This nonlinear response to temperature makes it difficult to predict if an insect will develop on average rapidly or slowly when it is exposed to naturally fluctuating temperatures in a certain microclimate. Developing rapidly could allow it to fit another generation into the warm season. This is good if that generation makes it to the overwintering stage on time. But it could be catastrophic if they do not. Additionally, too fast development could set an individual out of sync with potential mates, if the others develop more slowly.

Adult green-veined white butterflies (Pieris napi) and one small white (P. rapae). In order to meet each other and mate successfully, adult butterflies need to emerge (= “crawling out of the pupal shell”) in synchrony with with their mates. Photo: Bella Siemers.

Research questions:

1) How different can development time be in a natural microclimate landscape?

2) What are the consequences for adult synchrony and number of generations?

- We worked with a common butterfly species for which we know exactly how fast eggs, larvae and pupae develop at a certain temperature, says Caroline Greiser first author to the scientific article in Functional Ecology and a landscape ecologist at the Department of Physical Geography, Stockholm university. We simulated development of individuals across 21 hectares using near-ground temperature data from over 90 loggers. In other words, we pretended to put an egg at each logger site and calculated - based on hourly temperature fluctuations - how quickly the egg would develop into a larva... into a pupa and, finally, when the adult butterfly emerges. We simulated different scenarios using logger data from different years and trying out different starting dates.

One of over 90 microclimate loggers installed close to the ground and protected by a white plastic tube from sun and rain. The loggers measured hourly temperature data over 3 growing seasons. Photo: Caroline Greiser.

Some will never be able to meet and mate

The researchers found that butterflies developed at very different speeds across the area, with microclimate often gradually desynchronising neighbouring individuals, resulting in adults emerging up to 2 weeks apart from each other. With an approximate adult life span of 10 days, many individuals of the population will therefore never be able to meet and mate with each other. Microclimate variation also affected the decision to start another generation vs. going into winter sleep mode.

The researchers modelled temperature-driven developmental speed of individual butterflies from egg to adult in a natural microclimate landscape by combining hourly temperature data of > 90 loggers and the non-linear response to temperature. (Credit: Caroline Greiser and Olle Lindestad)

But there were also surprises: “fast” sites could become “slow” sites when temperatures started to exceed the optimum. In some scenarios, this mechanism served to synchronize individuals across the area/population.

Conclusions

We conclude that nonlinear reactions to temperature and microclimate variation must more often be incorporated into research on climate (change) biology.

The Kronängen study area south of Stockholm. A complex terrain and a mosaic of different forest types and open fields create a heterogenous microclimate landscape with cold and warm places existing side-by-side only meters apart. Photo: Mark Ghaly.

Contact:

Caroline Greiser