Borneo Ants and Macaranga Trees Face Oil Palm Plantation Changes

Rainforests are home to partnerships that may appear straightforward yet have developed over vast evolutionary timescales. In Borneo, one such association links ants with trees.

Scientists have long regarded this relationship as a textbook illustration of cooperation in the natural world. Fresh research, however, indicates that human activity is altering this equilibrium in unforeseen ways.

A new study examines the effects of oil palm plantations on the enduring relationship between ants and Macaranga trees. It shows that another insect has entered the system and begun to alter its dynamics.

The ancient Macaranga ant-tree partnership

Macaranga trees offer both food and shelter to Crematogaster ants.

The ants occupy hollow stems and consume specialised food bodies produced by the tree. In exchange, they defend their host by attacking herbivores and clearing away damaging vines.

This association has persisted for millions of years, helping both organisms endure in the highly competitive rainforest. Researchers frequently cite it as a classic case of mutualism.

A wasp takes over tree cavities

While investigating these trees, researchers identified an unexpected development. A wasp known as Dasyproctus agilis has begun making nests within the same stems used by the ants.

“While surveying these ant-plants, I noticed that many stems had been hollowed out in an unusual way. When we opened them, they were full of flies being eaten alive by wasp larvae,” said Dan Lestina, who led the study.

“Adult wasps hunt and paralyze the flies, then store them inside the plant’s cavities as food for their young.”

This behaviour has never previously been recorded in this plant family. It is also the first known instance of these wasps using habitats created for ants.

Oil palm plantations favour wasps

The team compared trees growing in several settings, including logged forests and oil palm plantations.

Their findings revealed a distinct trend. Wasps occurred in just one tree within logged forests, whereas they occupied far more trees in plantations.

This pattern indicates that plantation landscapes may encourage the wasps’ expansion.

Oil palm plantations contrast sharply with natural forests, with less complex vegetation and more uniform surroundings. They also experience altered temperature and humidity conditions.

Such conditions appear to suit generalist species such as this wasp. The insect can adjust to a range of environments and is even found as an agricultural pest.

“Human activities are transforming habitats worldwide, and this kind of shift in species interactions is exactly what we expect to see,” said study co-author Dr. Kalsum M. Yusah.

“We don’t yet know whether this wasp is native or introduced, but its spread is clearly linked to disturbed landscapes.”

Wasps displace ants

The research also identified another notable pattern: trees containing more wasps supported smaller ant colonies.

This points to possible competition for space between wasps and ants. Both depend on the limited cavities found inside the trees.

Precisely how this relationship operates remains uncertain. The wasps could colonise stems before ants arrive, or they might force ants out. Additional environmental influences could also contribute.

It might be assumed that a decline in ant numbers would result in greater plant damage. Yet the study did not detect a pronounced rise in herbivory.

The researchers suggest that this could reflect the young age of the trees examined. Smaller trees may not yet rely fully on protection from ants.

The consequences may be more pronounced in older trees. As trees mature, ants become more important, meaning that disruption could become increasingly apparent over time.

Land use disrupts mutualisms

This example draws attention to a broader concern: changes in human land use can interfere with biological relationships that have existed for long periods.

As habitats are altered, new species can enter and reshape established systems. Initially, these effects may be difficult to detect, but they can accumulate with time.

Macaranga trees play an important part in forest recovery. A reduction in their health could therefore slow regeneration in disturbed landscapes.

Researchers also highlight potential evolutionary consequences. Should trees cease gaining an advantage from accommodating ants, they may no longer invest in the structures that support them.

“When mutualistic benefits break down, it can drive long-term evolutionary change,” said Dr. Tom M. Fayle from Queen Mary University of London.

“If these structures become less valuable to the plants because wasps exploit them, the plants may stop investing in them. These subtle, long-term consequences of human activity are far less understood than straightforward biodiversity loss.”

A shifting ecological balance

The study demonstrates how vulnerable ecological relationships can be. Even a partnership that has remained stable can change when new pressures emerge.

The introduction of only one species, alongside habitat change driven by people, can transform an entire system. A relationship that functioned effectively for millions of years may now encounter fresh pressures.

As landscapes continue to be transformed, further interactions of this kind may emerge. Understanding them will be essential for safeguarding ecosystems in the future.