Nilgai inventory at Palo Alto Battlefield National Historic Park: Final report

DJ&A, P.C.

doi:10.36967/2296986

Nilgai antelope (Boselaphus tragocamelus) are invasive ungulates introduced to southern Texas, where they have since established free-ranging populations. One such population has been consistently observed within the western unit of Palo Alto Battlefield National Historical Park. Park managers are concerned about potential impacts to native vegetation resulting from nilgai antelope grazing and alteration of the historic character of the park. To provide Palo Alto Battlefield National Historical Park with the ability to make informed management decisions, baseline information regarding abundance, occupancy, and distribution of nilgai antelope was collected and analyzed by DJ&A, P.C. with support from the Caesar Kleberg Wildlife Research Institute at Texas A&M University, Kingsville. Using remote camera traps and population modelling techniques for estimating abundance of unmarked populations, 20 active camera trap stations were installed for one year, with two additional stations rotated through a subset of eight identified latrine sites to capture spatiotemporal patterns of use and group demographics. Captured images of nilgai antelope were analyzed to estimate abundance, occupancy, and predicted distribution of nilgai antelope in the park. Other wildlife species detected were also analyzed as data permitted. Total abundance of nilgai antelope within the study area was estimated to be 58 individuals (SE=1.8), with some seasonal variation in response to births, deaths, emigration, immigration, and potential changes in group size and behavior stemming from biological activities such as parturition and breeding. Independent detection event analyses resulted in 2,333 discrete events, with 2,190 of those containing nilgai antelope. Relative to nilgai antelope, 26 other wildlife species were detected including feral hog, collared peccary, raccoon, bobcat, coyote, and white-tailed deer in order of decreasing independent detection events. Of nilgai antelope, adult females were most commonly detected and subsequently were the primary drivers behind observed patterns of use across the study area. Detections occurred at all sites, indicating use of the entire study area, with central and northwestern portions maintaining the highest overall activity for all sex and age classes. Southeastern portions had notably low detections, indicating a lack of suitable habitat. Time series analysis of nilgai antelope activity showed minimal fluctuations in activity with three primary peaks in May, September, and January. Adult males exhibited relatively heightened activity in the fall leading into the breeding season. Nilgai antelope detections increased significantly with increasing cover of invasive grasses and grazed species and decreased as cover of native species and gulf cordgrass increased. This pattern indicates that habitats with increased prevalence of invasive graminoids may offer more suitable habitat than habitats with intact native vegetation such as coastal prairie within the study area. Continued restoration of native vegetation communities could result in reduced habitat suitability and overall carrying capacity for nilgai antelope within the western unit of Palo Alto Battlefield National Historical Park.

Nilgai inventory at Palo Alto Battlefield National Historic Park: Final report

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