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The effect of expected climate change on insect performance: physiology, behavior and life history

Final Report Summary - CLIMINSECTS (The effect of expected climate change on insect performance: physiology, behavior and life history)

I received the Career Integration Grant in March 2013. The main research goal was to study the effect of unfavorable climate on insect behavior and life-history, and to relate their response to thermal stress to the potential effects of climate change. Some of the questions I asked were: (1) Did insects get smaller in the last 100 years possibly due to global warming? (2) What is the effect of stressful physiological state, such as starvation and old age, on thermal tolerance? (3) Is it possible to disentangle between the effect of the temperature that insects are exposed to as larvae and adults on their performance? (4) What is the effect of temperature on longevity? (5) Is there a difference between a single event of stress (or mild stress) vs. repeated events of stress (or chronic stress)?

I used several insect species to answer these and other questions. Some of the work has been done on reptiles, as another group of ectotherms. The most important results are:
(1) Global warming and increasing temperatures were expected to lead to smaller body size. In contrast to most studies on vertebrates showing a decrease in body size in the last 100 years, we detected no such change when measuring over 4000 specimens of 29 beetle species. It could be that other factors, such as food availability, are more important, or that the variability among years is too high to detect such a pattern.
(2) Stressful physiological state led to flawed thermal tolerance in flour beetles. For instance, starvation and old age deteriorated thermal tolerance. The effect of starvation was reversible. Stress did not always lead to impaired thermal tolerance. For example, cold shock enhanced heat tolerance and in some cases the heat shock enhanced cold tolerance in flour beetles, exemplifying "cross-tolerance" between stressors. Understanding how animals trade-off between conflicting demands and whether stress is always harmful or can be beneficial under certain circumstances is important to understand how they could respond to changes in their environment.
(3) I have shown that growth and adult temperatures have considerably different effects on thermal tolerance. Temperature experienced as adults, prior to exposure to a more stressful temperature in the same direction (e.g. a mild warm temperature followed by a stressful warm temperature), induced acclimation and a better response to stress. Growth temperature played a different role: A higher temperature led to stronger heat AND cold tolerance. This result contradicts the beneficial acclimation hypothesis and does not point to acclimation.
(4) In a macroecological study on ~1000 reptile species, I have shown that reptiles in colder regions live longer, plausibly due to slower development and older age at first reproduction. The factors dictating longevity of different species and the basic reasons why animals age are still under debate and are definitely affected by climate.
(5) Repeated or chronic stress has a different effect on insect performance than a single mild stress. It is not surprising that chronic stress impaired performance more strongly than a mild stress, expressed for instance in reduced movement activity, lower tendency to mate and weaker reproduction intensity. However, in contrast to mild stress, chronic stress led to larger fat reserves, longer starvation tolerance and better cold tolerance. It suggests that while being stressed, insects shift their focus from reproduction to survival. This shifts is clearly costly, as it impairs reproduction, but might assist them to survive under harsh conditions.
(6) The current project led to many other publications that were not the main focus of the research proposal but are tightly related to insect responses to unfavorable temperature. Just to name an example, we studied how insects take into account conflicting demands in their habitat choice procedure. Specifically, the wormlion, a sand-dwelling insect, prefers shade that assures lower temperatures and risk of desiccation. However, shade is compromised if the sand is obstructed with stones or is shallow. The ability of insects to consider simultaneously three factors or more and reach an optimal decision (similar perhaps to metaphorically calculating a "weighted average") is surprising. Interestingly, while shade is preferred under warmer temperatures, the preference for light increases when wormlions are tested under cooler temperatures, demonstrating the interaction between illumination conditions and temperature and their joint effect on habitat preference by insects.

The results obtained have several important implications for insects under stressful conditions. First, insects could be less strongly affected by climate change than vertebrates, or that their size variability between years is too high to detect a change. Most studies till now focused on vertebrates. Insects, as a group, could plausibly adapt more easily to environmental change. Second, stress is mostly negative but under some circumstances stress can lead to beneficial responses, such as "cross-tolerance" to other stressors or increased survival due to a shift in the balance point in the trade-off between reproduction and survival. It is known in different animals that a low stress level can have a beneficial effect (termed "hormesis"), and it has implications for humans too. It makes it harder to predict whether environmental change would be harmful or not. The latter point holds true also for the potentially different and even contradictory effect of thermal stress experienced during distinct life stages of insects.

Inon Scharf, Department of Zoology, Tel Aviv University