Living in the city: Resource availability, predation, and bird population dynamics in urban areas

Abstract

This article explores factors that shape population structure in novel environments that have received scant theoretical attention: cities. Urban bird populations exhibit higher densities and lower diversity. Some work suggests this may result from lower predation pressure and more predictable and abundant resources. These factors may lead to populations with few winners and many losers regarding access to food, body condition, and reproductive success. We explore these hypotheses with an individual-energy-based competition model with two phenotypes of differing foraging ability. We show that low frequency resource fluctuations favor strong competitors and vice versa. We show that low predation skews equilibrium populations in favor of weak competitors and vice versa. Increasing the time between resource pulses can thus shift population structure from weak to strong competitor dominance. Given recent evidence for more constant resource input and lower predation in urban areas, the model helps understand observed urban bird population structure.

Introduction

Rapid urbanization has become one of the major concerns in conservation ecology (Miller and Hobbs, 2002). Within 30 years, more than half of the world population is expected to live in cities (United Nations 2003). In recent years, urban ecology has received increasing attention from ecologists, anthropologists, and social scientists interested in understanding the influence of human activities on urban ecosystems and their biological communities (Grimm et al., 2000). Whereas urbanization normally leads to increasing population densities, species diversity is normally lower in cities than in adjacent wildlands (Marzluff, 2001). While these patterns are well documented, we know much less about the drivers of these patterns (Marzluff et al., 2001). Recently, Shochat et al. (2006) called for adopting a mechanistic approach to urban ecological research. Although field experiments would comprise the main component of such an approach, theoretical and mathematical models will play important roles in helping frame and test new ideas in urban ecology. The work presented here focuses on the development of such theory.

Cities may represent an excellent laboratory for comparative studies on population dynamics as urban areas provide large-scale examples of changing environmental conditions (Grimm et al., 2000, Collins et al., 2000). Urban habitats (areas dominated by built structures, i.e. more than 10 buildings ${\mathrm{ha}}^{-1}$ and at least 620 humans ${\mathrm{km}}^{-2}$) and suburban habitats (areas with 2.5–10 buildings ${\mathrm{ha}}^{-1}$) (Marzluff, 2001) are normally characterized by extremely high food resource abundance. Indeed, despite high stress induced by a range of factors associated with urban life such as noise (Katti and Warren, 2004), chemical contaminants (Burger et al., 2004), and high densities of domestic predators (Sorace, 2002), urban habitats throughout the world are characterized by high population densities of different taxa (Marzluff et al., 2001, Beckmann and Berger, 2003a).

These differences between population densities in urban and wild settings may simply reflect the predictions of the resource-matching rule (Parker, 1978) across large spatial scales, i.e. population density increases proportionately with resource density. However, based on results from field experiments on optimal foraging behavior (Bowers and Breland, 1996, Shochat et al., 2004a), Shochat (2004) suggested that, in terms of abundance, urban bird populations lay over the carrying capacity of the urban environment, resulting in lower average individual body condition. Thus, urban bird populations are relatively higher than the resource-matching rule would predict, and Shochat (2004) suggested this situation might persist as a long-term equilibrium. Our aim in this paper is to apply a mathematical model to explore potential factors that may facilitate persistent resource overmatching in urban areas.

Our investigation focuses on how changes in population structure (as defined by frequencies of phenotypes with differing competitive ability) induced by changes in resource dynamics and predation might potentially contribute to maintaining higher than expected population densities in urban areas. Specifically, temporal variation in urban environments may be low compared with wildlands (Beckmann and Berger, 2003a, Catterall et al., 1998, Shochat et al., 2004b). It has been suggested that this difference, that so far has been ignored, plays an important role in shaping urban population structure (Shochat, 2004). In what Shochat called the “Credit Card Hypothesis”, he suggested that low variability in resource abundance allows the persistence of many weak competitors who remain in poor body condition who are less reproductively successful and who would not otherwise survive. We develop and analyze a competition model to determine whether, and under what conditions, reducing variation in resource abundance can induce such a shift in population structure and, in turn, allow for sustained resource overmatching.

Predation, too, can have a strong effect on both inter- and intra-specific competition (Caswell, 1978, Mittelbach, 1986, Butler and Wolkowicz, 1986, Abramsky et al., 1998). While it is agreed that cities are richer in food resources than wildlands, there is still some debate whether predation decreases or increases in cities relative to wildlands (Gering and Blair, 1999, Thorington and Bowman, 2003, Blair, 2004). We believe that the weight of existing evidence suggests that predation pressure on adult birds is lower (see discussion below). Thus, since predation in urban environments is a key issue, we investigate how changes in predation pressure affect resource competition and population dynamics in general. Specifically, through the analysis of the model, we tease out the relative importance of competition for resources and predation on structuring urban populations and generate some specific predictions about population structure and individual body condition that may be tested through comparative studies of urban and non-urban populations.