SICB 2019: What Factors Influence How Ants Cope With High Urban Temperatures?

What consequences does the urban heat island effect have for animals that live in cities? How might increased heat interact with other environmental factors, such as water availability, to affect survival or fitness of animals exposed to these higher temperatures? Dustin Johnson, a masters student in Zach Stahlschmidt’s lab at University of the Pacific chose to investigate these questions in a group of organisms many of us might take for granted, ants.

Ants are found across a incredibly variety of ecosystems and commonly exploit urban habitats. Johnson collected individuals of 11 species of ants that are abunant in cities in Central California and exposed them to conditions where water was freely available or restricted. He then tested how this water availability treatment, the ants’ body sizes, and their active temperature affected their critical thermal maxima, the temperature at which the ants lost coordinated movements. Johnson found that larger ants had higher thermal limits, and reduced water availability did decrease thermal limits in some, but not all, species.

In a follow up experiment, Dustin focused on two specific species which have divergent thermal ecologies: Prenolepis imparis, which has low active temperatures and thermal limits, and Formica moki, which has high active temperatures and thermal limits. These two species were exposed to water limitation for 8 and 32 hours. The thermal maximum of P. imparis, the less thermally resistant species, was reduced by both these limitation treatments, but the maximum of F. moki was only reduced by the most extreme water limitation treatment. These results show that the lethal thermal limits of ants are affected by both body size and their access to water, but that these effects can be quite different in different species. More broadly, water is an important resource for animals inhabiting urban environments, and reduced water availability make make species which exploit urban environments more susceptible to the negative impacts of higher temperatures in cities.