Corridor use and the elements of corridor quality: Chipmunks and fencerows in a farmland mosaic

Biological Conservation 68 (1994) 155-165

CORRIDOR USE A N D THE ELEMENTS OF CORRIDOR QUALITY: CHIPMUNKS AND FENCEROWS IN A FARMLAND MOSAIC Andrew F. Bennett Department of Conservation and Natural Resources. 123 Brown Street, Heidelberg, Victoria 3084, Australia

Kringen Henein & Gray Merriam Department of Biology, Carleton University. Ottawa. Canada, KIS 5B6

(Received 14 November 1992; revised version received 5 July 1993; accepted 5 July 1993)

INTRODUCTION

Abstract An important issue in developing practical conservation actions and in understanding how corridors function is that of identifying what constitutes a high quality corridor jot a particular species or assemblage. We studied the use of fencerow corridors by the eastern chipmunk Tamias striatus to identify the elements of corridor quality for an animal species. Chipmunks were trapped in Jour woods and 18 fencerows of varying wMth, habitat and linear continuity, in farmland near Ottawa, Canada. There were many recorded movements by individuals within and between fencerows and woods. Chipmunks used fencerows in at least two main ways, and the elements of corridor quality were found to differ for these two modes of use. Resident individuals lived within and along many fencerows, thus promoting continuity of the resident population between woods. They favoured Jencerows with tall trees and a woodland structure; neither fencerow width nor linear continuity accounted jot additional variation in the number of residents after habitat attributes were included in a stepwise regression model Transient chipmunks, those trapped once only in a fencerow, apparently used the fencerow network as a pathway through farmland Linear continuity offencerows was the most important correlate of the number of transients, with habitat attributes explaining additional variance in a regression model Fencerows with grassy vegetation alone were never used by chipmunks and, like the surrounding farmland, appear to be inhospitable habitat. The different ways that individuals may use corridors, and the differing elements of corridor quality, have implications for models of corridor use and metapopulation function, and for practical corridor management and restoration.

The concept of incorporating habitat corridors into strategies for the conservation of wildlife populations has recently received increased attention throughout the world (e.g. Recher et al., 1987: Adams & Dove, 1989; Harris & Gallagher, 1989; Bennett, 1990a; Johnsingh et al., 1990; Baranga, 1991; Merriam, 1991; O'Donnell, 1991; Saunders & Hobbs, 1991). This response reflects a growing concern about the effects of widespread habitat loss and fragmentation, and a recognition that practical steps must be taken to maintain and restore continuity to populations of animals that have become fragmented and isolated. There is a growing literature on the value of corridors to wildlife. Much of the information available consists of reports on the use of linear features such as roadsides, streams, hedges, fencerows and plantations as habitat by a wide range of species (e.g. see Bennett, 1990a, 1991 for reviews). However, evidence is also accumulating on the role of corridors in assisting the movements of animals through inhospitable landscapes, and facilitating continuity between otherwise isolated populations (Suckling, 1984; Johnson & Adkisson, 1985; Henderson et al., 1985; Burel, 1989; Bennett, 1990b, Dmowski & Kozakiewicz, 1990; Saunders & de Rebeira, 1991). The potential values of corridors for conservation are also being recognised by theoretical approaches and simulation models (Fahrig & Merriam, 1985; Urban & Shugart, 1986; Burkey, 1989; Henein & Merriam, 1990; Hanski & Gilpin, 1991; Lankester et al., 1991) that have demonstrated that the persistence and stability of small isolated populations will be enhanced by opportunities for dispersal, or by interaction within a metapopulation. Corridors have captured the imagination of the wider community and a diverse range of 'greenways', 'greenbelts', 'linear reserves', 'lifelines', 'buffer strips' and 'wildlife corridors' are being incorporated into land management plans and planning strategies (Dobbyns,

Keywords: Canada, chipmunk, Tamias striatus, fencerows, corridors, farmland.

Biological Conservation 0006-3207/94/$07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain 155

156

A. F. Bennett, K. Henein, G. Merriam

1983; Baudry & Burel, 1984; Harris & Gallagher, 1989; Little, 1990; Harris & Scheck, 1991). The restoration and provision of corridors to link isolated populations is an intuitively appealing concept and it is a practical action that is often within the scope and resources of individuals or small groups. However, the implementation of corridors has, in many ways, forged ahead of scientific understanding and empirical data on their function, conservation value and optimum design (Simberloff & Cox, 1987; Hobbs, 1992; Simberloff et al., 1992). One of the key issues that is central to practical conservation actions, and also to scientific understanding of corridor function and the dynamics of metapopulations, is that of identifying what constitutes quality in a corridor for a particular species or assemblage. Present understanding suggests that three main attributes of corridors--habitat, width, and linear continuity--are likely to be primary influences on their use by animals (Forman & Godron, 1986; Recher et al., 1987; Bennett, 1990a; Harris & Scheck, 1991; Harrison, 1992). Habitat within a corridor determines the availability and abundance of essential resources such as food, shelter, refuge from predators, and breeding sites. Width is a measure of the area available to animals. It also determines the relative amount of 'edge' versus 'interior' habitat, and influences the intensity of 'edge effects' such as micro-climate changes, weed invasion, and predation. Linear continuity refers to the spatial continuity of a corridor in the landscape. Corridor length, the number and severity of gaps or barriers, and the presence of alternative pathways, are potential influences on the ease of passage of an animal through the landscape. In this study we examine the use of fencerow corridors in farmland by the eastern chipmunk Tamias striatus, to identify the elements of corridor quality for a representative woodland animal. The eastern chipmunk is a small, diurnal, burrow-dwelling, sciurid mammal that is widespread and common in eastern North America (Snyder, 1982). In the Ottawa area chipmunks are active from April through October, and hibernate in winter when there is an extensive cover of snow. Although native to woodland, this species persists in many farm landscapes where wooded vegetation remains (Svendsen & Yahner, 1979; Henderson et al., 1985). Henderson et al. (1985) found that population sizes of chipmunks in small woods displayed marked temporal variation, and suggested that local extinctions may regularly occur. They artificially induced population extinctions in two woods and reported that these woods were rapidly recolonized by chipmunks moving from nearby wooded areas, apparently along interconnecting fencerow vegetation. Our study first considers the patterns of use of fencerow corridors by chipmunks in the farmland mosaic. Secondly, we examine the relative importance of three attributes of fencerows, their habitat, width and linear continuity, as factors determining the quality of corridors as perceived by chipmunks.

METHODS Study area The study was carried out in 200 ha of farmland at Manotick, approximately 20 km south of Ottawa, Canada. Small remnant woods, dominated by sugar maple Acer saccharum, white ash Fraxinus americana, eastern white cedar Thuja occidentalis, basswood Tilia americana and white elm Ulmus americana, occur amongst farm fields used mainly for pasture and crops of corn Zea mays and oats Avena sativa. Fencerow vegetation that occurs on narrow uncultivated strips of land along fencelines between fields provides a corridor network through the farmland. Vegetation in fencerows is highly variable in structure and plant species composition. It may range from long grasses to shrubs and vines or a woodland of mature trees, depending on the age and history of disturbance. Stone walls, rocks and fence rails add further structural heterogeneity to some fencerows (Fritz & Merriam, 1993). Within this farmland mosaic, trapping and radiotelemetry studies have both shown that chipmunks are confined to wooded or shrubby vegetation. They have seldom been recorded in, or moving through, grassy fields or crops in the study area (Wegner & Merriam, 1979; Henein & Merriam, unpublished data).

Trapping procedures Chipmunks were trapped in four woods and 18 fencerows in four trapping sessions between May and September 1989. Fencerows were selected to represent a range of distances from woods, and to encompass the range of vegetation types occurring in this area. Each trapping session covered a two-week period and was separated from the next session by two weeks. In the first week of each session, traps were set in the woods on four consecutive days, and in the second week in the fencerows on four consecutive days. We used Shermantype live-traps containing dacron wool bedding and a mixture of peanut butter, rolled oats and honey as bait. In the woods, traps were arranged in grids of five rows x five columns (two woods) or seven rows x five columns (two woods), with 20-m spacing between traps. In one wood, the trapping grid encompassed the entire area, but for others it was located in a corner or section adjacent to fencerows and open farmland. In each fencerow, a 100-m length was trapped using 10 traps spaced at 10-m intervals, with traps located in the centre of the fencerow vegetation. Traps were checked twice a day, morning and evening. This represents 4800 trap days. Captured chipmunks were identified, sexed, and their weight, age and reproductive condition noted. New animals were individually marked with a monel tag in each ear before release at the point of capture. Tags were replaced when lost; three fencerow animals lost both tags between captures and could not be identified. Chipmunks were classed as subadults (i.e. young of the year) if they were non-reproductive and less than 90 g at first capture. Those non-reproductive individuals

Corridor use by chipmunks in a farmland mosaic greater than 90 g at first capture were classed as 'uncertain age', although they are believed to have been almost entirely young of the year. The resident status of chipmunks in each fencerow or wood was assigned to one of three categories. The term 'resident' was used to describe those individuals that were recorded in the same fencerow or wood during two or more trapping sessions. This does not imply a long-term resident status as field studies were limited to a single spring/ summer season. 'Temporary residents' were those recorded during one session only, even if trapped several times; and 'transients' were those trapped in a particular fencerow or wood only once. Measurement of fencerow variables Habitat features of each fencerow were measured in two ways. First, the floristic composition of the vegetation was assessed by noting all woody plant species greater than 2 m in height occurring or projecting into a 5-m tranverse 'slice' of fencerow, centred on each trap site. These data were summed for the 10 sites in each fencerow to give a score from 1 to 10 for each plant species. Fencerows were then classified into groups having similar floristic composition using the Bray-Curtis association measure and the U P G M A fusion strategy within the software package PATN (Belbin, 1990). Secondly, structural attributes of the habitat were also measured for the same 5-m slice of fencerow and averaged over the 10 sites. The percentage cover, in 10% cover class intervals, was estimated for: tall trees (>10 m height), small trees (4-10 m), tall shrubs (1.54 m), vines and creepers ( 1 . 5 4 m), shrubs (<1.5 m), vines and creepers (<1.5 m), grasses, herbs, litter/bare ground, rocks, and logs (including fence rails). Width of fencerow vegetation was measured at each trap site at 1 m and 2 m above ground. Values at each height were averaged for the 10 sites in the fencerow. Because an identical length was sampled for each fencerow, width is also an index of fencerow area where trapping was carried out. Four variables that represent aspects of the linear continuity of fencerows were measured as follows: (i) the distance (m) to the nearest wood, measured along fencerows from the closest end of the fencerow trapline; (ii) the percentage of this distance that consists of gaps (see below) in the fencerow vegetation; (iii) the overall length (m) of the shortest path between two woods, along fencerows, that encompasses the particular fencerow: and (iv) the percentage of this path length that consists of gaps. Gaps were defined as lengths of fencerow greater than 5 m that lacked trees or shrub cover. Typically, they were sections of grassy or herbaceous vegetation. Analyses of corridor quality The relationship between the number of chipmunks and each of the fencerow variables was examined by simple correlation and non-parametric (Kruskal-Wallis) one-way analysis of variance. Variables were log-trans-

157

formed where this provided a better fit for correlations. Forward stepwise multiple regression analyses were then performed (SAS Institute, 1988) to develop models with combinations of one or more variables that explained the relative abundance of chipmunks in fencerows. Our underlying assumption in these analyses is that the number of chipmunks recorded in a particular fencerow is a measure of the quality of that fencerow as sensed by this species.

RESULTS Chipmunks in the farmland mosaic A total of 530 captures of 119 chipmunks (68 males, 51 females) was recorded during the study (Table 1). Chipmunks were resident in all four woods, and were trapped in 14 of the 18 fencerows that were studied (Table 2). Individuals of all age and sex classes were recorded in fencerows (Table 1). Many individuals were trapped both in woods and fencerows (Table 1). Of those recorded in fencerows, the percentage of individuals that were also trapped in woods ranged from 0 to 75% (~--38%), and was significantly negatively correlated with distance to the nearest wood ( r = - 0 . 5 2 , p<0.05, 13 d.f.). Thus, in fencerows close to woods there was a higher proportion of individuals that occurred both in fencerows and woods. However, more than half of all chipmunks trapped in fencerows (43/81) were not recorded in woods. The use of fencerows by chipmunks Patterns of residency of chipmunks (Tables 2 and 3) provide insights into the different ways in which fencerows are used by this species. Residents (those individuals trapped in the same fencerow in at least two trapping sessions) were recorded in 12 of the 14 fencerows where chipmunks were caught. More than half of these animals (12/22) were also recorded in woods, and at least four apparently occupied a home range that encompassed both a wood and adjacent fencerow vegetation (e.g. Male 1715/16 in Wood 1 and Fencerow A) as they were also Table 1. Number of individual chipmunks trapped in fencerows and woods (values in parentheses are percentages)

Adults Males Females Sub-adults Males Females Uncertain age Males Females

Total

Fencerows

26 19

16 (62) 13 (68)

21 (81) 13 (68)

11 (42) 7 (37)

29 29

25 (86) 18 (62)

12 (41) 19 (66)

8 (28) 8 (28)

13 3

9 (69) 0 (0)

Total individuals 119 Trap-days 4 800

Woods

8 (62) 3 (100)

81 (68) 76 (64) 2 800 1 920

Fencerows and woods

4 (31) 0 (0) 38 (32)

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A. F. Bennett, K. Henein, G. Merriam Table 2. Use of individual fencerows by chipmunks Note that some individuals were transient in more than one fencerow. Fencerows

Total captures Number of individuals Residency status Residents Temp. residents Transients

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

13 4

44 13

11 3

13 6

18 7

17 12

39 17

18 6

0 0

1 1

0 0

14 5

13 6

18 2

0 0

17 8

27 8

0 0

2 1 1

3 6 4

1 0 2

1 3 2

3 1 3

0 2 10

1 8 8

2 0 4

0 0 0

0 0 1

0 0 0

2 0 3

1 1 4

1 0 1

0 0 0

1 3 4

4 2 2

0 0 0

trapped in woods in at least two separate sessions. Individuals from all age and sex classes were recorded as fencerow residents (Table 3), including lactating females during the breeding season in May/June. The extent to which breeding in fencerows was independent of woods is not clear; of 10 lactating females that were trapped in fencerows, seven were also recorded in woods during the study. However, limited data from radio telemetry (Henein & Merriam, unpublished data) have shown that females are able to maintain a natal burrow and raise a litter in some fencerows independently of woods. Young chipmunks were detected in fencerows soon after emergence from the natal burrow and as they gained independence, and many (47%) were recorded only in fencerows. Of those trapped in both fencerows and woods, 50% ( 8 / 1 6 ) were first encountered in a fencerow. Individuals regarded as temporary residents were from all age and sex classes and comprised one third (27/81) of all chipmunks trapped in fencerows (Table 3). This category of residency is difficult to interpret with trapping data, but it was included to ensure the clearest distinction between true residents and transients. It probably includes genuine residents that either do not survive or avoid traps, as well as individuals that are only briefly present in the fencerow while moving through the landscape. For example, many individuals classed as temporary residents in fencerows were also trapped in woods (44%); but a similar number (44%) were known only from a single fencerow. At least half of these individuals were subadults (Table 3). Transients comprised a third of all chipmunks recorded in fencerows. Adult males appeared to be more transient in status than were other population classes. Only one adult male was resident in a fencerow, while 10 were transients. These relative proportions were more balanced for adult females and for subadults (Table 3). A contingency analysis of age and sex classes (excluding those of uncertain age) indicated that degree of residency was not evenly distributed among classes (log-likelihood ratio G -- 12.64, p < 0.05). Some transients in fencerows may occur there on forays from a home range in a nearby wood. However, more than half (17/32) of all transients were captured at a single fencerow only and nowhere else in the study

area. This suggests substantial mobility for some individuals, although high mortality may also contribute to these results. Trapping records revealed many movements by chipmunks, both within and between landscape elements in the farmland mosaic. Movements between landscape elements (i.e. excluding movements within a fencerow or wood) were made by at least 56% (45/81) of the chipmunks using fencerows, and these are presented diagrammatically in Fig. 1. Of the total of 80 recorded movements between landscape elements, 71% were between wood and fencerow, 26% from fencerow to fencerow, and 3% from wood to wood. Clearly, these recorded movements represent only a subset of the actual movements taking place within the landscape during this time, as substantial areas of woods 1, 2 and 3 and much of the fencerow network were not trapped (see Fig. 1). Further, traps were open for less than a third of all days during the study period. What determines fencerow quality for chipmunks There was marked variation in the number of captures, the number of individuals, and the resident status of individuals in each fencerow (Table 2). To account for this variation, measures of fencerow habitat, width and linear continuity were examined in turn as correlates of the use of fencerows by residents, transients and the total number of chipmunks. Models that provided the best explanation for variance in use of fencerows were then developed from combinations of these variables.

Habitat Floristic composition. Seven groups of fencerows were recognised, based on the frequency of occurrence of trees and tall shrubs at trap sites (Table 4). This classification largely reflects the frequency of occurrence of plant species commonly occurring in fencerows, such as white ash, hawthorn Crataegus sp., white elm, nannyberry Viburnum lentago, choke cherry Prunus virginiana, white oak Quercus alba, Virginia creeper Parthenocissus vitacea and grape Vitis riparia. Group 1 comprised 10 fencerows in which these species were common. Typically, these fencerows had a relatively continuous cover of trees and tall shrubs, from 5 to 9 m in width. They ranged from those having a woodland structure with old trees (e.g. fencerows L and

Corridor use by chipmunks in a farmland mosaic

were grassy strips lacking tall trees and shrubs, except for a single tree in fencerow O. Chipmunks were not evenly distributed between fencerow groups (Table 5). Only a single transient was trapped at those sites with few or no trees and shrubs (Groups 4, 5 and 6/7), but both residents and transients were present in the wider fencerows with a diverse and relatively continuous tree and shrub cover (Groups 1, 2, and 3) (Table 5). The number of residents showed the strongest variation among fencerow groups (Table 5). Structural attributes. Correlations between the number of chipmunks and structural attributes for each fencerow (Table 6) also suggested that fencerow habitat is an important correlate of corridor quality. Tree cover was an important feature with which residents and transients were both significantly correlated (Table 6). Residents were also significantly positively correlated with the cover of tall shrubs, vines (above 1.5 m), litter or bare ground, and the richness of trees and shrubs, and negatively correlated with grass cover. Thus, residents favoured fencerows with trees that resembled woods; they were least common, or absent, in those dominated by grasses or patchy low shrubs. There was a stronger relationship between the number of residents and these habitat attributes than there was for transients.

Table 3. Residence status for age and sex classes of chipmunks trappedin fencerows

Adult Male Female Subadult Male Female Uncertain age Male Female Total

Resident

Temporary resident

Transient

1 3

5 6

10 4

8 8

11 3

6 7

2 0

2 0

5 0

22

27

32

159

H with mature Q. alba) to those dominated by younger trees (especially F. americana) and shrubs (e.g. fencerows C and D). Group 2 comprised two fencerows that were rich in trees and shrubs, and Group 3 was a single fencerow with several species not occurring elsewhere (sugar maple, balsam poplar Populus balsamifera, European buckthorn Rhamnus cathartica). Fencerows in Groups 4, 5 and 6/7 were narrow (<5 m), had few trees and shrubs and a progressively greater dominance of grasses (Table 4). Fencerows I and O in groups 6 and 7 were combined for analysis, as both

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Fig. 1. Study landscape at Manotick, showing woods (W1-W4) and fencerows (A-R) trapped for chipmunks, and the numbers of recorded movements of individuals between landscape elements. Arrows indicate the direction of movement of chipmunks, n o t the pathway. Heavy solid lines show bi-directional movement, heavy dashed lines indicate uni-directional movement, and digits show the number of recorded directional movements between two landscape elements. Fencerows are shown by thin solid lines, and thin dashed lines indicate streams. Trap locations are shown by circles (woods) and tick marks (fencerows).

A. F. Bennett, K. Henein, G. Merriam

160

Table 4. Two-way table displaying the frequency of occnrrence of plant species at trap sites in fencerows

Species

Fencerow groupsa 1

2

N

B

D

F

G

L

P

J

R

5 8 8 10 10 10 2

5 6 3 7 8 9 10 9 7 10 9 4 1 8

4 4 4 4 6 8 3

5

9 7 2 3 6 2 1

10 10 7 6 8 10 5 3 3 7 6 4 7 10 4 2 7 4 3 3 4

9 9 3 4 1 2 5

7 2

2 2

1

1

1

5

3

1

4

3 5

4 7 2 5

2 7 6 8 2 3 3 10 7 6

4

8 1

2 3

1

2

Acer saccharum Acer rubrum Populus grandidentata Ostrya virginiana Prunus nigra Prunus serotina Thuja occidentalis Rhamnus frangula Picea glauca Prunus pensylvanica

4 3 3 2 2 4

1 2

Tilia americana Populus balsamifera Rhamnus cathartica Acer negundo

1

1

1

5 2

1 6 10 8 2

H

7

8

M

6/7

5

E

Salix sp. Malus sp. Salix discolor Spiraea alba Abies balsamea

C

4

Q

Fraxinus americana Crataegus sp. Ulmus americana Vitis riparia Viburnum lentago Parthenocissus vitacea Prunus virginiana Comus spp. Quercus alba Populus tremuloides

A

3

7 3 4 3

5

2 4

K

I

O

2

1 1 1 1

1 1

2

1

1

1 1

1

1

1 3

1

1

Mean floristic richness

10.3

16.5

10

6.0

6.0

0.5

a Fencerows (A-R) have been classified into seven groups (10 = present at each 5-m slice of fencerow centred on a trap site).

Width Fencerows varied from 1 to 10 m in width (g= 5.6 m) (Table 5). The number of residents in each fencerow was significantly positively correlated with fencerow width (Table 6), but there was no such relationship for the number of transients or total numbers of chipmunks. Linear continuity Linear continuity of fencerows was more important for transients than for residents (Table 6). The number of transients was significantly negatively correlated with

the length of pathway between woods, the proportion of gaps along this path, and the proportion of gaps along the length o f fencerow to the nearest wood. This suggests that the number o f transient chipmunks in a fencerow is likely to be greatest for shorter pathways with a low proportion of gaps. The distance to the nearest wood was not a significant correlate; the mean distance to the nearest wood for all fencerow sample sites was 170 m. Results from stepwise multiple regression analyses of the number o f chipmunks in fencerows are summarised in Table 7. The number o f residents was best predicted

Table 5. Occurrence of ehil~aunks in relation to flo "nstic groups of fencerows Values are means ± standard deviation. A3 and p values are from Kruskal-Wallis one-way analysis of variance.

Floristic group Fencerows Width (1 m) Residents Transients Total chipmunks

1

2

3

4

5

6/7

A,B,C,D,F G,H,L,N,P 6-5±1.8

M,Q

E

J,R

K

I,O

7-7±2.8

4.4

3.5±0.6

4-5

2.1±0-9

1.4±0-8 3.9±3-0 7.6±4.9

2.5±2.1 3.0±1.4 7.0±1.4

3-0 3.0 7-0

0.0 0.5±0.7 0.5±0.7

0.0 0.0 0.0

0.0 0.0 0.0

~

p

10.88 10.08 10.59

0.05 0.07 0-06

Corridor use by chipmunks in a farmland mosaic Table 6. Correlation coefllekuts for the relationship between use of feneerows by cbipnemks and fencerow habitat, width and linear continuity (*p < 0-05, **p < 0-01, ***p < 04101)

Attribute

Residents Transients

Habitat Floristic richness 0.52* Structural attributes Tall trees (> 10 m) 0.65** Small trees (4-10 m) 0.64** Tall shrubs (1.5-4 m) 0.50* Tall vines (1.5-4 m) 0.51" Low shrubs (<1.5 m) 0-19 Low vines (<1-5 m) 0.16 Grass -0.51" Herbs -0.39 Litter/bare 0.53* Logs 0.44 Rocks -0.02 Width Width at 1 m above ground 0.56* Width at 2 m above ground 0-61"* Linear continuity Distance to nearest wood (log) -0.11 % distance to nearest wood as gaps (log) -0.57* Path length between woods (log) 0.07 % path length between woods as gaps (log) -0.41

Total individuals

0.41

0-41

0.54* 0.53* 0.49* -0.13 0.25 -0.05 -0.35 -0-35 0.27 0-41 0.00

0.57* 0.54* 0.51" 0.11 0-25 0.04 -0.38 -0.38 0.28 0.34 0.05

0.19

0.23

0.21

0.31

-0-26

-0.21

-0-55*

-0.61"*

-0.54*

-0.40

-0.70***

0.64**

by two habitat attributes, tall trees and vines (>1.5 m), which together accounted for some 70% of the variance. Measures of width or linear continuity did not explain significant additional variation. In contrast, transients were best predicted by a model incorporating both linear continuity and habitat attributes, together accounting for 84% of the variation (Table 7). The proportion of non-wooded gaps in the fencerow pathway between woods was the most important factor, alone accounting for almost half of the variation in transient chipmunks trapped. For the total number of chipmunks in fencerows, only one significant variable was included in the model, the proportion of gaps in the pathway between woods, which explained some 40% of the variance.

161

DISCUSSION Corridor use and the elements of corridor quality The patterns of occurrence of chipmunks in woods and fencerows, together with movements revealed by trapping and concurrent observations by radio-telemetry (Henein & Merriam, unpublished data), point to the dynamic nature of the population in this farmland mosaic. Chipmunks were present in all woods and in many fencerows, and there was extensive movement within and between these landscape elements: from fencerow to fencerow, between fencerows and woods, and along fencerows between woods. The many habitat components scattered throughout the farmland mosaic contain a single, spatially dynamic, demographic unit, rather than a set of discrete populations in woods that interact through infrequent dispersal events. Movement of chipmunks through grassy fields and crops is possible, but the lack of captures in grassy fencerows in this study is consistent with other evidence (Wegner & Merriam, 1979; Henderson et al., 1985) that farmland vegetation is an inhospitable habitat that isolates woodland populations. In contrast, the network of wooded fencerows was extensively used by chipmunks and clearly provides sufficient habitat connectivity to permit population continuity throughout the farmland mosaic. An important finding is that the elements of corridor quality varied depending on the way in which the fencerow corridor was used. First, resident animals lived within fencerow vegetation, thus providing continuity of the resident population between woods along many of these landscape linkages. The relative abundance of residents was best predicted by features of the fencerow habitat. In contrast, presence of transient individuals (presumed to use fencerows primarily for movement through the landscape) was best predicted by a combination of linear continuity and habitat. Consideration of all chipmunks as a single group would have obscured these differing responses. Indeed, the model for 'total chipmunks' in fencerows had approximately half the explanatory power of separate models for residents and transients (Table 7). This study has also clarified the relative contributions of three physical attributes of corridors--habitat, width and spatial continuity--to corridor quality as sensed by these animals.

Table 7. Stepwise multiple regression analyses for number of chipmunks captured in fencerows in relation to fencerow variables

Dependent variable

Residents Transients Total

Independent variables, variance explained (r2) and regression coefficients(b) for significantsteps Step 1

Step 2

Step 3

Step 4

Tall trees 42.8%, b = 1.7 % gaps in path 48.3%, b = -3.5 % gaps in path 40-6%, b = -4-9

Vines (> 1.5 m) 27.4%, b = 0.4 Litter/bare 15.6%, b = -1.3

Tall shrubs 12.3%, b = 1.0

Path length 8.2O/o,b = -3.7

Total variance from significant steps

70.2% 84.4O/o 40.6%

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A. F. Bennett, K. Henein, G. Merriam

Habitat suitability may be a basic element of corridor quality, with minimum requirements for attracting and then supporting animals. With the exception of a single transient, chipmunks were not recorded in fencerows that were dominated by grasses, or patchy shrubs and grasses without tree cover (Floristic groups 4-7, Table 5). For residents, significant positive correlation with the cover of trees, tall shrubs and litter/bare ground is consistent with favouring fencerow habitat that most closely resembles that in woods (negative correlation with grass cover is the reciprocal relationship with wood-like habitat). The importance of woody habitat reflects the need by residents for a reliable daily supply of resources such as food, diurnal refuge and shelter, and burrows. Other studies of species resident within linear habitats (e.g. Yahner, 1983; Osborne, 1984; Recher et al., 1987; Lindenmayer et al., 1993) have also shown significant relationships between the relative abundance of residents of a species and the availability of certain habitat components. In contrast, transients that are simply passing through a corridor are not necessarily reliant on the corridor habitat for food and breeding sites; but, like chipmunks in fencerows, they may need vegetative cover or other habitat components (e.g. burrows) for refuge while moving (Merriam & Lanoue, 1990). Width of vegetation can influence corridor quality in several ways. First, with increased width the area of habitat in the corridor is larger. A greater area allows more home ranges, and also provides the opportunity for a greater abundance and diversity of resources. Several studies have implicated width as an important determinant in the use of linear landscape elements by particular species (Stauffer & Best, 1980; Shalaway 1985; Dickson & Huntley, 1987: Recher et al., 1987). In this landscape, fencerow width was a significant correlate of the number of resident chipmunks but not of transients (Table 6). However, width was highly significantly intercorrelated with habitat variables such as the cover of tall trees, small trees and vines, and could not add further to the predictive model for residents after habitat measures had been included. The widest fencerows were those that had the greatest tree cover, and wei'e most similar to woodland vegetation. Secondly, width is believed to be an important attribute because it determines the relative proportion of edge habitat and the intensity of edge effects in corridors. Changes in ecological processes such as predation, nest parasitism, weed invasion, windthrow and climatic exposure have been proposed to be more severe in narrow than in wide corridors (Ambuel & Temple, 1983; Simberloff & Cox, 1987; Soul6 & Gilpin, 1991). However, empirical data on edge effects in corridors and of their consequences for animals are sparse (e.g. Lynch & Saunders, 1991). The eastern chipmunk is regarded as a species that can thrive in edge habitats, and consequently edge effects are unlikely to have been important in this study. Gaps or barriers in a corridor could impede the passage of animals, and consequently the number and

extent of gaps may affect choice of route, or survival during movements across the landscape. The proportion of the pathway between woods comprised of gaps >5 m long was the most important single predictor of the numbers of transient chipmunks that were recorded. This is direct evidence that gaps in corridors may be an important element of connectivity. Limited field studies, mostly related to roads as barriers, suggest that the consequences of gaps and barriers can range from disruption of social organization to the total genetic isolation of divided populations (Bennett, 1991). What constitutes a gap will vary between species and depend on the scale and mode of movement, the habitat specificity, and, possibly more importantly, what process is being considered (Merriam et al., 1989). The importance for animals of other aspects of spatial continuity, such as circuitry, junctions and nodes (Forman & Godron, 1986; Forman, 1991) are less well understood and empirical data are scarce. Behavioural responses by animals to corridors were not explicitly considered in this study, and may also be an important determinant of corridor quality. For example, behavioural avoidance due to the presence of predators may inhibit an animal from entering a corridor.

Implications for predictive models Understanding the correlates of corridor quality is critical to the design and utility of metapopulation and corridor models (Henein & Merriam, 1990; Soul6 & Gilpin, 1991). Models can provide useful insights, make long-term predictions, and generate hypotheses for field testing. However, they are dependent on empirical data for the calibration of their parameters. They must simulate real processes and be validated and revised in the light of new data if they are to be relevant to practical conservation initiatives. In a deterministic model, Henein and Merriam (1990) showed that corridor quality is an important element in the measure of connectivity of a metapopulation, and that corridors with high mortality can act as sinks for undiscriminating dispersers, thus draining the regional population over time. However, data from this study suggest that chipmunks do discriminate between corridors of varying quality. Further empirical data are needed to confirm whether behavioural choice avoids mortality for particular species. Soul6 and Gilpin (1991) modelled single direct movements by animals in corridors. They suggest from their modelling that with increasing corridor width, the value of increased 'interior' habitat is offset by the tendency of individuals to wander unproductively within the corridor. However, the assumption of corridors in which animals can live and breed, as in this and other studies (Suckling, 1984; Bennett, 1990b), could lead to a different conclusion. Several empirical studies in which species were resident within corridors suggest that greater width may be advantageous because of the increased resources available to both residents and transients (Shalaway, 1985; Dickson &

Corridor use by chipmunks in a farmland mosaic

Huntley, 1987; Recher et al., 1987). Corridors that provide suitable breeding habitat for resident animals should enhance the success of a population over time in a fragmented landscape by providing an additional source of new individuals as well as a route between patches.

Implications for corridor design and management From a management perspective, it is useful to consider two types of corridors: (i) those that facilitate the movements of animals, but which may not be acceptable as living habitat; and (ii) those which both have habitat acceptable to resident animals and also are used for movement through the landscape. Both forms of corridor use can make a useful contribution to enhancing continuity of populations in fragmented landscapes. Corridors used primarily for movement are likely to have a high level of continuity between two habitats or resources used by a species. Foraging movements of birds to and from food resources (Johnson & Adkisson, 1985; Webster, 1988), and the natal dispersal of small mammals between habitat patches (Wiggett & Boag, 1989) are examples of direct movements over finite distances. As the distance to be travelled increases, the use and effectiveness of such corridors is likely to decrease due to behavioural avoidance or lack of vital resources. The minimum quality of corridor habitat required to foster such movements is not clear, but data for small mammals suggest that the relationship between size of an animal, movement distances, and time exposed while moving are not simply linear (Wiggett & Boag, 1989; Merriam & Lanoue, 1990), Corridors that provide habitat suitable for animals to live within also allow them to move through the landscape. A corridor may, at the same time, provide movement pathway and resident habitat for different individuals within a population (Suckling, 1984; Henderson et al., 1985; Bennett, 1990b, Merriam & Lanoue, 1990; this study), or for different species in a local assemblage (Newbey & Newbey, 1987; Saunders & de Rebeira, 1991). Corridors that function in these ways are likely to be more effective than simple movement corridors in facilitating population interchange and movement over greater distances relative to the size and scale of movement of the species. Landscape movement is a general term that includes exploration, foraging sorties, searches for mates, seasonal movements and dispersal events. These different types of movements have different consequences for population demography and conservation. The distinction made in this study between transient and resident chipmunks in fencerow corridors does not clarify the demographic roles of the movements made by these animals. Transients may achieve a much greater rate of interpatch movement, for example, but the movement of a single pregnant female that results in the successful recolonization of a habitat patch from an adjacent fencerow population may be demographically more effective than the movement of many

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transient juveniles at the beginning of the season of heaviest mortality. The relative effectiveness of different types of movements that are facilitated by corridors is an important area that requires attention. Plans for the management or restoration of corridors need to consider differences in costs, maintenance requirements and the effectiveness of corridor function associated with different forms of use. They must also consider the target species, or assemblage, for which the corridor is intended. Corridors that provide habitat, or preferably contiguous habitat patches, are the most desirable means of promoting population continuity in the landscape. These linkages may need ongoing management to maintain and enhance habitat quality so that the resources that animals require, either seasonally or over a lifetime, are available. For some species such corridors may require substantial areas (Harrison, 1992), but the populations they can support are an additional benefit to the advantages that accrue from increased movements of animals through the landscape. However, there are many corridors of varying habitat quality already present in the environment (e.g. fencerows, hedgerows, roadsides, riparian strips). These may fulfil the role of movement corridors and provide habitat for a range of species within present constraints of ownership and land-use patterns, and we may need simply to pay the maintenance if we wish to safeguard such populations in heavily-used landscapes. ACKNOWLEDGEMENTS We thank Russell Walton, John Wegner, Karl StuartSmith and Dave Omond for their assistance in the field; and Dr David Lindenmayer for useful comments on the manuscript. The involvement of A.F.B. in this study was made possible by the provision of six months' study leave from the then Department of Conservation and Environment, Victoria, Australia, for which he is most grateful.

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