Climate news does not get worse: new field data show total insect (and other arthropod) biomass in Central American rainforest has declined 10 to 60 times since the 1970s. Meanwhile also insectivores, like lizards, frogs and birds, are rapidly declining in the Luquillo rainforest of Puerto Rico. The driving force: climate warming.

Rapid insect decline in Puerto Ricon rainforest as a result of climate change. Top panels show total forest canopy insect biomass, bottom panels walking stick numbers. Left panels show development over time, right panel the correlation with temperature rise…

It’s been a while since we last updated our climate-ecology series, but this news is too big to ignore…

Comparing European and Central-American insect collapse

You probably recall last years shock study about rapid insect decline in North-West Europe. Inside German nature reserves flying insect populations declined by a staggering 76 percent in only 27 years, a German-Dutch research group wrote in PNAS.

Well, apparently, in the tropics the decline is even worse. At least in the tropical rainforests of Puerto Rice, two ecologists, Bradford Lister and Andres Garcia from Rensselaer Polytechnic University and Universidad Nacional Autónoma de México respectively conclude in the same journal.

There’s another big difference. The European insect decline study points in the direction of agriculture (insecticides) and other land use factors (habitat degradation, ecological inertia). In Puerto Rico though the strongest driver of the insect decline is climate change – that’s at least what their statistical model shows as the only very likely cause, able to replicate the monitored sharp species declines.

Rapid Central-American temperature rise

And to be more specific: it’s temperature rise, the researchers add, not cyclones (from which the forest can thus far relatively quickly recover) or the El Niño/La Niña Southern Oscillation (ENSO). Droughts are locally more complex: while the Amazon rainforest is clearly drying out (from the South) the Central-American rainforests are more vulnerable to direct heat stress, but also increasingly to droughts.

Nearby ground-based thermometers record the rapid warming: between 1978 and 2015, average temperatures at El Verde meteorological station rose at an average rate of 0.050 degrees Celsius per year – that’s exactly +2 degrees Celsius in just 37 years(!). At nearby Bisley Tower meteorological station the recorded warming is even faster: on average 0.055 degrees Celsius per year between 1993 and 2015.

Rapid insect decline rainforest Puerto Rice between July 1976 and 2012 and between January 1977 and 2013, as a result of climate change. Right graph shows an observed decline of arthropod biomass in the forest canopy of 86 percent in summer and 62 percent in winter, collected in sweep samples. The left graph shows the even more dramatic decline of insects on the forest floor, using sticky traps. There observed insect numbers declined by a truly staggering 97.3 percent in summer and 98.3 percent in winter…

Under future projections the various manifestations of climate change pose a combined additional threat to the rainforest, the researchers conclude: “[…] as climate warming continues, the frequency and intensity of hurricanes in Puerto Rico are expected to increase, along with the severity of droughts and an additional 2.6–7 °C temperature increase by 2099, conditions that collectively may exceed the resilience of the rainforest ecosystem.”

We’ll get back to it.

© Rolf Schuttenhelm | www.bitsofscience.org