In the latest study revealing the extent of declines in global insect populations, researchers working in Puerto Rico’s Luquillo rainforest have documented a “bottom-up trophic cascade” triggered by large declines in the biomass and abundance of arthropods, the family of animals that include insects and spiders.
Populations of insectivorous predators like lizards, frogs, and birds are also down.
Insect declines have been observed in other regions. This 2017 study on flying bugs in Germany reported greater than a 75% decline over 27 years (Hallman, 2017). But now, comparably large declines have been observed in a pristine research area in Puerto Rico. This area is, interestingly, protected as part of the El Yunque National Forest, the only tropical rain forest managed by the United States Forest Service.
It is also home to the Luquillo Long-Term Ecological Research program, where comprehensive insect data was collected in 1976. Researches conducted a new survey of arthropod populations in 2012 to compare with this historical dataset.
The results were alarming. For the sticky “catch traps” used to trap ground-dwelling insects, biomass decreased up to 60-fold, while sweep-net samples saw declines in biomass of 4-8 fold. Insectivorous predators were also in trouble; anole lizard biomass was down 30%, and mist net captures of birds were down about 50% (Guarino, 2018).
David Wagner, an invertebrate conservation expert quoted in a Washington Post article on the study, calls this “one of the most disturbing articles I have ever read.” This tropical food web “appears to have been obliterated from the bottom” up (Guarino, 2018), with the big changes linked clearly to loss of insect biomass. A key clue in support of this conclusion — the survey found that seed-eating bird populations are holding steady.
While other studies have hypothesized that pesticide exposure and other environmental toxins might be to blame, the remoteness of this area, and the fact that pesticide use has been on the decline in the region, leads the authors to propose that rising temperatures from our changing climate are driving sharp reductions in insect populations.
Climatic conditions don’t usually fluctuate much in the tropics, so insects in this region are sensitive to temperature changes. But, over the last 40 years global temperatures have been rising. In the Luquillo rainforest in Puerto Rico, the average high temperature has risen by 4 degrees Farenheit, apparently too much for many insects to handle.
Over 30% of our food supply relies on insect pollinators, raising the stakes if the now well-documented declines in insect populations around the world continue unabated.
Bradford C. Lister, Andres Garcia, “Climate-driven declines in arthropod abundance restructure a rainforest food web,” Proceedings of the National Academy of Sciences, Oct 2018, 201722477; DOI: 10.1073/pnas.1722477115.
Caspar A. Hallmann, Martin Sorg, Eelke Jongejans, Henk Siepel, Nick Hofland, Heinz Schwan, Werner Stenmans, Andreas Müller, Hubert Sumser, Thomas Hörren, Dave Goulson, Hans de Kroon, “More than 75 percent decline over 27 years in total flying insect biomass in protected areas,” PLoS One, 12:10, 2017, doi: 10.1371/journal.pone.0185809.
Ben Guarino, “‘Hyperalarming’ study shows massive insect loss,” Washington Post, October 15, 2018.