This study measured the population dynamics of black-tail jackrabbits (Lepus californicus) and desert cottontail rabbits (Sylvilagus auduboni) in the grasslands and creosote shrublands of McKenzie Flats, Sevilleta National Wildlife Refuge. The study was begun in January, 1992, and continued quarterly each year. Rabbits were sampled via night-time spotlight transect sampling along the roads of McKenzie Flats during winter, spring, summer, and fall of each year. The entire road transect was 21.5 miles in length. Measurements of perpendicular distance of each rabbit from the center of the road were used to estimate densities (number of rabbits per square kilometer) via Program DISTANCE. Results from 1992 to 2002 indicated that spring was the peak density period of the year, with generally steady declines through the year until the following spring. Evidence of a long-term "cycle" (e.g., the 11 year cycle reported for rabbits in the Great Basin Desert) did not appear in the Sevilleta rabbit populations.
The purpose of the study was to assess the dynamics of rabbit populations in the grasslands and creosote shrublands of the Sevilleta NWR. Rabbits are important herbivores in these habitats, and can influence NPP and plant species composition. In turn, these animals also provide high-quality prey for many of the Sevilleta's mammal and reptile carnivores and birds of prey. Density data on rabbits can be used to calculate herbivore pressure on the plant communities.
When the samples were collected: The samples were collected in winter, spring, summer, and fall, of each year. Rabbit populations were sampled during a single night during each of these four seasons per year. Dates of collection varied in some years, but generally the sampling was conducted in January, April, July, and October.
Sampling Design: The rabbits were sampled along 21.5 miles of roadway that was broken up into four "legs" of varying lengths.
Leg A: Black Butte southward to Five Points (5.7 miles).
Leg B: Five Points eastward to the turnoff before Palo Duro Canyon (4.1 miles).
Leg C: Palo Duro turnoff northward to the old McKenzie Headquarters site (6.1 miles).
Leg D: McKenzie Headquarters site northwestward to Black Butte (5.6 miles).
Measurement Techniques: The rabbit surveys were conducted at night using spotlights. Surveys began one hour after sunset, when no trace of sunlight or dusk remained. Beginning in 1998, samples were taken only during full-moon periods. A pickup truck was driven slowly (8-10 miles per hour) along the road of the 21.5 mile circuit. Two (or more) observers stood in the bed of the pickup truck, and scanned the left and right sides (respectively) of the road with spotlights, while the driver kept watch for rabbits directly in front in the road. During 1992, the spotlights were Q-Beam 500,000 candlepower spotting lights, with both flood and spot settings (spot settings were used during the rabbit sampling). From 1993 through 1996, Q-Beam spotlights with 1,000,000 candlepower were used. In 1997, new spotlights with 3,000,000 candlepower were used; these lights were set permanently on "flood", but illuminated well at distances previously reached by the spot settings of the less-powerful spotlights.
In addition to the spotlights used by the standing observers in the bed of the pickup truck, two spotlights mounted on the pillar posts of the truck's cab were turned on and set for the roadsides ahead of the truck; these lights, coupled with the high-beam setting of the truck's headlights, illuminated the road in front of the truck for approximately 100 meters. When a rabbit was observed, one person's spotlight illuminated the spot at which the rabbit was first seen. The second person's spotlight would track the rabbit, so that it was not counted twice. A meter tape was walked out from the center of the truck bed (which equalled the center of the road) in a perpendicular direction from the road to the location at which the rabbit was first observed. That distance was measured and recorded to the nearest meter.
If a rabbit was observed in the middle of the road, the distance was recorded as zero. Beginning in January, 2000, perpendicular distances to the rabbits were taken with a laser range finder, with accuracies of less than 1 meter (accuracies were tested before field use and confirmed to be <1m). Generally, rabbits within 100 meters of the road could be seen relatively clearly with all three types of spotlights. Other data recorded included (1) the odometer reading in miles from the beginning of the sample at Black Butte (odometers were reset to zero at the start of the sample), (2) whether the rabbit was on the Left or Right side of the road, and (3) the species of rabbit. In addition, incidental data were recorded on weather conditions, presence of clouds and moon, and the time at which the survey was begun, along with the times at which each Leg was begun and finished. Finally, the names of the people on the sampling crew were recorded.
Analytical Procedures: The perpendicular distance data were entered into Program DISTANCE to estimate the total density of rabbits in the study area. Values were computed as numbers of individuals per square kilometer In the analyses, if there were sufficient numbers of rabbits (>10 per leg), the difference legs were analyzed separately, and the resulting mean densities were estimated by averaging the four leg estimates. In the results tables below, these instances are indicated by the category, "MEAN". If sample sizes were too small to estimate the four legs separately, then all the rabbit observations were pooled together, and a density estimate for the entire 21.5 mile survey was calculated. These results are indicated by the category, "ALL".
The program DISTANCE command codes were as follows:
Stratum/label='DATE ENTERED HERE';
LEGS, DATE ENTERED HERE'/Effort=21.5;
DISTANCE DATA ENTERED HERE, SEPARATED BY COMMAS;
Est /key=uniform /adj=cosine /select=sequential /criterion=AIC /monotone=weak;
Est /key=uniform /adj=hermite /select=sequential /criterion=AIC /monotone=weak;
Est /key=hnormal /adj=cosine /select=sequential /criterion=AIC /monotone=weak;
Est /key=hnormal /adj=hermite /select=sequential /criterion=AIC /monotone=weak;
Density by sample;