Retracted: Consumer‐resource interactions along urbanization gradients drive natural selection*

Abstract

Urbanization is an important component of global change. Urbanization affects species interactions, but the evolutionary implications are rarely studied. We investigate the evolutionary consequences of a common pattern: the loss of high trophic-level species in urban areas. Using a gall-forming fly, Eurosta solidaginis, and its natural enemies that select for opposite gall sizes, we test for patterns of enemy loss, selection, and local adaptation along five urbanization gradients. Eurosta declined in urban areas, as did predation by birds, which preferentially consume gallmakers that induce large galls. These declines were linked to changes in habitat availability, namely reduced forest cover in urban areas. Conversely, a parasitoid that attacks gallmakers that induce small galls was unaffected by urbanization. Changes in patterns of attack by birds and parasitoids resulted in stronger directional selection, but loss of stabilizing selection in urban areas, a pattern which we suggest may be general. Despite divergent selective regimes, gall size did not very systematically with urbanization, suggesting but not conclusively demonstrating that environmental differences, gene flow, or drift, may have prevented the adaptive divergence of phenotypes. We argue that the evolutionary effects of urbanization will have predictable consequences for patterns of species interactions and natural selection.

Urbanization reshapes a food web

“…Indeed, trophic extensions of island biogeography theory are consistent with patterns of predator loss observed in many urban ecological communities (e.g. Start et al, ). Beyond dichotomies between predators and their prey, specialist predators are particularly likely to be lost from fragmented urban environments (Gravel et al, ).…”

“…Tall goldenrod is a common old‐field plant in much of Eastern North America and supports a diverse community of herbivores (including gall‐forming insects) and their specialist enemies (Root, ; Start et al, ). In this study, we focus on a relatively well‐understood and readily quantifiable portion of this food web—stem gallers and their enemies, including both bird predators and specialist parasitoids.…”

“…For instance, plant–pollinator interactions may become more or less frequent (Irwin, Warren, Carper, & Adler, ), competitive interactions can be rearranged (Francis & Schindler, ), and patterns of parasitism can be altered (Theodorou et al, ) by urbanization. More generally, it has been repeatedly demonstrated that high trophic‐level species tend to be absent from heavily urbanized environments (Figure ; Faeth, Warren, Shochat, & Marussich, ; Raupp, Shrewsbury, & Herms, ; Start et al, ), possibly because of an increased sensitivity of predators to fragmentation (Gravel, Massol, Canard, Mouillot, & Mouquet, ). Indeed, trophic extensions of island biogeography theory are consistent with patterns of predator loss observed in many urban ecological communities (e.g.…”

“…Among the many differences between urban and rural environments, cities are typically warmer, more homogeneous and less permeable, creating novel environmental conditions and reshaping key biological patterns (Alberti, ; Johnson & Munshi‐South, ; Johnson, Thomson, & Saini, ). Indeed, urban environments tend to support depauperate ecological communities, with some species being more sensitive to urbanization than others (Alberti, ; Start, Bonner, Weis, & Gilbert, ). However, while the responses of individual species (e.g.…”

“…Beyond the dependence of one species on another for sustenance, sampling effects alone may make some species more likely to occur in urban environments than others. Regionally rare species (including most high trophic‐level species) may not be present due to chance alone, because rare species are less likely to colonize patches in urban areas (Figure ; Start et al, ; Start & Gilbert, ). This presents the intriguing possibility that urban food webs may be depauperate in general and specifically lack high trophic‐level and rare taxa (Figure )—species which typically interact only weakly and with few other members of ecological communities (McCann, ).…”