哺 動 学 誌(J.Mamm.Soc.Japan)11(3・4):93-106 December1986 Vertical Space Use of Voles and Mice in Woods of Hokkaido, Japan* Hisashi ABE Institute of Applied Zoology, Faculty of Agriculture, Hokkaido University, Sapporo 060, Japan (Accepted February 5, 1986) ABSTRACT.- I observed the use of vertical stratification of woods by two species of voles, Clethrionomys rufocanus bedfordiae and C. rutilus mikado, and by three species of mice, Apodemus speciosus ainu, A. peninsulae giliacus and A. argenteus, in four different types of woods in Hokkaido, Japan. I set containers in the woods inside which a piece of bait was placed, including minute pieces of colored indigestible plastic, which were separated by color in each of four strata (under the litter, on the litter surface, and 50 cm and 150 cm in height above the litter), and at the same time performed live-trapping on the ground surface. All the faeces dropped in traps were then collected and analyzed to estimate the degree of vertical space use. All species but one used all four strata of habitats, but the vertical niche width (H') was narrowest in C. rufocanus and widest in A. argenteus, with subequal and intermediate widths in C. rutilus and A. speciosus. Data of A. peninsulae were insufficient to estimate the use of strata. As to habitat preference, C. rufocanus appeared in all four habitats, C. rutilus in three, A. speciosus in two, and the remaining two in one habitat. These trends of habitat preference agreed with those found in Hokkaido to date. Introduction Six species of murid rodents occur in Hokkaido. Except for a relict species, Clethrionomys rex, the ecological features of the other five species have been relatively well documented (OTA, 1984). However, the partitioning of vertical space by these species has never been examined. Recent studies have demonstrated that cricetid rodents in North America and squirrels in tropical rain forests of Gabon reduce competition by partitioning vertical space (SMITH & SPELLER,1970; MESERVE,1977; EMMONS,1980; BARRYet al., 1984). The present work was planned to observe and document the vertical space use of the above-mentioned five species in selected woods in Hokkaido. Study areas and Methods The following four habitats, which were different in flora and vegetational structure, were selected as study plots. A. Broad-leaved natural woods of a shelterbelt in Oyafuru, Ishikari, central * A part of this paper was presented at the Fourth International Theriological Congress , Edmonton, 13-20 August, 1985. 94 H. Abe Hokkaido. This is a 1.6•~0.8 km2 wind shelterbelt which is situated in rice-fields and runs in a north-south direction. The study plot was located near the south end of the shelterbelt. Ulmus davidiana and Fraxinus mandshurica were the main tall trees in the study plot ; Juglans mandshurica and Mrrus bombycis constituted the middle layer of the woods, and Vitis coignetiae was a common vine found on the trees. The shrub layer was composed of Sambucus racemosa, Alnus japonica and saplings of the trees. Filipendula kamtschatica, Polygonum sachalinense, Lilium cordatum, Sasa senanensis, ferns and sedges composed the ground cover. B. Acacia (Robinia pseudoacacia) shelterbelt in Maeda, Teine, Sapporo. This is a 4.5•~0.8 km2 wind shelterbelt that runs in an east-west direction and is situated in farmland. The study plot was located near the west end. This is a plantation consisting of acacias, along with a few Populus nigra and Fraxinus mandshurica. Sasa senanensis dominated the undergrowth, but part of the ground was covered with shrubs of Sbiraea salicifolia. C. Picea glehni forest on a dune in Odaito, Nemuro, eastern Hokkaido. This is a natural forest (a) mainly consisting of P. glehni mixed with a few other species, Betula ermani, Sorbus commixta, Quercus mongolica and Abies sachalinensis. The ground is sparsely covered with low mosses, sedges and Maianthemum dilatatum. The outer two sides (b) of the forest (a) are swampy and densely covered with Phragmites communis and Calamagrostis langsdorffi, accompanying with sparse, low trees of Alnus hirsuta, A. japonica and P. glehni. D. Broad-leaved natural forest in Iwamizawa, central Hokkaido. This is a part of the broad-leaved natural forest on the hill range of Sho-toshibetsu, Iwamizawa. The number of tree species was most abundant in this plot. Acer mono and Tilia japonica were dominant species, and Sorbus alnifolia, Kalopanax septembobus, Acer japonicum, Magnolia kobus, Ostrya japonica, Quercus mongolica, Magnolia obovata, Maachia amurensis, Betula maximowiczii, Tilia maximowicziana and Morus bombycis were also common. Cephalotaxus harringtonia and Euonymus oxyphyllus were common shrubs. Sasa senanensis was the most dominant ground cover. Rodent populations in the four plots were sampled using Sherman live traps placed at 10 m intervals in 70 by 100 m to 100 by 100 m grids (Table 1). Oat grains were used as bait. The traps were in operation for five days in each plot. In the plot of Nemuro, two lines of traps, each with 10 traps, were also set for two days in swampy areas (b) at 10 m from the outer traplines of the main plot (a). This supplementary trapping was carried out to compare rodents living in two very different habitats (a and b). Trapping was performed in 1983 from late June to early July in Oyafuru, from early to middle August in Maeda, from middle to late August in Nemuro and from middle to late October in Iwamizawa. Each captured animal was weighed, sexed, examined for reproductive activity, toe-clipped for identification and released at the point of capture. Population density was estimated according to ZIPPIN's method (ZIPPIN,1956) and corrected by the average range length of each species in each plot. Vertical space use was measured by the following method. Containers were set in the study plots which contained a piece of bait including minute pieces of colored indigestible plastic, which were separated by color and placed in each of four strata (under the litter, on the litter surface, and 50 cm and 150 cm in height above the litter), Vertical Space Use of Voles and Mice in Hokkaido 95 Table 1. Collecting data on rodents in four study plots. *Each set consisted of four vertical stations . •õ1: No. of mice trapped. 2: No. of mice per hectare, which was estimated by Zippin's method corrected by average range length. 3: Total no. of mice captured. 4: No. of bait-takings counted regardless of the number of layers used by an individual at a time. Numerals in ( ) : Percent to the total capture. Cb : Clethrionomys rufocanus bedfordiae, Cr : C. rutilus mikado, As : Apodemus speciosus ainu, Aa : A. argenteus, Ap : A. peninsulae giliacus. and at the same time live-trapping was performed on the ground surface. Then, all the faeces dropped in traps were collected and analyzed in the laboratory to estimate the degree of vertical space use. The minute plastic pieces were madeby cutting 0.25-0.4 mm thick plastic strings to a length of about 0.5-1 mm, or 0.1 mm thick tape into pieces of about 0.2-1 mm2. The bait, 1•~1 cm2 and 3-5 mm in thickness, was a baked cake made of 90 percent wheat flour and 10 percent minute plastic pieces by weight. The container was a triangular pipe made of 3 mm thick plywood board measuring 7 cm (width)•~17 cm (length). An L-shaped attachment was used to secure the container to tree trunks at the heights 50 cm and 150 cm. Before . conducting the feeding experiments, it was ascertained that the flat baits placed in the above-ground containers had not fallen down to the ground by usual wind. A container was placed in each of the four strata in the woods, and one set of four containers was placed between two trap stations in a trap line. Then 77 sets and 308 containers were distributed in Oyafuru, 80 and 320 in Maeda, 63 and 252 in Nemuro, and 81 and 324 in 96 H. Abe Iwamizawa. Feeding experiments were carried out together with trapping for a population census. To complement the results of the feeding experiments, black papers powdered with talc and placed in the containers for a period of five days after the experiments were collected and footprints were examined. However, the numbers of footprints on the papers from the lower two strata (on the litter surface and beneath the litter) were too numerous and the pattern of footprints was too complicated to read accurately ; therefore, the footprints could not be used for the analysis of vertical use of habitats. As the numbers of footprints in the upper two strata were few, I set traps at the stations with these footprints to ascertain the species to which the footprints belonged. Microhabitats were characterized by measuring the following 24 parameters in a circle of 2.5 m radius centered on each trap. The number of broad-leaved and coniferous trees classified by the diameter at breast height as <15 cm, 15-30 cm, 30-60 cm, and >60 cm, and the number of them classified as having a height of 1.5-5 m, 5-15 m, and >15 m were counted. Densities of lower vegetation at the strata of <0.5 m and 0.5-1.5 m were classified into three categories by a standard number of Sasa bamboos, i. e., l : <50/m2, 2 : 50- 100/m2, 3 : > 100/m2. The densities of other plants were also classified according to this standard. Vegetation cover was classified as the maximum coverage at strata of <0.5 m, 0.5- 1.5 m, 1.5-5 m, 5-15 m, and >15 m into three categories, i.