Study on arthropods in puerto rico: insect mortality in the rainforest

Rising temperatures are causing massive damage to tropical insects. Even the animals that depend on them for food are disappearing.

It too lives in Puerto Rico’s rainforests: the iridescent blue morpho Photo: reuters

Insect mortality is not just rampant in Europe. As a new study by biologists Bradford Lister and Andres Garcia shows, arthropods – of which insects are the most species-rich class – are also disappearing en masse from tropical regions. At the same time, lizards, frogs and birds are dying in the rainforests because they lack food. The results from Puerto Rico, published in the U.S. scientific journal PNAS, also suggest that climate change is deadly for insects, especially in the tropics.

Insect mortality is increasingly regarded as one of the greatest ecological problems of our time. A small but growing number of publications now document their worrying decline. In 2017, a team of researchers led by Caspar Hallmann caused a stir when they demonstrated a decline in insect biomass in German protected areas of over 75 percent within 27 years. The current study now underscores the urgency of countermeasures at the global level.

Although tropical forests harbor the vast majority of insects, the authors say, little is known about trends in actual abundance. To change this, they compared today’s arthropod abundance in Puerto Rico’s Luquillo rainforest with figures from the 1970s.

Even then, Lister had set traps at various heights and crawled through the branches with nets to measure the amount of the small forest dwellers. He did the same with his colleague Garcia from 2011 to 2013. Afterwards, the experts again determined the biomass of their catch.

Dramatic decline

No matter which method they chose: The results were startling. Between 4 and 8 times fewer animals ended up in the nets, and as many as 30 to 60 times fewer in the sticky traps. Biologists describe this as highly alarming.

Although the study only takes into account two points in time and a small number of locations and trapping methods, it is based on a smaller amount of data than the much-discussed German study. Nevertheless, by referring to earlier measurements, it provides new evidence of the worldwide insect die-off.

4 to 8 times fewer arthropods landed in nets and 30 to 60 times fewer in sticky traps than in 1976.

Lister and Garcia looked not only at the decline of arthropods themselves, but also at its effect on other animals. The simultaneous decline in lizards, frogs and certain bird species is directly related to the death of arthropods as they feed on them, he said. For example, the number of insect-eating yellow-flanked toads, a small green bird species, dropped by 90 percent between 19, he said. In contrast, he said, there was no decline in red-flanked towhees, which feed purely on plants.

Thus, the study confirms what biologists have long been calling attention to: Insect decline has far-reaching consequences. The so-called cascading effects on other organisms certainly take place in many other tropical ecosystems as well, Lister and Garcia argue.

Global warming kills insects

Their statistical calculations show another trend: During the period in which insects and their predators have become rarer, the average temperature in the Luquillo rainforest has risen by a good 2 degrees. Previous studies on insect mortality had identified changes in land use and pesticide use as the main causes.

However, these are practically absent in the rainforest area studied: It has been protected as El Yunque National Forest for over 100 years and is considered to be comparatively little influenced by humans. Pesticide use in Puerto Rico has even declined by 90 percent since the 1990s.

Garcia and Lister therefore bring global warming, in particular, into the discussion about the causes of insect mortality. Now, one might think that insects in the tropics are used to heat. But since they cannot regulate their body heat, they are adapted to a narrow temperature spectrum. Temperature extremes in particular would affect the animals. In temperate climates, on the other hand, their tolerance threshold is higher.

The authors conclude that the impact of climate change on tropical ecosystems is probably much greater than previously assumed. In doing so, they do not contradict the state of research, which points to several causes of insect mortality. Insects – and entire ecosystems – are particularly threatened where pesticides, climate change and other factors coincide.

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