If you doubt the dire warnings the Intergovernmental Panel on Climate Change (IPCC) have issued about the consequences of a 2ºC (3.6ºF) increase in global temperatures, visit Puerto Rico's rainforests. Exactly that much warming has caused a catastrophic collapse in the invertebrate species that form the basis of the food web there, with a predictable impact on the vertebrates that feed on them.
Global warming is expected to raise temperatures in polar regions much more than the tropics. However, biologists have raised fears that the consequences of even small tropical warming could be exceptionally dire. In temperate zones, plants and animals experience large swings in temperature between summer and winter, so an extra few degrees may prove to be something they are relatively well evolved to survive. On the other hand, temperatures in tropical rainforests have been stable for so long, biologists have predicted that species living there would be unable to cope with even modest warming.
Dr Brad Lister of Rensselaer Polytechnic University and Dr Andres Garcia of Universidad Nacional Autónoma de México realized a natural laboratory exists to test this theory. In Puerto Rico's Luquillo rainforest, a combination of local fluctuations and the background worldwide warming have raised average daily maximum temperatures by 2ºC (3.6ºF) since the 1970s, accompanied by dramatic increases in extreme high temperature days.
Better still, the rainforest was extensively surveyed for arthropods in 1976, the class of animals that includes insects and millipedes. By replicating the sticky traps and ground level nets used in the previous survey, Lister and Garcia were able to determine how arthropod numbers have changed in 40 years.
The results were devastating, according to Lister and Garcia in Proceedings of the National Academy of Sciences. The mass of insects caught in the forest canopy was just one-eighth of what it had been previously. Close to the ground, things were even worse with a 60-fold drop in the weight of invertebrates collected.
Insect-eating frogs, bird, and lizards experienced a matching decline, producing a ripple effect through the whole ecosystem. More than two-thirds of land-based animal species are arthropods, so their decline inevitably affects everything else.
“Our results suggest that the effects of climate warming in tropical forests may be even greater than anticipated," Lister said in a statement.
The authors confirmed their findings by investigating a rainforest reserve in Mexico, where temperatures have risen even more sharply, and found an eight-fold decline in arthropod biomass.
Insect numbers are falling worldwide for many reasons, but the rainforest is well protected and insecticide use is falling in Puerto Rico. The authors argue that since all 10 previously most common species were affected, it rules out many specific causes and leaves warming as the primary suspect.