Population Ecology of Peromyscuc leucopus, Microtus pinetorum, Blarina brevicauda, Peromyscus nuttali, and Rattus norvegicus in an Abandonded Landfill
F.W., P.C., A.P. Oak Ridge High School Students
The purpose of this study was to observe the population ecology of small mammals in an abandoned landfill. Eight species were live trapped in aluminum traps and ear tagged while specific data were recorded. These species included 52 white-footed mice, 230 pine voles, 103 short-tailed shrews, 24 golden mice, 96 Norway rats, 15 harvest mice, 5 eastern chipmunks, and 1 least shrew. The cumulative home ranges indicate animal separation into micro-habitats in the field. Population densities, reproductive cycles, and immigrations show possible movement patterns of the animals throughout the grid. The landfill appears to be recovering as it provides a sufficient habitat for a sizeable population.
*Please Note: These pictures were taken from 1990-1992. This was before the outbreak of the Hauntavirus (1994). We are not currently conducting a mammal survey. If we were to start any type of mammal survey, we would follow all safety precautions as recommended by Measuring and Monitoring Biological Diversity: Standard Methods for Mammals (1996).
The number of abandoned landfills is currently growing at an alarming rate in the United States. In light of this fact, the study of the recovery of an abandoned landfill is important environmentally. The study detailed in this paper involves population dynamics and the interactions of small mammals in an abandoned landfill. The landfill was graded and seeded according the Environmental Protection Agency criteria in 1982. It is currently in old field succession with brambles and small pines beginning to replace the herbaceous cover over approximately twenty-five percent of the half hectare study plot. This field is part of the CRESO study site at the ACWS.
Using Sherman live traps, small mammals were captured, tagged, and released. Species, tag number, age, weight, sex, size of testes or nipples, pregnancy, trap station, and fate (new, recapture, retag) were recorded. Five species made up the majority of mammals caught: Peromyscus leucopus, Microtus pinetorum, Blarina brevicauda, Peromyscuc nutalli, and Rattus norvegicus. In this study, the populations were surveyed through population densities as well as home ranges, which outline the general area each population is found. Reproduction and immigration cycles were also determined for each species.
MATERIALS AND METHODS
The site chosen for a mammal study was a recovering landfill with enough cover to protect the ground from sun and wind while providing a good source of food to the mainly herbivorous small mammals. The plot was one hundred by fifty meters (half a hectare) with stakes at 10 meter intervals. Mammals were trapped for three consecutive days every other week from April of 1991 through October of 1992. Sherman folding traps were positioned next to a runway and baited with sunflower seed. The traps were set in the early evening around 7:00pm and were retrieved in the morning around 7:00am. The mammals were then ear tagged using size ff fingerling fish tags. The species, tag number, age (adult, subadult, or juvenile), weight, sex, size of testes or nipples, pregnancy, trap station, and fate were then recorded and the mammal released at the point of capture. Blarina brevicauda were marked by toe clipping. Sex for this species was not determined. Home range map charts and minimum number alive (MNA) were recorded after each three-day trap session. MNA included all mammals caught during a trapping week plus those animals caught in subsequent weeks. Mammals were considered residents if they were present for more than two trap sessions. The trappability of residents was defined as the number caught at one time divided by the number known to be present at that time (Wolff 1985b).
One thousand two hundred fifty small mammals were captured in a total of 11,190 trap nights. Trap nights were defined as the number of traps set multiplied by the number of nights in a trap session. Individual captures were 52 Peromyscus leucopus, 230 Microtus pinetorum, 103 Blarina brevicauda, 24 Peromyscus nutalli, 96 Rattus norvegicus, 15 Reithrodontomys humulis, 5 Tamias striatus, 1 Sorex minutus (Figure 1). This survey only deals with Peromyscus leucopus, Microtus pinetorum, Blarina brevicauda, Peromyscus nutalli, and Rattus norvegicus because not enough data was available to be statistically significant with the other species.
The average home range area of each adult was determined and compared between the sexes of each species. Home ranges consisted of areas that formed polygons and were not on the edge of the grid. The average home ranges for P. leucopus were 306.0 square meters for the females and 325.0 square meters for the males, for M. pinetorum 136.0 square meters for the females and 190 square meters for the males, for B. brevicauda 203.0 square meters, for R. norvegicus 330.0 square meters for the females and 308.0 square meters for the males, and for P. nutalli 250.0 square meters for the females and 175 square meters for the males. A note must be made that the numbers were few (twelve R. norvegicusand nine P. nutalli) when computing the home ranges for the last two species.
The adult male-female ratio was compared for each species (Table 1). The ratio differed not only by species, but also by age. Due to the few Microtus pinetorm and Peromyscus nutalli juveniles captured, no ratio could be determined. The ratio between sexes did not differ, and the weights of all species except R. norvegicus remained the same from 1991 to 1992 (Table 2).
Population density was computed for all five species from 1991 to 1992 (Figure 2). Densities ranged from a high of 92 for M. pinetorum in late summer and a low of 0 for P. nutalli in mid-fall. Population density for all but one species increased in the summers of 1991 and 1992. P. nutalli was the only species to increase during the spring of 1992. Also, the total population density of P. nutalli increased in 1992. The population density of the other four species decreased in 1992.
Reproductive cycles were determined per month for all species except for B. brevicauda (Figure 3). Males with medium or large testes, pregnant females and non-pregnant females with medium, large, or lactating nipples were considered in reproductive condition. M. pinetorum were in and out of reproductive condition throughout the year while P. leucopus, P. nutalli, and R. norvegicus showed definite reproductive cycles in the spring and summer.
Immigrants that attained residence were compared with the total number of captured adults per month for all species except B. brevicauda (Figure 4). Because of the high trappability (89% P. leucopus, 96% M. pinetorum, 85% P. nutalli, 80% R. norvegicus), immigrants were considered to attain residency after two trap sessions. M. pinetorum showed movement in the late summer and fall for both years while P. leucopus and P. nutalli showed movement during the summer and spring for both years.
Due to the small size and disrupted nature of the field, few small mammals were expected. However, this was not the case, with 526 individuals representing eight species being trapped (Figure 1). M. pinetorum was the most populous species trapped in the field, followed by B. brevicauda, R. norvegicus, P. leucopus, and P. nutalli.
Male-female ratios seemed to determine possible behaviors among the different populations (Table 1). It was found that the male-female ratios differed not only by age, but also by species. As the population ofP. leucopus had approximately the same number of males and females, it seemed to show monogamous behavior. Also, the home ranges were a close one-to-one ratio, which further suggests monogamous behavior among the species. M. pinetorum had a close one-to-one ratio in this study, and this fact would suggest monogamy among the population. However, male home ranges of M. pinetorum were larger than the female home ranges, and this fact suggests polygamy among the population. These observations can be explained because M. pinetorum have been known to participate in monogamy, polygamy, and polyandry (Wolff 1985a). Although sex cannot be differentiated in B. brevicauda, it was possible to hypothesize that one sex had a greater home range than the other because when home ranges were tabulated, it was noted that most of the home ranges were either greater than 200 square meters or less than 100 square meters. The greatest ratio between males and females was found in Rattus norvegicus. More than twice the number of adult males was caught than females. This fact coincides with the females having a larger average home range than males. The greater male population suggests polyandry among the species. The mating behavior pattern for P. nutalli could not be determined due to small sample size.
The average adult weights for each species were tabulated by year in order to see any possible health changes (Table 2). There appeared to be no great difference in weights except in the weights of R. norvegicus. There is not enough data to determine the cause of this decrease in weights.
The cumulative home ranges of the five species show niche separation. P. leucopus inhabit the surround edge of the field which is dominated by brambles, Virginia pines, and some sericea, goldenrod, and grasses. M. pinetorum inhabit the middle of the field, which is dominated by sericea, goldenrod, and grasses. P. nutalli inhabit another edge of the field that is dominated by brambles. R. norvegicus and B. brevicauda have home ranges that encompass the entire field.
Population density was computed as the number of animals found per hectare in each month (Figure 2). The possible explanation for the increase in population density in the spring and summers of 1991 and 1992 could be the influx of immigrants onto the grid (Figure 4). The overall population density of P. nutalli increased in 1992. This may be due to the increasing growth of brambles, which are an ideal habitat for P. nutalli (Burt 1964). The population density of the other four species decreased in 1992. One study (Wolf 1985b) suggests that P. leucopus tend to fluctuate, on a four to six year cycle. The P. leucopus population in this study may also fluctuate and Wolff's study would explain the downward trend for the P. leucopus population. The decrease in population of M. pinetorum, B. brevicauda, and R. norvegicus, awaits further study.
Reproductive cycles differed by species (Figure 3). M. pinetorum cycled in and out of reproductive condition throughout the year but peaked in late fall of 1991. They appear to be going out of the reproductive cycle in late October, 1992. P. leucopus peaked in the late fall of 1991 and appear to be peaking in the fall of 1992. Along with P. nutalli, P. leucopus were out of reproductive condition in the winter. This fact coincides with the decrease in population density in the winter. If the animals are not reproductively active, then they may not move about as much and are less likely to be trapped. R. norvegicus peaked in the spring of 1991. Even though their reproductive condition peaked again in the spring of 1992. R. norvegicus population has been in decline since the fall of 1992. It is not known why there were few captures from July of 1992 until the end of this study.
Immigration proportions were defined as the number of immigrants divided by the number of total adults captured (Figure 4). Immigration of P. leucopus was apparent in the late summer of 1991 and spring and summer of 1992. This fact coincides with the population being reproductively active in the spring, summer, and late fall of both 1991 and 1992. M. pinetorum, on the other hand, have appeared to have immigrants throughout the year. Considering that the population's reproductive condition fluctuated throughout the year, the immigration pattern is valid. However, this does not explain the decrease in population density in the late winter and early spring. P. nutalli appeared to move primarily in mid-winter, late spring and summer in 1992. The population density has increased in 1992, so the immigration patterns may increase due to the influx of immigrants in the second year of trapping. R. norvegicus appeared to move only in the late spring, summer, and fall of 1991. Because the population density has decreased dramatically in 1992, no immigrations patterns were recognized in 1992.
B. brevicauda were not expected to be captured since they are insectivorous. Although they were the third most populous species (Figure 1), the method of tagging by toe clipping is less reliable than ear tagging because of the possibility of B. brevicauda losing their toes to natural causes. Also, without dissection, the sex of B. brevicauda is very difficult to determine. Therefore, it was difficult to integrate information on B. brevicauda into this study.
This study of small mammals in an abandoned landfill was conducted for two years and indicates that populations are able to survive in an abandoned landfill habitat. This conclusion can be drawn from the evident similarities between these data and other data from researchers studying field habitats of small mammals. In particular, Dr. Jerry Wolff's data of small mammal populations from Mountain Lake Biological Station in Giles County, Virginia correspond with our data.
While basic population structure is beginning to emerge, the underlying forces behind species interactions are much more difficult to ascertain. For example, M. pinetorum, P. leucopus, and P. nutalliappear to be dividing the old field niche spatially, but what factors are responsible for the separation? Is there food competition or territory competition between species? Also, as the field succeeds into a pine forest, what effects will this succession have on the overall population density? We hope to examine these more difficult questions with additional research.
Burt, William Henry. 1952. A Field Guide to Mammals. Houghton Mifflin Co. Boston.
Wolff, Jerry O. 1985a. Behavior. Pages 350-351 in Robert H. Tamarin, ed. Biology of New World Microtus. Boston.
Wolff, Jerry O. 1985b. Comparative population ecology of Peromyscus leucopus and Peromyscus maniculatis. Canadian Journal of Zoology. 63: 1548-1555.