General Ecology of a Rural Population of Norway Rats (Rattus Norvegicus) Based on Intensive Live Trapping

Article excerpt

ABSTRACT.-

We used intensive live trapping over a 1-y period to investigate the general ecology of a population of Norway rats (Rattus norvegicus) inhabiting a barn in east-central Illinois. At the start of our study in April 1986, the population contained 10 adult females and three adult males. The population increased and exhibited two peaks of about 100 individuals, one peak in late June and the other in late October 1986. Reproduction ceased during the late autumn and winter, and the population declined to only one adult male and one adult female by spring 1987. Increases in the number of rats represented young born at the barn, not adult rats moving into the population; decreases in population size likely resulted from predation rather than dispersal. Females first captured as adults persisted longer at the barn than did males and females first captured as juveniles or subadults; the few males first captured as adults persisted the shortest time of all age and sex classes. Young males gained body mass more rapidly than did young females. Wounding and parasitism by botflies occurred at relatively low levels. Our data indicate that a rat population with negligible immigration and seasonal breeding can exhibit dramatic changes in numbers, and that live-trapping at weekly intervals can yield high recapture rates useful for examining growth rates, survival and other basic life history characteristics.

INTRODUCTION

Despite the frequent use of laboratory strains of Norway rats (Rattus norvegicus) in studies of behavior and physiology (Gill et al, 1989), and the importance of wild rats as pests in agricultural and urban areas (Pratt et al, 1977), reservoirs of human pathogens (Gratz, 1984; Childs et al, 1988) and threats to the conservation of native species (e.g., ground-nesting birds: Atkinson, 1985), there have been few studies of natural populations of this species, particularly in rural areas. Further, many studies of wild rat populations have relied solely on kill trapping (Leslie et al, 1952; Blanchard et al, 1985; Butler and Whelan, 1994). Although kill trapping provides satisfactory census data (Emlen et al, 1949) and allows detailed examination of rats for wounds and reproductive characteristics (e.g., embryo counts and placenta! scars), it makes impossible the gathering of information from individuals over time (e.g., survival, growth rates and movements). Mark-recapture studies allow monitoring individuals over time; because rats are considered pests, however, most mark-recapture studies of them have been conducted in conjunction with kill-trapping (Davis, 1948; Farhang-Azad and Southwick, 1979), permanent removal of rats from populations (Glass et al, 1989) or poisoning (Bishop and Hartley, 1976). Additionally, mark-recapture studies typically have involved live trapping at monthly intervals (Bishop and Hartley, 1976; Farhang-Azad and Southwick, 1979; Stroud, 1982; Glass et al, 1989) or every 2 wk (Davis, 1948). Trapping success and recapture rates reported in these studies were low.

In this study, we used intensive live-trapping (3 d per wk) over a 1-y period to monitor in detail a population of rats living at a barn and adjacent silo in Illinois; the rats were not subjected to control measures, such as kill-trapping or poisoning, at or near the study site and we did not remove individuals from the study site for detailed examination in the laboratory. Here we provide data on the trappability, dynamics and structure of the population. We also describe patterns of survival, growth, wounding and botfly parasitism of rats from our study population.

METHODS

The main trapping site was a barn at the University of Illinois Biological Research Area 6 km northeast of Urbana, Champaign County, Illinois (40°15'N, 88°28'W). Thirty m to the north of the barn was a farmhouse occupied by one person and a large dog, and beyond this were cornfields. Seventy-seven m to the east of the barn was a macadam road across which there was deciduous forest. …