Abstract
Urbanization can alter the composition of arthropod communities. However, little is known about how urbanization affects ecological interactions. Using experimental colonies of the black bean aphid Aphis fabae Scopoli reared on Vicia faba L, we asked if patterns of predator-prey, host-parasitoid and ant-aphid mutualisms varied along an urbanization gradient across a large town in southern England. We recorded the presence of naturally occurring predators, parasitoid wasps and mutualistic ants together with aphid abundance. We examined how biotic (green areas and plant richness) and abiotic features (impervious surfaces and distance to town center) affected (1) aphid colony size, (2) the likelihood of finding predators, mutualistic ants and aphid mummies (indicating the presence of parasitoids), and (3) how the interplay among these factors affected patterns of parasitoid attack, predator abundance, mutualistic interactions and aphid abundance. Aphid abundance was best explained by the number of mutualistic ants attending the colonies. Aphid predators responded negatively to both the proportion of impervious surfaces and to the number of mutualistic ants farming the colonies, and positively to aphid population size, whereas parasitized aphids were found in colonies with higher numbers of aphids and ants. The number of mutualistic ants attending was positively associated with aphid colony size and negatively with the number of aphid predators. Our findings suggest that for insect-natural enemy interactions, urbanization may affect some groups, while not influencing others, and that local effects (mutualists, host plant presence) will also be key determinants of how urban ecological communities are formed.
Highlights
Urbanization is the defining feature of recent history; today over 50% (>90% in developed countries) of people live in urban environments (United Nations 2014)
When analyzing parasitized mummies as the response variable, we removed the first sampling period from the dataset since no mummies were found on this period, we modelled this as a function of the proportion of impervious surfaces, plant richness, distance to the town center, predator abundance, aphid numbers, and number of ants
We found that the presence of mutualistic ants, predators and parasitoids varied as a function of aphid numbers on the plants
Summary
Urbanization is the defining feature of recent history; today over 50% (>90% in developed countries) of people live in urban environments (United Nations 2014). There are some dramatic physical changes from increased densities of roads, buildings and other sealed structures and microclimatic changes such as the urban heat island effect (Bradley and Altizer 2007; Faeth et al 2011). Together, these changes affect the likelihood of encountering species at higher trophic levels (Faeth et al 2005; Egerer et al 2017). Understanding how such extreme anthropogenic habitat changes may affect patterns of ecological interactions is perhaps most tractable with arthropod model systems (McIntyre 2000; Bang and Faeth 2011), but experimental studies in urban ecosystems are scarce
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