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Disentangling the Predation Paradox: Human Food Subsidies, Elevated Predator Densities, and Non-lethal Effects of Predators Along an Urbanization Gradient

Principal Investigator/Project Leader: 
Department of Project: 
Environmental Conservation Dept.
Project Description: 

Predation is considered a key limiting process, and management actions for declining species, such as forest-dwelling songbirds, are often aimed at reducing impacts of predation. This is of particular concern in areas undergoing urbanization, since densities of potential predators tend to increase dramatically with urbanization. However, an increasing number of studies report no difference in predation rates in urban versus wildland areas, despite these increases in potential predators - a phenomenon sometimes called the 'predation paradox.' Thus, in forests along urban to rural gradients, it remains unclear: should management focus more on enhancing habitat, controlling predators, controlling basal resources, or combined approaches? We aim to address this question by investigating two key hypotheses that may account for the so-called predation paradox. Hypothesis 1: One oft-cited explanation for the decoupling of predation rates and predator densities is that predators haveshifted their diets toward human-provided food resources, releasing songbirds from predation. But the assumption that highly abundant predators in cities are eating human-provided food is rarely tested. Building on our previous study of forest-nesting songbirds and their predators in western Massachusetts, we will use stable isotope analysis of predator diets to test the hypothesis that potential nest predators are consuming greater quantities of human foods with increasing levels of urbanization in the surrounding landscape.

Hypothesis 2: Another possibility is that not all the negative impacts of predators are captured by examining predation rates. A species can be negatively impacted by predators through either direct predation or non-lethal effects (NLEs). NLEs are any change in an individual's behavior caused by "fear" of predation. Birds often adjust activities such as nestling feeding rates on short time scales in response to changes in perceived predation risk. Systematic reviews have found that avian species in urban areas tend to produce smaller and lighter offspring than their wildland counterparts. This can lead to reduced fledgling survival in areas with high human-development. We hypothesize that NLEs may be an important mechanism producing lowered nestling condition with urbanization. We will test this hypothesis by experimentally altering predation risk environments through playbacks to House Wren (Troglodytes aedon) nests in nest-boxes across an urban-to-wildland gradient. Previous work with house wrens and other cavity nesting species has established that non-lethal effects can be induced experimentally through playbacks, but these experiments have not been carried out along urban to rural gradients. While testing these two hypotheses does not exhaustively address the potential explanations for the predation paradox, it does examine key mechanisms that have rarely been addressed in the literature on urbanization and biodiversity. The outcomes of these two interconnected studies will provide information to assess whether actions to reduce predator densities would be an effective means of managing urbanizing forests for declining songbird species.

Goals / Objectives

In our study, we aim to provide insight into how urbanization alters the effects of predators on declining forest bird species. The project aims to use both observational and experimental methods across an urbanization gradient in western Massachusetts in order to:

Objective 1: Test whether diets of typical nest predators shift toward human provided food resources

Objective 2: Conduct a manipulative experiment of non-lethal effects of predators on prey reproductive fitness. Together with our previous work, these two objectives will contribute new insights into the mechanisms by which human food subsidies and elevated predator densities affect the productivity and success of nesting songbirds in areas impacted by urbanization. Resolving these issues will aid in decisions about managing declining forest bird species by addressing whether actions aimed at reducing predator densities will have positive effects on songbird productivity.

Environmental Conservation topics: