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133 6 Eco-Evolution on an Urban Planet What role do humans play in the evolution of Earth? Can the emergence and rapid development of cities change the course of Earth’s evolution? Can they determine the probability of crossing thresholds that will trigger abrupt change on a planetary scale? A great challenge for urban ecology in coming decades is to understand the role humans play in eco-evolutionary dynamics. If, as emerging evidence shows, rapid evolutionary change affects ecosystem functioning and stability, current rapid environmental change driven by urbanization might have significant implications for ecological and human well-being on a relatively short time scale. Humans are major drivers of microevolutionary change. At the same time, novel interactions between human and ecological processes may produce unprecedented expressions and opportunities for innovation. Understanding the mechanisms by which cities mediate eco-evolutionary feedback will provide important insights into how to maintain ecosystem function on an urbanizing planet. Increasing evidence shows that cities are driving significant changes in wildlife, including plants, fungi, and other organisms. Songbirds are becoming tamer and bolder in response to the novel urban environments (Atwell et al. 2012) and are changing their tunes to keep urban background noise from masking their acoustic signals (Desrochers 2010; Dowling, Luther, and Marra 2011). Recent studies indicate that spiders are getting larger and even doing better in cities than in their natural habitats (Lowe, Wilder, and Hochuli 2014). Fish have adapted to cope with poisons in urban waters (Whitehead et al. 2010), while earthworms are well able to tolerate contaminants in soil (Kille et al. 2013). Seeds 134 Chapter 6 produced by weeds that grow in our parking lots do not travel very far compared to those growing in the countryside (Cheptou et al. 2008). Humans are selective agents that determine which species can live in cities and cause organisms to undergo rapid evolutionary change. The urban habitat is not simply altering biodiversity by reducing the number and variety of native species. Urban-driven changes in biodiversity are more subtle and, to a certain extent, unexpected. Many organisms, including insects, birds, fish, mammals, and plants, are adapting to the new environment by changing their physiology, morphology, and behaviors. Microevolutionary change is taking place not only in tropical forests, but also in our backyards. The challenge for urban ecologists is to determine whether these changes might affect ecosystem function at the planetary scale. This question can be answered only by significant studies over the long term. Yet building on the emerging evidence, it is possible to articulate hypotheses linking urbanization to rapid evolution and to examine the role that urbanization could play in evolutionary dynamics. Expanding the New Synthesis Eco-evolutionary feedbacks—reciprocal interactions between ecological and evolutionary dynamics on contemporary time scales—were hypothesized over half a century ago (Pimentel 1961), but only recently have they been tested empirically (Schoener 2011). There is significant evidence that changes in ecological conditions drive evolutionary change in species traits that, in turn, alters ecological interactions (Endler 1986; OdlingSmee , Laland, and Feldman 2003). Yet despite the remarkable progress in studying eco-evolutionary feedbacks over the past decade, empirical studies are still limited, and potential implications for environmental change and the evolution of species are only beginning to emerge (OdlingSmee , Laland, and Feldman 2003; Post and Palkovacs 2009; Stockwell 2003). In particular, we do not know what role human activity plays in reciprocal interactions between ecological and evolutionary processes. Earlier assumptions about the different time scales of ecological and evolutionary processes have shaped the unidirectional character of most empirical eco-evolutionary studies and can partly explain our lack of curiosity about the human role in shaping the evolutionary trajectory of Planet Earth. But recent evidence that significant evolutionary change Eco-Evolution on an Urban Planet 135 does occur on a short time scale urgently challenges both ecologists and evolutionary biologists to redefine the dynamic interplay between the two fields and to understand interactions between human agency and ecoevolutionary feedback across different levels of biological organization. Humans are major drivers of microevolutionary change (Hendry, Farrugia, and Kinnison 2008; Palkovacs et al. 2012). In human-dominated environments, selection pressures acting on traits can affect population dynamics by changing organisms’ rates of survival or reproductive success , leaving a genetic signature that might affect community dynamics and ecosystem functions (J. N. Thompson 1998). Phenotypic trait changes resulting from changes in gene frequencies might affect population dynamics through changes in demographic rates (Pelletier, Garant, and Hendry 2009). Genetic signatures have been observed...


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