Estimating Cottontail Abundance from Livetrapping Data

Estimates of cottontail (Sylvilagus floridanus) abundance were derived from several estimating equations applied to data obtained by livetrapping a confined population of known size on the Olentangy Wildlife Experiment Station, Delaware County, Ohio, in 1961. The same methods of estimation were also applied to field data obtained in 1963 at Robert Allerton Park, Monticello, Illinois. Findings suggested that probability of capture differed among members of the populations and, perhaps, changed after capture. The multiple census methods published in 1938 by Schnabel and in 1943 by Schumacher and Eschmeyer yielded estimates well below the numbers of rabbits in the populations. A method which utilized the frequency of capture, based on techniques of maximum likelihood estimation (MLE) for the Poisson distribution, also apparently tended to underestimate rabbit abundance. A simplified equation derived for MLE for the geometric distribution produced useful estimates and is suggested for estimating rabbit numbers from data obtained by livetrapping. Intercepts of linear regression lines fitted to logarithmic plots of data on frequency of capture also appeared to yield adequate approximations of the numbers of rabbits in the zero (uncaptured) class. The mathematical bases for using these methods are subject to limitations which are discussed. The purpose of this paper is to present and compare estimates of cottontail abundance from a confined population of known size to provide a better idea of the relative accuracy of several estimating equations. Over the years several methods of estimating animal abundance from data obtained by livetrapping have been suggested. In most instances, estimating equations have been recommended on the basis of mathematical models, with no attempt made to test their accuracy by estimating known numbers of animals under field conditions. Recent papers on trap response of cottontails (Geis 1955, Huber 1962) and on problems of rabbit population studies (Eberhardt et al. 1963) suggest that a variety of factors may affect trapability of cottontails and thus bias population estimates based on capturerecapture data. The relative accuracy of several estimating equations was evaluated by comparing estimates derived from live'A contribution from Federal Aid in Wildlife Restoration Projects Ohio W-103-R and Illinois W-66-R; the U. S. Bureau of Sport Fisheries and Wildlife; the Ohio Department of Natural Resources, Division of Wildlife; the Illinois Department of Conservation; and the Illinois Natural History Survey, cooperating. Work performed in part under contract AT(451)-1350 between the Atomic Energy Commission and the General Electric Company. 87 WRIGHT, P. L. 1948. Preimplantation stages in the long-tailed weasel (Mustela frenata). Anat. Rec. 100(4):593-607. 1950. Development of the baculum of the long-tailed weasel. Proc. Soc. Expt. Biol. and Med. 75:820-822. 1953. Intergradation between Martes americana and Martes caurina in western Montana. J. Mammal. 34(1):74-86. 1963. Variations in reproductive cycles in North American mustelids. Pp. 77-97. In A. C. Enders (Editor), Delayed implantation. University of Chicago Press, Chicago, Illinois. 309pp. . 1966. Observations on the reproductive cycle of the American badger (Taxidea taxus). Pp. 27-45. In I. W. Rowlands, Editor, Comparative biology of reproduction in mammals. Symposia Zool. Soc. London, No. 15. Academic Press, London. 527pp. , AND R. RAUSCH. 1955. Reproduction in the wolverine, Gulo gulo. J. Mammal. 36(3): 346-355. Received for publication August 22, 1966. he purpose of this paper is to present d compare estimates of cottontail abunance from a confined population of known ize to provide a better id a of the relative ccuracy of several estimating equations. ver the years several methods of estimatg animal bunda ce from data obt ined ceived for publication August 22, 1966. sity of Chicago Press, Chicago, Illinois. . . bservations on the reproductive of the American badger (Taxidea taxus). -45. In I. W. Rowlands, Editor, Comi e biology of reproduction in ma mals. i g have been suggested. In ces, esti ating equations have This content downloaded from 157.55.39.207 on Thu, 20 Oct 2016 04:05:46 UTC All use subject to http://about.jstor.org/terms 88 Journal of Wildlife Management, Vol. 31, No. 1, January 1967 trapping data with the actual number of rabbits in a confined population; the same methods of estimation were applied to field data. We wish to express our appreciation to Wayne B. Nelson, Department of Mathematics, University of Illinois, for review of the mathematical concepts embodied in the method of maximum likelihood estimation, to Mrs. Helen C. Schultz and Dr. Glen C. Sanderson of the Section of Wildlife Research, Illinois Natural History Survey, for critically reading the manuscript, and especially to Edward A. Leite, Ohio Division of Wildlife, Ashley, for allowing us to cite his unpublished data.