Several IWS employees presented at the 2019 Western Section of The Wildlife Society conference, held at the south end of Yosemite, February 4 - 8. Following are abstracts and materials from our participating authors:
IMPACT OF MOUNTAIN LION PREDATION ON PRONGHORN POPULATIONS IN NORTHEASTERN CALIFORNIA: ARE LIONS USING A CHANGING LANDSCAPE AGAINST NOVEL PREY?
David Garcelon (presenter), Brian Hudgens, Jonathan Ewanyk, Matthew Brinkman
Pronghorn in northeastern California underwent a large population decline in the early 1990's. The population did not recover to its former numbers and continued to show a 5% annual decline. From 2014-2016, we initiated an investigation into what factors might be limiting growth in the population. Over that period we captured and placed GPS telemetry collars on 48 adult female and 42 juvenile pronghorn. Vital rates were investigated and we found that the pregnancy rate (88%) was within the range found in other stable populations, and juvenile survival (44%) was equal to or higher than most other published studies. However, adult female annual survival was low (69%). We were able to assign 59% (n = 17) of known mortality to predation, of which 80% were attributed to mountain lions. Mountain lions are ambush predators and pronghorn generally occupy open stage-steppe habitats, making ambush less probable. With the expansion of juniper woodland into sage habitat, lions may be benefiting from increased opportunity to close the distance on an otherwise unapproachable prey species. Current data suggests that if mountain lion predation impacts were removed, pronghorn population growth rates would be positive.
CANOPY EFFECTS ON EARLY STAGE VITAL RATES FOR NORTHERN RED-LEGGED FROGS AND POTENTIAL IMPACTS TO POPULATION GROWTH RATES.
Kelcy McHarry (presenter), Brian Hudgens, Jessica Abbott
Habitat characteristics at breeding sites may influence amphibian vital rates and population persistence, but the impact may differ among populations. Our research investigated the effect of canopy cover on early stage vital rates for several populations of Northern red-legged frogs (Rana aurora). We evaluated if the effect of canopy cover differed among populations, and if so, whether the effect varied with ambient air temperatures. We reared eggs and conducted mark-recapture surveys for tadpoles in enclosures with contrasting canopies. We built matrix models to evaluate if observed effects of canopy cover on tadpole survival impacted population growth rates. Egg hatch success did not differ between canopy treatments or among sites. The effect of canopy cover on tadpole survival varied among populations but there was no evidence that there was an interaction between canopy cover and local air temperature. For the populations where a canopy cover effect was detectable, population growth rates varied from approximately stable assuming tadpole survival observed in the open canopy treatments, to 30% annual decreases assuming tadpole survival observed in closed canopy treatments. Our results demonstrate that canopy cover availability can influence tadpole survival and alter population trajectories.
EFFECTS OF TEMPERATURE AND HYDROPERIOD ON NORTHERN RED-LEGGED FROGS.
Lindsey Gordon (presenter), Brian Hudgens, Jessica Abbott, Kelcy McHarry, Melissa Harbert
Climate change has been implicated in the decline and extirpation of numerous species and threatens the stability of biological communities around the globe. Amphibians, a taxonomic group dependent both on temperature for developmental processes and water for breeding habitats, face a diversity of threats involving climate change. The northern red-legged frog (Rana aurora), a California Species of Special Concern, will likely be impacted by climate change through increased temperatures and decreased precipitation in the western United States. The Institute for Wildlife Studies started collecting demographic data in northern California and central Oregon beginning in 2016 to examine how temperature and hydroperiods, seasonal patterns of water levels influence R. aurora tadpole development. We conducted a mark-recapture study of R. aurora tadpoles contained in field mesocosms at seven field sites to track tadpole development through metamorphosis. Temperature and drying rates, influencing water levels, varied among sites and years. We found that temperature influenced developmental rates based on the time of season. Likewise, changes in water depth affected development of late stage tadpoles. Understanding how temperature and hydroperiods impact larval growth is an important component for managing and potentially mitigating the effects of climate change for amphibian conservation.
DIET COMPOSITION OF MOUNTAIN LIONS ON THE MODOC PLATEAU.
Jonathan Ewanyk, David Garcelon, Micaela Gunther
Diet composition of mountain lions (Puma concolor) varies regionally and is central to understanding the dynamics of predator-prey interactions. In a 2014-2016 study, the Institute for Wildlife Studies found that the majority of known mortalities for collared adult female pronghorn (Antilocapra americana) on the Modoc Plateau were attributed to mountain lions. To better understand prey selection by mountain lions in this montane desert ecosystem, 21 mountain lions (14 male, 7 female) were fitted with GPS collars and monitored for hunting behavior. From February 2016 through August 2018, we investigated 258 GPS clusters (spatially aggregated points) to determine feeding events of large prey items. We found mule deer (Odocoileus hemionus) to be the primary prey, followed by feral horses (Equus caballus), coyotes (Canis latrans), livestock, pronghorn, and elk (Cervus canadensis). Mule deer were observed in the diet of all study animals except for one male, which predominately preyed on feral horses. Diet composition in this high desert ecosystem may be influenced by habitat characteristics, which are currently being investigated as part of a broader study. We hope to elucidate the relationship between habitat and prey selection for this cosmopolitan carnivore prior to impending wolf recolonization of the study area.
AMPHIPOD PREDATION ON NORTHERN RED-LEGGED FROG EMBRYOS.
Melissa Harbert, Brian Hudgens
This study tested whether amphipods (Crangonyx sp.), traditionally thought to be detritivores, prey on northern red-legged frog (Rana aurora) embryos at the Humboldt Bay National Wildlife Refuge in California (HBNWR). Following observations of embryo disappearance associated with amphipods, we performed experiments in which embryos were subjected to one of four treatments: enclosed chambers with all predators excluded, enclosed chambers with only amphipods present, chambers with fine mesh screen allowing access by all small predators including amphipods, and chambers open to all predators. Trials with the last 2 treatments were repeated at a nearby site with no amphipods, Big Lagoon. We found that average predation rates of viable embryos were similar among open/screened chambers at HBNWR and open chambers at Big Lagoon (21-25%), while the average predation rate in amphipod-only chambers was 15%. Predation rates were <= 1% for predator exclusion chambers at HBNWR and screened chambers at Big Lagoon. We conclude that embryo predation by amphipods does occur and has the potential to be as important of a mortality factor as predation by all other macroinvertebrate/vertebrate predators. However, the significance of this interaction is likely to be highly context-dependent and affected by factors such as amphipod density/food availability and the size/cohesiveness of eggmasses.
PRONGHORN FAWN SURVIVAL AND POPULATION DYNAMICS IN NORTHEAST CALIFORNIA.
Colton Wise, David Garcelon, Brian Hudgens
California's Modoc Plateau population of pronghorn (Antilocapra americana) has experienced a steady decline since the winter of 1992-1993. Declines in ungulate populations are typically associated with factors causing high fawn mortality, such as severe weather conditions, decreases in habitat quality, or increased predation. To better understand how fawn survival impacts population dynamics for this area we monitored 53 fawns with VHS or GPS collars in 2015, 2016, and 2018. The overall fawn survival rate for the first four months after birth was 44%. Overall fawn survival was similar for each year (2015 = 44%, n=24; 2016 = 44%, n=18; and 2018 = 45%, n=11), as was survival between agricultural (45%, n=30) and non-agricultural habitat (43%, n=23). Survival for female fawns was 57% (n=24) and 34% (n=29) for males. Fawn survival rates in our study area were generally higher than reported in studies from other areas. Modeling suggests that fawn survival alone does not explain the steady population decline. We are examining other vital rates, such as pregnancy rates and adult survival, and landscape characteristics to better elucidate the factors influencing the decline of the pronghorn population in this area.
IDENTIFYING MOVEMENT BARRIERS FOR PRONGHORN IN THE MODOC PLATEAU.
Brian Hudgens, David Garcelon, Justin Brice, Colton Wise
Habitat fragmentation has long been recognized as contributing to wildlife population declines and loss of biodiversity. Most research addressing the issue of habitat fragmentation has focused on identifying landscape features that facilitate movement between disjunct habitat patches, such as corridors. However, the hallmark of a fragmented landscape is the imposition of movement barriers. Pronghorn are especially sensitive to movement barriers, particularly fencing. We used location data from 45 adult pronghorn inhabiting the Modoc Plateau tracked from 2014-2016 to identify movement barriers at two spatial scales. At the landscape scale, we used habitat suitability to identify areas where large regions of unfavorable habitat isolate patches of favorable habitat. Within patches of highly suitable habitat, we identified regions with barriers to pronghorn movement based on directional movement analysis. At a more local scale, we identified which fences within a region pose the most significant barrier to pronghorn movement based on the number of approaches to a fence segment and the probability that an approaching animal crossed the fence segment. Each of these analyses identified different kinds of movement barriers, and represent complimentary approaches to directing efforts to reconnect fragmented landscapes.
TREE HOLLOW ROOST SELECTION BY THE TOWNSEND'S BIG-EARED BAT AND OTHER BATS ON THE NORTH COAST OF CALIFORNIA.
(Independent Master's Thesis project by IWS employee)
Amon Armstrong, Joseph Szewczak
The state of California lists the Townsend's big-eared bat (Corynorhinus townsendii) as a species of special concern because of population declines due mainly to habitat loss and disturbance. This species roosts in open cavity structures, typically caves, throughout the western United States. On the north coast of California, this species and others will roost in basal hollows of trees. This study seeks to increase knowledge of the characteristics of basal hollows and landscape associations that correlate with their use by bats. We measured 140 tree hollows at 9 sites in Del Norte, Humboldt, and Mendocino counties and collected guano for one year (2017-2018). We used the weight of monthly guano collected in basal hollows as an index of bat use. Species were identified by DNA analysis at Northern Arizona University's "Species from Feces" lab. In an initial test of 15 guano samples, 47% of bats identified were Townsend's big-eared bats. The top ranked generalized additive model (n = 27 hollows), using tree hollow measurements as predictors versus guano mass response, indicated more bat activity in hollows with higher and wider openings and larger enclosures above the opening. Quantifying basal hollow roost preferences will support forest management and conservation practices. View poster PDF