No abstract

Alteration of wide‐ranging wildlife migrations can drastically impact the structure and function of ecosystems, yet the causes and consequences of shifting migration patterns remain largely unknown. Management decisions made in one portion of a landscape may induce spatial and temporal shifts of wildlife use in another, creating tension among private, state, and federal lands with varying missions. Recent declines in migratory behavior have initiated studies focused primarily on spring migration, but the timing and benefits of autumn migration have received limited attention due to the difficulty in assessing the extreme asynchrony in autumnal events, although nutrition during this time period is crucial to winter survival and reproduction. Here, we used five years of data from 73 female elk (Cervus canadensis) which utilize a landscape managed by 4 federal agencies, a state, and private landowners, to identify the driving factors behind the initiation of fall migration in two subpopulations, one of which migrates to a protected area where hunting is prohibited. Most elk departed summer range prior to frost or snow, with 67% of elk that used the protected area migrating prior to the onset of archery hunting season (1 September), preemptively avoiding risk, while no elk from the other subpopulation left prior to archery season. However, departure from productive summer range prior to frost or snow, nearly two months before vegetation senescence, led to an important tradeoff. Early migrants gave up 0.30% of late summer‐fall integrated NDVI (iNDVI) per day when they moved to the safety of the protected area, leading to an average difference of 15.81% in iNDVI between an individual departing on 30 August (the mean departure date) vs. an individual departing on 1 November. Our results suggest that in areas where migratory ungulates span multiple jurisdictions, the benefits of migratory behavior may be dramatically impacted by unevenly distributed anthropogenic disturbance. As this is a common scenario globally, our work highlights the urgent need to improve our understanding of subtle changes in migratory behavior, both spatially and temporally, which may erode the resilience of migration to future change.

Increases in Yellowstone National Park, USA, bison (Bison bison) numbers and shifts in seasonal distribution have resulted in more frequent movements of bison beyond park boundaries and development of an interagency management plan for the Yellowstone bison population. Implementation of the plan under the adaptive management paradigm requires an understanding of the spatial and temporal structure of the population. We used polythetic agglomerative hierarchical cluster analysis of radiolocations obtained from free‐ranging bison to investigate seasonal movements and aggregations. We classified radiolocations into 4 periods: annual, peak rut (15 Jul‐15 Sep), extended rut (1 Jun‐31 Oct), and winter (1 Nov‐31 May). We documented spatial separation of Yellowstone bison into 2 segments, the northern and central herds, during all periods. The estimated year‐round exchange rate (4.85‐5.83%) of instrumented bison varied with the fusion strategy employed. We did not observe exchange between the 2 segments during the peak rut and it varied during the extended rut (2.15‐3.23%). We estimated a winter exchange of 4.85‐7.77%. The outcome and effectiveness of management actions directed at Yellowstone bison may be affected by spatial segregation and herd affinity within the population. Reductions based on total population size, but not applied to the entire population, may adversely affect one herd while having little effect on the other. Similarly, management actions targeting a segment of the population may benefit from the spatial segregation exhibited.

Knowledge of Yellowstone bison (Bison bison) parturition patterns allows managers to refine risk assessments and manage to reduce the potential for transmission of brucellosis between bison and cattle. We used historical (1941) and contemporary (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) weights and morphometric measurements of Yellowstone bison fetuses to describe fetal growth and to predict timing and synchrony of parturition. Our method was supported by agreement between our predicted parturition pattern and observed birth dates for bison that were taken in to captivity while pregnant. The distribution of parturition dates in Yellowstone bison is generally right-skewed with a majority of births in April and May and few births in the following months. Predicted timing of parturition was consistently earlier for bison of Yellowstone's northern herd than central herd. The predicted median parturition date for northern herd bison in the historical period was 3 to 12 days earlier than for 2 years in the contemporary period, respectively. Median predicted birth dates and birthing synchrony differed within herds and years in the contemporary period. For a single year of paired data, the predicted median birth date for northern herd bison was 14 days earlier than for central herd bison. This difference is coincident with an earlier onset of spring plant growth on the northern range. Our findings permit refinement of the timing of separation between Yellowstone bison and cattle intended to reduce the probability of transmission of brucellosis from bison to cattle. JOURNAL OF WILDLIFE MANAGEMENT 69(4):1716-1730; 2005