Moose Research in Rocky Mountain National Park

Since being introduced outside Rocky Mountain National Park in the late 1970s, moose populations in the park have grown rapidly. Today, moose are present in every major drainage in the park.

Article Highlights

• Moose are contributing to imbalances between herbivores and the vegetation they eat. This imbalance has led to degradation of the park’s wetland ecosystems.
• Moose were originally extremely rare, transient visitors to this area and experts have concluded they did not have established populations. Moose were introduced by the State of Colorado in 1978-79 outside the park and populations have grown rapidly.
• Moose research is focused on population estimates, habitat use, and vegetation impacts. These data will help inform current vegetation management, restoration opportunities, and potential moose management strategies in the future.

A bull moose in Sprague Lake, Rocky Mountain National Park.

NPS Photo

Shiras moose (Alces alces shirasi) are large herbivores now present in Rocky Mountain National Park (RMNP). Shiras moose are the smallest of four subspecies of North American moose, with adult males (bulls) weighing up to 1000 pounds and females (cows)
weighing up to 750 pounds.

Native or Not? Moose in RMNP

A comprehensive review of paleontological, archaeological, and historic records found little evidence of moose in or near RMNP₃. Historical accounts from the 1860s document one instance of a moose that was shot and killed near present-day Estes Park₇. Experts have concluded moose within this region were infrequent and transient visitors and not established populations.

In 1978 and 1979, Colorado Division of Wildlife (now known as Colorado Parks and Wildlife (CPW)) introduced 12 moose each year into North Park, west of RMNP, to increase wildlife viewing and hunting opportunities. In 1980, the first moose was observed in the park, in the Kawuneeche Valley₁₀. Until 2011-2012, moose in RMNP were rarely seen outside the Kawuneeche Valley and the park’s west side.

A Growing Population with Growing Concerns

Today, moose are present in all watersheds in RMNP, making them frequently observed on both the east and west sides of the park. In some instances, these typically solitary animals have been seen in unusually large herds in the summer in RMNP. While moose in other populations do occasionally congregate, this behavior tends to occur in winter when moose seek a break from deep snow₈. These observations demonstrate that the park’s moose population is increasing, which aligns with statewide trends. From 2014 to 2021, CPW statewide population estimates jumped from ~2,400 to ~3,500 animals, representing a 45% increase in 8 years₄.

Moose are wetland specialists, meaning they rely on the food and habitat found in wetland ecosystems and riparian corridors. During the summer months, moose predominantly forage on riparian willow which accounts for 91% of their diet₅. Moose in the Kawuneeche Valley can eat up to 60 pounds of vegetation per day₆.

The extirpation of wolves and grizzly bears from this region means moose in the park have few predators to regulate their population. This, paired with their willow-heavy diet, means that moose significantly impact already-degraded willow populations in the park. The impact of moose browse may be equal to, or greater than, the impact of elk browse on willow communities.

During summer, willow makes up 91% of a moose’s diet compared to only 15% of an elk’s diet₂. A large moose consumes about 45 pounds of willow a day while a large elk only consumes about 3 pounds a day.

Willow are a focus for park managers because they are key components of willow-beaver wetland ecosystems. Willow are a critical food and dam building material for beaver, whose dams cause the overbank flooding necessary to create and maintain willow habitat and riparian wetland complexes. Healthy wetland ecosystems are biodiversity hotspots and provide ecosystem services such as improving water quality, buffering droughts and floods, and reducing spread of wildfire. RMNP is actively trying to restore willow wetlands at a landscape scale as part of the Elk and Vegetation Management Plan (EVMP) and the Kawuneeche Valley Restoration Collaborative (KVRC).

A moose herd in the Colorado River District. While moose in other populations do occasionally congregate, this behavior tends to occur in winter when moose seek a break from deep snow.

NPS Photo

Collaborative Research Addresses Important Management Questions

Research by park ecologists and partners helps further our understanding of the park’s moose population and its impact on willow communities and other vegetation, like subalpine fir.

Aerial moose count surveys using thermal infrared helps researchers estimate the park’s moose population. Aerial surveys provide counts of moose within the survey area and statistical models turn counts into population estimates. Preliminary results from surveys in 2019 - 2020 estimate an average population size of 149 animals (2019) and 143
animals (2020) within the survey area₁. While surveys covered roughly 65% of the park (268 miles₂), they did not include all areas of known moose habitat like Wild Basin or Paradise Park. The above numbers are an underestimate of the parkwide population, which is unknown at this time.

Aerial surveys and GPS collar data₉ also helps managers model how the park’s moose population might change over time. The ratio of bulls, cows, and calves identified during aerial surveys, paired with yearly survival data from collars, result in an estimate that the park’s moose population grew 5% annually in 2019 and 2020₁.

Left image
A thermal infrared image of a bull moose near a river channel.
Credit: NPS Photo

Right image
A thermal infrared image of a cow moose and a calf.
Credit: NPS Photo

In addition, GPS collar data helps managers understand which habitat moose occupy. By pairing moose locations with vegetation height, distance from water, slope, and other environmental characteristics, researchers can better predict habitat preference and use this information to identify areas of the park where moose are more likely to establish home ranges.

The ratio of bulls, cows, and calves identified during aerial surveys, paired with yearly survival data from collars, result in an estimate that the park’s moose population grew 5% annually in 2019 and 2020₁.

GPS collar data also captures how moose move around and utilize the habitat they occupy. Pairing this space-use data with population data can help researchers estimate the cumulative impact moose are having on the park’s willow populations. Vegetation surveys conducted as part of the EVMP document browse impacts by both moose and elk and help monitor willow and other vegetation conditions on the ground.

A time series (2020-2022) of GPS collar data that shows the space use patterns of two moose in and around Rocky Mountain National Park.

NPS Animation.  Map data ©2022 Google.

Knowledge and data from moose research will help inform current vegetation management plans and will support potential moose management strategies in the future.

Park biologists monitoring a cow moose during a collaring event in 2017.

NPS Photo

Resources and References

1. Abouelezz, H.G. and Hobbs, N.T. In review. A high-altitude thermal infrared method for estimating moose abundance and demography in Rocky Mountain National Park, USA. Wildlife Biology.
2. Baker, D. L., and Hobbs, N. T. (1982) Composition and Quality of Elk Summer Diets in Colorado, The Journal of Wildlife Management 46 (3), 694-703.
3. Brunswig, R. (2015) Archaeological and Paleontological Evidence for the Prehistoric and Early Historic Presence of Bison (Bison sp.) and Moose (Alces alces) in Rocky Mountain National Park and the North Central Colorado Rocky Mountains. Internal Report.
4. Colorado Parks and Wildlife. Moose Statistics. Retrieved 2023, from www.cpw.state.co.us.
5. Dungan, J. D., and Wright, R. G. (2005) Summer diet composition of Moose in Rocky Mountain National Park, Colorado, Alces 41, 139-146.
6. Dungan, J. D., Shipley, L. A., and Wright, R. G. (2010) Activity patterns, foraging ecology, and summer range carrying capacity of moose (alces alces shirasi) in Rocky Mountain National Park, Colorado, Alces 46, 71-87.
7. Estes, M. (1939). Memoirs of Estes Park. The Colorado Magazine, 16(4), 121-132. https://www.historycolorado.org/sites/default/files/media/document/2018/ColoradoMagazine_v16n4_July1939.pdf
8. Peek, J. M., LeResche, R.E., and Stevens, D.R. (1974) “Dynamics of Moose Aggregations in Alaska, Minnesota, and Montana.” Journal of Mammalogy 55 (1): 126–37. https://doi.org/10.2307/1379262.
9. Rocky Mountain National Park (2017). Moose Annual Survival, Population Estimation, Habitat Use and Carrying Capacity on the Summer Range in Rocky Mountain National Park. Fact Sheet.
10. Stevens, D.R. (1988) Moose in Rocky Mountain National Park. Internal Report.