Trophic structure in a rapidly urbanizing planet

Abstract

The human population is rapidly urbanizing, and the negative impacts of urban cover on biodiversity and ecosystem function are expected to increase. Trophic dynamics have been hypothesized to change with urbanization, with consequences for biodiversity and function. Here, I review recent progress in this area by focusing on how urbanization affects dietary sources, trophic interactions and the functional ecology of synanthropic species. Urbanization affects primary autochthonous production in terrestrial and aquatic ecosystems by replacing and fragmenting natural areas with impervious cover, increasing nutrient supply, changing hydrological regimes, and altering the composition and seasonality of primary producers. The responses of primary production differ between climatic regions or across hydrological regimes. Urbanization can also change the availability of subsidies (autochthonous vs. allochthonous resources) because many urban species feed on human food (anthropogenic subsidies) and because of changes in the plant composition and physical characteristics of riparian zones. Urbanization can change the composition of consumers by decreasing the abundance of apex predators, releasing mesopredators, as well as the introduction of non‐native omnivores. Few direct experiments have been conducted on trophic interactions in urban ecosystems. They broadly suggest that urbanization weakens herbivory and predation, but that it might increase competition between synanthropic and urbanophobic species. However, the outcomes of these interactions are highly context‐specific. The reliance of synanthropic species on anthropogenic subsidies appears to be an important aspect of urban trophic ecology. However, more research is needed to understand how dietary flexibility, especially in relation to anthropogenic subsidies, contributes to the physiology and population dynamics of synanthropes. Urbanization can dramatically change trophic dynamics in the urban ecosystem with implications for biodiversity patterns, management and conservation. However, it is clear that a broader and more mechanistic understanding of the urban food webs is needed. This can be accomplished through inclusion of functional trophic metrics in monitoring efforts, the use of stable isotope food web metrics, the use of multi‐trophic‐level experiments and a more detailed study of the functional ecology of synanthropes. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13114/suppinfo is available for this article.

Impacts of urbanization on insect herbivory and plant defences in oak trees

“…Urbanization significantly decreased leaf chewer damage, and this finding was remarkably consistent across sites throughout most of the geographic distribution of Q. robur . The observed reduction in leaf chewer damage in urban sites relative to rural sites contradicts initial work on this subject proposing that herbivory should be higher in urban habitats, presumably because urban conditions limit investment in anti‐herbivore defences or weaken predator top–down control of herbivore populations (reviewed by Dreistadt et al 1990, Raupp et al ). Interestingly, our results are not alone as a few recent studies have similarly reported decreases in insect herbivory, particularly by leaf chewers, with urbanization (Herrmann et al 2012, Bode and Gilbert 2016, Kozlov et al 2017).…”

“…Leaf burst in this deciduous species usually occurs during April in southern Europe and in May for northern Europe, whereas leaf senescence and leaf drop typically start in September for northern Europe and in October for southern Europe. In its native range, Q. robur supports a diverse community of specialist (and a few generalist) insect herbivores such as leaf chewers, miners and gallers (Southwood et al 2005, Tack et al , Tack and Roslin 2011, Castagneyrol et al 2012, Giffard et al 2012, Moreira et al 2017, 2018a).…”

“…Plant–herbivore interactions also play a pivotal role in ecosystem functioning (Stam et al 2014, Turcotte et al 2014). Nonetheless, studies on herbivory, particularly by insects, in urban settings or along transitions between urban and rural environments have received relatively little attention (Dreistadt et al 1990, Raupp et al ). The few studies conducted over the last two decades have found contrasting patterns of insect herbivory on trees and herbaceous plants; some studies have reported higher insect herbivory in urban areas relative to rural habitats (Cuevas‐Reyes et al 2013, Turrini et al 2016) whereas other studies have found the opposite pattern (Bode and Gilbert 2016, Kozlov et al 2017).…”

“…The few studies conducted over the last two decades have found contrasting patterns of insect herbivory on trees and herbaceous plants; some studies have reported higher insect herbivory in urban areas relative to rural habitats (Cuevas‐Reyes et al 2013, Turrini et al 2016) whereas other studies have found the opposite pattern (Bode and Gilbert 2016, Kozlov et al 2017). Unfortunately, the causes behind these patterns are not always clear (Raupp et al ). Further work on the underlying ecological factors driving urbanization effects on insect herbivory is needed to understand the mechanisms behind changes in species interactions and ecological function in urban environments.…”

“…Most previous studies have directly evaluated the effect of urbanization on insect diversity or abundance, and seldom explored the abiotic or biotic factors that may drive urbanization effects (Raupp et al ). Apart from any direct effects of urbanization on insect herbivory (e.g.…”