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.

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