Ecology and Genetics of Rodent Populations on the Southern Delmarva Peninsula and Virginia Barrier Islands





Nancy D. Moncrief, Raymond D. Dueser, C. Randolph Carlson, and John H. Porter





Dept. of Mammals (NDM), Dept. of Fisheries and Wildlife (RDD and CRC), and Dept. Environmental Sciences (JHP)
Virginia Museum of Natural History (NDM), Utah State University (RDD and CRC), and University of Virginia (JHP)


Abstract

Ecology and Genetics of Rodent Populations on the Southern Delmarva Peninsula and Virginia Barrier Islands

We are conducting a multidisciplinary study (which simultaneously combines genetic markers, demographic studies, and topographic surveys) at two island sites and, for comparative purposes, two sites on the adjacent Delmarva peninsula. At these four sites we are using capture-recapture techniques to monitor rodent populations in order to detect differences and similarities in densities, survival, body mass, reproductive output and dispersal. We pursue answers to questions such as: Do population of small mammals fluctuate in abundance on the southern Delmarva Peninsula and adjacent barrier islands? Are fluctuations in abundance correlated between mainland and island populations? Can causal factors be determined (or at least some potential causal factors eliminated) to explain these fluctuations? Are insular populations more frequently subject to extinction than mainland populations? Do mainland populations more frequently experience immigration than island populations? Do the many differences between island and mainland populations that have been documented by other studies also exist on the Virginia barrier islands? If not, why not?

To monitor rodent populations at each of the four sites, we are using live traps (100 per site) permanently positioned at coordinates marked with wooden stakes at 10 m intervals. Trapping sessions (4 nights) are conducted on all grids at least once per month May-October. Each time an individual is captured, its weight and reproductive condition are recorded, along with the trap location on the grid. On initial capture, the animal is uniquely marked (using an ear tag), and a small piece of skin (3 mm2 ear clip) is removed for subsequent genetic analyses.

Grids were established on Myrtle and Ship Shoal islands in October 1995. In the spring of 1996, trapping grids were established at two mainland sites: in March at Eastern Shore of Virginia National Wildlife Refuge and in May at the Shirley Field, near Oyster Virginia.

Results of trapping activities for October 1995 through May 1997 are summarized in the four tables below. For each site, the total number of individuals of each species captured during each trapping session is shown. Abbreviations for scientific names are as follows: Mm = Mus musculus, Mp = Microtus pennsylvanicus, Op = Oryzomys palustris, Pl = Peromyscus leucopus, Rn = Rattus norvegicus, Bb = Blarina brevicauda, Sf = Sylvilagus floridanus. Traps were Fitch-type live traps made with number ten cans, except those marked with an asterisk, which were Sherman live traps. Two different trapping sessions were conducted at the mainland sites during each month in July 1996 and August 1996.

Myrtle Island 

# trap- nights month Mm Mp Op 300 Oct 95 42 8 34 300 Mar 96 3 0 0 400 May 96 4 14 0 102 Jun 96 7 24 9 200 Jul 96 1 13 0 400,32* Aug 96 5 23 6 400 Oct 96 6 14 11 400 May 97 4 6 6

Ship Shoal

# trap- nights month Mp Op 300 Oct 95 18 16 294 Mar 96 10 0 291 May 96 17 1 198 Jun 96 44 1 200 Jul 96 40 2 400,64* Aug 96 43 7 388 Oct 96 27 70 400 May 97 23 12

Eastern Shore of Virginia NWR # trap- nights month Mm Mp Op Pl Bb -- Oct 95 127,60* Mar 96 0 6 0 5 0 400 May 96 0 8 0 0 0 300 Jun 96 0 10 2 0 0 298 Jul 96 0 3 3 2 1 400 Jul 96 0 1 0 1 0 400 Aug 96 0 0 8 1 0 400 Aug 96 2 1 12 0 1 392 Oct 96 6 0 5 5 4 400 May 97 19 23 12 15 0

Shirley Field

# trap- nights month Mm Mp Sf Cp -- Oct 95 132 Mar 96 0 1 0 0 300 May 96 0 0 0 0 300 Jun 96 0 6 0 0 299 Jul 96 1 6 0 0 400 Jul 96 0 5 0 0 400 Aug 96 0 2 0 0 400 Aug 96 0 1 0 0 400 Oct 96 0 4 0 1 400 May 97 0 37 1 0

These data are being analyzed and trapping efforts are ongoing. Trapping sessions are planned for June, July, August, September, and October 1997.


Literature Cited


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Submitted by moncrief@amazon.evsc.virginia.edu