Temporal and Spatial Dynamics of Small Mammal Populations
The Small Mammal Population Study (Sev008) is a baseline SevLTER dataset initiated in 1989 with the inception of the project in order to examine the spatial and temporal distribution of populations. This research is coupled with measures of primary productivity, and climate to ascertain: 1) the influence of climate on population dynamics; and 2) the role of animals as trophic drivers regulating vegetation dynamics. A serendipitous result of these studies has been the role of small mammals in understanding human disease epidemics in the Southwest, especially Sin Nombre Hanta Virus. The original study had two primary goals: 1) Monitor long-term population dynamics of the nocturnal desert rodent communities found in major habitat types and across transitions on Sevilleta NWR, and 2) Archive concurrently collected museum voucher specimens which provide long term morphological, reproductive, parasite and genetic data from the small mammal communities found at each locality.
Sev008 sampling occurred at 6 sites representing 5 habitat types the Sevilleta from 1989-1998: Juniper Savanna/Arroyo Riparian at Two22, Plains-Mesa Sand Scrub at Rio Salado Grass and Chihuahuan Desert Scrub at Rio Salado Larrea on the west side of the Refuge; Pinyon-Juniper Woodland at Goat Draw, Desert Grassland at Black Grama Core and Chihuahuan Desert Scrub at the Creosote Core site on the east side. Systematic collection of voucher material coincided with population monitoring efforts at these sites over the same 10 year period. All museum specimens are maintained in the Museum of Southwestern Biology. In 1998, after 10 years of study, the footprint was reduced to Rio Salado Larrea, Black Grama Core, Creosote Core and Goatdraw sites. Today we continue to collect data at our intensively integrated study sites on the east side of Sevilleta: Black Grama Core and Creosote Core, making these datasets especially valuable after 22 years of monitoring.
Permanent capture-mark-release trapping webs are used to estimate density (number of animals per unit area) of each rodent species at each site. The method makes use of concepts from distance sampling, i.e., point counts or line-intercept techniques. The method makes no attempts to model capture-history, therefore it is not necessary to follow individuals through time (between sessions). Distance sampling methods allow for sighting or detection (capture) probabilities to decrease with increasing distance from the point or line. The modeling of detection probability as a function of distance forms the basis for estimation. Trapping webs are designed to provide a gradient of capture probabilities, decreasing with distance from the web center. Each web consists of 12 trap lines radiating around a center station, each line with 12 permanently-marked trap stations. In order to increase the odds of capturing any animals inhabiting the center of a web, the center station has four traps, each pointing in a cardinal direction, and the first four stations of each trap line are spaced only 5 m apart, providing a trap saturation effect. The remaining eight stations in a trap line are spaced at 10 m intervals. The web thus establishes a series of concentric rings of traps (Figure X). Traps in the ring nearest the web center are close together, while the distances separating traps that form a particular ring increase with increasing distance of the ring from the web center. The idea is that the web configuration produces a gradient in trap density and, therefore, in the probability of capture. Three randomly distributed trapping webs were constructed at each site. The perimeters of webs were placed at least 100 m apart in order to minimize home range overlap for individuals captured in the outer portion of neighboring webs.
Each site containing three webs is sampled for three consecutive nights during spring and fall. During each trapping session, one Sherman live trap (model XLF15 or SFAL, H. B. Sherman Traps, Tallahassee, FL) is placed, baited with rolled oats, and set at each permanent, numbered station (four in the center) on each web, for a total 444 traps over three webs. Traps are checked at dawn each day, closed during the day, and reset just before dusk. Habitat, trap station number, species, sex, age (adult or juvenile), mass, body measurements (total length, tail length, hind foot length, ear length), and reproductive condition (males: scrotal or non-scrotal; females: lactating, vaginal or pregnant) are recorded for each initial capture of an individual. Each animal is marked on the belly with a permanent ink felt pen in order to distinguish it from other individuals during the same trapping session. The trap station number for an initial capture relates to a particular trapping ring on a web and, therefore, to a particular distance from the center of the web. The area sampled by a ring of traps is computed based on circular zones whose limits are defined by points halfway between adjacent traps along trap lines (Figure X); an additional 25 m radius is added to the outer ring of traps in order to account for home range size of individuals caught on the outer ring. Area trapped and number of individuals caught for each ring of traps is the basis for estimating the probability density function of the area sampled. The program DISTANCE produces the estimators used to calculate density.
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