Researchers studying the effects of termites in an old-growth tropical forest in Malaysian Borneo found that both termite numbers and activity increased during the El Niño drought of 2015-2016, resulting in higher leaf litter decomposition and soil moisture compared to a test plot where the termite population had been artificially suppressed.

These improvements in soil condition also resulted in an increase in seedling survival, suggesting that termites will have an important role to play in maintaining plant diversity in the future, given that the severity and frequency of droughts is predicted to increase with a changing climate.

Why termite number and activity increase during drought is unclear at present, but researchers think that the conditions possibly made the termites’ tunnels drier and less water-logged, making moving through the environment easier.

Droughts can transform tropical forests. They kill trees and slow their growth. They also affect microbes that are critical to keeping the forest soil healthy. But termites, better known for their intricate mound-building skills and for chewing through wooden furniture, help tropical forests withstand drought, a new study has found.

Tiny termites are known to play big roles in tropical forests: they dig tunnels through the forest floor, and eat wood and leaf litter, moving moisture and mixing nutrients through the soil. Despite these roles, termites are an understudied group in ecology, says Louise Amy Ashton, assistant professor at the University of Hong Kong and lead author of the study published in Science.

Termites’ impacts on forests, especially during drought, which is increasing in severity and frequency across the tropics, has not been experimentally studied before. Ashton and colleagues had the opportunity to do just that during the El Niño drought of 2015-2016.

The team set up experimental plots within an old-growth tropical rainforest in the Maliau Basin Conservation Area in Sabah, Malaysia. In four of those plots, the researchers suppressed termite activity by both physically removing all termite mounds, and by applying insecticides and using poisoned toilet-paper rolls as bait to get rid of most wood- and leaf-litter-eating termites.

“The insecticide is active against all insects and other arthropods,” co-author Theodore Evans, an associate professor at the University of Western Australia, told Mongabay in an email. “However, the concentration used was so low, a lot of bait has to be eaten to reach a lethal dose, and so will work better against social species [like termites].

“Also, toilet paper is almost pure cellulose, so will be eaten only by animals that target cellulose, and undecomposed at that,” he added.

The team monitored termite activity in these “suppressed” plots and compared it with four control plots — similar plots where termite activity was not interfered with — both during and after drought.

Overall, termite activity was considerably lower in the plots where termites had been suppressed compared to the control plots. At the same time, other invertebrate groups hadn’t been affected much, meaning that any difference between the suppressed and control plots could be attributed to the termites themselves. “Up to now we didn’t have the methods to target suppression of termites,” co-author Kate Parr, a professor at the University of Liverpool, said in an email. “Our novel methods have enabled us to target the specific role of termites.”

During the course of their experiments, the team found that the number of termites in the control plots were more than double during drought compared to post-drought or when normal rains had resumed. Moreover, during drought conditions, increased termite activity led to considerably higher leaf litter decomposition, increased soil moisture, and greater diversity in soil nutrient distribution (which influences plant diversity in turn) in the control plots compared to the suppression plots. These differences were less stark in the post-drought conditions — suggesting that termites had an especially important role to play during drought.

So what caused termite numbers and activity to increase during drought? The researchers haven’t identified the exact cause, but they think that drought conditions possibly make the termites’ tunnels drier and less water-logged, making moving through the environment easier. The dry conditions could have also reduced competition from fungi, the other main decomposers in tropical forests.

“Fungi need to have water and food in the same place in order to survive and grow, which is why fungi do well in wet environments,” Evans said. “During dry periods, the food and water are separated, with the food (wood and litter) on the (dry) soil surface, and the water at depth (ground water). Termites carry water to their food and nests (they have special water sacs for this purpose), and so can be active and continue feeding during dry periods.”

The drought may also have suppressed the activity of ants, one of the main predators of termites, Parr said. This could, in turn, have had a positive effect on termites.

The increased activity of termites also had knock-on effects throughout the forest, the team found. The improved soil conditions created by the termites during drought translated to a 51 percent increase in seedling survival on the control plots compared to the suppression ones. This suggests that termites will have an important role to play in maintaining plant diversity in the future, given that the severity and frequency of droughts is predicted to increase with climate change.

What the study also shows is that “pristine rainforests have lots of termites that contribute to ecosystem health and resilience to drought,” said co-author Paul Eggleton, an entomologist at the Natural History Museum, London. “However, disturbed forests have fewer termites, and so will have lower ecosystem health and resilience.”

Parr said that since the team did not suppress all termites in their plots, the findings were, in fact, “a huge underestimate.”

“The true importance of termites is likely much greater!” she said.

Banner image of tropical rainforest in Sabah, Malaysia, by Rhett A. Butler/Mongabay.

Citation:

L. A. Ashton et al. Termites mitigate the effects of drought in tropical rainforest. Science, 2019 DOI: 10.1126/science.aau9565