A recent report published in the Proceedings of the National Academy of Sciences has revealed an alarming decline in the insect populations of Puerto Rico. The authors of the study point to climate change as a factor in the loss of tropical insect life.
The study focused on Puerto Rico’s large national forest, El Yunque. This forest is the only tropical rain forest in the National Forest system and has been protected for even longer. Spanish King Alfonso XII named the jungle a 19th-century royal preserve, and just a few decades later Theodore Roosevelt made it a national reserve.
Bradford Lister, a biologist from Rensselaer Polytechnic Institute, first studied Puerto Rican rainforest insects in 1976 when he and a team of colleagues traveled there to measure the resources. Those resources included the insectivores as well as the insects, including the birds, frogs, and lizards. When Lister went back to El Yunque almost 40 years later with his colleague Andres Garcia, an ecologist at the National Autonomous University of Mexico, they could see the decline in birds and butterflies even before they took any measurements.
Lister and Garcia measured the forest’s insects and arthropods, a species classification of invertebrates that includes spiders and centipedes. While the very first microscopes were developed to study insects, these researchers needed a method of study that could handle a larger subject capacity. They trapped the arthropods in plates covered in sticky glue placed on the ground and in the tree canopy. They also swept nets over the brush to collect insects there.
Every collection method showed a significant decrease in the number of insects and arthropods in the forest. The catch rate in the sticky traps fell by 60 times between 1976 and 2013. The net sweeps only captured between and 12.5% and 25% of the sweeps in previous years.
Lister and Garcia also set out to measure the population of anole lizards, which eat arthropods, in El Yunque. Compared with counts from visits in 1976 and 1977, the anole biomass dropped by over 30% and some anole species have disappeared completely from the interior forest. Other insect-eating frogs and birds have also dropped significantly in numbers, indicating that the food web is collapsing from the bottom up.
According to Garcia and Lister, this obliteration is a result of climate change. The average high temperature in the rainforest has increased by four degrees Fahrenheit over the same 40-year period as the arthropod decline. Even this slight change in temperature is detrimental to tropical species, as they are adapted to live in certain temperatures and cannot regulate their internal heat to adjust.