Mammal Study 30: 131–137 (2005) © the Mammalogical Society of Japan

Effects of habitat fragmentation on the presence of Japanese , lis, in suburban forests

Tomomi Kataoka1,* and Noriko Tamura2

1 Laboratory of Ecology, Faculty of Agriculture, Hokkaido University, Kita 9 Nishi 9, Sapporo, Hokkaido 060-8589, Japan 2 Tama Forest Science Garden, Forestry and Forest Products Research Institute, Todori 1833, Hachioji, Tokyo 193-0843, Japan

Abstract. Habitat fragmentation affects population density, resulting in a higher probability of and lower rates of recolonization of habitat patches in some holarctic tree squirrels. Several local populations of the Japanese , Sciurus lis, seem to have decreased during the 1990s. We tested the hypothesis that the contracting distribution of Japanese squirrels is connected with habitat fragmentation via forest destruction for human land use. We investigated whether woodlot characteristics and degrees of woodlot isolation affected the presence of squirrels in suburban fragmented woodlots in western Tokyo. Japanese squirrels were present in 12 of 76 woodlots. Squirrel presence was not detected in all 23 woodlots east of a single north-south trending road within the study area. This result suggested that the road prevented squirrels from dispersing from the mountainous range west of the study area into eastern woodlots. In 53 woodlots west of the road, woodlot size and preferred habitat had positive effects on the probability of squirrel presence, whereas isolation variables did not affect its presence. We suspected that the habitat fragmentation via forest destruction for human land use might be one of the causes leading to the recent diminishing in local populations of Japanese squirrels.

Key words: distribution, habitat fragmentation, Japanese squirrels, Sciurus lis.

Habitat fragmentation is defined by Saunders et al. reviews see Bright 1993; Bowers and Matter 1997; (1991) and Andrén (1994) as the process of subdividing Krohne 1997). The vulnerability of a species to habitat a continuous habitat into smaller and usually more iso- fragmentation is thought to be due to ecological charac- lated pieces, resulting in remnant vegetation patches sur- teristics, such as site tenacity, dispersal ability, coloniza- rounded by a matrix of different vegetation types and/or tion capability, and level of specialization in habitat use land uses. An important and large-scale cause of habitat (Van Apeldoorn et al. 1994; Delin and Andrén 1999). fragmentation is expansion and intensification of human Studies on the effects of habitat fragmentation on pop- land uses such as forest management, agriculture, urban ulations of holarctic tree and flying squirrels (genera Sci- development and road construction (Mader 1984; Bayne urus, Tamiasciurus and Glaucomys) in North America and Hobson 1998; Primack 2000). and Europe suggest that different species tolerate dif- In many species, it has been demonstrated ferent levels of forest fragmentation. For example, grey that habitat fragmentation reduces population densities squirrels, S. carolinensis, and southern flying squirrels, and dispersal rates and extends dispersal distances. G. volans, are less likely to be present in small and iso- Therefore, it is expected that smaller habitat patches will lated forest fragments than in larger forests which were have higher probabilities of local extinction and lower close to many woodlots (Fitzgibbon 1993; Swihart and rates of recolonization following local extinction (for Nupp 1998; Nupp and Swihart 2000). On the other hand,

*To whom correspondence should be addressed. Present address: Tama Forest Science Garden, Forestry and Forest Products Research Institute. E-mail: [email protected] 132 Mammal Study 30 (2005) fox squirrels, S. niger, and American red squirrels, T. to forest fragmentation. Therefore, we investigated the hudsonicus, seem better adapted to fragmented habitats degree to which habitat fragmentation changed in and less likely to be negatively affected by forest western Tokyo, eastern Honshu, from 1970 to 2000. fragmentation than other squirrel species (Sheperd and Secondly, using distributional data in our study area, Swihart 1995; Swihart and Nupp 1998; Bayne and we tested whether woodlot occupancy by the Japanese Hobson 2000; Nupp and Swihart 2000). Swihart and squirrel was affected by woodlot size, area covered with Nupp (1998) used a demographic model to demonstrate suitable vegetation, and degree of isolation from the that characteristics of squirrel species vulnerable to frag- mountainous range or adjacent woodlots. mentation were a strong preference for forest habitat, reduced ability to move through unsuitable habitat types, Study area and lower rate of population growth. The Japanese squirrel, S. lis, is a holarctic We surveyed the distribution of Japanese squirrels that was recorded as a common species in Japan. How- from 1996 to 1997 in an area of 22400 ha in western ever, since 1991, certain populations in western Honshu Tokyo, Japan (35°40'N, 139°15'E). The west side of the were considered threatened and it had probably become study area is part of the mountainous range dominated extinct in Kyushu (Mammalogical Society of Japan by Mt. Takao (599 m above sea level, Fig. 1). Two 1997). In suburban areas of eastern Honshu, the dis- expanses of tree-covered hills range eastward from the tribution has recently contracted (Shiozawa et al. 1985; mountainous range. The hills were mainly covered with Furuuchi et al. 1990; Okazaki 1993; Kamiya and deciduous broad-leaved forest dominated by two de- Noguchi 1995). Because the Japanese squirrel is con- ciduous oaks, Quercus serrata and Q. acutissima, and sidered a habitat specialist that greatly prefers natural included Japanese red pine, Pinus densiflora, fir, Abies mixed-species forests and pinewoods to artificial planta- firma, and evergreen oaks, Q. glauca and Q. acuta, in tions of conifers (Tamura 1998; Yatake et al. 1999), we patches. Coniferous forests were mainly artificial plan- hypothesized that local and the shrinking tations consisting of Japanese cypress, Chamaecyparis distribution of this species were results of vulnerability obtuse, and Japanese cedar, Cryptomeria japonica. Iso-

Fig. 1. The distribution of isolated woodlots >1 ha in the west of Tokyo in 1997. Presence/absence of Japanese squirrels in woodlots was detected by our field surveys from 1996 to 1997. Number of woodlots is shown in parentheses. The western side of the study area is part of the mountainous range dominated by Mt. Takao. Kataoka and Tamura, Japanese squirrels in fragmented habitats 133 lated woodlots were defined as: areas covered with any 1997. In three of these woodlots, we observed individ- of the forest vegetation described above, entirely sepa- uals directly. In the other 73 woodlots where we could rated by different distances and without contact at the not detect squirrels from direct observation, we con- level of canopy layer (Fig. 1). firmed squirrel presence by their characteristic feeding Investigation of aerial photographs (scale 1 : 30000) signs on Japanese walnuts, Juglans ailanthifolia. Wal- and topographic maps (scale 1 : 25000) from 1970 to nuts are the preferred food of the squirrel (Kato 1985; 2000 (published by the Geographical Survey Institute in Tamura 1997). When the Japanese squirrel feed on the Ministry of Land, Infrastructure and Transport of walnuts on trees, two pieces of walnut shell with the Japan) revealed that these hills have undergone intensive edges nicked by their incisors fall to the ground. How- development via human activities, represented in parti- ever, large Japanese field mice, Apodemus speciosus, the cular by increased road construction and expansion of other potential walnut predators, show the evidently residential area concomitant with a rise in the popula- different feeding signs from the squirrel’s signs, which tion of the study area. In 1970, 56.6% of woodlots were leave whole walnut shells having chewed a hole through >100 ha, and 5.2% of woodlots were <20 ha (Fig. 2). the shell by their incisors. Thus, we confirmed Japanese After 1970, smaller woodlots increased slowly, and by squirrel presence by signs of split walnut shells that were 2000, they composed 60.9% of all woodlots. A 15 m clearly nicked and which had not decayed. If no walnut wide east-west expressway was built in 1968, and a 4-lane trees were present within a woodlot, we installed at least 12 m wide north-south paved highway was built in 1985 one metal mesh box feeder (16 × 12 × 23 cm) per 10 ha through the study area (road A and B in Fig. 1). Two woodlot size to make walnuts available to Japanese rivers (top and bottom in Fig. 1) make it difficult for squirrels. Box feeders were set on tree branches at 1.5–2 squirrels to disperse into the study area from the north m above the ground, because this level is too high for or the south. field mice to climb. We checked the feeders weekly for feeding signs on walnuts to confirm squirrel presence. If Materials and methods walnuts survived more than six months in the feeders, we assumed Japanese squirrels to be absent from the woodlot. Presence/absence of squirrels We focused our efforts on surveying isolated woodlots Woodlot size and preferred habitat >1 ha, because the minimum home range size reported Aerial photographs and maps (published by the Geo- for the Japanese squirrel is 3.7 ha (Yatake and Tamura graphical Survey Institute and by Bureau of City Plan- 2001). We surveyed squirrel presence in all 76 isolated ning in Tokyo Metropolitan Government) were used to woodlots of the study area from July 1996 to November measure woodlot size (WS) using a planimeter and the

Fig. 2. Percentages of isolated woodlots in six classes of woodlot size from 1970 to 2000 in the study area. Woodlot sizes were measured using aerial photographs and topographic maps. Number of woodlots is shown on each column. 134 Mammal Study 30 (2005) freeware, Scion Image. Open fields and building sites woodlots with and without squirrels were analyzed using were eliminated from measures of WS. Red pine, walnut Mann-Whitney’s U test. We checked correlations among and deciduous forests mixed with evergreen broad- these size and isolation variables, and used univariate leaved trees or coniferous trees are preferentially used by and multiple logistic regressions to assess the effects of the Japanese squirrel (Kato 1985; Tamura 1998; Yatake size and isolation as independent variables on squirrel et al. 1999). We discriminated these forest vegetation presence in a woodlot. types in each woodlot by stereoscopy of aerial photos and from our fieldwork. We also measured the total area Results of preferred habitat for squirrels per woodlot (PH) using the planimeter and the same freeware as for woodlot size. Japanese squirrels were present in 12 of 76 isolated woodlots (Fig. 1). All 12 of these woodlots with squirrel Degree of woodlot isolation were located west side of road B, and the ratio of wood- We measured three variables of woodlot isolation lots with squirrel differed between west and east sides of from aerial photographs and topographic maps, and from the road (P = 0.01, Fisher’s exact probability test). How- our fieldwork. Distance to nearest woodlot (DW) was ever, woodlots east and west of road B did not vary in estimated as the shortest distance between two woodlots size (WS and PH) or in terms of their isolation variables of >1 ha, excluding the mountainous range. Distance to (DW and DLW, Table 1). To assess the possibility that nearest large woodlot (DLW) was estimated as the short- road B prevented squirrels from dispersing from the est distance to a woodlot of >20 ha. Yatake and Tamura mountainous range and western woodlots to east side of (2001) found that home range sizes of female and male the road, we conducted the following analysis for a sub- Japanese squirrels to be 10 ha and 20–30 ha, respec- sample of 53 woodlots west of the road. tively. Home ranges of females were exclusive, whereas Two size and three isolation variables were compared home ranges of males overlapped with other male and between groups of woodlots with and without squirrels female home ranges. Therefore, we assumed all wood- using Mann-Whitney’s U test (Table 2). Woodlots with lots larger than 20 ha to be potential source habitats for squirrels in WS and PH were significantly larger (mean ± squirrels. The distance to nearest mountainous range SD, 122.9 ± 124.2 ha and 54.0 ± 64.7 ha, respectively) (DM) was estimated as the shortest distance between the than woodlots without squirrels (12.6 ± 12.8 ha and 2.0 ± mountainous range and each woodlot. 2.9 ha, P < 0.001). The minimum size of woodlots with squirrels was 20.8 ha containing 7.9 ha of PH. West of Statistical analysis road B, squirrel presence was detected in all woodlots The ratio of woodlots with and without squirrels larger than 100 ha, containing 12.2–183.3 ha of PH. Of between eastern and western woodlots in relation to road the isolation variables, DLW of woodlots with squirrels B was analyzed using Fisher’s exact probability test. was significantly shorter (30.2 ± 46.9 m) than that of Comparison of two size (WS and PH) and three iso- woodlots without squirrels (179.2 ± 260.1 m, P < 0.01). lation variables (DW, DLW and DM) between groups of Univariate logistic regression in woodlots west of road

Table 1. Comparison of variables of woodlots east and west of road B.

Eastern woodlots (n = 23) Western woodlots (n = 53) Mann-Whitney’s U test Variable Mean ± SD Mean ± SD Mean rank Mean rank (U value) (Range) (Range) WS (woodlot size [ha]) 31.6 ± 29.5 37.5 ± 74.6 n.s. 44.7 35.8 (3.5–116.8) (2.6–449.2) (466.0) PH (preferred habitat within a 7.3 ± 11.7 13.8 ± 37.1 n.s. 42.7 36.7 woodlot [ha]) (<0.1–47.9) (<0.1–183.3) (514.0) DW (distance to nearest woodlot of 136.0 ± 218.5 95.0 ± 166.5 n.s. 37.7 38.9 >1 ha [m]) (12.5–750.0) (10.0–925.0) (590.5) DLW (distance to nearest large 250.1 ± 339.0 145.4 ± 237.6 n.s. 40.3 37.7 woodlot of >20 ha [m]) (12.5–875.0) (10.0–925.0) (569.0) n.s.; not significance Kataoka and Tamura, Japanese squirrels in fragmented habitats 135

Table 2. Comparison of variables of woodlots with and without squirrels west of road B.

Woolots with squirrels (n = 12) Woodlots without squirrels (n = 41) Mann-Whitney’s U test Variable Mean ± SD Mean ± SD P value Mean rank Mean rank (Range) (Range) (U value) WS (woodlot size [ha]) 122.9 ± 124.2 12.6 ± 12.8 ** 46.3 21.4 (20.8–449.2) (2.6–64.7) (15.0) PH (preferred habitat within a 54.0 ± 64.7 2.0 ± 2.9 ** 46.1 21.4 woodlot [ha]) (3.8–183.3) (<0.1–12.2) (17.0) DW (distance to nearest woodlot of 76.3 ± 130.8 100.6 ± 176.7 n.s. 21.2 28.7 >1 ha [m]) (12.5–440.0) (10.0–925.0) (176.5) DLW (distance to nearest large 30.2 ± 46.9 179.2 ± 260.1 * 17.0 29.9 woodlot of >20 ha [m]) (12.5–175.0) (10.0–950.0) (126.0) DM (distance to nearest moutainous 2.0 ± 2.2 2.3 ± 2.1 n.s. 24.8 27.6 range [km]) (<0.1–6.2) (<0.1–6.6) (220.0) **; P < 0.001, *; P < 0.01 and n.s.; not significance.

Table 3. Univariate and multiple analyses by logistic regression Discussion models relating to the probability of squirrel presence in woodlots west of road B (n = 53). While Japanese squirrels were observed in several Regression woodlots west of road B, we did not confirm squirrel Regression model R2 adj coefficient1 presence in woodlots east of the road throughout our WS (woodlot size)2 0.092 (**) 0.73 field surveys (Fig. 1). However, Okazaki (1993) ob- PH (preferred habitat within a woodlot)2 0.462 (**) 0.77 served Japanese squirrels in some of the woodlots east WS + DW (distance to nearest woodlot 0.003 (n.s.) 0.74 of road B before and after the road construction in the of >1 ha) 1980s, the implication being that relatively recent extinc- WS + DLW (distance to nearest large –0.004 (n.s.) 0.74 tions have occurred in eastern woodlots of the study area. woodlot of >20 ha) In this study, there were no differences in any of the size WS + DM (distance to nearest –0.141 (n.s.) 0.73 or isolation variables between woodlots west and east of mountainous range) the road (Table 1). Mader (1984) suggested that the PH + DW 0.004 (n.s.) 0.79 fragmentation of 6 m or wider highway with certain PH + DLW 0.001 (n.s.) 0.77 traffic was an effective and stable hindrance to migra- PH + DM 0.055 (n.s.) 0.77 tion and gene flows between habitat patches. In eastern 1 Only coefficient of the last variable is shown in multiple regression. woodlots of the study area, none of the variables of size Effect of the first variable was always significant (P < 0.01). 2 There or isolation between woodlots (WS, PH, DW and DLW) was a positive correlation between WS and PH (r = 0.94, P < 0.01). seemed to have any effect on the occurrence of squirrels, **; P < 0.01 and n.s.; not significance. because road B was likely to act as a barrier to the dis- persal of squirrels from the mountainous range to eastern B (Table 3) revealed that WS and PH had significant woodlots. positive effects on squirrel presence (P = 0.004 and The mean size of woodlots with squirrels was signifi- 0.003, R2 = 0.73 and 0.77, respectively). There was also cantly larger than woodlots without squirrels west of a strong positive correlation between these two variables road B (Table 2), and woodlot size had a significantly (WS and PH, r = 0.94, P < 0.01). However, in multiple positive effect on the presence of Japanese squirrels logistic regressions, none of isolation variables (DW, (Table 3). The mean preferred habitat area in woodlots DLW or DM) provided additional power to either of the with squirrels was also significantly larger than in wood- size variables (WS or PH) in explaining squirrel presence lots without squirrels (Table 2), and preferred habitats (Table 3). had a significantly positive effect on squirrel presence (Table 3). Studies on the Eurasian red squirrel, S. vul- garis, showed that woodlot size and/or preferred habitat within a woodlot significantly affected the probability 136 Mammal Study 30 (2005) of squirrel presence in different fragmented landscapes Fujiwara for giving us useful information about squirrel (Wauters 1997). In the Netherlands, for example, both occurrence and woodlot location in the study area. 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