13

Predicting Habitat Distribution of the Alien Formosan Squirrel
Using Logistic Regression Model
Asako MIYAMOTO1, Noriko TAMURA2, Ken SUGIMURA1 and Fumio YAMADA3
1

Department of Forest Management, Forestry and Forest Products Research Institute (FFPRI)
P.O.Box16, Tsukuba Norin Kenkyu Danchi-nai, Ibaraki 305-8687, Japane-mail: asakom@ffpri.affrc.go.jp
2
Tama Forest Science Garden, (FFPRI), 1833-81, Todori, Hachioji, Tokyo 193-0843, Japan3
Department of Wildlife Biology, (FFPRI), P.O.Box16, Tsukuba Norin Kenkyu Danchi-nai,
Ibaraki 305-8687, Japan

Abstract
There is growing concern about the impact on the habitat of the native Japanese squirrel (Sciuruslis) in relation to the spread of the Formosan squirrel (Callosciurus erythraeus thaiwanensis), an alien
species that was introduced and became naturalized in Kamakura City, Kanagawa Prefecture, in the
1950s. We used a logistic regression model to predict the probability of occurrence of the Formosansquirrel throughout Kanagawa Prefecture. Wooded areas, the proportion of cultivated fields surrounding
a wood and the proportion of evergreen trees in a wood, factors that influence the presence of Formosansquirrels, were used as environmental variables in the model. As a result, we found that woods
suitable for the Formosan squirrel were distributed not only around Kamakura, which Formosansquirrels currently inhabit, but throughout Kanagawa Prefecture. Furthermore, a comparison of its
current habitat and its predicted suitable habitat suggested that the assemblage of large woods along the
Sagami River and Sagami Bay would be conductive to increasing its population and expanding its
distribution to western Kanagawa. Because there are forests in western Kanagawa which are extensive
and are expected to be a suitable habitat for the Formosan squirrel, we should prevent Formosansquirrels from invading the west side of the Sagami River.
Key words: alien species, Formosan squirrel, Geographic Information System (GIS),
logistic regression model, potential breeding site

1. Introduction
The distributions of many species are restricted byfactors such as climate, environmental conditions, andgeographic limitations. However, humankind has
considerably changed the process of natural evolution
by relocating countless species of animals. Therefore, today, many species, either intentionally or
accidentally, have been introduced into sites that were
not their original habitats (Primack & Kobori, 1997).
Alien species damage native species and ecosystems
on a global scale. Thus, to prevent further loss of
biological diversity, we need to study and control the
spread of alien species as quickly as possible.
Several studies have focused on changes in how
alien species affect the number of native species andtheir habitats (Yamada et al., 1999; Sugimura, 2002).
However, most alien species have not been sufficiently investigated with respect to effective countermeasures to prevent their intrusion and expansion.
The Formosan squirrel (Callosciurus erythraeus
Global Environmental Research8(1)/2004: 13-21
printed in Japan

thaiwanensis), is an arboreal mammal that originatedin Taiwan (Fig. 1). This species was first naturalizedin Japan after it escaped from the zoological garden on
Izu-Oshima Island, Tokyo, in the 1930s (Udagawa,1954). After that, it spread throughout Izu-OshimaIsland, and was then transported and became established among several sites such as urban forests, parks
and Islands along the Pacific Coast in the western
Kanto Region (Fig. 1). The mechanism of its introduction to Kanagawa Prefecture is not clearly known:
whether some squirrels that were sent to the Enoshima
Botanical Garden from Izu-Oshima Islands in 1951
escaped and spread rapidly in Kamakura City, or if
squirrels kept as pets in Kamakura City before WorldWar II escaped and became naturalized (Ono, 2001).
In Kamakura there are many historic sites where
Formosan squirrels are fed by tourists, who admire theanimals’ graceful movement. Unfortunately, thisspecies injures garden trees by extensive bark stripping and damages houses and telephone cables by
gnawing, causing significant economic losses. In©2004 AIRIES14

A. MIYAMOTO et al.

Kamakura, residents’ properties have suffered fromextensive damage, and city administrators have takensome action to prevent the squirrel’s territorial expansion, for example, by city administrative actions suchas regulations prohibiting feeding proposed in 2002.
These actions, however, have so far met with little
success. If conditions remain suitable, the squirrel
may easily spread beyond its current habitat.
As of 2001, the Formosan squirrel had expanded
its distribution mainly to the eastern part of Kanagawa
Prefecture, from Koajiro on the Miura Peninsula to
Totsuka in Yokohama City and Akabane in Chigasaki
City. The species has not yet been sighted west of the
Sagami River, an area inhabited by the Japanesesquirrels, and the distributions of the two species have
not yet overlapped (Sonoda & Tamura, 2003).
However, if the Formosan squirrel continues toexpand its habitat, the two species may come into con-Fig. 1

flict over food resources and breeding sites in the near
future.
In the present study, we have clarified the environmental variables that influence the presence of the
Formosan squirrel and made a logistic regression
model using those variables in Kanagawa Prefecture.
Furthermore, we have compared the results of a fieldsurvey with the predictions of the squirrel’s occurrence and identified areas in which stronger countermeasures are required to prevent expansion of thesquirrel’s range.

2. Logistic Regression Analysis
Tamura et al. (2004) investigated the effect ofenvironmental variables on the presence of the
Formosan squirrel using logistic regression analysis in
the Ofuna area (Fig. 2).

Formosan squirrel, Callosciurus erythraeus thaiwanensis (Photo by N. Tamura) and distribution of its habitat
in Japan.

Fig. 2

Location of study area and Formosan squirrel’s habitat in 2001, Kanagawa
Prefecture. The habitat data with 1 km grid ( ) was adapted from Sonoda andTamura (2003). The area which is surrounded by the dotted line () indicates
the Ofuna area. We used this area’s squirrel presence/ absence data and environmental data to create our logistic regression model.

Predicting Habitat Distribution of the Alien Formosan Squirrel Using Logistic Regression Model

The logistic regression model is given by15

environmental variable was fitted individually in thelogistic model, which showed that the wooded area
had a greater effect on squirrel presence than any of
the other variables. Therefore wooded area wasalways included in the model. Since some of theenvironmental variables were intercorrelated (Table 2),
they could not be put into the model simultaneously.
As a result, the model which was the best predictor ofsquirrel presence was the following one, which
included wooded area, proportion of cultivated fields
surrounding the wood, and the proportion of evergreen
trees in each wood.
Probability (Y=1)=exp(β0+β1x1+β2x2+β3x3)/(1+exp(β0+β1x1+β2x2+β3x3))
where Y = probability of Formosan squirrel
occurrence, x1= wooded area, x2= proportion ofcultivated fields surrounding the wood, x3= proportionof evergreen trees in the wood and β0 = − 4.632, β1 =3.607, β2 = 3.648, β3 = 1.704, Cox & Snell R2 = 0.404.
This model showed statistical significance at P
value < 0.001. In Figure 3 the relationships between
the observed data of squirrel presence/absence andpredicted probability of its occurrence are shown. Of
the 100 woods analyzed, the squirrel inhabited 42
woods. The model classified 87.9% of woods without squirrel presence correctly and 78.6% of thewoods with squirrel presence correctly (cut-level at
P=50%). It seemed that the predicted probability of
Formosan squirrel occurrence almost matched theobserved data. The three environmental variables
included in the model had a positive effect on occu-

logit(p)= β0+β1x1+β2x2+· · · · ·+βnxn , andlogit(p)=log(p/(1-p)) ;hence
p=exp(β0+β1x1+β2x2+· · · · ·+βnxn)/(1+exp(β0+β1x1+
β2x2+· · · · ·+βnxn))
where p is the probability of occurrence of an event,β0 is a constant, βi represents the regression coefficients and xi represents the different environmental
variables.
Logistic regression analysis showed that the areaof a wood (wooded area), the proportion of cultivatedfields surrounding the wood, and the proportion ofbroad-leaved evergreen trees in the wood influenced
the presence of the Formosan squirrel.
However, according to a vegetation map publishedby the Ministry of the Environment, broad-leaved
evergreen trees used as the variable in the model arerare in Kanagawa Prefecture. Therefore, it is not
appropriate to include that variable in a broad-scaleassessment of Kanagawa Prefecture.
Thus in the present study, we made another modelusing the data from 100 woods in the Ofuna area.
Logistic regression analysis was used to determine theeffect of ten environmental variables related towooded area, distance between woods, vegetation ineach wood, and land use surrounding the wood byusing a stepwise procedure (Table 1). First each

Table 1 Environmental variables used to model Formosan squirrel occurrence.
log (Wooded area)log (Distance to nearest neighboring woods)
log (Distance to < 5 ha woods with squirrels)
Vegetation typeevergreen trees
deciduous trees
othersSurrounding land use
cultivated fields
residential areas
roads
othersa) logarithmic transformationb) arcsin transformation
Xi : variables selected to be entered into the model.

a)
a)
a)
b)
b)
b)
b)
b)
b)
b)

X1
X3
X2

Table 2 Pearson correlation coefficients between 10 environmental variables. (n = 100 in all cases)
Variables
1 log (Wooded area)
2 log (Distance to nearest neighboring woods)
3 log (Distance to < 5 ha woods with squirrels)
4
Evergreen trees
5
Deciduous trees
6
Other vegetation
7
Cultivated fields
8
Residential areas
9
Roads
10
Other land use

1

2

3

4

5

6

7

8

9

-0.230-0.6570.091
0.288-0.201
0.056-0.073
0.081
0.152

0.442
-0.046-0.1000.069-0.2300.274
0.013-0.110-0.145-0.2200.193-0.1830.112-0.001-0.007-0.043-0.7400.103-0.066-0.1370.068-0.5950.057
0.040-0.018-0.048-0.1370.036
0.132-0.007-0.677-0.367-0.200-0.057-0.284-0.197

16

A. MIYAMOTO et al.

Fig. 3

The relationship between observed data of squirrel presence/absence andpredicted probability of its occurrence in each wood. Observed data were plottedin order of predicted probability, with absence and presence of squirrels shown
separately.

pancy. This suggests that the probability of occurrence of the Formosan squirrel will increase if thewoods are large, more completely surrounded bycultivated fields, or contain more evergreen trees.
In the next section, we will predict the probabilityof occurrence of the Formosan squirrel in Kanagawa
Prefecture.

3. Probability of Occurrence of the
Formosan Squirrel throughout Kanagawa
Prefecture
The study area, ranging from 0 m to 1,470 m inelevation, was located in Kanagawa Prefecture (Fig. 2).
Of the total land area of 168,257 ha, 24.1% wascovered with forest, 17.1% was agriculture land, and58.8% was urban and other areas. The topography
was divided into three general areas: the Tanzawa
Mountains in the west, the plains and the tablelands inthe central part, and the Tama Hills area and Miura
Peninsula in the east. Large forests remain in thewestern Tanzawa mountain area. In the central andeastern parts, most of the land is urban, composed ofhousing sites, cultivated fields and small fragmented
forests (Fig. 4 (a), (b)). Over 800 m above sea level,
the vegetation consists of deciduous trees such as
Fagus crenata, F.japonica, and Quercus crispula.
Below that, vegetation consists mainly of broadleaved trees such as, Machilus thunbergii, Castanopsis
cuspidata var. sieboldii, Q.myrsinaefolia, andCyclobalanopsis salicina (Kanagawa Prefecture,1979).
To obtain the probability of Formosan squirrel

occurrence, the wooded area, the proportion of cultivated fields surrounding a wood, and the proportion of
evergreen trees in a wood were calculated using Geographic Information System (GIS) TNTmips Ver.6.6(MicroImages, Inc) and ArcView Ver.3.2 (ESRI) by
the following method.
First, we created a forest map showing the distribution of woods from an actual digital vegetation map
published by the Ministry of the Environment (19941999), with a vector format with a scale of 1:50,000.
From the viewpoint of habitat utilization by the
Formosan squirrel, the 130 vegetation/land coverclasses that were determined by plant sociologicalclassification, were divided into two classes: wooded
and non-wooded. Then, the wooded area and the proportion of evergreen trees within it were calculated.
Next, we calculated the proportion of cultivatedfields surrounding the woods from the forest map anda land use map, the 1:10,000 scale Detailed Digital
Information Metropolitan Area Map (1994) from theGeographical Survey Institute, with a grid resolutionof 10 m. The Detailed Digital Information Map
includes 16 land use classes. Of these, freshwater
bodies and the sea were excluded from the analysis.
The remaining land use classes were divided into four
categories: cultivated fields, residential areas, roads
and others (Table 3). We overlaid the forest map on
the land use map and created a 10 m buffer zone
around each wood. Then we calculated the proportion of cultivated fields in each of these buffer zones.
Using this data set and the logistic regression
model, we calculated the probability of occurrence of
the Formosan squirrel in each wood.

Predicting Habitat Distribution of the Alien Formosan Squirrel Using Logistic Regression Model

Fig. 417

Distribution of woods and land use in the study area (a) and fragmented woods in Kamakura City (b). In the eastern andcentral parts of the study area, most of the land is urban, composed of housing sites, cultivated fields and small fragmented
woods.
Table 3 Land use classes.
Land use class
Cultivated fields
Residential areas
Roads
OthersOriginal land use class
forests, cultivated fields, wastelandresidential areas (low, middle, high), business districts, industrial areasroads
emerging residential areas, open land , public facilities areas

We found that 583 woods (25.8% of the total) had
a high probability of squirrel occurrence (probability >0.8). The habitat potential of a wood increased withincreasing wooded area. Inhabitable woods (probability > 0.8) accounted for 12.0% of the total numberof woods of less than 5 ha in area , 44.8% of all woods
5-10 ha in area and more than 90% of all woods over
10 ha in area (Table 4).
Furthermore, woods suitable as Formosan squirrel
habitat (probability > 0.8) were distributed not onlyaround Kamakura, which the species currently
inhabits, but all over the study area (Fig. 5).
The average sizes of the Formosan squirrel’s homeranges are 0.72 ha for females and 3.83 ha for males.
Hence, if a wood of at least 5 ha exists, a breedingpair of this species has a chance to establish a homerange (Tamura et al., 1987). This is much smaller
than the average sizes of home ranges of the Japanesesquirrel, which are 12.1 ha for females and 22.1 ha formales (Tamura, 1998).

It seems that the ecological adaptation of the
Formosan squirrel to small fragmented and isolated
woods is advantageous for its expansion in urban
areas. In the study area, there are few large woods
because of land development, but small fragmented
woods between urban areas remain on the convoluted
tableland, the Tama Hills area in the northeastern part,and in a small hilly area in the Miura Peninsula. The
Formosan squirrel seems to be able to expand its
habitat by utilizing these woods.
A number of studies have suggested that grovesand hedgerows may facilitate the movement of animals from one wood to another, acting as steppingstones or corridors for dispersing individuals (Burel,
1989; Demaynadier, 1998; Gibbs, 1998). We cananticipate that the Formosan squirrel will expand its
habitat by utilizing small fragmented woods in urban
areas as stepping-stones.
Therefore, it will be
important to monitor the distribution of the species insmall fragmented woods in the future.

18

A. MIYAMOTO et al.

Table 4 The distribution of predicted probability of Formosan squirrel occurrence by wooded area class.
Predicted probability of
Formosan squirrel occurrence
<=0.2
<=0.4
<=0.6
<=0.8
<=1.0
Total number of woods

Fig. 5

Wooded area (ha)
<=5
715
328
233
220
204
1700

<=10
9
20
46
84
129
288

<=20
0
2
3
11
107
123

<=30
0
0
1
2
50
53

>30
0
0
0
0
93
93

Total number of
woods
724
350
283
317
583
2257

Probability of Formosan squirrel occurrence in each wood, using the logistic regression model.

4. Distribution of Potential Breeding Sites
From the analysis above, we found that woods
suitable for Formosan squirrel occurrence (probability> 0.8) were distributed all over the study area. We
have to pay attention to such potential habitat, andfurthermore, we have to pay special attention to areasin which many woods or forests are concentrated,because such areas may have a greater potential topromote a squirrel population increase. To identify
such areas, we performed the following analysis.
In a previous study of the relationship between thepresence of Formosan squirrels and environmental
variables, distances between woods did not have asignificant effect on the presence of this species(Tamura et al., 2004). Therefore, it is possible that
this species moves easily among woods that are within
the extent of its daily movement. According to
Tamura et al. (1987), the largest linear dimension of
the female home range is 163 m. So we consider
that if the distance between woods is less than 163 m,
females can move among them and have a chance toestablish their home ranges and reproduce. For thatreason, we connected potential habitat (woods with aprobability > 0.8) to others, which were within 163 m,
and defined such assemblages as potential breedingsites. In some cases, there was no other potential
habitat around one potential habitat. Such an independent potential habitat was also regarded as apotential breeding site. By carrying out such anPredicting Habitat Distribution of the Alien Formosan Squirrel Using Logistic Regression Model

analysis, we could take into account the assumptionthat small fragmented habitats serve as sources ofincreasing population.
From an analysis based on the above definition, we
identified 233 potential breeding sites (Fig. 6).
Potential breeding sites that were composed of at least
two potential habitats accounted for 33.5% of the total
number of potential breeding sites. Although eachhabitat was small, connecting two or more habitats
created comparatively large areas. Such potential
breeding sites were most numerous along the Sagami
River, in the Tama Hills area in the northeast, and on
the Miura Peninsula. Potential breeding site No.1 inFig. 6 (total wooded area of 2,118 ha) on the Miura
Peninsula, which extends from Kamakura to
Yokosuka, was the largest area east of the Sagami
River.
The largest potential breeding sites in the study
area were Tanzawa (Fig. 6, No.2; 17,723 ha) andHakone (Fig. 6, No.3; 5,403 ha) in the west. Both ofthese sites are a parts of a continuous forest that
extends outside of the study area, so the actual size ofthe breeding site is larger. As a result, it is apparent
that the area west of the Sagami River is a large
continuous site suitable as Formosan squirrel habitat.

Fig. 619

5. Important Areas for Population Control
Using the results from the previous section and thecurrent habitat data on the Formosan squirrel, we
identified important areas that might act as sources of
species expansion in the future. The current habitat
data were obtained from a questionnaire survey conducted in Kanagawa Prefecture in 2001 (Sonoda &Tamura, 2003) and a field survey in 2002 (Tamura, inpress).
In a few cases, we have not established the presenceof squirrels in all woods which were included in one
potential breeding site. In the other cases, we foundthem throughout the area of the breeding sites. As aconsequence, 30 of the potential breeding sites
included woods in which the Formosan squirrel waspresent (pink-trimmed sites in Fig. 6).
According to previous surveys on Formosansquirrel distribution in Kanagawa Prefecture from the
1950s to the 1990s (Shiozawa et al., 1985; Huruuchi
et al., 1990; Kamiya & Noguchi, 1995), the Formosansquirrel’s habitat was included in one potential
breeding site (Fig. 6, No.1), which was continuouswith Kamakura. However, by 2002, this species was
distributed outside site No.1. Thus, the Formosansquirrel expanded its habitat from site No.1, theDistribution of potential habitats, potential breeding sites and current breeding sites. Potential habitat ( ) indicates
probability of Formosan squirrel occurrence > 0.8 in each wood. Potential breeding sites ( ) indicate assemblages ofpotential habitats (see text). Current breeding sites ( ) indicate potential breeding sites which include the Formosansquirrel’s current habitat in 2002.

20

A. MIYAMOTO et al.

largest site east of the Sagami River as a source ofincreasing the population. At present, this species isalso found at other potential breeding sites along the
Sagami River and Sagami Bay. It seems that thesepotential breeding sites will be especially importantfor population control in the future.
If the Formosan squirrel invades the west side of
the Sagami River, we predict that this area, which iscurrently a habitat for native Japanese squirrels, will
provide the Formosan squirrel with an excellent large
habitat. Larger woods, which are able to support
more individuals, could therefore be important as
recolonization sources (Fitzgibbon, 1993). If theJapanese squirrel competes with the Formosan squirrel for habitat, as occurs between the Red squirrel S.
vulgaris L. and the Grey squirrel S. carolinensis in
Great Britain (Usher et al., 1992; Fitzgibbon, 1993), it
is possible that the Japanese squirrel’s habitat, and its
survival, will be threatened.
To prevent intrusion west of the Sagami River, it isimportant to monitor the areas, in which many woods
are concentrated and might be used as breeding habitats, such as woods along Sagami Bay (Fig. 6, No.4)
and along the Sagami River (Fig. 6, No.5), because it
is possible that the Formosan squirrel will invade thewestern area via these woods.
To prevent the squirrel’s expansion, an intensive
effort will have to be made to exterminate a considerable number of animals. From the results shownin Fig.6, we propose a method by which the Formosansquirrel would be removed from the small pockets it
inhabits in the peripheral areas of its current habitat.

6. Steps toward Effective Control Measures
We used three environmental variables in themodel: wooded area, the proportion of cultivatedfields surrounding a wood and the proportion of evergreen trees in a wood. Since, in all, wooded area
was the most effective variable for squirrel presence,
the results of our model showed that larger woods had
a greater probability of squirrel occurrence. To
support a population and its food resources, wooded
area is one of the most important factors. However,
we should take into account not only the size of thehabitat but also habitat quality, such as vegetation type,because it determines the total number of individuals
that the woods can sustain. A model made with anarrow range of data may not necessarily apply toother areas that have different environments. We
must conduct our analysis in consideration of the local
environment, including vegetation type and foodresources.
According to Kinoshita (1989), the Formosansquirrel inhabited about 60% of the land area in
Kamakura City in 1988, not only the hilly wooded
areas but also residential areas, where the squirrels
co-existed in close contact with residents. In fact, thespecies is seen near houses and roads in residential

areas located in the center of Kamakura, because it
easily utilizes man-made environments such as parks
and home gardens, and there are many facilities forfeeding in sightseeing places or around houses. In
addition to natural factors such as wooded area,
vegetation type and location of woods, it seems thathuman impact has affected the expansion of thisspecies.
Because this intrusion of an alien species isinfluenced by human impact, we should be able toimprove the accuracy of the model by adding humanimpact factors and obtain results which better reflect
the actual condition.
Moreover by using GIS, we could predict thepotential habitat for Formosan squirrels by integrating
ecological data, which is detailed but laborious toobtain, with broad-scale environmental data, such as
land use and distribution of woods. These results
based on the integration of specialized knowledge andspatial data, would help in making decisions regarding
land use and resource management. We expect toimprove the model further by improving the accuracyof the GIS data and learning more about the ecologicalcharacteristics of these animals.

Acknowledgements
We gratefully acknowledge the contributions of Mr.
Minotani and Mrs. Takashima and their biology
students at Shonan High School and Fujisawa HighSchool, respectively, for collecting the field data.
This research was partly funded by the Global
Environment Research Fund of the Japanese Ministryof the Environment.
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(Received 5 December 2003, Accepted 3 February 2004)