Influence of Supplemental Food on Local Populations of Peromyscus leucopus

American Society of Mammalogists

Influence of Supplemental Food on Local Populations of Peromyscus leucopusAuthor(s): Lonnie P. Hansen and George O. BatzliSource: Journal of Mammalogy, Vol. 60, No. 2 (May, 1979), pp. 335-342Published by: American Society of MammalogistsStable URL: http://www.jstor.org/stable/1379805 .

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INFLUENCE OF SUPPLEMENTAL FOOD ON LOCAL POPULATIONS OF PEROMYSCUS LEUCOPUS

LONNIE P. HANSEN AND GEORGE 0. BATZLI

ABSTRACT.-The influence of supplemental food on the movement and demography of the white-footed mouse (Peromyscus leucopus) was studied on two experimental and two control grids established within a 24-ha tract of deciduous forest. Preliminary trapping for 1 year prior to supplemental feeding indicated only minor differences between grids. After supplemental feeding, mice bred somewhat earlier in the spring on the experimental plot, but densities, survival, movement, reproductive intensity, and weights were not influenced by supplemental food. These parameters varied more between years than between the experimental and control plots within a year. We concluded that natural food supply was not limiting population densities at the time of supplemental feeding. However, increased food availability (mast crop) may have affected winter survival, thus causing the increased densities observed on all grids during the second year.

Two lines of evidence indicate that food supply affects the density of woodland

species of mice (Peromyscus spp.). Several researchers reported a positive correlation between the availability of seeds and densities of Peromyscus (Jameson, 1953, 1955; Pearson, 1953; McCarley, 1954; Gashwiler, 1965; Batzli, 1977). Others demonstrated that density does increase in response to provision of supplemental food (Bendell, 1959; Fordham, 1971; Smith, 1971), although the magnitude of this response differed as did the mechanisms proposed for its cause. Bendell (1959) identified increased survival of young mice as the major response to food, whereas Fordham (1971) cited increased reproduction and adult survival.

Increased density could result from immigration as well as increased reproduction and survival, so we designed an experiment to examine the influence of supplemental food on the movement and demography of P. leucopus within a large woodlot. We hypothesized that density would increase in areas with supplemental food owing to immigration, increased reproduction, and decreased mortality. This did not occur, a result which we interpret as evidence that the natural food supply did not limit the population density during the period of our experiment.

METHODS

Field work took place in Trelease Woods, a 24-ha remnant of prairie grove located 6 km NE

Urbana, Champaign Co., Illinois. Trelease Woods is a moist woodland consisting mostly of sugar maple (Acer saccharum), hackberry (Celtis occidentalis), white ash (Fraxinus americana), slip- pery elm (Ulmus rubrum), basswood (Tilia americana), red oak (Quercus rubra), and buckeye (Aesculus glabra). Sugar maple and paw paw (Asimina triloba) made up much of the understory. Boggess (1964) described the woody vegetation in detail.

We established a 9-ha grid with 256 trapping stations at 20-m intervals and began trapping in

January 1975. Monthly trapping periods consisted of 1 to 3 days prebaiting and 3 days of trapping. Longworth live-traps were baited with cracked corn and provided with cotton nesting material.

Cages (Getz and Batzli, 1974), permanently anchored within 5 m of a grid marker, protected the traps from disturbance. A border strip of 20 m (which approximated average movement of the mice within a trapping period) was added to each side of the grid to determine the effective area trapped. We calculated minimum density estimates by dividing the number of mice known to be present by the effective area trapped.

We marked the mice by attaching numbered monel tags to their ears. Age, sex, weight to the nearest gram, reproductive condition, stage of molt, trap location, and any abnormal physical characteristics of the mice were recorded at each capture. Age categories were based on weight (juveniles, <14 g; subadults, 14 to 18 g; adults, > 19 g). Medium or large scrotal testes indicated

J. Marmm., 60(2):335-342, 1979 335



336 JOURNAL OF MAMMALOGY Vol. 60, No. 2

FIG. 1.-Minimal densities of trappable mice on two plots given supplemental food (experimental) and on two control plots. Summer and winter months are shaded.

reproductive activity in males (Sadleir, 1974). Prominence of the teats and absence of hair im-

mediately surrounding them denoted lactation; weight increases of at least 4 g between trapping periods and swelling of the abdomen indicated pregnancy. Because the most substantial weight changes occur near parturition (Millar, 1975), early pregnancies could not be detected.

After trapping for a year to establish population characteristics, we delineated four subplots of 0.36 ha each within the 9-ha plot and provided supplemental food on two. We used Purina mouse chow (#5020) because P. leucopus grew and reproduced well on it in the laboratory (Hansen, 1978). Large metal cans shielded bottles of mouse chow from the weather, and both were inserted into protective cages similar to those used for traps. Only animals 2.5 cm or less in diameter had access to the supplemental food. The experimental grids contained feeding stations at 10-m intervals, which provided several feeding stations to each home range and thereby minimized the possibility of social interactions preventing access to supplemental food

by subordinate mice. Feeding stations were filled one to three times a week depending on weather conditions and removal of food. Furthermore, additional food was usually broadcast when servicing feeding stations. Feces left in the bottles and rapid removal of the mouse chow, especially during the summer, indicated that P. leucopus used the supplemental food.

In March 1976, biweekly trapping intervals were initiated on the experimental and control subplots. Monthly trapping continued on the 9-ha grid as a whole. Beginning in August 1976, because of high populations, we discontinued trapping the entire grid and trapped the subplots plus one line of traps surrounding each subplot (total of 4 ha). This scheme was continued until the end of the study in December 1976.



May 1979 HANSEN AND BATZLI-PEROMYSCUS POPULATIONS 337

FIG. 2.-Breeding intensity by adult female P. leucopus as indicated by pregnancy. No pregnancies were detected from November through February. Numbers over bars give sample sizes.

Trapping procedures captured a high proportion of known residents (>0.8) during all trapping periods except winter (0.69 and 0.76 on experimental and control, respectively). Hilborn et al. (1976) have shown that population enumerations are slight underestimates when probability of capture is high (>0.8). Therefore, we conclude that our enumerations probably seriously under- estimated only the winter population.

The average distance moved by mice within a trapping period was generally at least 20 m. As a result, most mice captured in the line surrounding each of the experimental plots probably had access to the supplemental food. Therefore, to reflect the supplemented subpopulation more accurately, we included data from the trap line immediately surrounding each plot as part of the plot for comparison of population characteristics.

RESULTS

Although total densities tended to be slightly higher on one of the control plots, numbers of P. leucopus using the plots did not differ substantially, even under the influence of supplemental food (Fig. 1). The differences in densities between years and among seasons were much greater than those among areas. Except for one control plot, densities peaked in November and December of 1975. In 1976 spring densities were about twice those for 1975, and numbers increased throughout the summer, peaking in August and September at much higher densities than at any previous time.

In 1975, pregnancies occurred earlier and in greater numbers on the control plots than on the experimental plots (Fig. 2). In 1976, however, pregnant females appeared approximately 1 month earlier on the experimental plots. The proportion of females pregnant throughout the summer and fall was similar between plots in both 1975 and 1976. The apparent difference in June 1975 was not statistically significant (x2 = 1.00,




FIG. 3.-Survivorship curves and mean expectation of life (ex+?95% confidence intervals) for mice first captured as juveniles (<14 g).

P > 0.25). It appears, therefore, that the only influence supplemental food had on reproductive performance which was detected by our methods was to advance the onset of the reproductive season.

Survivorship of mice first captured as juveniles or subadults was followed from the first capture until disappearance. As indicated by survivorship curves and life expec- tancy values (Leslie et al., 1955), survival remained highest on the control plots throughout the study (Fig. 3). More variability occurred between years than among areas, survival being somewhat lower in 1976. This in part was due to a slight un- derestimate of survival in 1976 because a few mice were still alive at the time of last trapping. Nevertheless, supplemental food had little influence on juvenile survival. An identical comparison of subadult survival revealed similar results except that survival did not differ as much between years. Expectations of further life were slightly greater (1.5 weeks) for mice first captured as subadults than for juveniles. Adult sur-

TABLE 1.-Movement between control and experimental plots and the surrounding grid be- fore (1975) and after (1976) placing supplemental food on the experimental plots.

Experimental Control

No. moving No. moving Proportion Proportion

Onto Off onto Onto Off onto

1975 Within trapping periods 37 44 0.46 45 48 0.48 Between trapping periods 37 30 0.55 44 40 0.52

1976 Within trapping periods 26 23 0.53 27 30 0.47 Between trapping periods 71 83 0.46 65 65 0.50




FIG. 4.-Mean weights (-95% confidence intervals) for adult male P. leucopus.

vival, measured by percent survival per 28 days, varied but did not appear to be influenced by supplemental food. Adult survival was slightly higher in January and

February of 1976 than in 1975 but higher in the fall 1975 than in 1976. Details of survival patterns can be found in Hansen (1978).

If food supply were limiting, mice should move into an area provided with supplemental food. We used live-trapping results to determine if such a response occurred on the experimental plots. The use of trapping data could produce spurious results if mice failed to enter traps when supplemental food was available. Trappability on the

experimental and control plots differed significantly (0.82 and 0.96, respectively; X2 = 4.55, P < 0.05) only during the spring. At this time raccoons attempting to obtain the

supplemental food disturbed trapping procedures and thus lowered trappability on the experimental plots. This disturbance, however, was not sufficient to prevent the use of trap capture as an indicator of movement. Movements onto and off of experimental and control grids did not significantly differ (X2 tests) throughout the entire




study (Table 1), suggesting that dispersion patterns did not change in response to the supplemental food.

Finally, if food availability affected the condition of the mice, the body weight of mice provided with supplemental food should be greater than control animals. We

only considered the weights of adult male P. leucopus because reproduction compli- cated trends in female weights. Mean weights of experimental mice did not differ

significantly from those of control mice at any time (Fig. 4). Furthermore, there were no significant differences in weights of mice between years. Mean weights of mice on all plots were lowest in midwinter and peaked in the spring.

DIscUSSION The only influence of supplemental food on P. leucopus detected by this study was

an earlier onset of breeding in the spring. Because reproduction in Peromyscus spp. may be closely linked to the food supply (Sadleir et al. 1973; Millar, 1975), and food

supply should be lowest during the early spring (Gorecki and Gebczynska, 1962; Batzli, 1977), supplemental food might be expected to advance the time of breeding. Similar observations on breeding have been made in other studies in which supplemental food was provided populations of Peromyscus (Fordham, 1971; Hansen and Batzli, 1978) and Apodemus (Watts, 1970; Flowerdew, 1972, 1973), but these studies also found effects on survival and density of the populations.

Although some investigators have proposed that dispersal of Peromyscus during spring and summer regulates population density (Sadleir, 1965; Healey, 1967; Metz- gar, 1971), dispersal did not affect the density of P. leucopus in Trelease Woods. During the breeding season, young mice in Trelease Woods did move away from the nest (76% of juvenile males and 11% of juvenile females moved >80 m from the site of first capture); however, immigration onto the study plots roughly balanced emigra- tion from the study plots (Table 1).

One might argue that we did not find differences between the experimental and control plots because mice on both plots had access to the supplemental food. That hypothesis also would explain the increased densities in 1976. In another study (Han- sen and Batzli, 1978), we found that P. leucopus provided with supplemental food weighed significantly more than control mice. Mice emptied the feeders very rapidly in Trelease Woods, yet weights of experimental mice gave no indication they were better fed than control mice. Although there was no differential movement of mice onto the experimental plots, mice could have moved onto the experimental plots, removed food and stored it off the area where it would be accessible to control mice. If this had occurred, we would expect a difference in weights of mice between years because we did not feed the first year. Body weights in 1976 were similar to those for 1975 (Fig. 4), so we conclude either that control mice did not have access to supplemental food, or that it had no advantageous effect.

Evidently, natural food supplies in 1976 were sufficient to maintain high densities without supplemental feeding. Mast is often the most important food item for P.

leucopus (Batzli, 1977). Acorn production of red oak, the most abundant mast produc- ing tree in Trelease Woods, was high (1,733 kg/ha) during 1975 but extremely low (140 kg/ha) during 1974 (John Edgington, pers. comm.). Poor adult survival in January and February of 1975 coincided with a poor acorn crop the previous autumn, and good adult survival in January and February of 1976 coincided with a good acorn crop in 1975. These differences in winter survival together with slightly earlier and more intensive breeding (Fig. 3) produced spring densities in 1976 twice those in 1975 (Fig. 2). Reproduction and survival did not differ during the summer of the 2 years, and the late summer population was twice as high in 1976. The decline in density in December 1976 was at least partly artificial. Severe winter weather, cold temperatures




and heavy snowfall, caused low trappability. Because it was the last trapping period, individuals missed then could not be caught later, and could not be included in the

density estimate. We conclude from these results that during years when natural food is abundant,

food availability is not a proximate factor regulating the density of P. leucopus. Ap- parently natural food supplies were abundant during the period of supplemental feed-

ing. The magnitude of the summer peak in density appeared to be dependent on overwinter survival, which was correlated with the previous year's mast crop.

ACKNOWLEDGMENTS

We are indebted to Drs. F. R. Cole, L. L. Getz, S. P. Havera, J. R. Karr, G. C. Sanderson, J. N. Thompson, and Mr. J. Edgington for assistance and helpful discussion during the study. E. Brighty provided invaluable field assistance throughout the study. Financial support was provided by the Department of Ecology, Ethology and Evolution, University of Illinois Research Board, School of Life Sciences Biomedical Research Grant, a Sigma Xi Grant-in-Aid, and an NSF Doctoral Dissertation Improvement Grant (DEB 76-16752) to E. Brighty.

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Illinois Natural History Survey, 279 Natural Resources Building, Urbana, IL 61801, and De- partment of Ecology, Ethology and Evolution, University of Illinois, Urbana, IL 61801. Sub- mitted 4 April 1978. Accepted 26 July 1978.



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Influence of Supplemental Food on Local Populations of Peromyscus leucopus