What Are Integrated Population Models?
This article breaks down population dynamics and how this tool informs conservation management action for the Bi-State Sage-grouse.
By Amy Sturgill, Bi-State Sage-Grouse Data and Communications Coordinator
Bi-State Sage-grouse are a geographically isolated and genetically distinct population of Greater Sage-grouse, found along the California-Nevada border in the southwestern extent of the species’ range. Known as a Distinct Population Segment, these birds are evaluated independently from other sage-grouse populations under the U.S. Fish and Wildlife Service’s (USFWS) Endangered Species Act (ESA). Because the Bi-State population is small and disconnected from other sage-grouse populations, they may be more susceptible to population declines—or even extinction. As a result, a concerted, collaborative effort to understand this population’s trend, and to conserve Bi-State numbers, has been underway for nearly twenty years.
Pictured here is the Bi-State region on the California-Nevada border and it’s six Population Management Units for Bi-State Sage-grouse.
In 2014, research staff working with the U.S. Geological Survey (USGS) developed an Integrated Population Model (IPM) framework to increase our understanding of Bi-State Sage-grouse populations. They sought to identify the number of individuals in the population, the factors affecting population changes, and to predict the trajectory of the species over time. This understanding has important implications for on-the-ground conservation and management actions.
When the Bi-State population was considered for listing under the ESA in 2018, the IPM and its ability to provide a more critical understanding of sage-grouse population dynamics, was a crucial tool in aiding the USFWS to ultimately reject issuing a listing decision.
IPMs are a type of model that incorporates population count data (how many individuals we see in a certain population through on-the-ground monitoring) with demographic data (information on survival and reproductive success attained through monitoring individuals within a population). Because population growth is directly tied to survival and reproductive success, it is necessary to understand that these rates may influence overall population growth or decline. In the last two decades, IPMs have been used to better understand population dynamics in a multitude of species including fish, waterfowl, songbirds, and polar bears. Today, an IPM framework has become an essential tool for assessing sage-grouse populations and determining conservation actions necessary to maintain healthy populations in the Bi-State area.
Historically, Bi-State Sage-grouse population information was derived solely from annual counts of males on leks during the breeding period in the spring. This monitoring data is widely used as an index of population size, but it neither accounts for total abundance nor improves our understanding of the processes driving change within the population. To gain a better understanding of why sage-grouse populations change, Bi-State partners began using an IPM framework to assess population trends that combined annual lek count data with monitoring information from radio-marked birds.
Each spring and fall, USGS technicians conduct intensive on-the-ground monitoring to collect information needed to inform the IPM. USGS wildlife biologist Rebecca Kelble and her crew members wander through the night, spotlights in hand, to locate and capture sage-grouse for this monitoring effort. "Every year the field technicians put in long and irregular hours to collect accurate and precise data,” Kelble said. “The technicians' dedication and hard work provide the baseline information for these models that are so useful to land and wildlife managers. It makes trekking through the sagebrush day after day in all weather conditions worth it."
Captured birds are marked with either Very High Frequency radio transmitters or Global Positioning System transmitters, which track their movements and aid in the determination of survival rates, female sage-grouse nesting rates, successful hatches, and chick survival. Sage-grouse populations exhibit cyclical patterns that are typically tied to climate. During years with ample moisture, sage-grouse populations typically fare well. During years of drought, mortality is typically higher and reproductive success lower, causing population declines. These fluctuating trends can make it difficult to tell the true health of a population. The IPM is one way to see beyond the population cycles and obtain a clear picture of population trends over time.
In the Bi-State, modeled estimates from the IPM indicate there are approximately 3,300 birds distributed across six Population Management Units (PMUs). While the overall population exhibits a stable growth trend, this pattern varies between PMUs. Some are experiencing population growth while others are experiencing declines. One population stronghold, the Bodie Hills PMU, contains nearly half (46%) of all sage-grouse in the Bi-State. This subpopulation is thought to be four times larger than it was 24 years ago with a low likelihood (2.4%) of experiencing extinction in the next ten years. The success of this group is likely due to its high-elevation habitat, relative to other PMUs, and the abundance of wet meadows, which help soften the impacts of drought.
The Pine Nut PMU, on the other hand, is one of the smallest subpopulations and, as the IPM highlighted, is in peril. The subpopulation contains approximately 33 total sage-grouse and a decreasing growth trend suggesting a 69.7% chance of extinction within the next ten years. There are a number of reasons for this decline. For example, the Pine Nut mountains experience a rain shadow effect from the nearby Sierra Nevada mountain range, making the area hotter and drier than many other areas in the Bi-State. This effect leads to lower environmental resistance and resilience to stressors like wildfire and invasive annual grasses. Additionally, there is a large population of wild horses in the region that may be further degrading habitat conditions. These combined stressors have likely negatively influenced reproductive success in the Pine Nut subpopulation leading to overall population decline.
When a demographic rate such as low reproductive success is identified as the underlying factor affecting a population’s trend, this information can be used to develop targeted management and conservation actions to alleviate the potential stressors. In the Pine Nut PMU, for example, the connection of that population decline to low reproductive success allows managers to examine the specific environmental stressors and to target actions like wildfire suppression, meadow restoration, and maintaining wild horse populations at Appropriate Management Levels.
Another example of IPM-informed on-the-ground management action is evident in the South Mono PMU. Using the IPM framework, USGS researchers identified the cause of sage-grouse declines in the Parker Meadow subpopulation: many eggs from nesting sage-grouse were infertile, driving the group toward critically low levels. The culprit was likely due to the decreasing number of birds in this subpopulation and the resulting dwindling genetic variability. Modeled results from the IPM suggested there was a 70% chance this subpopulation would disappear within five years.
In response, members of the Bi-State Technical Advisory Team took action. In a collaborative effort between USGS, the USFWS, California Department of Fish and Wildlife, Los Angeles Department of Water and Power, and the Bureau of Land Management, a translocation effort began, moving sage-grouse from the Bodie Hills PMU to Parker Meadows in one of the first attempts at sage-grouse translocation. Over the next three years, males, pre-nesting females, and females with broods were translocated annually. These efforts increased the number of birds and improved the genetic diversity in Parker Meadows. The result was a 179% increase in hatchability and a 180% increase in population growth. While this subpopulation is not quite out of the woods, it is currently recovering largely due to the information and understanding gleaned from the IPM.
