Prairie dogs (Cynomys spp.) are burrowing rodents considered to be ecosystem engineers and keystone species of the central grasslands of North America. Yet, prairie dog populations have declined by an estimated 98% throughout their historic range. This dramatic decline has resulted in the widespread loss of their important ecological role throughout this grassland system. The 92,060 ha Sevilleta NWR in central New Mexico includes more than 54,000 ha of native grassland. Gunnison’s prairie dogs (C. gunnisoni) were reported to occupy ~15,000 ha of what is now the SNWR during the 1960’s, prior to their systematic eradication. In 2010, we collaborated with local agencies and conservation organizations to restore the functional role of prairie dogs to the grassland system. Gunnison’s prairie dogs were reintroduced to a site that was occupied by prairie dogs 40 years ago. This work is part of a larger, long-term study where we are studying the ecological effects of prairie dogs as they re-colonize the grassland ecosystem. With this project, we would like to further investigate the impact that Gunnison’s prairie dogs have on the landscape. Gunnison’s prairie dog monitoring data has been collected from the beginning of the reintroduction project, but little information has been collected on how grassland species respond to the sudden presence of prairie dogs on the refuge.
This project will help determine if the prairie dog reintroduction has had positive impacts on the grassland ecosystem. Prairie dogs benefit grasslands in many ways, but their role as ecosystem engineers directly impacts other species by creating new habitat that would not be present without prairie dogs. We have documented physical landscape changes, but we have not specifically documented benefits to other grassland species. This work will help determine if the reintroduced prairie dog populations on Sevilleta NWR are now acting as a keystone species in a grassland ecosystem by monitoring small mammal populations to see if species richness, diversity, and density are different on prairie dog colonized areas versus non-colonized areas.
Trapping Location and Design:
Trapping will be done on the 16ha Prairie Dog Relocation Study Plots. There are 4 of them- A, B, C, and D. Each plot will have 169 traps placed in a grid covering 9 hectares. Using the vegetation quad map, there will be a trap placed at 1 meter to the north at each of the following vegetation plots 11-17, 20-26, 29-35, 38-44, 47-53, 56-62, 65-71. This accounts for 49 of the traps. There will also be a trap placed in between each veg plot, with rows running North/South, which accounts for 42 more of the traps. Then making a complete row in between the North/South vegetation quad rows, will account for the remaining 78 traps. To locate the veg plots, each are marked with a rebar and short white PVC. There is a numbered tag on each PVC corresponding to the map.
Flag each trap with a numbered pin flag to designate trap numbers. This is important in ensuring that all traps are checked and closed each day.
Trapping period will be one plot a week for 4 nights.
The traps are set each evening for four nights. This entails setting and baiting the traps at a given locality on Monday afternoon, then checking the traps at dawn on Tuesday (night 1), Wednesday (night 2), Thursday (night 3), and Friday (night 4). Each trap is baited with a handful of steamed, crimped oats tossed into the trap after it is placed on the ground; a few oats are left outside the trap entrance to entice passers-by. The ground needs to be smoothed out with a foot to make sure that the trap is level and not unbalanced.
Each morning, traps are checked as follows: the worker walks up and down the transects and closes open traps as you go along. Traps are not reopened until the late afternoon/early evening, at which time additional bait is also put in. When a closed trap is encountered, it is first checked to see if an animal is present by carefully and just slightly opening the door of the trap and looking inside. Be aware that kangaroo rats can jump out while doing this, so use caution. Sometimes, although a trap may appear empty, a tiny rodent may be hiding under the treadle (especially in the large traps). To check for this, one must reach into the trap and lightly push down the treadle. If the treadle will not go down, there is likely a mouse underneath. If no animal is in the trap, the trap is left closed until the afternoon. If a trap has an animal, the worker processes the animal at the stake and takes the relevant data. While checking for animals on Friday morning (night four), traps are picked up, emptied of seed, and returned to storage boxes, ready for placement at another locality the following week. Importantly, traps MUST BE counted as they are placed into storage boxes in order to insure that no traps (or animals) are left on the plot. If rain falls on the baited traps, they may require cleaning and drying back at the field station before storage or use the following week.
Removing rodents from trap
For each capture, the trap number is recorded first. Next, a given animal is shaken from the trap into a plastic gallon ziploc bag. This is accomplished by wrapping the opening of the ziploc bag over the door end of the trap. Make sure that they bag is tight so the rodent can’t squeeze out between the bag and the trap. Open the front door through the bag and lock open. Roll the trap upside down and swing it in an arc downward. As soon at the rodent enters the bag, close the bag off with your hand so the rodent cannot reenter the trap. With kangaroo rats, you often do not need to shake the trap to get the animals out. Instead, put the Ziploc bag on trap as normal and open trap door, but hold the trap angled upward instead of down and the rodent should come out on its own. Hold tight on the bag though because sometimes they come out rather quickly.
If a trap is triggered, but appears empty, don’t assume there is no animal in trap. Small species such as pocket mice can hide under the treadle. Make sure and lightly press down on the treadle to make sure it goes all of the way down. If not then there is most likely a rodent under treadle. You can also open up the back door to look under treadle, but use caution as to not let rodent escape.
If another animal (lizard, bird, rabbit, prairie dog) is caught in the trap, they can simply be released. However, make sure and mark on data sheet that the trap was closed due to bird/lizard/rabbit. If you do find a trap that was triggered by wind or large animal and is in fact empty, make sure and mark on the datasheet that that trap number was triggered but empty.
Handling and Processing rodents
In the bag, the processor positions the rodent with its head in the corner of the bag. Hold its head down with one hand from the outside of the bag, pressing gently on the back of the skull. Then reach in the bag with the other hand and grasp the animal with the thumb and forefingers by the loose skin around the back of the neck and shoulders, and then remove it for inspection.
First off check to see if the rodent is tagged or marked. If it is then you will mark that individual as a recapture on data sheet. After recording the ear tag number or other marking and the species of animal, it can be released. If it is not marked, then it will need to be marked and processed.
Dipodomys spp, Onychomys spp, Neotoma spp, Peromyscus spp, and any other large species you may catch will be uniquely marked with one ear tag. Ear tags should be placed at the very base of the ear on its interior edge (or the front of the ear). Putting it on the external side or back of the ear allows the rodent to rip the ear tag off more easily, by scratching at it with its hind legs.
Other species such as, Perognathus spp, Spermophilus spp, and other small rodents that have too small of ears to place an ear tag, will be marked with sequential individual numbers on their chest, using permanent markers. A different color must be used for each night (blue for 1st night, black for 2nd night, and red for 3rd night). Small rodents do not need to be marked the 4th night, but large rodents do need to be ear tagged. Start with number 1 and increase as necessary for catches.
Next, each animal is identified to species, sexed, and aged. Specific measurements are taken only for those genera which required them for species identification:
Peromyscus: Total length, tail, foot, ear;
Onychomys: Total length, tail, foot.
Perognathus, and Reithrodontomys: Total length, tail.
All measurements are taken to the nearest millimeter using a plastic ruler. The species is recorded by a 4-letter code that represents the first 2 letters of the genus and the first 2 letters of the species.
Sex and reproductive status is then determined by examination of the genitalia (lactating/vaginal/pregnant/scrotal). Look for enlarged scrotum, enlarged nipples, or an enlarged vaginal opening. If none of these are apparent, then the rodent is non-reproductive. Females will still have visible nipples when non-reproductive.
ADULT MALES reproductive status:
-Scrotal (ST): the scrotum can be fully enlarged or partially enlarged.
- Non-reproductive (N)
ADULT FEMALES reproductive status:
-Vaginal (V): in estrus; vagina is obviously swollen and looks large and puckered, vaginal plug can be present or absent
-Pregnant (P): heavier weight, can palpate babies
-Lactating (L): nipples (at least one) reddish and/or enlarged
Before releasing the individual, it is then weighed to the nearest gram, using a Pesola scale clipped to the base of the animal’s tail. Larger animals can easily get off of scale so it is easier to put them back in the bag and weigh them inside the bag. Make sure and weigh bag after rodent is released and subtract from first weight to get actual weight of rodent.
Animals which perished during captivity on plots are noted in the comments on field data sheets as 'D.I.T' (Dead In Trap).
This data is collected each summer, starting in 2013, by an student in the Sevilleta LTER Research Experience for Undergraduates Summer Program. Ear tagging started taking place in the summer of 2014.
Data Collector History
Ty Werdel 2013
Betsy Black & Andrew Velselka 2014