Comparison of dispersal distances and survivorship rates of juvenile rabbits (Oryctolagus cuniculus) between urban and rural communities in Illinois: A study in fragmented population dynamics.
Introduction
In Illinois, the domestic rabbit (Oryctolagus cuniculus) is found in its natural habitat, brushy fields and forests, as well as in densely populated urban settings. In the Chicago metropolitan area, O. cuniculus has learned to coexist with humans, populating parks, cemeteries, gardens and lawns. I have encountered the rabbit in both the downtown area where I live and in the rural farm community where I spent much of the summer of 2000 in frontline defense of natural areas from road development. It was during this extensive time of field observation I became aware of immense differences in the populations of rural rabbit communities in comparison to those found in urban settings. Differences vary from individual size and range to group interaction and colonization. Due to the species’ worldwide abundance, economic importance and reputation as an agricultural pest, there have been numerous studies regarding characteristics and behavioral patterns of O. cuniculus (Cowan, D.P. and D.J. Bell 1986; Lockley, R.M. 1961). I believe due to their abundance and highly successful adaptability, however, rabbits are often overlooked as a valuable source of information regarding the population dynamics of adaptive or fragmented species.
An important species’ characteristic and an element of any population dynamic is juvenile dispersal. Dispersal can affect population dynamics by influencing genetic health, demographic structures and establishment of new populations (Hassell et al. 1995). A significant amount of dispersal research pertaining to O. cuniculus is gender comparative. Male rabbits naturally distance themselves to avoid conflict (Mykytowycz 1958) while females will only fight to compete for burrows if they are in limited supply (Kunkele 1992). Relatively few studies, however, are dedicated to comparative juvenile dispersal patterns between differing environments or within heavily fragmented communities such as those found in downtown Chicago. Do differences in juvenile dispersal characteristics exist between urban and rural populations of O. cuniculus? If so, do these differences provide means for increased genetic variation and optimal habitat exploitation thereby insuring continued success of the species in the urban environment?
Objectives
This research will address two major aspects of juvenile dispersion of O. cuniculus, dispersal distance and survivorship rate. I will conduct a comparative study of juvenile rabbit dispersal distances between two study areas, inner city and rural, to determine if a significant difference exists between them. Additionally, I will calculate radio telemetry estimates of survival for each population and determine if a difference exists. Intense field studies will be used to collect necessary data for comparisons. I will use the results of this study to ascertain the value of dispersal distance as a derived trait for species survival amongst fragmented populations of O. cuniculus in inner city environments.
Methods
This experiment will be conducted through a series of field research investigations performed in two different study areas. One site is located within the Lakeview neighborhood on Chicago’s north side. Addison Street and Belmont Avenue bound this area to the north, south, Lakeshore Drive, and Clark Street to the east and west. This 240-acre tract primarily consists of densely constructed mid-rise homes and apartment buildings and ranks as one of the most densely populated neighborhoods in the United States. The second site is the Ditzler Farm located in the agricultural community of Rockford, Ill. The 200-acre combination of relatively undisturbed, low-lying grassland, hardwood forest and small creek provide ideal habitat for many species, including O. cuniculus.
Although capable of reproduction year round, most rabbit breeding activity takes place in the first half of the year with gestation lasting about 30 days. The need to locate nesting females, therefore, dictates this study will begin with the first signs of spring. Due to the abundance and ease of locating rabbits in both study areas, much of the initial monitoring will be accomplished through field observations. Physical sign surveys will be used to identify study areas comprising the most suitable habitat for rabbit occupancy and to detect any evidence of rabbit presence, such as burrows and excrement. Actual presence of O. cuniculus will then be confirmed by visually monitoring prospective sites during evening and nighttime hours.
Upon identification of occupied burrows in each study area, Hav-a-Hart traps (Woodstream Corporation, Lititz, PA) will be set and monitored in 20-minute intervals in order to live-trap the residents of each burrow. Current residents will be marked to eliminate any confusion between mature residents and the juveniles to come. Rabbits in each population will be sexed, ear tagged for easy identification and re-released in the immediate vicinity of their home burrow. Live trapping should continue until ear tags readily identify a majority of the adult population of each burrow. Following the identification of mature rabbits, great care will be used to monitor burrow activity for parturition, growth and maturation of young.
As juvenile rabbits begin to regularly exit the burrow at approximately 4-weeks postpartum, Hav-a-Hart traps will once again be used to trap the juveniles. Each will be sexed, ear tagged for easy identification and fitted with peritoneal radio transmitters (Global Tracking Systems Incorporated, Coleman, Alberta Canada). The transmitters will have intermittent transmission capabilities, one-year battery life and emit a mortality signal upon animal demise. Twice a week for the following 28 weeks I will hone in on each animal’s precise location using a hand-held, 3-element Yagi antenna, AVM instrument receiver and a hand held GPS device (AVM Instrument Company, Colfax, CA).
In an effort to minimize gender bias related to dispersal distance and survivorship, equal numbers of each gender in the two areas will be used in this study. Maximum distances traveled from the birth nest for each rabbit over the course of the study will be recorded as the dispersal distance. Mortality will also be noted as it occurs throughout the study period. At the end of this 28-week period, it is likely most of the rabbits will reach sexual maturity, i.e. attain adulthood, and the study will conclude. The project will be repeated until data has been collected on a minimum of 15 individuals in each of the two study areas.
Statistical analysis of the dispersal distances for the rural and urban populations of O. cuniculus will be conducted utilizing two tests. Initially, I will test the homogeneity of the variances using the Independent-Samples T test where dispersal distance will be the test variable and population, the grouping variable. The results of the Independent-Samples T test will determine the second statistical test conducted. Should the variances prove to be homogenous, a one-way ANOVA will be used to compare the mean dispersal distances of O. cuniculus between the two locations. Otherwise, a non-parametric test that does not require homogeneity of variance, such as the Mann-Whitney U test will be used. Using mortality data, radio telemetry estimates of survival will be calculated along with their confidence limits. Overlap of the range of confidence limits between the two populations, or the lack thereof, will be used to determine whether a statistical difference exists between the survivorship of the two populations.
References
Cowan, D.P. and D.J. Bell (1986). Leporid social behaviour and social organisation. Mammal Review 16:169-179.
Hassell, M.P., Miramontes, O., Rohani, P. & May, R.M. (1995). Appropriate formulations for dispersal in spatially structured models: comments on Bascompte & Solé, Journal of Animal Ecology. 64:662-664 (1995).
Kunkele, J. (1992). Infanticide in wild rabbits. Journal of Mammology 73:317-320.
Lockley, R.M. (1961). Social structure and stress in the rabbit warren. Journal of Animal Ecology 30:385-423.
Mykytowycz, R. (1958). Social behaviour of an experimental colony of wild rabbits (Oryctolagus cuniculus L.) 1. Establishment of the colony. CSIRO Wildlife Research 3:7-25.