Clark's nutcrackers and whitebark pine: resilience of a keystone mutualism in an altered ecosystem
CURRENT RESEARCH Landscape scale movement, habitat selection and resource tracking by the Clark’s nutcracker, a conifer seed disperser: Satellite-tracking in Wyoming, Washington and Montana
In the face of widespread environmental change, understanding and promoting resilience and stability of plant-animal seed disperser mutualisms is key to effective conservation strategies. Whitebark pine (Pinus albicaulis) is an obligate mutualist of Clark's nutcrackers (Nucifraga columbiana) because its seedlings sprout almost exclusively from nutcracker seed caches . Evidence suggests that declining whitebark pine communities are leading to reduced local nutcracker populations, which would lead to reciprocal whitebark pine declines. Because nutcrackers are highly mobile, facultative migrants, it is difficult to accurately monitor local population trends. Therefore, information on landscape scale space use is essential for more accurate predictions of nutcracker metapopulation stability, and local and range-wide resilience of the nutcracker-whitebark pine mutualism.
To address this information gap, in 2014, Dr. Taza Schaming began the first study to satellite-tag Clark's nutcrackers in the Greater Yellowstone Ecosystem, Wyoming. In 2018, she expanded this long-term satellite-tracking project into the North Cascades, Washington, and in 2021 Schaming will collaborate with Glacier National Park personnel to satellite-tag and track nutcrackers in northern Montana. To date, she has followed the movements of 14 Clark's nutcrackers for up to 3 1/2 years each. Satellite-tracking will increase our understanding of how local nutcrackers track resources, how far they emigrate, and how long they stay in habitats of different type and quality. In addition, because these satellite-tags function for multiple years, we will be able evaluate movement as a function of conifer cone crop levels and other changing environmental variables. Finally, satellite-tracking nutcrackers in geographically distinct regions, northwest Wyoming, the North Cascades and Glacier National Park, enables comparisons of nutcracker resource tracking, landscape scale long distance movements, and habitat selection in regions with different habitat types and health. Managing both nutcrackers and whitebark pine presents a complex conservation challenge. These results will improve local managers’ ability to identify Clark's nutcracker habitats and connectivity between distant habitats, aiding in the design of effective local and range-wide restoration and conservation strategies for both species.
Satellite-tagged Clark's nutcrackers in northern Washington 2018 pre-seed harvest and seed harvest seasons
All high accuracy points (<250m, 250-500m, and 500-1500m) for the seven satellite-tagged Washington Clark’s nutcrackers, including locations between the February 2018 date of trapping and November 1, 2018, the last date of the seed harvest season. Each color just represents a different bird.
Maps of satellite-tagged Clark's nutcrackers in northern Washington
August 17, 2018. The map shows all the high quality locations from the last 30 days of the seven satellite-tagged Clark's nutcrackers in Washington. The birds were originally trapped and tagged in Winthrop, WA in February 2018. (This informal screenshot will soon be translated into an elegant map;)
August 17, 2018. The map shows all the high quality locations from the last 100 days of the seven satellite-tagged Clark's nutcrackers in Washington. (This informal screenshot will soon be translated into an elegant map;)
Map of satellite-tagged Clark's nutcrackers from the Greater Yellowstone Ecosystem
Photo credit: Taza Schaming
The map shows all the high quality locations from the seven satellite-tagged Clark's nutcrackers in Wyoming, as well as photos of the habitat where four of the birds overwintered in winter 2015-2016. Three of these four birds then returned to Jackson the following summer.
PRELIMINARY RESULTS, WYOMING Preliminary results suggest that the satellite-tagged birds regularly moved over an enormous area outside of the fall seed harvest season (averaging 17,440 ± 4,098 ha, n = 7 birds, n = 1 year). This is far larger than the estimates determined via radio-tracking (195 ± 27 ha, n = 56, n = 2 years). It remains to be seen whether Clark’s nutcrackers use such a large area in all years, or in regions with other dominant habitat types. Additionally, despite the moderate whitebark pine cone crop in 2015, the majority of tagged birds (71%, n = 5) emigrated. This was surprising because nutcrackers were previously only thought to irrupt during widespread cone crop failure. Birds I tracked flew to Montana and Utah, up to 650 km away, where they overwintered. All but one returned to Wyoming in summer 2016. It is unknown if these long-distance movements, or the roundtrip annual travel, are common in other years, or in other parts of the range. Six of the satellite-tagged birds continued to transmit through 2016. The last high accuracy location data for five birds suggests they currently remain relatively close to Jackson, Wyoming. The sixth bird was last located in central Utah, where it had stayed throughout 2016. These data have important conservation implications. One, Clark’s nutcrackers use a much larger area than previously documented for their everyday existence. They use a larger area than expected throughout the year, not only during the fall harvest season, and they use large swaths of space even during a year with an average, healthy whitebark pine cone crop. The birds also use a mosaic of habitat types throughout the year. The long distance flights to non-whitebark pine habitat in Utah also suggests that that individuals do not show high fidelity to a region or to specific habitats for fall foraging. This means that if whitebark pine stands decline below a threshold, nutcrackers may largely disappear from the ecosystem, and the Greater Yellowstone may lose the important seed dispersal ecosystem function of this bird. Alternatively, the birds may travel long distances so regularly that even if the majority of individuals move out of the ecosystem due to low whitebark pine cone crops, enough may move through the region that when the cone crop is adequate, they stay and disperse seeds.
Photo credit: Taza Schaming
BACKGROUND Clark's nutcrackers are keystone species in western North America. They are obligate mutualists of whitebark pine, and facultative mutualists of multiple conifers, playing an important role in forest regeneration and seed dispersal for at least ten conifer species. Nutcrackers shape the ecosystems in which they live; annually, individual nutcrackers are estimated to store up to 98,000 seeds in thousands of separate locations. Seeds not retrieved for food are able to germinate. Clark's nutcrackers disperse seeds up to 32.6 km, effectively moving seeds longer distances than wind, rodents and all other North American seed hoarding birds, enabling rapid migration of seeds, and contributing to gene flow across and between habitat islands. They move seeds across altitude and elevation, as well as into disturbed areas. In the face of current climate change, the long-distance dispersal of conifer seeds, and thus the continued association between Clark's nutcrackers and conifers, may be critical in mitigating against local genetic bottlenecks and inbreeding depression by bolstering effective population size, and facilitating rapid colonization of newly available ideal habitats.
Whitebark pine, a candidate for the Endangered Species List, is a critical component of montane ecosystems in the western U.S., where it contributes to biodiversity and ecosystem function. Whitebark pine is considered a keystone species because many animal species depend on its high-fat, high-energy nuts. In particular, survival and reproduction of the endangered grizzly bear (Ursos arctos) is linked to whitebark pine cone abundance. Whitebark pine is also valuable for watershed protection because it delays snowmelt which leads to decreases in spring flooding, then decreases in summer droughts. Currently, whitebark pine is disappearing range-wide due to infection by the non-native Cronartium ribicola, which causes white pine blister rust, and outbreaks of mountain pine beetles (Dendroctonus ponderosae), which have been worsened by climate change.
Evidence suggests that declining whitebark pine communities are leading to reduced local Clark's nutcracker populations. A downward trend in nutcracker populations would have reciprocal effects for whitebark pine population regeneration, and consequently for the viability of subalpine ecosystems in the western U.S. To sustain whitebark pine communities, Clark's nutcracker populations must be maintained. This requires information on landscape scale space use in Clark's nutcrackers.