Reduced thermal variability in cities and its impact on honey bee thermal tolerance.

Karina Sánchez-Echeverría,Gerardo Sánchez-Rojas,Luis Mendoza-Cuenca,Iriana Zuria,Ignacio Castellanos

doi:10.7717/peerj.7060

Abstract

Urbanization is one of the most significant land cover transformations, and while climate alteration is one of its most cited ecological consequences we have very limited knowledge on its effect on species’ thermal responses. We investigated whether changes in environmental thermal variability caused by urbanization influence thermal tolerance in honey bees (Apis mellifera) in a semi-arid city in central Mexico. Ambient environmental temperature and honey bee thermal tolerance were compared in urban and rural sites. Ambient temperature variability decreased with urbanization due to significantly higher nighttime temperatures in urban compared to rural sites and not from differences in maximum daily temperatures. Honey bee thermal tolerance breadth [critical thermal maxima (CTmax)—critical thermal minima (CTmin)] was narrower for urban bees as a result of differences in cold tolerance, with urban individuals having significantly higher CTmin than rural individuals, and CTmax not differing among urban and rural individuals. Honey bee body size was not correlated to thermal tolerance, and body size did not differ between urban and rural individuals. We found that honey bees’ cold tolerance is modified through acclimation. Our results show that differences in thermal variability along small spatial scales such as urban-rural gradients can influence species’ thermal tolerance breadths.

Highlights

Urbanization is one of the most profound drivers of current environmental change (Shochat et al, 2006; Grimm et al, 2008)
We investigated whether reduced thermal variability and an increase in ambient temperature caused by urbanization can influence the limits of thermal tolerance in the honey bee (Apis mellifera L.), one of the world’s most important crop pollinators (Klein et al, 2007)
The narrower ambient temperature range in the urban landscape did not result from differences in mean daily maximum temperatures among urban and rural sites (t = −0.54, d.f. = 8, p = 0.610) (Fig. 2B), but from differences in higher mean daily minimum temperatures in the urban areas compared to the rural sites (t = 3.48, d.f. = 8, p = 0.008) (Fig. 2C)

Summary

Introduction

Urbanization is one of the most profound drivers of current environmental change (Shochat et al, 2006; Grimm et al, 2008). More than half of the human population lives in cities, and this figure is expected to rise in the upcoming decades, in developing countries of Africa, Asia and Latin America, resulting in an expansion of urbanized areas worldwide (United Nations, 2014). It is important to investigate species’ responses to such rapid changes in order to understand the present and future impacts of urbanization. Many species can occur in both rural and urban environments, and the consequences of the urban environment on the responses of these populations (e.g., behavioral and physiological) are poorly understood (Diamond et al, 2017; Banaszak-Cibicka et al, 2018)

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Conclusion