The Sawtooth National Forest (SNF), with its discreet forest segments and unique flora and fauna, presents unique challenges to sensitive species. Changing forest structure due to harvesting, fire management, grazing, juniper encroachment, pathogens, and climate change, coupled with increased human recreational usage, could further threaten these species. By studying population health indicators in a top predator, the impact of changes may be revealed before large scale population declines occur.
The Northern Goshawk (Accipiter gentilis; hereafter ‘goshawk’) has historically maintained a strong population within the SNF. This forest predator is currently listed as a sensitive species by the Forest Service and there is insufficient data to determine the trend of the population. This lack of population trend data and the perceived risk to the population from the disturbances mentioned above has created a renewed interest in better understanding the ecology of the goshawk within the SNF.
The SNF is comprised of a patchwork of various mixed conifers and Aspen that together make up only 20% of the shrub-steppe dominated landscape3. The SNF classifies many of these forest structures as not functioning correctly or at risk, due to insects, disease, and fire management practices. This naturally distributed patchwork of forest habitat surrounded by non-forested rangeland could put forest species’ populations at risk due to the principles of island biogeography. The primary food base associated with successful reproductive rates in goshawks across their range, squirrels of the genera Scuirus and Tamiasciurus, is absent from two major sections of the SNF. The implementation of successful conservation measures within the SNF requires more specific knowledge of how goshawks utilize and are dependent upon the unique flora, fauna, and changing structure of this forest habitat.
Research Questions
My research will focus on a set of known historic and newly found goshawk nest locations within the SNF. The core research questions are the following:
1. What prey do goshawks depend upon in the South Hills? Dietary analyses of goshawks have been performed in many parts of the world and have consistently found a diet dominated by mammals, both numerically and by biomass6. They have further shown the top prey item to be tree squirrels of the genera Scuirus and Tamiasciurus6. As mentioned earlier, these genera are absent from the South Hills. It is important for this study and management in general, to establish the dietary composition the South Hills goshawk population requires. I hypothesize that mammal biomass consumed by goshawks is consistent with other studies, and is fulfilled by greater consumption of ground squirrels. Diet will be quantified using nest cameras installed in eight occupied nests.
2. Does prey abundance affect goshawk site selection and/or nest productivity? If so, which prey elements are the key determining factors? Prey densities have been shown to have a significant influence on the variation in goshawk reproduction7. An Arizona study found that four species, two tree squirrel species and two avian species, accounted for 89% of the variation in goshawk reproduction. As goshawks compensate for the lack of tree squirrels, it is important to determine which species have the greatest impact on productivity. I hypothesize the same avian species will have important influence, but ground squirrel abundance will become the most significant factor influencing nest productivity. Alternatively, large avian species such as grouse may have the potential to provide sufficient biomass to strongly influence productivity. Prey abundance will be measured using distance sampling methods. Abundance by species will be analyzed against nest productivity measured through observations.
3. Do habitat structural changes and disturbance affect nest productivity? Due to the lack of trend data on the population as cited above, establishing a baseline productivity measure for comparison to historic and future surveys is important to estimate the overall health of the population. Segmenting productivity by adult age, prey base, habitat quality and disturbance measures will help identify the most significant management variables to focus on9. I hypothesize that territories with more stable forest structure, with low natural and human disturbance, will have higher productivity than those in transitional and heavily-used forests. Stability per nest site will be established using GIS and ground-truth measurements in each nest area versus average forest control sites.
4. Does habitat structure or prey abundance affect the sex ratio of successfully fledged young? Assuming that productivity varies with habitat quality, analyzing the sex ratio of successful offspring can help determine to what degree sex ratio is random (moves toward 1:1), based on diet or weather, or another life history strategy. A male dominated sex ratio could be an early indicator of population stress. I hypothesize that diet and habitat quality will be the most significant factors influencing the sex ratio of successful fledglings; in other words, stable habitats with high prey abundance will fledge a balanced sex ratio, where lower quality habitats will fledge a male-dominated sex ratio. Sex ratio will be determined by morphological measurements of nestlings and validated with DNA techniques utilizing feather samples.
Conclusions
Addressing these key research questions will enable more effective identification and prioritization of conservation measures for the greatest conservation impact. It will also provide a detailed picture into the current state of the population including occupancy, nesting success, and productivity.