American badger & Burrowing owl Habitat Suitability Assessment Study
The Midpeninsula Regional Open Space District (Midpen) was seeking to improve the management of American badgers (Taxidea taxus; badger) and burrowing owls (Athene cunicularia; BUOW) throughout their preserves. To achieve this, Midpen needed to better understand how they could evaluate and implement science-based management to benefit the occurrences of these two highly sensitive species. This process included identifying exactly where badgers and burrowing owls were present at their preserves, the habitat characteristics that best supported their occurrences, information about the existing populations of each species, and the relationship between the species based on their mutual association with grassland habitats.
Our role in this project was to construct a Habitat Assessment and Linkage model using the habitat characteristics associated with existing badger and burrowing owl location data throughout the Peninsula, and then apply the model to the District Preserves to create a map depicting levels of habitat suitability for each species.
Field-based surveys for each species were conducted in areas with a high probability of occurrence and control areas (low probability of occurrence) to test the habitat model accuracy and to identify other factors that may be affecting badger or burrowing owl presence in the Preserves. The Linkage model was then validated by field methods that included tracking along 1 km long transects, camera arrays, and roadkill surveys.
While surveying and monitoring, we also gathered genetic data to develop estimates of population distribution, size, and presence, and to provide species and land management recommendations to Midpen.
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Creation of a District-wide Habitat Suitability Assessment for badgers and BUOWs
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The habitat characteristics associated with known occurrence locations for badgers and burrowing owls in the Peninsula, including San Mateo, Santa Clara, and Santa Cruz Counties were used to create a model of suitable habitat for both species within the District’s properties.
Model creation included the following steps:
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Compiling a comprehensive database of badger and BUOW occurrence records from a variety of sources.
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Using GIS data to create Habitat Suitability and Habitat Linkage models for badgers and BUOWs.
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Generating transects for empirical testing of model results.
Badger Monitoring and Research: badgers leave extensive and recognizable signs, such as burrows (dens) due to their prolific digging activity to capture prey, establish diurnal shelter, and construct natal dens. These areas of activity can provide reliable indicators of the timing (based on the estimated age of the overturned soil) and location of badger presence, as well as characteristics of habitat used for foraging and denning.
To validate the Habitat Assessment model for badgers, we employed two types of surveys, walking transect surveys, and camera monitoring.
Camera Trapping
Camera monitoring was used for:
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Identifying active badger burrows where badgers were present.
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Surveying potential linkages between grassland fragments for badger use and frequency of transits.
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Identifying individuals within select areas to estimate the overall regional badger population, the number of badgers overlapping in select areas, and the number of badgers using identified linkages
Genetic analysis
Our project partner, Dr. Jessie Quinn, at Dr. Ben Sacks lab at UC Davis, conducted a genetic analysis to determine the genetic population structure of the badger population within the Peninsula and the surrounding Bay Area. DNA microsatellite loci was used to determine the minimum number of individuals, sex, genetic diversity, and population genetic structure within Midpen properties, and between the peninsula and proximate regions (East Bay, Central Coast, Marin/Sonoma).
Study results include
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The linkage [AS1] model was developed by creating a Cost Surface model for badgers to reflect the cost of movement through the study area. The habitat variables used for developing the model included vegetation, habitat types, soil, hydrology, land use, and roads from GIS layers. Each habitat variable was reclassified to reflect the suitability of a habitat feature for badger presence (denning) and movement using ArcMap 10.2. This resulted in a model that reflected a range of highly suitable habitats with low costs for movement for badgers to poor habitats with high movement costs for badgers within the study area. Badger locations collected from the camera data and transects were overlaid onto the cost surface and linkage model to validate them and to discern which linkage badgers were utilized.
The map resulting from the Cost Surface model showed a fragmented landscape for badgers on the Peninsula. Large swaths of habitat within the study area consist of highly unsuitable habitats, such as steep, forested ravines with dense vegetation understory. The ravines bisect a majority of the available highly suitable habitat for badgers, such as grasslands. Other areas of suitable habitat for badgers are bisected by high-use roads, which could restrict badger movement across the landscape due to mortality from vehicles, potentially isolating individuals or populations. The highly fragmented landscape highlights the importance of identifying connections between suitable habitats and increasing the permeability of the landscape for badgers to find resources and mates and for juvenile dispersal from their natal areas.
More information about the project can be found in the interactive story below.
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https://storymaps.arcgis.com/stories/40523de82f3042629f0e5f58112f3038
