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Pacific Pocket Mouse Monitoring Plan for MCB Camp Pendleton: Short Term Studies and Long Term Goals

Pacific pocket mouse (Perognathus longimembris pacificus) Photo by Cheryl Brehme
Prepared by C.S. Brehme and R.N. Fisher, U.S. Geological Survey, Western Ecological Research Center in collaboration with a multi-agency expert scientific panel: Burnham, K., Meserve, P., Spencer, W., Miller, W., Pavelka, M., and D. Deutschman

The Pacific pocket mouse (Perognathus longimembris pacificus, PPM) is one of 19 subspecies of the little pocket mouse in the heteromyid rodent family.  ‘Pocket’ refers to external cheek pockets that the mice use to temporarily store small seeds while foraging, minimizing energetic costs and water loss.  These mice are extremely small with adults only weighing 5-8 grams (about the same as a silver quarter).  PPM were historically rare and patchily distributed along coastal southern California and were thought to be extinct until rediscovered on Dana Point in 1993. They were federally listed as endangered on September 29, 1994 and subsequently found in three locations within MCB, Camp Pendleton in 1995 (North San Mateo, South San Mateo, and North Santa Margarita). These four locations comprise the only currently known extant populations of this subspecies.

Principal Investigator:  Robert N. Fisher
Project Lead: Cheryl S. Brehme

Project Details

The U.S. Geological Survey was contracted to develop a scientifically valid, effective, and cost-effective monitoring program for the PPM on Camp Pendleton in 2007. The primary goals of the monitoring program are to document trends in the status of the Pacific pocket mouse (Perognathus longimembris, PPM) on base, identify results criteria for management action, and evaluate the effectiveness of management actions.

In 2007, the USGS, representatives from MCBCP, and a scientific review panel reviewed literature on PPM ecology and previous studies. A two-day workshop was held on September 6th and 7th in which many independent and agency scientists, consultants, and land managers gave additional input. The major consensus along with workshop materials and panel comments are presented in “Pacific Pocket Mouse Monitoring Plan for Marine Corps Base Camp Pendleton: Short Term Studies and Long Term Goals” (2009).

North and South San Mateo Assessment
Live-trapping at South San Mateo- Denise Clark --Photographer: Cheryl Brehme

The first study in 2008 was to immediately and comprehensively assess the status of the two smaller North and South San Mateo PPM populations. Completed in 2008, the USGS documented a successfully reproducing and well distributed population at the South San Mateo site. However, no PPM were captured at the North San Mateo site where a combination of stressors were theorized to be responsible for the apparent decline in PPM numbers. These included a reduction in food resources and suitable habitat at North San Mateo, along with greater pressure from human impacts and invasive species. The US Fish and Wildlife Service is currently working on habitat management for PPM at this site. This 2008 study also generated important information on habitat survey methods and detection probabilities that will be important for monitoring program development.

Methodology Study
After reviewing all available information, the USGS and the panel agreed the current sampling methodology of live-trapping is time intensive, destructive to habitat, and likely causes stress to the captured animals.  The use of this method as a primary sampling tool limits the amount of area that can be surveyed effectively in any given year, thus reducing the robustness of any spatial long-term trend indices. Because of this, a study was warranted to determine if an alternate accurate sampling method can be found that will increase probability of detection and cost effectiveness, as well as decrease negative impacts to the species and its habitat.  The availability of alternative detection methods will allow more flexibility in designing and carrying out studies for this species.

In 2009, we evaluated several different detection methods (Sherman traps, tracking tubes, and canine scent detection) by these criteria on MCBCP.  We found that all three sampling methods evaluated were successful in detecting PPM with advantages and disadvantages to each. 

Tritia Matsuda collecting reference tracks from a PPM --Photographer: Cheryl BrehmePacific Pocket Mouse in tracking tube --Photographer: Cheryl BrehmeWERC biologists processing a pocket mouse.

Short video of two pocket mice in trap/tube array 

For long-term monitoring of PPM, the optimal method will have high probability of detecting PPM, low impact to PPM, and be reasonable in cost.  The use of tracking tubes best met these criteria of the sampling methods evaluated in this study.   PPM tracks are definable by both size (hindfoot and forefoot width and length) as well as toe pattern.  The tubes can be left out and checked at occasional intervals (such as weekly).  Live-trapping is much more intensive, requiring three visits within each 24 hour period, but is needed for understanding population demographics, such as reproductive success.  Canine scent surveys are recommended as the most suitable method for discovery, as dogs can cover over large spatial areas over a short period of time.  However, validation of positive canine detections by tracking tubes, live-trapping, or scat DNA tests is required.  An assay to identify PPM and other small mammal species from scat was developed in coordination with the Center for Conservation Biology, University of Washington.

Discovery- Canine Scent Surveys 

Canine Scent Survey --Photographer: Cheryl BrehmeIn 2010, we used two canine scent detection teams  (“Sampson” and “Alli” with Heath and Julie as professional handlers)  to survey 538 km of potentially suitable PPM habitat around established PPM populations and across the Base.  This was for the purpose of delineating more accurate population boundaries for long-term monitoring and for discovery of any potentially unknown populations.  Canine scent detections were validated using tracking tubes and/or Sherman live-traps.  From this effort, we were able to better establish detailed sampling boundaries for this species and helped to discover a 5-fold increase in amount known occupied habitat for the South San Mateo PPM population.

The following video shows Sampson searching for PPM, sitting to indicate a detection, pointing with his nose, and being rewarded by playing with a ball.  Julie finds a PPM sized scat at the site where Sampson pointed.

Video of dog searching for pocket mouse scent

See Conservation Canines; University of Washington;

PPM Monitoring Protocol

After carefully evaluating these and other data, USGS scientists in collaboration with Camp Pendleton and the scientific panel , designed a relatively simple, multi-scaled, habitat-based, adaptive monitoring program for PPM.  This program tracks trends in overall occupancy of PPM on base, as well as within each of the three populations (Santa Margarita, South San Mateo, and North San Mateo) and includes a relative density index.

The program design is a habitat-based occupancy monitoring scheme in which a large number of plots are surveyed on a yearly basis.  The primary metric within and across populations will be proportion of area occupied (PAO) by PPM accounting for imperfect probabilities of detection.  Covariate data, to include habitat variables, environmental variables, presence of other species, and PPM density indices, will be included in models to determine which factors are significant in predicting occupancy and/or influence probabilities of detection.  Multi-year analysis will allow us to monitor extinction and colonization rates among sample plots.  This program can directly incorporate management actions and be used to inform management recommendations.  Because the population sizes vary by greater than 68 fold (13 ha, 105 ha, and 885 ha), the sample plot size and number of sample plots were optimized for overall precision both within and across the three populations.

Specifically, this program will use sample plots multi-scaled from (12.5 m2) to (100 m2).  Therefore, PPM occupancy and response to habitat covariates can be analyzed at multiple spatial scales: 1.0 ha, 0.25 ha, 0.063 ha, and 0.016 ha with total sample sizes of 50, 200, 800, and 3200 across the three populations, respectively.  It also allows us to calculate and track an index of PPM density over time from the number of occupied (12.5 m2) smallest plots within each (100 m2) larger sample plot.

Monitoring will occur across all plots continually throughout the annually active period for PPM- May and June (minimum of 60 days), greatly increasing the probability of detecting PPM within a sample plot.  We will use tracking tubes as our primary method of detection, which are accurate, cost-effective, and, with weekly sampling, will minimize disturbance of PPM habitat.  While 80 % of plots will be permanent (i.e. sampled every year) for greatest power to detect trends, 20 % of plots will be randomly chosen each year to generate a more detailed spatial coverage within PPM habitat over time.  This will also result in sampling a greater number of plots across a wider range of habitat and landscape conditions over time to better model habitat relationships and help inform management.

A small number of core sample plots within each population will be sampled for a longer period of time (Mar-Aug/Sept) using tracking tubes and two periods of live-trapping.  Data from the core plots will be used to determine annual times of emergence, peaks in activity, period(s) of reproduction, and torpor, as well as survivorship between years.  This will be beneficial in understanding PPM demography and phenology in relation to climatic variables, spatial occupancy, and aid in predictions of responses to climate change.

Future Directions

This year will be the first for implementation of this program.  We are excited to test the tracking tubes over large scale and hope to find predictors of PPM occurrence over multiple spatial scales. We hope to establish a strong feedback loop between monitoring and management actions that will help managers make informed decisions for military training and the recovery of this critically endangered species. MCBCP is currently in the process of producing a Management Plan for this species that will tie into this feedback loop. All elements of the protocol will be re-evaluated for MCBCP objectives and power to detect PPM population trends after the first 3 to 5 years of monitoring.

USGS Contact For This Project
Robert Fisher
(619) 225-6422
San Diego Field Station
4165 Spruance Road Suite 200
San Diego, CA 92101
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