Wildlife Research at Bonanza Creek
Provided by: Knut Kielland - Institute of Arctic Biology, UAF
The Bonanza Creek LTER sites are situated in an intact ecosystem supporting an abundance of wildlife species, many of which are scarce in the Lower 48. Many of these species are of profound economic and cultural importance for rural and urban residents of Interior Alaska.
Ecosystem studies of mammalian herbivory
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Studies of mammalian herbivory, particularly by of moose and snowshoe hares, have been part of the Bonanza Creek LTER project for over 20 years. Winter browsing by moose entails the turnover of up to 50% of the annual twig production of preferred forage such as willows, and a delay in green-up by approximately 8-10 days. Moose browsing opens up the canopy resulting in warmer and drier soils, which in turn have significant effects on other soil characteristics including organic matter composition and stable isotope chemistry. For example, enhanced evaporation from the soil surface in the presence of browsing results in a more pronounced salt crust, which in turn has negative effects on spruce germination and growth. Enrichment of soil 13C-carbon signatures illustrates feedbacks between herbivory, canopy structure, evapotranspiration and geochemistry.
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Fig. 1. Effect of mammal browsing on inorganic soil carbon concentrations and d13C signature in early succession. |
Feces from moose can account for approximately 30% of aboveground N input in willow communities. Moreover, herbivory significantly accelerates decomposition as
exemplified by a 50%, increase leaf litter carbon flux in browsed stands compared to those protected from browsing.
Fig. 2. Increase in decomposition of leaf litter from feltleaf willow following winter browsing by moose. |
These edaphic changes are augmented by herbivore-mediated changes in vegetation succession. Because moose and hares prefer to browse willows over alder, herbivory affects the relative abundance of these species and over time contribute to a shift of dominance of willows to dominance of N-fixing alder. After nearly 2 decades of herbivore exclusion the willow stands that constituted the vegetation of at the start of these experiments have now completely succeeded to alder. Willow stands protected from browsing (inside the exclosures), however, appear to have reached a stable equilibrium vis á vis alder. Thus, herbivory appears to accelerate successional change.
Fig. 3. Changes in alder and willow abundance (expressed as leaf litter biomass ratio) in the presence and absence of herbivory on the Tanana River floodplain. |
The Tanana River is the largest tributary of the Yukon River and exhibit large seasonal variation in river stage (nearly 3 m) and discharge (2x). The channels are constantly changing due to continuous erosion and deposition of new terraces. These terraces are rapidly colonized by willows. Thus, the riparian landscape emanate from a constant tug-of-war between the physical processes of deposition, allowing willow communities to flourish, and the biotic effects of herbivory which transform willow communities into alder stands.
Our modeling efforts, using spatially explicit vegetation models, show how the interaction of fluvial dynamics and herbivory act to produce the observed patterns of vegetation distribution on the landscape.
Fig. 4. Ratio of willow to alder in the riparian zone along the Tanana River in relation to browsing intensity and fluvial dynamics. Response surface depicting the landscape composition was based on simulations using the Alaska Frame-based Ecosytsem Code (ALFRESCO). |
Whereas mammalian herbivores concentrate on deciduous species in early succession, seedlings of all tree species in Alaska's boreal forest may be subject to herbivory. For example, regenerating spruce seedlings are typically browsed heavily by snowshoe hares. This browsing typically result in curtailed height growth as well as high rates of mortality.
Fig. 5. Relationship between height and basal diameter of white spruce seedling/saplings
in the presence (blue) or absence (pink) of browsing by snowshoe hares. C2 Exclosure 2009. |
When hares are abundant they impart high rates of mortality on spruce seedlings, suggesting that if masting in white spruce (which typically occurs every 10 years) coincides with the peak of the hares cycle (which also has a period of about 10 years) a large proportion of the current spruce cohort may die, consequently resulting in a delay of the establishment and dominance of spruce in succession.
Population studies
Since 1999 we have monitored populations of snowshoe hares on two 9 ha live-trapping grids (one in black spruce forest and one in a riparian shrub community). The grids consist of 50 traps arranged in a 5 x 10 grid with a 50 m inter-trap distance. The live-traps are Havahart size 3 and Model 1079. Hares are trapped 4 times per year (spring, summer, autumn and winter) with each trapping session lasting 4 nights. Captured snowshoe hares are sexed, weighed (±5g), and the right hind foot is measured (mm). Newly captured hares are tagged in each ear with No. 3 Monel tags from the National Band Company. Snowshoe hare abundance are estimated using maximum likelihood estimators (Otis et al. 1978) assuming population closure.
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Abundance estimates (N) are used to inform a recent model of population density (d) (Efford 2004) that neither relies on the size of the trapping grid nor estimation of the effective trapping area. The method relies on the estimated population size (N) and capture probability, as well as the mean distance between successive captures, which provide an estimate of the scale of individual movements. These closed population capture-recapture data are used as inputs to the simulation model of the trapping process based on: 1) spatially-defined trap locations, 2) the magnitude of individual hare movements, 3) the overall capture probability, and 4) the spatial scale of the detection function describing capture probabilities (Efford 2004). Our frequent trapping schedule allows us to estimate population growth rates, sex and age composition, and apparent recruitment.
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Since 2008 we have been conducting an in-depth study of seasonal patterns of hare mortality and the sources thereof using radio telemetry. Over the initial 2 years of this study we have collared approximately 150 hares, and we try to maintain approximately 45-50 collared hares continuously on the trap grids. Survival of hares may be nearly 100% during the summer, but declines sharply during late fall when hares molt, food sources change, and the population, mostly composed of young-of-the-year individuals, face a new world with the onset of winter. Sources of mortality appear to be related to the density of vegetation. Thus, hares on the spruce grid are mostly killed by lynx, while those on the riparian grid which has less cover, are more often killed by avian predators such as goshawks.
Fig.6. Proportion of snowshoe hares surviving near the peak of the population cycle. Annual survival was derived from radio-collared individuals on the riparian (Rip) and Black spruce (Spruce) trapping grids, June 2008 - May 2009. |
As part of the snowshoe hare survival studies we also conduct investigations regarding lynx ecology. Whereas all the major predators on snowshoe hares (lynx, coyotes, goshawks, and Great Horned owls) are common in our study area, lynx appear to be the major mammalian predator on hares.
Fig. 7. Male lynx released in Bonanza Creek instrumented with ARGOS satellite collar. |
To gain a better understanding of how lynx use their habitat we capture them in cage traps and foot-snares and instrument them with transmitters that communicate with ARGOS satellites or via GPS technology. In Bonanza Creek, lynx typically use a core area of about 30-40 km2, but during breeding season males have been recorded to travel extensively, sometimes covering nearly 20 km in a single day.
Fig. 7. Locations of lynx in along the Tanana River based on ARGOS satellite data, showing a typical core area using during summer 2008 (left panel) and exploratory movements associated with breeding March 2009 (right panel). Note difference in scale between panels |
Relevant publications:
Kielland, K., K. Olson and E. Euskirchen. 2010. Demography of snowshoe hares in relation to regional climate variability during a 10-year population cycle in interior Alaska. Can. J. For. Res.
Angell, A. and K. Kielland. 2009. Establishment and growth of white spruce on a boreal forest floodplain: interactions between microclimate and mammalian herbivory. Forest Ecology and Management 258:2475-2480
Feng, Z., R. Liu, D. L. DeAngelis, J. P. Bryant, K. Kielland, F. Stuart Chapin, III, R. K. Swihart. 2009. Plant Toxicity, Adaptive Herbivory,
and Plant Community Dynamics. Ecosystems 12:534-547
Butler, L.G., K. Kielland. 2008. Acceleration of vegetation turnover and element cycling by mammalian herbivory in riparian ecosystems. Journal of Ecology 96:136-144.
Butler, L.G., K. Kielland, T.S. Rupp, and T.A. Hanley. 2007. Interactive controls of herbivory and fluvial dynamics over vegetation patterns along the Tanana River, interior Alaska. J. Biogeography 34:1622-1631.
Belant, G., K. Kielland, E.H. Follmann, and L. Adams. 2006. Interspecific resource partitioning in sympatric ursids. Ecological Applications 16:2333-2343.
Kielland, K., J.P. Bryant, and R.W. Ruess. 2006. Mammalian herbivory, ecosystem engineering, and ecological cascades in taiga forests. Pages 211-226, In: F.S. Chapin, III, M.W. Oswood, K. Van Cleve, L. Viereck, and D. Verbyla (editors), Alaska's Changing Boreal Forest, Oxford University Press, New York, NY.
Rexstad, E. and K. Kielland. 2006. Population dynamics of small mammals in interior Alaskan forests. Pages 121-132, In: F.S. Chapin, III, M.W. Oswood, K. Van Cleve, L. Viereck, and D. Verbyla (editors), Alaska's Changing Boreal Forest, Oxford University Press, New York, NY.
Kielland, K. 2001. Stable isotope signatures of moose in relation to seasonal forage composition: a hypothesis. Alces 37:329-337.
Kielland, K. and J.P. Bryant. 1998. Moose herbivory in taiga: effects on biogeochemistry and vegetation dynamics in primary succession. Oikos 82:377-383.
Kielland, K. and Osborne, T. 1998. Moose browsing on feltleaf willow: optimal foraging in relation to plant morphology and chemistry. Alces 34:149-155.
Ruess, R.W., R.L.,Hendrick, and J.P. Bryant. 1998. Regulation of fine root dynamics by mammalian browsers in early successional Alaskan taiga forests. Ecology 79:2706-2720.
Kielland, K., J.P. Bryant, and R. Ruess. 1997. Mammalian herbivory and carbon turnover in early successional stands in interior Alaska. Oikos 80:25-30.
Rossow, L.J., J.P. Bryant, and K. Kielland. 1997. Effects of above-ground browsing by mammals on mycorrhizal colonization in an early successional taiga ecosystem. Oecologia 110:94-98.
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