|
| Physiological intolerance of high temperatures places limits on organismal responses to the temperature increases
associated with global climatic change. Because ants are geographically widespread, ecologically diverse, and
thermophilic, they are an ideal system for exploring the extent to which physiological tolerance can predict responses
to environmental change. Here, we expand on simple models that use thermal tolerance to predict the responses of ants
to climatic warming. We investigated the degree to which changes in the abundance of ants under warming reflect
reductions in the thermal niche space for their foraging. In an eastern deciduous forest system in the United States with
approximately 40 ant species, we found that for some species, the loss of thermal niche space for foraging was related to
decreases in abundance with increasing experimental climatic warming. However, many ant species exhibited no loss of
thermal niche space. For one well-studied species, Temnothorax curvispinosus, we examined both survival of workers and
growth of colonies (a correlate of reproductive output) as functions of temperature in the laboratory, and found that the
range of thermal tolerances for colony growth was much narrower than for survival of workers. We evaluated these
functions in the context of experimental climatic warming and found that the difference in the responses of these two
attributes to temperature generates differences in the means and especially the variances of expected fitness under
warming. The expected mean growth of colonies was optimized at intermediate levels of warming (2–48C above ambient);
yet, the expected variance monotonically increased with warming. In contrast, the expected mean and variance of
the survival of workers decreased when warming exceeded 48C above ambient. Together, these results for T. curvispinosus
emphasize the importance of measuring reproduction (colony growth) in the context of climatic change: indeed, our
examination of the loss of thermal niche space with the larger species pool could be missing much of the warming impact
due to these analyses being based on survival rather than reproduction. We suggest that while physiological tolerance of
temperature can be a useful predictive tool for modeling responses to climatic change, future efforts should be devoted to
understanding the causes and consequences of variability in models of tolerance calibrated with different metrics of
performance and fitness. | |
|