17th October 2018

New study finds massive loss of insects in Puerto Rico

A study by the Rensselaer Polytechnic Institute finds that insect populations in Puerto Rico have crashed since the 1970s. The fall may be caused by rising tropical forest temperatures.

Insects of Puerto Rico.

Since the mid-1970s, average temperatures in the tropical forests of northeastern Puerto Rico have risen by 2°C. Meanwhile, the biomass of arthropods – invertebrate animals such as insects, millipedes, and sowbugs – has declined by as much as 60-fold, according to new findings published this week in the journal Proceedings of the National Academy of Sciences (PNAS).

This study supports a new IPCC warning of severe environmental threats from a 2°C rise in the global temperature and the need for the world's nations to aim for a 1.5°C limit. Like some other tropical locations, the study area in the Luquillo rainforest has already reached or exceeded 2°C and the researchers believe the consequences are potentially catastrophic – not just for insects, but the entire food chain.

"Our results suggest that the effects of climate warming in tropical forests may be even greater than anticipated," says Brad Lister, lead author and a faculty member in the Department of Biological Sciences at Rensselaer Polytechnic Institute. "The insect populations in the Luquillo forest are crashing, and once that begins the animals that eat the insects have insufficient food, which results in decreased reproduction and survivorship and consequent declines in abundance."

Data was collected between 1976 and 2013 by the authors and the Luquillo Long Term Ecological Research program at three mid-elevation habitats in the protected Luquillo rainforest. During this time, mean maximum temperatures increased by 2.0°C.

Luquillo rainforest

Major findings included:

• Sticky traps used to gather organisms on the ground and in the forest canopy showed a collapse in arthropods, with biomass catch rates falling by as much as 60-fold between 1976 and 2013.

• The biomass of arthropods collected by ground-level sweep netting also declined as much as eight-fold from 1976 to 2013.

• As arthropods declined, simultaneous decreases occurred in Luquillo's many insect-eating lizards, frogs, and birds.

• The authors also compared estimates of arthropod abundance they made in the 1980s in the Chamela-Cuixmala Biosphere Reserve in western Mexico with estimates from 2014. Over this time, mean temperature increased by 2.4°C, while arthropod biomass declined eightfold.

Cold-blooded animals living in tropical climates are particularly vulnerable to climate warming, since they are adapted to relatively stable year-round temperatures. Given their analyses of the data, which included new techniques to assess causality, the authors conclude that climate warming is the major driver of reductions in arthropod abundance in the Luquillo forest. These reductions have precipitated "a major bottom-up trophic cascade and consequent collapse of the forest food web."

Given that tropical forests harbour two-thirds of the Earth's species, these results have profound implications for the future stability and biodiversity of rainforest ecosystems, as well as conservation efforts aimed at mitigating the future effects of climate forcing.

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