South Moravian forests have more caterpillars than tropics, scientists find

Illustrative photo: Helga Kattinger, Pixabay / CC0

The Czech Republic, as well as the rest of the world, has been facing a serious decline in the numbers of insects. But how many insect individuals are really out there? And do their numbers differ across the continents? An international team of scientists led by Czech ecologist Vojtěch Novotný set out to explore these questions. They collected data for their study from forests on three continents, evaluating them for 14 years. The results were recently published in the scientific magazine Ecology Letters.

Vojtěch Novotný, photo: Yerpo, Wikimedia Commons, CC BY-SA 3.0

I discussed the study with Vojtěch Novotný, head of the Ecology Department at the Biology Centre of the Czech Academy of Sciences:

“We have been trying to evaluate the species diversity of tropical rainforests in the first place. Everybody knows that these are highly diverse places but surprisingly we don’t actually have a very specific idea how many insects for instance are living in one hectare of tropical forest. That is still unknown.

“Our research focused on a number of different groups of insects, not only counting species but also on what they are doing in this forest ecologically: What are their requirements for survival? What is their impact on plants? What is their impact as food for birds and other vertebrate species? So the first big topic of our study was the complex ecology of a rainforest.

Tropical forest, illustrative photo: Sam Beebe, CC BY 2.0

“The second one was to explain why there are so many species in the tropics compared to the forests in the temperate zone. That’s why we did a lot of comparative studies of tropical and temperate zone forests across different continents.”

The research was carried out in nine different locations on three different continents. How did you select the locations?

“First, when we want to do a tropical versus temperate zone comparison, we have to take into account the whole history of evolution of ecosystems on Earth.

Photo: Petr Klimeš, Archive of the Biology Centre of the Czech Academy of Sciences

“Every continent is a sort of biological laboratory on its own. That’s why we wanted to have comparisons in Asia, so we chose Japan in the temperate zone, and then compared it with the tropics, which is New Guinea, in our case.

“Likewise, we have the Czech Republic as an example of a European ecosystem and then we went to Cameroun for the tropical counterpart.  Finally it was the United States and Panama in the Americas.”

And how exactly did you collect the data? What did the research look like and how many people did the project involve?

“The main problem with doing research on insects living on trees is that all the interesting action takes place up in the canopy."

“The main problem with doing research on insects living on trees is that all interesting action takes place up in the canopy. So tropical forests as well as temperate zone forests are actually very difficult ecosystems for us entomologists to study.

“So we were guided first to the locations which have so-called canopy cranes. It is a construction built in the middle of a forest and instead of lifting material for building construction, they lift researchers up in a little gondola.

Canopy crane in Papua New Guinea, photo: Archive of Vojtěch Novotný

“So you can guide that crane and be lifted up to the canopy and there is about one hectare of forest within reach for research. Unfortunately there are very few cranes like that. We were able to use one in Japan and Panama.

“And finally, and that was the most adventurous part of our study, we built one such crane in Papua New Guinea. So now we have a crane of our own and can use it full time.”

How long did it take you to collect the data?

“It took us over six years. We are still continuing some collections but the main set of data is ready.

Photo: Martin Volf, Archive of the Biology Centre of the Czech Academy of Sciences

“At every location we also have to think about the seasonal change. Especially in the temperature zone, when leaves are flushing in spring, that’s our busiest season, when all the insects are there to eat the young leaves.

“When you come later in the season, towards the summer and autumn, you can find completely different insect species there. In the tropics you have a rainy season and a dry season, again with a change of insects.

“With the teams ranging from ten to twenty people at each location we were able to obtain enough samples for the study.

What are the main outcomes of your study? And did some of these outcomes come as a surprise?

Tropical mountain forest in Papua New Guinea, photo: Petr Klimeš, Archive of the Biology Centre of the Czech Academy of Sciences

“We had a working hypothesis that insects in the tropics are especially rich. That’s why biologists are so interested in the tropics.

“But when we actually look closely at the relationships between insects and their vegetation, we found out that for many insect groups, we can actually predict the number of insect species from the diversity of vegetation.

“So the reason why tropics look so diverse to us is simply that we have a very impoverished vegetation in our forests in Europe, as well as in the temperate zone Asia and America.

“It’s because of the relatively recent glacial periods which really decimated the number of tree and plant species which are growing in these temperate zones.

Photo: C. Redmond, Archive of the Biology Centre of the Czech Academy of Sciences

“So we are still living the consequences of this biological disaster of the Ice Ages and that’s why we have, in comparison to the tropics, so few insect species here.”

One of the things that you have discovered is that that there are far more caterpillars in the temperate forests than in the tropical rain forests. Why is that? What has the greatest influence on the number of insects in forests?

“Yes, that was another interesting finding. So far I have been talking about the number of species, about the variety of insects. But when we look at the number of individuals, then surprisingly we find that in certain periods of the year, in the spring, forests here are actually supporting a higher number of insects, higher number of caterpillars, than the tropics.

Broad-leave trees in the temperate zone, photo: Petr Klimeš, Archive of the Biology Centre of the Czech Academy of Sciences

“So the lush vegetation of the tropics is mostly inedible, because it’s heavily protected both chemically and mechanically, for instance by spikes or hairs. It is made very difficult for insects to feed on the vegetation.

“While here in the temperate zone, except for coniferous, all the broad-leave trees have to shed leaves in the winter. They have no other choice than to flush new leaves in the spring, and these are very attractive for insects.

“So suddenly there is much more food at a given time in spring in the temperate zone forest than at any time in the tropical forest.”

Why is it important to collect such data? What purpose does it serve?

Tropical forest, photo: Petr Klimeš, Archive of the Biology Centre of the Czech Academy of Sciences

“I would say there are two answers to this question. One is, because people are curious about their surroundings. We want to know things, even if they are of no immediate use to us.

“Of course we have always been wondering about the tropical forests. They are very attractive psychologically to us because they are so exotic. So I would say that’s one reason.

“And then there is also a practical reason. There is a very close connection between the diversity of vegetation and the impact insects can have on this vegetation.

“For instance, here we have bark beetles, which are destroying large areas of the forests in central Europe. That happens extremely rarely in tropical forests. That’s because they would have a very hard time to achieve such an impact in a diverse forest.

“If our forestry goes for mixed species forests, they will be much better protected against insect outbreaks.”

“This is a clear direction that if our forestry goes for mixed species forests, they will be much better protected against an outbreak of insects such as we are experiencing right now. So that’s one example of how this research can guide our thinking here in Europe.”

Here in Europe, we are facing a serious decline in insect species. Have you yourself noticed this decline while carrying out your research?

“I think there is no doubt that there is a serious decline in insects in Europe, which has been converted into a human-managed ecosystem.  But it is a hot topic of ecological research right now whether the tropics are facing a similar decline or not.

Palm field, photo: Hans Eiskonen, Pixabay / CC0

“I would say this really depends on the region considered. When you go to Malaysia or Indonesia with huge oil palm fields, than yes. These definitely reduce the numbers of insects and are generally a disaster for the diversity of the landscape.

“We don’t have anything like this in Papua New Guinea. That’s still a much healthier landscape with 70 percent of the forest. So personally we have not seen any decline of insects there over the 25 years we have been carrying our research there.

“Unfortunately we have not been studying these questions. But from what we have seen, the lesson is that as long as we keep a healthy landscape, insects are very adaptable and can survive well in these kinds of diverse landscapes. If we convert everything to huge agricultural fields, it will lead to a disaster.”

South Moravian forest, photo: Martin Volf, Archive of the Biology Centre of the Czech Academy of Sciences

When we spoke five years ago you said you would be trying to answer the question how important insects are. What would happen in a forest if suddenly all the insects disappeared? Did you find the answer to your question?

“Yes. As it is in science, it is not a definitive and complete answer. It is a partial answer, a step towards a better understanding of the problem.

“We have done some interesting experiments in the tropics, where we have small areas of cleared vegetation and we let this vegetation regenerate naturally into a forest vegetation.

“We had areas where we experimentally excluded insects by spraying them with insecticides. The areas without insects ended up being much less diverse botanically than the areas where we let insects in.

Scientists count caterpillars, photo: Martin Volf, Archive of the Biology Centre of the Czech Academy of Sciences

“So it looks like insects function as a sort of diversity police. That’s because without insects, plants which are very successful and can grow fast will dominate the vegetation and outcompete the other plants.

“With insects, if some plants are getting more common, they will feed especially on these common plants and they will limit their growth and unintentionally give a chance to rare plants to survive.