The complexity of woody encroachment and its effects on the tolerance thresholds of grasslands songbirds

Background/Question/Methods

Across the globe, grasslands are experiencing a loss of ecosystem services due to numerous threats, such as overgrazing, conversion to agriculture, and woody encroachment. As a result of these changes, 73% of grassland birds in the Great Plains are experiencing population declines. In North America, an estimated 20% of grasslands remain undeveloped, however, these areas continued to be threatened by woody plant encroachment. In fact, woody plant encroachment is a prevalent concern for grasslands worldwide and one often discussed in literature as a major threat to grassland songbirds. However, current management practices are having limited success combatting woody encroachment on a large scale. Further research on how woody encroachment effects grassland species is needed to focus management practices and maximize the effectiveness of management efforts. In this study, we investigate the role different height classes of woody vegetation play in influencing grassland bird occurrences and determine potential threshold responses of grassland birds. We conducted point counts for three years at four Wildlife Management Areas in western Oklahoma followed by vegetation sampling. We identified thresholds and zones of change in species abundance and frequency using three different height classes of woody vegetation to determine if different woody plant structures on the landscape alter the responses of grassland birds.

Results/Conclusions

Our results indicate contrasting responses by individual bird species dependent on woody vegetation height. Threshold points also varied between height classes for individual species. These results suggest that not all woody vegetation is the same and should not be considered equally when developing management strategies. The knowledge gained from this project will help to identify landscapes where current management practices could be maximized.