We perform science for a number of reasons. In the medical field the focus may be on understanding the methods used by pathogens and how to combat those agents or evaluating the health impacts of contaminants. In the ecological field we often focus on the mechanisms operating within an ecosystem to better understand the natural processes and the impact of humans on those processes. This knowledge can help us to predict trends within a population, but also to recommend management actions which address the underlying influences on those trends instead of simply focusing on the trend itself.
My thesis research on Northern Goshawk Breeding Ecology includes a few of these objectives. The primary purpose of the study is to establish management recommendations which can help ensure that the ecosystem health is maintained. This health is balanced upon and measured by the presence of a strong apex predator population, one of which is the Northern Goshawk. To establish these recommendations we must first advance our knowledge of the species and how it uses this unique forest habitat. This knowledge advancement is the core of my thesis research.
The context of my study area forest is very important to this prioritization. In most forests the Northern Goshawk's role may not be as critical as in the South Hills of the Sawtooth National Forest. In the South Hills the forest is very heavily naturally fragmented and geographically isolated from other forests. The result, through the ecological function of island biogeography, is lower species diversity. Species diversity on islands is related to the size of the island and the distance to the next island or mainland. The South Hills forest is essentially an island of forest habitat removed from other islands by a sea of sagebrush, grassland, agricultural fields, and human development. With lower species diversity, the role of each species in the ecosystem can be amplified and the stability of the ecosystem much more easily disrupted by external forces either natural or anthropogenic (human caused). A side effect of this "island" influence, and one reason why my thesis is novel, is the natural absence of tree squirrels within this forest. It is estimated that tree squirrels disappeared from the area at least 10,000 years ago through unknown causes. Tree squirrels have been shown to be the top dietary choice of Northern Goshawks around the world and their abundance being the most influential factor in Northern Goshawk nest success.
My preliminary results show that during the core breeding season goshawks are consuming at least 2/3 of their diet in Belding's Ground Squirrels. This may not seem like such an significant shift, until you realize that unlike tree squirrels, Belding's Ground Squirrels are only available for 3-4 months of the year. They are still hibernating when nest site selection is made by breeding goshawks and they estivate (summer hibernation) before the goshawk fledglings have become independent. This explains why my second finding indicates that avian prey abundance and not squirrel abundance is the most influential on goshawk nest occupancy in the South Hills. In fact, mammalian abundance was dropped from all of my top predictive models. This is not to say that they are not critical for success, I believe they are, but simply that their abundance is not a good predictor of nest occupancy.
But lets get back to the management recommendations. Within my annual report I was asked to provide a list of management recommendations. While there are no expectations that they all will be fully implemented, I still took this responsibility very seriously. It would be easy to say that all human influences and disturbance should be removed. This would not only be ignored, it does not represent what my study findings show. I believe in wild untouched places and truly wish that we had more of them available. However, humans have touched and influenced nearly every place on earth. If we are not willing to sit back for 10,000 years just to see what happens, our previous influence warrants continued management in many areas. The South Hills are no exception. When I began this exercise I was expecting a much stronger view against disturbance and active management, but surprisingly that is not what I recommended.
Disturbance: Disturbance has been widely shown in many studies of many birds to have a detrimental effect on nest productivity. However, many of the South Hills goshawks have been well habituated to noise. One nest is just 50m from the busiest campsite in the area and is successful most years. Another has an ATV dead-end turn around at the base of the tree. Many nests are within 200m of a road or ATV trail. This speaks more to the number of roads and trails than to the goshawks preference for these roads. Some of the most successful nests were in high disturbance areas. However, the two goshawk nests that we discovered that failed were both subject to high disturbance (anthropogenic and natural). The first was within 100m of the busiest road in the South Hills and the other had a herd of elk calving at the base of the tree when we found it abandoned. Not sure either was the cause of the failure. The most significant act of disturbance that we observed was the result of firewood collectors cutting down a nest tree! The nest was not occupied at the time, but had been improved this season so it was still a relevant resource for the population. In the end, the only management actions that I have recommended to address disturbance is blocking the dead-end ATV trail a bit further from the tree and labeling nest trees to prevent accidental cutting. Both of these have been accepted and will be implemented as soon as we gain access to the area this spring.
Forest Succession:The largest anthropogenic disruption in the South Hills is the disturbance of the fire regime. Humans have been very successful in preventing fire in this natural landscape. The results is that much of the forest, especially Aspen, is the same age with a similar structure. Species diversity in general is dependent upon a mosaic of age classes and structures. Goshawks to date have generally benefited from this action as they prefer late stage forest structure (mature trees with relatively open understory). However, most of the mature Aspen in the South Hills are reaching the end of life. They are regenerating, but these regenerated forests will not be available to goshawks for decades. Most all of the goshawk nest stands that are Aspen are over 50% dead. Hence there is a predicted looming gap between the death of the stands available today and the emergence of new regenerated stands. In a natural fire regime, stand succession would not be synchronized and thus stands would always be available in all of the separate successional stages. The management recommendation here is to not only introduce fire to the habitat, or at least not be so aggressive when extinguishing natural fire, but also to accelerate forest succession through thinning. Thinning a young stand of trees can accelerate the development of that stand into a mature stand by shortening the development time by up to a decade. Thinning is not new in the South Hills; three of the successful nests last year were located in stands which had been thinning during the past decade. Thinning is already within the forest service budget, this just shifts the choice of stands to those that could also benefit goshawk success. Once again, the goal is not to return all stands to mature, but to disrupt the age synchronization currently pervasive in the area.
|Aspen stand in transition - unfriendly to goshawks.|
|Previously thinned Lodgepole Pine stand (open understory) - friendly to goshawks|
Prey abundance: One of the reasons that the goshawk is a good local management indicator species is that they are a generalist predator. They are dependent and consume a broad range of species including mammals and birds. Managing the forest for these species usually requires work to increase the health in general. Goshawks do feed heavily on cavity nesting birds such as woodpeckers. Cavity nesting birds are heavily dependent upon diseased, dying, and dead trees. Too often we remove as many dead trees as we can, possibly limiting this resource. I recommended not removing standing dead trees located within or near goshawk nest stands.
The surprising end result is a collection of relatively painless management actions which could help improve the ecosystem services in the South Hills - painless restrictions on a few ATV trails, labeling nest trees to prevent accidental destruction, thinning of some new stands to accelerate forest succession, and leaving standing dead trees in existing stands. We will see what insights my second field season brings. I eagerly await my return to the field.