Reference: Lister BC, Garcia A (2018) Climate-driven declines in arthropod abundance restructure a rainforest food web. Proc Natl Acad Sci 201722477. doi: 10.1073/pnas.1722477115
Featured image: Photo of el Yunque forest taken by Kevin Avilés-Rodríguez
El Yunque a Puerto Rican treasure chest of culture and nature
One of my favorite places to visit in my home of Puerto Rico is the tropical forest known as El Yunque. El Yunque is the largest primary forest on the island of Puerto Rico, due to its designation as a Crown Reserve by the Spanish King Alfonso XII in 1876. This special protection saved most of the forest from the massive agricultural landscape change that happened across the island around the 1900’s. Because of this, El Yunque is a trove of culture and nature. Within the forest live numerous species of mammals (16), birds (35), reptiles (18) and amphibians (16).
Long term climate change in a tropical rainforest
Because El Yunque is one of the largest and most pristine forest in Puerto Rico, scientist have been studying it for many years. In particular, scientist have documented how long-term weather patterns affect the plants and animals that commonly live in it. Researchers working at a field station within El Yunque show that the temperatures within two areas of the forest have been steadily increasing over the last 36 years (Figure 2). In addition to the steady warming of the forest, this area has endured six major hurricanes and eight severe droughts, weather conditions whose frequency and severity are affected by climate change.
Figure 2: Steady increase of temperatures at two sites in El Yunque Tropical Rainforest. Both forest sites had warmed by 2 degrees Celsius by the year 2015.
A 36 -year decline of insects, frogs and birds
How might these changes in temperature affect the forest animals? To answer this question, insects are ideal subjects to study because they are often small, abundant, and very sensitive to temperature changes. In fact, tropical species, because of the steadiness of temperature through the seasons, are fairly sensitive to changes in temperature. To collect data on the number of insects, the authors used canopy and ground insect traps and weighted the total mass of insects captured. They found that in 2013 there were 36 times less insects that in 1976 (Figure 3). They also wanted to explore whether this decline in insect abundance was related to the overall increase in temperature within the forest. To answer that question, they compared annual samples of insects captured at the canopy with time of collection and the average maximum temperature of that year. The found that insect abundances are decreasing with time and that this pattern is also related to increasing forest temperatures (Figure 4).
Figure 3: Summer and winter comparison of insect abundances in years 1976-1977 and 2012-2013. Insects were captured at both the canopy and ground of the forest to compare insect abundances in both parts of the forest habitats.
Figure 4: Patterns of insect abundance decline with time and temperature. Insect abundance is lower in the more recent, warmer years.
This change in temperature can also affect other animals. For example, frogs, like insects, are very sensitive to temperature changes, since they need to keep their skin moist in order to breathe through their skin. Additionally, most of the frog species that live in El Yunque are terrestrial, meaning that they live on the land and depend on rain and moisture collected on plants and soils to maintain moist skin. Insects are also an important part of frog and birds’ diet in the forest, meaning that climate change could also be affecting other species by decreasing the amount food available. During their research the authors also show that frogs and birds have lower abundances in more recent years and warmer years (Figure 5).
Figure 5: Patterns of coqui frogs and bird abundances decline with time and temperature. Coqui frogs have a more rapid decline in abundances in warmer years.
Insect decline in the world
This study was very cool due because they were able to use long-term data to show patterns of climate change and their effect across many groups of animals. There is a strong need for more long-term studies to discover patterns of change among communities across the world. Recently, another study in Germany also showed a dramatic decline in insect abundances through time. Insects are one of the most diverse species in the planet, and they play important roles in the maintaining healthy ecosystems. What appears to be small changes to small things have dramatic impact across the globe.