Theses and Dissertations at Montana State University (MSU)
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Item Long-term response of willow to beaver reintroduction and moose browsing in the southern Absaroka-Beartooth Wilderness(Montana State University - Bozeman, College of Agriculture, 2022) Dines, Rachael Helen; Chairperson, Graduate Committee: Bok SowellWillow is an important component in the southern Absaroka Beartooth Wilderness (ABW) riparian areas because it provides critical beaver habitat and is an essential food source for moose populations. Forest Service managers have monitored long-term trends in willow condition and herbivore populations in the southern ABW. Beavers were extirpated in the mid-1900's and then reintroduced beginning in 1986. Moose numbers increased in the early 1900's and willow over-browsing followed until the population declined after habitat loss from the 1988 Yellowstone fires. The objective of this study was to examine willow condition in response to changes in beaver and moose populations in the southern ABW. I used annual stream-side surveys to record the number and location of beaver colonies from 1986 to 2021 and aerial imagery to describe changes in willow canopy cover. I used a binomial regression to evaluate the relationship between willow canopy cover and beaver colony density and longevity to estimate if long-term beaver presence increased willow canopy cover. I used linear regression to analyze indices of moose relative abundance, willow plots recording height and browsing, and a long-term browsing exclosure to evaluate the effect of moose population trends on willow height. Beaver have remained at carrying capacity over the last 20 years, and average willow canopy cover increased from 16% in 1981 to 37% in 2019. The probability of willow occurrence increased the longer beaver occupied an area and with colony density (colonies/km). Moose relative abundance declined from 1987 to 2021, while average willow height increased from 113cm in 1988 to 190cm in 2021 and browsing decreased from 53% in 1989 to 3% in 2021. Browsing was an important factor limiting willow height. Moose abundance had a positive linear relationship with average browsing pressure and negative linear relationship with average willow height. This study shows that the southern ABW was able to recover from historic over-browsing and beaver extirpation. The long-term effects of beaver reintroduction and reduced moose numbers in a historically degraded environment enhanced willow, indicating that under the right circumstances, willow communities have the potential to recover from a previously degraded state.Item Ecosystem pioneers: beaver dispersal and settlement site selection in the context of habitat restoration(Montana State University - Bozeman, College of Agriculture, 2018) Ritter, Torrey Daniel; Chairperson, Graduate Committee: Lance McNewThe activities of beavers (Castor canadensis) provide a variety of benefits to stream systems by capturing and storing water and sediment, expanding riparian areas, and increasing habitat heterogeneity. Understandably, land and wildlife managers are interested in using beavers as a habitat restoration tool. However, streams targeted for restoration are often degraded and lack recent beaver activity, and therefore represent suboptimal habitat. The habitat selection process for beavers in suboptimal and unmodified habitats provides a natural analogue to beaver restoration, but the process is not well-understood. I radio-marked juvenile beavers and conducted beaver-use surveys in tributary streams of the Gallatin and Madison River drainages in southwest Montana to investigate dispersal, survival, and settlement site selection by beavers colonizing novel areas. My objective was to study beaver ecology in the context of beaver restoration to improve identification of suitable project locations. Beaver colony densities in the study area were low or average, though colony densities in suitable habitat were generally high. There was evidence of delayed dispersal, and as the density of active beaver colonies increased the probability of dispersal decreased. Radio-marked beavers that dispersed settled quickly and dispersal distances were highly variable. Most beavers settled in active colonies or other beaver-modified habitats, and colonization of unmodified stream segments was rare. My top habitat selection models indicated new settlement sites were located in stream segments characterized by low gradients, dense woody riparian vegetation close to the stream, and relatively narrow stream channels. Stream channels at new settlement sites were more variable both in cross-sectional and longitudinal depth and were more heavily influenced by secondary channels than unsettled sites. My results suggest beavers select for pre-engineered habitat over unoccupied stream segments, and in novel areas habitat conditions that facilitate stable dam construction appear most important. When choosing project locations, restoration practitioners should consider local beaver colony locations and densities to assess the potential for dispersers to reach the restoration site. Stream segments that provide dam resiliency and hiding cover should be targeted for initial restoration efforts, and pre-engineering of habitat prior to beaver occupancy may increase the probability of successful colony establishment.Item Effects of beaver reintroduction and ungulate browsing on aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range(Montana State University - Bozeman, College of Agriculture, 2013) Runyon, Molly Jean; Chairperson, Graduate Committee: Bok SowellUngulate browsing and lack of overstory disturbance have historically prevented aspen regeneration on the Northern Yellowstone Winter Range (NYWR). Aspen clones regenerate if sprouts are produced that grow into recruitment stems (>2 m tall) and replace the mature overstory. Beaver were reintroduced to the Eagle Creek drainage on the NYWR in 1991 in an attempt to facilitate recovery of riparian aspen communities by removing aspen overstory and increasing sprouting. However, intense ungulate browsing, primarily from the Northern Yellowstone elk herd, was preventing aspen recruitment in Eagle Creek in 2005. Wolf predation has contributed to a 56% decrease in this elk herd from 2005 to 2012. I investigated the effects of beaver reintroduction and ungulate herbivory on aspen recovery in the Eagle Creek drainage in 2012. Aerial photos taken of Eagle Creek in 1990, 2005, and 2011 showed that although beaver activity stimulated aspen sprouting, the mature overstory of many aspen stands has not been replaced 21 years after beaver reintroduction (p>0.05). Sprouting and recruitment were investigated using 4-m radius circular vegetation plots (n=31) established in aspen stands throughout Eagle Creek in 1997 and monitored annually until 2012. Beaver activity stimulated increased sprouting in 71% of these plots, and 77% of the plots had > or = 1 recruitment stem in 2012. Prolonged flooding and high browsing levels contributed to lack of recruitment in 23% of the plots (p<0.05). In 2012, 75% of the paired plots associated with aspen exclosures had unfenced aspen stems with an average stem height > or = 2 m. Recent increases in aspen recruitment in Eagle Creek indicate that aspen communities are regenerating. This is likely the result of decreased browsing pressure on aspen saplings from 2005 to 2012. These findings are consistent with the predictions of a density-mediated trophic cascade following wolf reintroduction.Item Restoring aspen riparian stands with beaver on the northern Yellowstone winter range(Montana State University - Bozeman, College of Agriculture, 2007) McColley, Samuel David; Chairperson, Graduate Committee: Bok F. Sowell.Aspen (Populus tremuloides) on the Gardiner Ranger District, Gallatin National Forest, have declined over the last half-century. In an attempt to reverse this trend, beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991. Beaver promote aspen suckering through their dam and lodge building activities. In 2005, I assessed the long-term effects of beaver on aspen stands and the associated riparian area in the Eagle Creek Drainage. Aerial photographs taken in 1990 and 2005 were used to compare changes in riparian area vegetation where beaver were reintroduced. Aspen canopy cover decreased (P<0.05) from 43% to 25% on Eagle Creek (29 ha) between 1990 and 2005. Willow (Salix spp.) cover increased (P<0.05) from 10% to 14% and alder (Alnus incana) cover and water surface area doubled during the same period. Aspen recovery was estimated by comparing vegetative changes among control sites with <10% beaver use (n = 5), active beaver sites (n = 6), sites abandoned for 1-3 years (n = 7), sites abandoned for 4-6 years (n = 4), and sites abandoned for 7-11 years (n = 5).