Hierarchical habitat selection by the Acadian Flycatcher: implications for conservation of riparian forests
Marja H. Bakermans, MS
Advisor: Amanda Rodewald
Riparian forests represent critical habitat for forest birds throughout midwestern United States, yet they continue to be fragmented and degraded due to agricultural practices and urban development. Knowledge of the ecology and management of riparian communities comes primarily from studies conducted at small spatial scales. However, recent work demonstrates that the surrounding matrix can affect diversity, abundance and reproductive success of breeding birds within a forest patch. In this thesis, I used the Acadian Flycatcher (Empidonax virescens), a species of concern in the Midwest, to examine multi-scale impacts of land uses on riparian forests. Habitat selection by Acadian Flycatchers was investigated at three distinct scales: territory (1 ha), stand (3-5 ha), and landscape (314 ha) in 36 riparian forests in central Ohio. I addressed potential mechanisms behind landscape-scale patterns by examining understory arthropod biomass, vegetation structure, and breeding productivity across a spectrum of rural and urban land uses.
Acadian Flycatchers were surveyed three times per year during June 2001-2002 in mature deciduous riparian forests along an urbanization gradient. Landscape metrics (e.g., % urban cover and riparian forest width) were quantified within 1 km of each study site (e.g., % urban cover range = 0-53%; forest width range = 56-565 m) while vegetation characteristics and understory arthropod biomass were measured within each riparian forest and Acadian Flycatcher territory. In addition, I target-banded 11 male Acadian Flycatchers to map territories and monitor productivity. Habitat selection at the territory scale was evaluated by comparing features measured within the territory with average stand values. Two sets of a priori candidate models were developed representing plausible factors affecting abundance and productivity of Acadian Flycatchers at both the stand and landscape scale. Candidate models were ranked based on Akaike’s information criterion (AICc).
Results suggest that the criteria used by Acadian Flycatchers to select habitat change with spatial scale, suggesting that this species selects habitat in a hierarchical manner. Percent urban cover in the landscape best explained abundance and productivity of Acadian Flycatchers. Both abundance and productivity of Acadian Flycatchers were negatively associated with percent urban land cover while numbers of predators increased with urban development. Acadian Flycatchers were 3 times more likely to be detected in more rural (£ 1% urban development) riparian forests compared to more urban (≥ 10% urban development) riparian forests. Interestingly, as urban cover in the landscape increased, Acadian Flycatchers selected wider riparian forests than those selected in rural landscapes. Vegetation structure and understory arthropod biomass had little explanatory power at the stand scale but were central in territory selection. Vegetation density was lower and understory arthropod biomass was greater within the territory than throughout the stand.
In short, I believe that increased numbers of predators coupled with reduced breeding productivity and site fidelity in urban riparian forests may be an underlying mechanism affecting Acadian Flycatcher landscape patterns. Therefore, my work suggests that planning at the landscape scale is one of the most important factors for Acadian Flycatchers in riparian forests in central Ohio. Specifically, riparian forests in rural areas are preferred by Acadian Flycatchers and should be safeguarded. I suggest that midwestern riparian forests in rural landscapes be ≥ 100 m wide while riparian forests in urban landscapes should be ≥ 150 m wide. Next, manipulating vegetation characteristics within the stand can further enhance habitat quality for Acadian Flycatchers. For instance, removal of invading exotic shrubs may create a relatively open understory to accommodate multiple territories for Acadian Flycatchers and other avian species. This study is the first to examine multi-scale habitat selections of a Neotropical migrant across rural and urban landscapes. Further research is needed to (1) determine if manipulations of selected vegetation features affect habitat selection and productivity, (2) investigate if predator control increases selection of urban riparian forests, and (3) determine if conspecifics influence habitat selection of Acadian Flycatchers.