Landscape Ecology **: *** ***, ****.

***

**** ****** ******** **********. ******* in the Netherlands.

Comparing the landscape level perceptual abilities of forest sciurids in

fragmented agricultural landscapes

Patrick A. Zollner

Department of Life Sciences, Indiana State University, Terre Haute, IN 47809, USA; Present address: USDA

Forest Service, North Central Research Station, 5985 Highway K, Rhinelander, WI 54501-9128, U.S.A. (e-mail:

abo95v@r.postjobfree.com)

Received 26 January 1999; Revised 22 September 1999; Accepted 14 October 1999

Key words: chipmunks (Tamias striatus), connectivity, dispersal, fox squirrel (Sciurus niger), gray squirrel (Sciurus

carolinensis), habitat isolation, inter-patch movements, perceptual range

Abstract

Perceptual range is the maximum distance from which an animal can perceive the presence of remote landscape

elements such as patches of habitat. Such perceptual abilities are of interest because they in uence the probability

that an animal will successfully disperse to a new patch in a landscape. Furthermore, understanding how perceptual

range differs between species may help to explain differential species sensitivity to patch isolation. The objective

of this research was to assess the perceptual range of eastern chipmunks (Tamias striatus), gray squirrels (Sciurus

carolinensis), and fox squirrels (Sciurus niger) in fragmented agricultural landscapes. Animals were captured in

remote woodlots and translocated to unfamiliar agricultural elds. There they were released at different distances

from a woodlot and their movements towards or away from the woodlot were used to assess their ability to perceive

forested habitat. Observed perceptual ranges of approximately 120 m for chipmunks, 300 m for gray squirrels, and

400 m for fox squirrels, suggest that differences in landscape-level perceptual abilities may in uence the occurrence

of these species in isolated habitat patches.

Introduction landscape by different species (With, 1994; Crist and

Wiens 1995; Diffendorffer et al. 1995; With and Crist

Interspeci c differences in patterns of landscape use 1995, 1996; Wiens et al. 1997; Firle et al. 1998; Had-

often can be attributed to behavioral phenomena (Ims dad 1999). Such differences can alter the dynamics

1995). For example, habitat specialists may be reluc- and structure of populations (Crist and Wiens 1995),

tant to traverse large areas of matrix habitat (Laurance often increasing the susceptibility of isolated popula-

1990; Rail et al. 1997; Heinen et al. 1998), and tions of poor dispersers to local extinction (Petterson

such behavior can affect the persistence of a frag- 1985; Andren 1994). Thus, by re ning our knowl-

mented population (Andren 1994; Laurance 1995; edge of what animals know about their surroundings

With and Crist 1995). Mortality risks during disper- and how they make decisions as they move through

sal, as well as life history traits such as vagility and landscapes (Crist and Wiens 1995; Roitberg and Man-

body size, may also in uence the distribution of an- gel 1997; Pither and Taylor 1998; Turchin 1998), we

imals in fragmented landscapes (Taylor et al. 1993; should increase our understanding of how the spatial

Lidicker and Koenig 1996; Zollner and Lima 1999a). con guration of habitat affects different species (Ims

Indeed, several experiments have documented dif- 1995; Gustafson and Gardner 1996; Zollner and Lima

ferential use of and movement through a common 1999a; Haddad 1999).

The ability of animals to perceive habitat at a

The U.S. Government s right to retain a non-exclusive,

distance is a behavioral mechanism that may be an im-

royalty-free licence in and to any copyright s acknowledged.

portant component of dispersal success in fragmented

524

landscapes (Lima and Zollner 1996; Zollner and Lima success in fragmented landscapes, as well as provide

1997). An animal s perceptual range will determine information on a mechanism that may contribute to

the ease with which it can locate habitat patches and observed difference in patterns of patch occupancy.

hence the time spent searching in a hostile matrix for Thus, the objective of this work was to examine

such habitat (Zollner and Lima 1999a). Consequently, the perceptual ranges of these three species, eastern

a species sensitivity to habitat fragmentation may be chipmunks, eastern gray squirrels, and fox squirrels

to a great extent a function of its perceptual range. Un- in fragmented agricultural landscapes of east central

fortunately, empirical information on the perceptual Illinois and west-central Indiana.

abilities of vertebrates is rare and based on a few sin-

gle species studies performed in different landscapes

(Yoemans 1995; Zollner and Lima 1997; Andreassen Methods

et al. 1998; Gillis and Nams 1998; Zollner and Lima

1999c). Thus, a study comparing the perceptual abil- General methods

ities of several species within a common landscape

The ability of chipmunks and squirrels to orient to-

and relating these abilities to each species occurance

wards forested habitat from a distance was used as a

in isolated habitat patches should clarify the in uence

behavioral assay of their ability to perceive forested

of perceptual range on dispersal success.

habitat at a distance. These abilities were assessed by

Forest-dwelling-sciurids have received consider-

capturing chipmunks and squirrels at distant woodlots

able attention in studies of habitat fragmentation, and

and moving them to an unfamiliar, bare, fallow eld,

they appear to be sensitive to the effects of patch

which was devoid of fence rows. At these novel elds

isolation (Henderson et al. 1985; Verboom and Van

animals were released at several distances (determined

Apeldoorn 1990; Fitzgibbon 1993; Van Appeldoorn

by pilot work; Zollner unpublished data) from the edge

et al. 1994; Wauters et al. 1994; Sheperd and Swi-

of a mature woodlot. The orientation of the movement

hart 1995; Rushton et al. 1997; Heinen et al. 1998).

path at each release distance was measured to assess

However, species may differ in their sensitivity to the

perceptual range of each species (Zollner and Lima

effects of isolation, and these differences are likely to

1997; Zollner and Lima 1999c). Critical to this work

re ect a species mobility in matrix habitat or some

is the assumption that the perception of a woodlot is

correlate such as body size (Swihart and Nupp 1998).

equivalent to movement towards the woodlot. Few en-

Among the forest-dwelling sciurids of eastern North

vironments would appear as hostile to a chipmunk or

America, it is clear that gray squirrels (Sciurus car-

squirrel as a barren eld, and survival in such an en-

olinensis) are more sensitive to patch isolation than

vironment requires locating forested habitat as soon

fox squirrels (Sciurus niger; Table 1). A third for-

as possible. Thus, it is reasonable to assume that if

est dwelling sciurid common to this area the eastern

these animals perceived forested habitat they should

chipmunk (Tamias striatus) is also negatively effected

have attempted to reach it.

by isolation of habitat (Table 1). However, the sensi-

I captured chipmunks with Sherman live traps and

tivity of chipmunks to patch isolation relative to the

squirrels with Tomahawk live traps in mature woodlots

other two species is unclear (Table 1). Working in

5 29 km from the release site. This 5 km mini-

east-central Illinois Rosenblatt et al. (1999) only found

mum distance and movement barriers (roads, streams,

chipmunks in the three largest and best connected

etc.) between capture and release sites minimized the

patches of ten which they surveyed, while gray and

chance that animals had prior experience at the release

fox squirrels were present in six and nine patches re-

site. I used adult males and females, and all pregnant

spectively. However, Nupp and Swihart (1998) found

or lactating females were excluded from the experi-

chipmunks in all four patches that they surveyed in

ments. Traps were checked twice each day, once in

northwestern Indiana, including one patch that was

the morning and once in the evening. Prior to their re-

870 m from the nearest woodlot. More extensive sur-

lease, animals were provisioned with seeds and housed

veys in northwestern Indiana indicate that chipmunks

overnight in their traps in a small, unheated shed.

are less sensitive to patch isolation than either gray

Releases were accomplished using a standard re-

squirrels or fox squirrels (Nupp and Swihart in re-

lease mechanism . The mechanism was constructed of

view). An assessment of the perceptual abilities of

a 40 cm long piece of PVC pipe that was either 6.5 cm

each of these three species should help clarify the role

(chipmunks) or 13 cm (squirrels) in diameter. A metal

that perceptual range plays in in uencing dispersal

525

Table 1. Reported sensitivities of three woodland sciurids to patch isolation.

Effect of patch isolation Reference

Fox squirrels

Fox squirrels were insensitive to levels of fragmentation in this study. Nupp and Swihart (in review)

Fox squirrels were present in 31 of 37 patches surveyed and are apparently

capable of crossing agricultural matrix.

Wide distribution of fox squirrels (present in 9 out of 10 woodlots) indicates Rosenblatt et al. (1999)

high mobility or prolonged persistence in isolated patches.

Fox squirrels are ubiquitous in non-urban woodlots of row-crop dominated Rosenblatt (in review)

east central Illinois; attributed to their superior dispersal ability.

Relative to gray squirrels, fox squirrels move greater distances, and visit

more patches when translocated.

None of 49 radio-collared fox squirrels moved between isolated patches Sheperd and Swihart (1995)

across agricultural matrix.

Some fox squirrels did move 200 500 m away from woodlots through

fencerows and one patch was colonized by a non-collared squirrel from

at least 800 m away.

Spatially explicit simulation found interpatch movements by fox squirrels Swihart and Nupp (1998)

were not constrained by patch isolation.

Empirical surveys found fox squirrels in all 18 patches examined even the

most isolated ones.

Gray squirrels

Gray squirrels are less likely to be found in patches > 500 m from other Fitzgibbon (1993)

woodlots and not connected by hedgerows.

Sites that historically contained gray squirrels lost them as landscapes were Nixon et al. (1978)

reduced to less than 20% forested coverage.

Among 54 habitat variables analyzed, the best predictor of gray squirrel

presence was the amount of forested habitat within 23.31 km2 of a site.

Gray squirrels were restricted to continuous forests and large sites (only Nupp and Swihart (in review)

present in 7 of 37 patches surveyed).

The best- t logistic regression model of gray squirrel presence was posi-

tively associated with patch area and negatively associated with isolation.

Gray squirrels were present in only 6 of 10 patches surveyed and were Rosenblatt et al. (1999)

absent from isolated rural woodlots.

Gray squirrels are restricted to towns and riparian forests in row-crop dom- Rosenblatt (in review)

inated east central Illinois, which was attributed to their hesitancy to move

across open elds.

Relative to fox squirrels gray squirrels move shorter distances, and visit

fewer patches when translocated.

Successful introduction of gray squirrels to woodlots where they were ab- Rosenblatt (1999)

sent for 20+ years supports the hypothesis that absence was maintained by

isolation.

Spatially explicit simulation found interpatch movements by gray squirrels Swihart and Nupp (1998)

were constrained by patch isolation.

Empirical surveys found gray squirrels in 4 of 18 patches examined but not

in the isolated ones.

526

Table 1. Continued.

Effect of patch isolation Reference

Eastern Chipmunks

Eastern chipmunks reside in fencerows and use them as movement path- Bennett et al. (1994)

ways between patches.

Two chipmunks were observed moving > 500 m between patches pre-

sumably through fencerows and numerous other long movements were

documented within fencerows.

One individual regularly crossed 70 m of open eld. Forsyth and Smith (1973)

Spatially explicit simulation found that patch connectivity was the most Henein et al. (1998)

important factor determining persistence of eastern chipmunk populations

in fragmented agricultural landscapes.

Chipmunks were observed to move as far as 1560 m between woodlots and

this travel was presumed to be largely through fencerows.

Henderson et al. (1985)

Chipmunks probably never crossed areas of matrix larger than 20 60 m

although one individual may have moved as far as 460 m across elds.

Eastern chipmunks were present in 4 out of 4 patches surveyed including Nupp and Swihart 1998

one isolated patch.

Chipmunks appear to be negatively in uenced by forest fragmentation as

survival rates for individuals living in patches were signi cantly lower than

for those in continuous forest.

Eastern chipmunks were present in 32 out of 37 patches and no signi cant Nupp and Swihart (in review)

model of presence/absence could be developed based on landscape metrics.

Chipmunks never crossed roads with clearances > 30 m and roadways > Oxley et al. (1974)

90 m act as barriers.

Eastern chipmunks were only detected in large forest tracts that were well Rosenblatt et al. (1999)

connected to other forested areas (3 out of 10 patches surveyed).

Fencerows provide corridors for movements by chipmunks and reduce Wegner and Merriam (1979)

isolation of woodlots.

Chipmunks were never captured in agricultural elds.

spike (30 cm long) was placed through two holes in Baumgartner 1940). Next, a unique tag was attached to

the pipe at one end, and driven into the ground to se- an animal s ear. A tracking spool (1.7 g, 180 m, denier

cure the mechanism. This secured end of the pipe was two-ply nylon No. 2 quilting bobbin; Barbour Threads

covered with an opaque cap that prevented the animal Inc., Anniston, AL, USA) was then glued to the an-

from exiting the release mechanism. The other end of imal s back, and the loose end of this spool was tied

the pipe was left open until the animal was placed in- to the release mechanism (Boonstra and Crane 1986;

side it, at which time the pipe was sealed with a plastic Key and Woods, 1996). I followed the spool-and-line

cap. technique described by Key and Woods (1996) with

At the time of release, I transported chipmunks and a few modi cations: no animals used in this study

squirrels to the study in opaque boxes that prevented were anaesthetized, and rather than wrapping tracking

them from visually assessing their surroundings. The spools in adhesive tape, I placed them in small dark

actual release locations were placed in straight lines brown unin ated rubber ballons. After the spool was

parallel to the edge of the woods at different distances securely attached (20 30 s) each animal was lowered

from the woods for each species. Along these parallel into the PVC pipe facing the back, and a plastic cap

lines the release sites were spaced such that no animals was placed over the open end of the pipe.

were released within 70 m of each other on any given The release itself was done remotely from a dis-

day. At the release site, I removed animals from their tance of 60 m, so that my presence did not in uence

traps and restrained them in either a heavy black cotton animal movements. This remote release was accom-

bag (chipmunks) or a wire handling cone (squirrels; plished by pulling on a string, thereby removing the

527

plastic cap and opening one end of the release mecha- tions with Mardia-Watson-Wheeler (MWW) pairwise

nism. After removing the caps, I immediately left the tests (Batschelet 1981). No test demonstrated any sex-

study site and did not return until the following day. related differences in orientation towards the woods

(MWW test, 2 0.1), hence data

While releasing animals and leaving the study site, I

was at the same distance from the woods as the release from both sexes were combined before performing

mechanism, so my presence should not have biased statistical analyses.

the animals to move towards or away from the woods.

Additionally, remote observations through a spotting Species-speci c methods

scope during pilot work indicated that chipmunks and

Eastern fox squirrels

squirrels remained inside the release mechanism for

During April May, 1997, I captured 47 fox squirrels

30 45 min after it was opened (P. A. Zollner pers.

in a mature oak hickory woodlot in east-central Clark

obs.).

County, Illinois. These animals were released at a site

The tracking spool left a trail of thread record-

26 km away in north-central Edgar County, Illinois.

ing an animal s movements after it exited the release

This release site was a large 132-ha eld bordered to

mechanism. The day following release, stick ags

the south by a second growth oak hickory forest, to

were placed in the ground along each thread trail at

the north by a road, and to the north, east and west

approximately 3 m intervals and at all points where the

by additional, large agricultural elds. Other than the

animal turned sharply. Each trail was followed until

forest along the southern edge of the release site, the

the thread ended or it reached the woods (chipmunks

nearest trees were 1.8 km away. All fox squirrels were

only). Trails that did not reach the woods were on aver-

age ( SE) 147.1 m ( 3.6) long. Animals occasionally released in this eld between 10:00 and 14:00. Fox

squirrels were released 300 m (15 squirrels), 500 m

broke the thread before travelling the full 180 m al-

(16 squirrels), and 800 m (16 squirrels) from the

though all trails included in these results exceeded

forested southern edge of the eld. During April May,

100 m in length. After tracking, I used a sighting com-

1998, while studying gray squirrels (see below), I

pass (Brunton Sight Master 80NL) and eld tape to

captured an additional 24 fox squirrels, which were

measure the bearing and distance from the point of

released 300 m (8 squirrels), 400 m (8 squirrels) and

release to (i) each ag in the trail and (ii) the nearest

500 m (8 squirrels) from the forested southern edge

point along the woodlot edge.

of the eld. These additional releases of fox squirrels

I assessed perceptual abilities by determining

allowed for the de nition of fox squirrel perceptual

whether the animals locations after traveling a pre-

range at the same scale used for gray squirrels (see

scribed distance (chipmunks 50 m; squirrels 100 m)

below).

were oriented towards the woods. These prescribed

distances were shorter than the minimum distance to

Gray squirrels

the woods (chipmunks 60 m, squirrels 300 m) but long

During April May, 1998, I captured 28 gray squir-

enough to allow an animal to orient after dashing out

rels in a mature oak hickory woodlot in south-central

of the release mechanism. The use of this minimum

Clark County, Illinois. These squirrels were all of

distance ensured that animals did not reach the woods

the gray color morph, although there is no reason to

as a result of random wandering (Goodwin et al. 1999;

expect melanistic animals would have behaved differ-

Zollner and Lima 1999b). The angle to each animals

ently (Gustafson and Van Druff 1990). These animals

location after travelling the prescribed distance was

were released at the site used for the fox squirrel re-

calculated from the recorded movement pathways us-

leases (see above), which was approximately 29 km

ing trigonometry. V-tests were used to assess whether

away in Edgar County, Illinois. All gray squirrels were

these angels were signi cantly oriented towards the

released between 10:00 and 14:00. Squirrels were re-

woods for each species. The V-test is a modi cation

leased at 300 m (8 individuals), 400 m (10 individuals)

of a Rayleigh test which examines whether observed

and 500 m (10 individuals) from the forested southern

angles are statistically clustered around a hypothesized

edge of the eld. Only 8 gray squirrels were released

angle (Batschelet 1981). I also used V-tests to deter-

at 300 m because this species was dif cult to capture

mine whether the locations of the last points to which

locally, and it was apparent that they were orienting

animals were tracked were oriented towards the home

towards the woods from 300 m.

woodlot (site of capture). Furthermore, I examined the

possibility of sex-related effects on angular orienta-

528

Eastern chipmunks

The chipmunk releases took place at two sites over the

course of two different seasons. This was necessitated

by dif culty in capturing enough animals in a single

season and changes in the crop rotations between the

years at the release sites. Tests indicated no difference

between releases at these two sites, hence data were

combined for all analyses (see below). Overall, 21

chipmunks were released 60 m from the woods, while Figure 1. Angular orientations of fox squirrels released during

1997. Fox squirrels were released 300, 500, and 800 m from the

20 chipmunks were released at both 120 and 180 m

woods; angular orientations were assessed after 100 m of travel.

from the woods. The solid square in the center of each panel represents the site where

During May 1996, I captured 22 chipmunks in animals were released and the trees show the direction to the woods.

The angular orientation of each fox squirrel is depicted as an open

a mature oak hickory woodlot in northwestern Vigo

circle on the unit circle. Vectors indicate average angle and degree

County, Indiana. These animals were released at a

of orientation and are displayed only for cases with statistically

site 6.4 km away in west-central Vigo County. This signi cant orientation towards the woods.

release site was a 10-ha eld bordered to the east by

a mature oak hickory forest, to the north and south

by additional agricultural elds and to the west by a

road beyond which there were more agricultural elds.

Other than the eastern edge of the release site, the

nearest trees were 250 m away. As with the squirrels,

all chipmunks were released between 10:00 and 14:00.

Fourteen chipmunks were released 60 m and 8 chip-

munks were released 120 m from the forested eastern

edge of the eld. Figure 2. Angular orientations of fox squirrels released during

1998. Fox squirrels were released 300, 400, and 500 m from the

During October November 1997, I captured 39

woods; and angular orientations were assessed after 100 m of travel.

chipmunks in a mature oak hickory woodlot in eastern All symbols are as in Figure 1.

Clark County, Illinois. These animals were released

at a site approximately 5 km away in central Clark

County, Illinois. This release site was 7 km northwest P 0.1; 800 m releases, V-test: u =

a mature oak hickory forest, to the east and west by

0.03, P > 0.1).

additional elds and to the south by a road beyond

which more elds were located. Other than the north- Releases conducted during 1998 suggest that the

ern edge of the release site, the nearest other trees were perceptual range of fox squirrels was between 400 and

360 m away. All chipmunks were released in the eld 500 m (Figure 2). The angular orientation of fox squir-

between 10:00 and 14:00. Chipmunks were released rels released 300 m from the woods was signi cantly

oriented towards the woods (V-test: u = 2.23, P

Releases conducted during 1997 demonstrated that the

0.1).

perceptual range of fox squirrels was between 300

Fox squirrels were released 300 and 500 m from

and 500 m (Figure 1). The angular orientation of fox

the woods during both 1997 and 1998. Mardia

squirrels released 300 m from the woods was signi -

cantly oriented towards the woods (V-test: u = 4.93, Watson Wheeler pairwise comparisons of fox squirrel

529

Figure 4. Angular orientations of chipmunks released during 1996

Figure 3. Angular orientations of gray squirrels released during

and 1997 (see text). Chipmunks were released 60, 120, and 180 m

1998. Gray squirrels were released 300, 400, and 500 m from the

from the woods; and angular orientations were assessed after 50 m

woods; and angular orientations were assessed after 100 m of travel.

of travel. All symbols are as in Figure 1.

All symbols are as in Figure 1.

P > 0.1) or 120 m (MWW test, 2 = 5.06, d.f. =

data found no year-speci c difference in the angular

2, P > 0.1) from the forest edge. Thus, data from

orientations of fox squirrels released at 300 m (MWW

test, 2 = 3.01, d.f. = 2, P > 0.1) or 500 m (MWW each site were pooled for chipmunks released at 60

test, 2 = 3.71, d.f. = 2, P > 0.1) from the forest and 120 m during all subsequent analyses; all 180 m

releases were done at the second site.

edge. After combining the data from the two years,

The perceptual range of eastern chipmunks was

fox squirrels released 300 m from the woods were

between 120 m and 180 m (Figure 4). The locations

still signi cantly oriented towards the woods (V-test:

u = 5.29, P 0.1). Finally, fox squirrels

0.0001; 120 releases, V-test: u = 2.64, P

they were captured (500 m releases, V-tests, u = 0.51,

P > 0.1; 800 m releases, V-tests, u = 0.15, P > 0.1). 0.1); these chipmunks also failed to show a signi -

cant orientation towards the site at which they were

captured (V-test, u = 0.81, P > 0.1).

Gray squirrels

The perceptual range of gray squirrels was between

300 and 400 m (Figure 3). Gray squirrels released Discussion

300 m from the woods were signi cantly oriented to-

wards the woods (V-test: u = 2.85, P 0.1; was less than that of fox squirrels. This ordering of

500 m releases, V-test: u = 0.46, P > 0.1). Fi- perceptual ranges is inversely related to the reported

nally, gray squirrels not signi cantly oriented towards sensitivities of these two species to habitat isolation

the woods also failed to show a signi cant orientation (Table 1), which suggests that differences in percep-

towards the site at which they were captured (400 m tual abilities contribute to the occurrences of these

releases, V-test, u = 0.06, P > 0.1; 500 m releases, species in fragmented agricultural landscapes. Prox-

V-test, u = 0.32, P > 0.1). imately, the differences in the perceptual abilities of

these species may also be related to differences in

Eastern chipmunks body size (Gillis and Nams 1998). The ultimate ori-

gin of these differences in perceptual abilities may be

Recall that circumstances dictated the use of two sites

related to historical differences in habitat occupied by

over two eld seasons for the chipmunk releases (see

these species. Historically, fox squirrels were common

above). Mardia Watson Wheeler pairwise compar-

at the interface of the eastern deciduous forests and

isons of chipmunk locations found no site-speci c

the prairie, while gray squirrels were found in interior

differences in the angular orientations of chipmunks

forest habitat (Allen 1943; Smith and Follmer 1972;

released at 60 m (MWW test, 2 = 1.49, d.f. = 2,

530

Swihart and Nupp 1998). Fox squirrels presumably be good approximations of the maximum perceptual

have a longer evolutionary history with large areas of range for each of these species.

open habitat, while gray squirrels have been exposed Perceptual range in uences dispersal success most

to open habitat only since the recent clearing of forests when species face an intermediate probability of suc-

for agriculture. This is consistent with the observation cessful dispersal. Fahrig (1988) demonstrated this pat-

that fox squirrels forage as patch transients while tern for simulated animals facing different proportions

gray squirrels forage as patch residents (Steele and of suitable habitat in a landscape. This same principle

Weigl 1992). is likely to apply to other factors that affect dispersal

The observed perceptual range of chipmunks sug- success. For example, species using highly effective

gests they should be more sensitive to the effect of search strategies will nd patches quickly no matter

fragmentation than either squirrel species, but disper- what their perceptual range (Zollner and Lima 1999a).

sal success is likely to be in uenced by a variety of Alternatively, species facing very high mortality risks

factors (see below). Chipmunks clearly are sensitive may never successfully disperse because they will die

to the effects of habitat isolation (Wegner and Merriam before reaching new habitat even when endowed with

1979; Henderson et al. 1985; Heinen et al. 1998; Nupp vast perceptual abilities (Swihart and Nupp 1998).

and Swihart 1998; Rosenblatt et al. 1999), however Such differences may explain why some simulations

their sensitivity relative to that of the squirrels remains have found dispersal success to be sensitive to percep-

unresolved (Rosenblatt et al. 1999; Nupp and Swihart tual range (Fahrig 1988; Pulliam et al. 1992; Turner

in review). This ambiguity may in part be attributable et al. 1993) while others have indicated that perceptual

to differences between study sites such as the pro- range is inconsequential (Liu et al. 1995; Swihart and

portion of the landscape containing forested habitat, Nupp 1988).

the quality of the habitat for chipmunks, or the range A key assumption in this experiment was that if

of isolation values investigated. Additional factors the animals could perceive forested habitat they would

such as the occurrence of fence rows in these land- move towards it. This is a reasonable assumption since

scapes might confound comparisons because fence all of these species are woodland resident animals

rows containing resident populations of chipmunks (Snyder 1982; Koprowski 1994a, 1994b) that face an

(Bennett et al. 1994) may not be included in calcu- increased risk of predation in open habitat (Bowers

lations of patch isolation. Finally, differences might and Ellis 1993; Bowers et al. 1993; Lima 1998). This

also be attributable to geographic variation in either increase in risk is demonstrated by the observation

historical habitat or land use patterns. Note that the that all of these species will forage in open elds, but

perceptual ranges reported here are consistent with the only when they are close to forested habitat or other

sensitivities to patch isolation observed by Rosenblatt cover (Lima and Valone 1986; Sheperd and Swihart

et al. (1999) who s work occurred in close geographic 1995; McAdam and Kramer 1998). Further support

proximity to these study sites. for this assumption was provided by the observation

The perceptual range which each of these species that several chipmunks released at 180 m actually dug

effectively experience during dispersal may not always shallow tunnels, presumably to reduce their risk of

be as great as the values reported here. All three of predation while lost. In contrast, chipmunks released

these species are known to disperse during times of at 60 and 120 m moved directly towards the woods and

the year when crops are present in the elds, and never dug such tunnels. This protocol also assumed

visually obstructive crops may reduce the ability to that the release sites were unfamiliar to the subjects.

perceive distant habitat (Zollner and Lima 1997). Ani- This assumption is supported by several lines of evi-

mals might minimize the perceptual constraints which dence: (i) all translocation distances exceeded reported

crops impose by climbing trees prior to dispersal, but homing abilities of all species (Hungerford and Wilder

any such gains would only apply to the immediate 1941; Seidel 1961; Bendel and Therres 1994); (ii) no

vicinity of their point of origin and not their entire homeward orientation was detected for any of these

search path (Zollner and Lima 1999c). Furthermore, releases; and (iii) no marked animals were recaptured

dispersal is typically done by juveniles which may at trapping sites. Finally, pilot observations indicate

have more limited perceptual ranges than the adults that animals remain in the release mechanisms for at

used in these experiments (Zollner unpublished data). least 0.5 hours after the cap is removed. Thus distress

Nonetheless, the estimates presented here are likely to caused by handling the animals (Goodwin et al. 1999)

should have subsided prior to the recorded move-

531

ments. Furthermore, such stress should only increase Acknowledgements

the desire of these woodland resident animals to move

towards the forest if they perceive it (Zollner and Lima J. Deloughry, M. Evrard, T. Fitzpatrick, N. Hogan, J.

1999b). Irwin, S. Lima, D. Smith, F. Stepp, M. Young and

Perceptual range is certainly not the only factor Indiana State University kindly provided permission

affecting dispersal success. Landscape characteristics to use their properties in my experiments. K. Crane

such as the presence of fence rows or the composi- and E. Sprague provide assistance with the eld work.

tion of the matrix may be important for dispersing E. Gustafson, S. Lima, D. Rosenblatt, R. Swihart, and

chipmunks and squirrels (Wegner and Merriam 1979; V. Quinn commented on early versions of this man-

Fitzgibbon 1993; Bennett et al. 1994; Sheperd and uscript. Funding for this work was provided by the

Swihart 1995). Mortality risks are also known to in- Theodore Roosevelt Fund of the American Museum

crease during dispersal (Larsen and Boutin 1994; Van of Natural History, Indiana State University, and NSF

Vuren 1998), and chipmunks and squirrels may face grant IBN-9221925.

different mortality risks while moving through agri-

cultural elds (Smith and Follmer 1972; Swihart and

Nupp 1998). Such differences in mortality would de- References

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