Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

Abstract

Urban landscapes are characterized by high proportions of impervious surface resulting in higher temperatures than adjacent natural landscapes. In some cities, like those at cooler latitudes, trees may benefit from warmer urban temperatures, but trees in many cities are beset with problems like drought stress and increased herbivory. What drives patterns of urban tree health across urbanization and latitudinal temperature gradients? In natural systems, latitude–herbivory relationships are well‐studied, and recent temperate studies have shown that herbivory generally increases with decreasing latitudes (warmer temperatures). However, the applicability of this latitude–herbivory theory in already‐warmed urban systems is unknown. In this study, we investigated how the interaction of urbanization, latitudinal warming and scale insect abundance affected urban tree health. We predicted that trees in warmer, lower latitude cities would be in poorer health at lower levels of urbanization than trees at cooler, higher latitudes due to the interaction of urbanization, latitudinal temperature and herbivory. To evaluate our predictions, we surveyed the abundance of scale insect herbivores on a single, common tree species Acer rubrum in eight US cities spanning 10° of latitude. We estimated urbanization at two extents, a local one that accounted for the direct effects on an individual tree, and a larger one that captured the surrounding urban landscape. We found that urban tree health did not vary with latitudinal temperature but was best predicted by local urbanization and herbivore abundance. We did not observe increased herbivore abundance in warmer, lower latitudes cities, but instead herbivore abundance peaked in the mid latitudes of our study. This study demonstrates that urban landscapes may deviate from classical theory developed in natural systems and reinforces the need for research reconciling ecological patterns in urban landscapes.

Can Cities Activate Sleeper Species and Predict Future Forest Pests? A Case Study of Scale Insects

“…These scale species develop invasive traits, proliferate, and become chronic urban tree pests due to the UHI effect [49][50][51]. Other scale species on the same trees under the same conditions do not display invasive phenotypes or become pests [52,53]. Melanaspis tenebricosa and P. quercifex are consistent with characterizations of sleeper species as they are innocuous native species that become pests with environmental change, here, the UHI effect.…”

“…The strongest path indicated that impervious surface cover increased temperature which, in turn, increased scale density. Subsequent research on urban red maples corroborated these results [53,56,61,87].…”

Weak latitudinal gradients in insect herbivory for dominant rangeland grasses of North America

“…Although latitude can be a useful integrator of several axes of global variation in climate, relatively few studies investigate the hypothesized underlying climatic drivers behind latitudinal patterns in herbivory (e.g., Adams & Zhang, 2009;Gao et al, 2019;Moreira et al, 2015;Zhang, Zhang, & Ma, 2016), while even fewer consider nutrient availability that can have different spatial patterns than climate variables (e.g., Lynn & Fridley, 2019;Moreira, Castagneryrol, Abdala-Roberts, & Berny-Mier y Teran JC, Timmermans BGH, Bruun HH, Covelo, F, Glauser G, Rasmann S, Tack AJM., 2018). In addition, it is possible that latitudinal variation in insect herbivore damage and plant resistance to herbivory are driven by resource availability, trade-offs in plant growth and defenses (Kim, 2014), and that they depend on herbivore specialization (Dyer & Forister, 2019), as well as land use or urbanization (Just, Dale, Long, & Frank, 2019). Though climate is surely a strong driver of latitudinal variation in herbivory, deviations from this expected pattern are likely due to unmeasured edaphic and plant-trait controls on consumption rates.…”

Plant Diversity Along the Urban–Rural Gradient and Its Relationship with Urbanization Degree in Shanghai, China

“…The former mainly links to the socio-economic dimension of urbanization and the latter focuses more on urban biophysical and environmental factors. These two kinds of indices are more straightforward and easily obtained from urban census data or retrieved from remotely sensed data, which have been widely utilized to study urbanization and their relationships to, or impacts on urban biodiversity [72,[100][101][102][103]. Compared with most of the previous studies, our study directly used the urban land use proportion to quantify the urbanization degree and established a quantitative relationship to plant species diversity that can help easily understand and predict the influences of urbanization on plant diversity; therefore, this study can provide insight to optimize ecosystem services in future urban green space planning and design [104][105][106].…”