Basic Science and Technol-

ogy Planning Project (G2000046807) sponsored by the Ministry of Science and

Technology of China, and the National Natural Science Foundation of China

(40171091). We thank Keming Ma, Liding Chen, and Lingyun Gao for their help in

the field. Assistance by Ningning Kong and Shujuan Li in data acquisition and thedatabase development process is gratefully acknowledged. We thank Rusty Feagin,

Matt Simmons, and Jose Cuervo for reviewing an earlier version of this manuscript.

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Utilities of edge-based metrics for studying landscape fragmentation