Urban warming trumps natural enemy regulation of herbivorous pests

Abstract

Abstract. Trees provide ecosystem services that counter negative effects of urban habitats on human and environmental health. Unfortunately, herbivorous arthropod pests are often more abundant on urban than rural trees, reducing tree growth, survival, and ecosystem services. Previous research where vegetation complexity was reduced has attributed elevated urban pest abundance to decreased regulation by natural enemies. However, reducing vegetation complexity, particularly the density of overstory trees, also makes cities hotter than natural habitats. We ask how urban habitat characteristics influence an abiotic factor, temperature, and a biotic factor, natural enemy abundance, in regulating the abundance of an urban forest pest, the gloomy scale, (Melanaspis tenebricosa). We used a map of surface temperature to select red maple trees (Acer rubrum) at warmer and cooler sites in Raleigh, North Carolina, USA. We quantified habitat complexity by measuring impervious surface cover, local vegetation structural complexity, and landscape scale vegetation cover around each tree. Using path analysis, we determined that impervious surface (the most important habitat variable) increased scale insect abundance by increasing tree canopy temperature, rather than by reducing natural enemy abundance or percent parasitism. As a mechanism for this response, we found that increasing temperature significantly increases scale insect fecundity and contributes to greater population increase. Specifically, adult female M. tenebricosa egg sets increased by approximately 14 eggs for every 18C increase in temperature. Climate change models predict that the global climate will increase by 2-38C in the next 50-100 years, which we found would increase scale insect abundance by three orders of magnitude. This result supports predictions that urban and natural forests will face greater herbivory in the future, and suggests that a primary cause could be direct, positive effects of warming on herbivore fitness rather than altered trophic interactions.

Herbivore regulation in urban agroecosystems: Direct and indirect effects

“…In particular, the number of flowers within gardens directly negatively affected aphid densities, but counter to our predictions did not directly affect parasitism. This contrasts with previous studies of parasitism in rural agricultural systems where parasitism increases with agroecosystem floral availability (Jonsson, Wratten, Landis, Tompkins, & Cullen, 2010), but supports other findings that local factors including floral availability have no direct effect on parasitism in urban systems (Hanks & Denno, 1993;Dale & Frank, 2014;Lowenstein, Gharehaghaji, & Wise, 2017). Flowers may, however, instead attract other aphidophagous predators that are negatively affecting aphid densities.…”

“…Moreover, the seasonal fluctuations that affect population-level resource availability and environmental stressors are combined with direct effects from human activities in cities that alter resources (Faeth et al, 2005). The altered patterns in resources and stressors due to temporal change and anthropogenic change can impact ecological predictions organized around direct versus indirect effects, resource concentration versus natural enemy regulation, at local versus landscape scales in urban systems (Dale & Frank, 2014). Thus even though we are beginning to understand the local and landscape factors that regulate herbivores through parasitism in urban systems (Fenoglio et al, 2013;Pereira-Peixoto et al, 2016), we still lack an understanding of local, landscape, and temporal factors in urban agroecosystems compared to rural agricultural landscapes.…”

Persistence and survival of the spider Nephila plumipes in cities: do increased prey resources drive the success of an urban exploiter?

“…This supports previous research that has found increases in insect abundance in urban areas. In particular, urban warming and impervious surfaces increase abundance of pest species (Dale and Frank 2014;Meineke et al 2013) and smaller fragments support higher abundances of herbivorous insects (Gibb and Hochuli 2002). Many other factors can also increase insect abundance in cities, including vegetation diversity (Raupp et al 2010), artificial night lighting (Eisenbeis et al 2009), water sources for breeding (Rubio et al 2013) and loss of predator diversity (Christie et al 2010).…”

Urban warming reduces aboveground carbon storage

“…For example, urban warming increases densities of two native scale insect species that survive better [23] and produce more eggs [24,25] at hotter urban sites.…”