NOTES

Thirteen-fold change in population size of Peromyscus leucopus

OWEN J. SEXTON Department of Biology, Washington University, St. Louis, MO, U .S.A . 631 30

and University of Michigan Biological Station, Pellston, MI, U .S.A . 49769

JOHN F. DOUGLASS Division of Biological Sciences, University of Michigan, Ann Arbor, MI, U.S.A. 48109

ROBERT R . BLOYE Glen Eyrie, Box 6000, Colorado Springs, CO, U.S.A. 80934

AND

ANDREA PESCE Division of Biological Sciences, University of Michigan, Ann Arbor, MI, U.S.A. 48109

Received December 18, 198 1

SEXTON, 0. J., J. F. DOUGLASS, R. R. BLOYE, and A. PESCE. 1982. Thirteen-fold change in population size of Peromyscus leucopus. Can. J . 2001. 60: 2224-2225.

A live-trapping study of a population of white-footed mice Peromyscus leucopus was carried out during the midsummers of 1976, 1977, 1979, and 1981. The trapping grid (60 x 120 m) was located in a lakeside forest in the northern portion of the lower peninsula of Michigan, U. S . A. Extremes in estimated population size were 1 1 individuals in 1979 and 139 in 198 1 .

SEXTON, 0. J., J. F. DOUGLASS, R. R. BLOYE et A. PESCE. 1982. Thirteen-fold change in population size of Peromyscus leucopus. Can. J . 2001. 60: 2224-2225.

Une population de souris h pattes blanches, Peromyscus leucopus, a fait l'objet d'une Ctude dkmographique au cours de 1976, 1977, 1979 et 1981. Des pikges permettant de capturer les souris vivantes ont Ctt installts h la mi-ttk de chaque annCe, dans u,ne foret au bord d'un lac (zone d'tchantillonnage de 60 X 120 m), dans la partie nord de la Nninsule basse du Michigan aux Etats-Unis. La densit6 minimale a CtC estimke h 11 individus en 1979, et la densit6 maximale, h 139 individus, en 1981.

[Traduit par le journal]

Introduction Anthony et al . (198 1) recently reviewed the literature

dealing with amplitudes of population fluctuations in the white-footed mouse Peromyscus leucopus and indicated that they are relatively small compared with those seen of other species of small rodents. Successive class exercises in live trapping of small mammals have provided evidence for a minimum 13-fold increase in population size of P. leucopus inhabiting a hardwood forest in the northern part of the lower peninsula of Michigan over 2 years.

Location, materials, and methods Our trapping program was carried out by students in four

classes of Vertebrate Ecology at the University of Michigan Biological Station, Cheboygan Co., MI, U.S.A., during the summers of 1976, 1977, 1979, and 198 1. The trapping grid (60 X 120m) was established on Grapevine Point, a blunt

approximately 3 m above Douglas Lake (elevation, 217 m); it represents part of an exposed terrace formed by wave action of post-Pleistocene Lake Algonquin. The forest is a mesic hardwood forest (Table 1) which is recovering from lumbering and fires (Pettingill 1974).

The grid was permanently marked with stakes placed at each

TABLE 1. Frequency of trees (dbh* r 10 cm) and shrubs (dbh 2.5-9.9 cm) on the Grape Vine Point study area

Species Trees Shrubs

Sugar-maple Acer saccharum 43 82 Basswood Tilia americana 26 Beech Fagus grandifolia 9 Bigtooth aspen Populus grandidentata 9 White ash Fraxinus americana 7 White birch Betula papyrifera 4 Hop-hornbeam Ostrya virginiana 2

peninsula extending into Douglas Lake. The site is level and *dbh, diameter at breast height.

0008-430 1 1821092224-02$0 1.00 /O 0 1982 National Research Council of Canada/Conseil national de recherches du Canada

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NOTES

TABLE 2. Summary of the population estimates of Peromyscus leucopus trapped on a grid in a mesic forest in the northern part of the lower peninsula of Michigan, U.S.A.

Estimates are based on procedures in Overton and Davis (1969)

Year

Parameters 1976 1977 1979 1981

M 5 17 8 8 1 x 1 15 6 43 m 6 24 8 74 N 30 27 11 139

N at the lower 95% confidence level 2 15 4 100

N at the upper 95% confidence level 584 45 25 187

comer. Row and column intervals were each 10 m (7 rows, 13 columns). A single Sherman live trap was placed at the most favorable site within 1 m of each of the 9 1 intersections of rows and columns.

All classes followed the same trapping procedures. The traps were baited with peanut butter and opened just before dusk; they were examined and closed just after sunrise. Captives were identified as to species and sex, toe-clipped individually for later recognition, and released in the immedi- ate vicinity of their point of capture. Each year, trapping was conducted during two periods, one in late July and one in early August. The first and second trapping periods each year were each 5 nights in duration and were separated by 2 days of no trapping. Exceptions were made in this sequence if heavy rains occurred. Graduate students in mammalogy (J . F. D . , R . R . B . , and A.P.) accompanied each team during the initial trapping period to ensure compliance with the standardized procedures.

Our estimate of population size during the first trapping period was based upon techniques described in Overton and Davis (1969): N = Mmlx, where N is the number of individuals estimated to be present in first capture period; M is the number of individuals captured, marked, and released during first period; m is the total number of individuals captured during second period; and x is the total number of marked animals from first period recaptured during second period.

Results The 4 years' estimates of the population size of

Peromyscus leucopus are presented in Table 2. The lowest estimated value ( 1 1 individuals) was obtained in 1979 and the highest (139) was obtained in 1981. We assumed that the functional area sampled by the 6 x 120m grid was 70 x 130 m because a boundary strip equal to one half of the distance to the next trap was added to the core area. The estimated density of P . leucopus at the site in 198 1 was 153 mice per hectare.

Woodland jumping mice (Napaeozapus insignis) were captured regularly at the site each year, and short-tailed shrews (Blarina brevicauda) and eastern chipmunks (Tamias striatus) were only captured occa- sionally.

Discussion Anthony et al. (198 1 ) trapped in two locales in central

Pennsylvania for P . leucopus. They estimated that population size at one site ranged from 6 to 35 mice per hectare over a 2.5-year period. Similar values for the second area were 6 to 14 mice per hectare.

Terman (1966) examined data from the North Ameri- can census of small mammals to ascertain the amplitude of population changes. He concluded that populations of P . leucopus exhibited a low range of fluctuations and a low average size. This author also reviewed briefly the literature dealing with other studies on fluctuations in various species of Peromyscus. He concluded that outbreaks are relatively rare in Peromyscus.

We observed a 13-fold increase in population size which occurred over an interval of no more than 2 years. The population density, 153 mice per hectare, is four times higher than similar estimates indicated by Anthony et al . (198 1 ) . Our study indicates that popula- tion levels of P . leucopus at least can occasionally attain high levels under still undefined circumstances.

Acknowledgements We are grateful to the students of Vertebrate Ecology

for their enthusiasm in carrying out this project and to the University of Michigan Biological Station for providing facilities.

ANTHONY, R. G., L. J. NILES, and J. D. SPRING. 1981. Small mammal associations in forested and old-field habitats-a quantitative comparison. Ecology, 62: 955-963.

OVERTON, W. S., and D. E. DAVIS. 1969. Estimating the numbers of animals in wildlife populations. In Wildlife management techniques. Edited by R. H. Giles, Jr. Wildlife Society, Washington, DC. pp. 403-455.

PETTINGILL, 0 . S., JR. 1974. Ornithology at the University of Michigan Biological Station and the birds of the region. Spec. Publ. No. 1. Kalamazoo Nature Center, pp. 1- 118.

TERMAN, C. R. 1966. Population fluctuations of Peromycus maniculatus and other small mammals as revealed by the North American census of small mammals. Am. Midl. Nat. 76: 419-426.

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