Date of Award

Fall 2017

Project Type

Dissertation

Program or Major

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

First Advisor

John A Litvaitis

Second Advisor

Adrienne I Kovach

Third Advisor

Thomas D Lee

Abstract

By altering plant species composition and reducing plant species richness, invasion of non-native (alien) plants can reduce caterpillar abundance in bird habitats and simplify the bird community, but whether these cascading effects of alien plants extend further to affect bird productivity has not been quantified. This study is the first to quantify how reduced caterpillar abundance associated with alien plant invasion affects the reproductive success of a breeding passerine, the common yellowthroat (Geothlypis trichas trichas). The study was conducted in three transmission line rights-of-way where plant composition in yellowthroat territories was dominated by a near monoculture of alien shrubs (“ALIEN” site), by a mixture of native and alien shrubs (“MIXED” site), or by only native shrubs (“NATIVE” site). At each site, I estimated total caterpillar abundance in each yellowthroat territory based on the native and alien shrub species composition. I then determined if differences in shrub composition in territories affected the types of arthropods that adult yellowthroats fed nestlings, or if differences affected yellowthroat productivity or nestling growth, plasma carotenoid concentrations, and carotenoid-based plumage color.

Caterpillar abundance was lowest on the alien shrubs autumn olive (Eleagnus umbellata) and glossy buckthorn (Frangula alnus) and on native red maple (Acer rubrum) and northern arrowwood (Viburnum dentatum). Caterpillar abundance was greatest on native birch (Betula spp.) speckled alder (Alnus incana spp. rugosa), willow (Salix spp.), meadowsweet (Spiraea latifolia), and sweetfern (Comptonia peregrina). Differences in the relative abundance of the specific native and alien shrub species composing territories resulted in territories at the MIXED site supporting the greatest total estimated caterpillar abundance. Territories at the ALIEN site supported the lowest caterpillar abundance that was 62% and 75% lower than that at the NATIVE and MIXED sites, respectively. Shrubland bird richness at the MIXED and NATIVE sites was ≥ 2x that at the ALIEN site, indicating that most shrubland birds avoided the ALIEN site for breeding. Differences in caterpillar abundance among sites did not result in difference in yellowthroat productivity, but adults at the ALIEN site fed nestlings a lower proportion of caterpillars and increased their foraging effort to feed nestlings a greater proportion of alternative prey (e.g., bees/flies, butterflies/moths, odonates, and spiders) than adults at the other sites. Caterpillar availability to birds seemed to be reduced only where alien shrubs grew abundant enough to reduce both the abundance and diversity of native shrubs within a shrubland. Overall, nestling growth, plasma carotenoids, and plumage color score were greatest at the NATIVE site where nestlings were fed a large proportion of caterpillars and an intermediate proportion of alternative prey, and lowest at the MIXED site where nestling diets were composed of a large proportion of caterpillars and the lowest proportion of alternative prey. Conversion of shrublands from native shrubs to near-monocultures of alien shrubs may equate to habitat loss for shrubland-dependent passerines that are unable to adapt to conditions of low shrub diversity and low caterpillar abundance. Species such as common yellowthroats that readily feed nestlings grasshoppers and spiders may experience no reduction in annual reproductive success in habitats where alien shrubs reduce caterpillar abundance, but elevated foraging costs of adults may result in negative fitness effects that can only be identified by monitoring the same individual birds over multiple breeding seasons.

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