Temporal changes with life long effects

Submitted by editor on 14 December 2018.Get the paper!

Walking through central European meadows among hills and mountains, in the middle of spring, we are amazed by the striking beauty of the plethora of flowers blooming, and the hundreds of insects feasting on the blossoms. Watching butterflies, we notice them stopping on a flower for a moment to refuel then flying away. Others may hop from one flower to another feeding much longer.

Clouded Apollo butterflies Parnassius mnemosyne spend particularly large part of their life on feeding. Some may be observed in patches rich in their favoured nectar plants for hours, switching from one flower to the next. They visit a few flower species within a habitat and a flight period much more often than others. Sometimes even relatively scarce flowers are among the favoured, while some of the most abundant are avoided. Although they visit only a handful of nectar species most of the time, they occasionally feed on a wide array of flowering plants. Favourites may change across habitats and flight periods within the same place.

 

♀ clouded Apollo feeding on the thyme Thymus odoratissimus, an occasionally visited nectar source in our study site. Note the yellow hair on the head and scarce hair on the back distinguishing her from ⚣. Photo: J. Kis.

Nectar plant composition changes considerably throughout their 20–40 days flight period, but individual lifetime is much shorter, thus different butterflies living in different periods within their reproductive season encounter different food resources. Nectar sources are similarly variable in the same habitat across years. How can such choosy insects exploit fast-changing resources? Are individuals capable to change diet when a favoured resource becomes scarce, or those living apart in time simply visit the actually more rewarding flowers and the population diet shift is shaped merely by different individual consumption patterns?

                      We addressed these questions by tracking individually marked Clouded Apollos in a small colline meadow surrounded by a forest. We recorded nectar plant visits and abundances regularly.

                          Study site, Visegrádi-hegység, central Hungary. Photo: J. Kis.                     

We plotted population visit intensities and abundances of the most frequented nectar plants, as well as individual visit histories over the flight period. We calculated similarity indices to compare the diets of individuals to the actual population diet, i.e. the pooled diet of all individuals during the focal butterfly’s lifetime, as well as to the full diet of the population through the entire flight period. We also related pairwise diet similarities to the extent of time overlaps between each pair of individuals.

 

The four most visited flowers in the study site, their abundance and visitation during the clouded Apollo flight period and a few examples of individual visit histories. Photos: J. Kis.

 Population diet shift over the flight period was the result of both individuals swapping resources and differences among individual diets in different periods of the season following changes in flower availability. Individual specialisation was partly explained by the time windows within they lived, although a considerable diet variation still remained. Butterflies with longer time overlaps during their life had more similar diets than those living in very different periods. They specialise on the most rewarding nectar sources for short time periods, although continuously sampling less profitable flowers. Thus they are able to change diet when in need, and specialise again to new resources, showing sequential specialisation.

Ultimately, the relative pace of environmental change compared to individual lifespan may be a key factor in resource use plasticity. Our findings shed light on how short-lived insects exploit their continuously changing resources. It also helps understanding the behavioural mechanisms of plant-pollinator interactions.

 


Clouded Apollo feeding on the pink Dianthus giganteiformis, a favoured nectar source in our study site. Note the thick light grey hair all over the body distinguishing him from ⚢. Colour dots on forewing tips allow easy identification. Photo: J. Kis.

The aiuthors through Viktor Szigeti

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