Oil Palm Plantations Disrupt Ant-Macaranga Tree Mutualism in Borneo

Rainforests can contain partnerships that appear straightforward yet have been shaped over vast evolutionary timescales. In Borneo, one such association links ants with trees.

For many years, researchers have regarded it as a textbook illustration of natural cooperation. Fresh research now indicates that human activity is altering this equilibrium in unforeseen ways.

A new study examines how oil palm plantations are influencing the enduring relationship between ants and Macaranga trees. It shows that another insect has entered this system and begun changing its dynamics.

Ancient ant-tree partnership

Macaranga trees offer Crematogaster ants both food and a place to live.

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 survive in the highly competitive rainforest. Scientists frequently cite it as a classic case of mutualism.

A wasp invading the tree cavities

While investigating these trees, scientists identified a new development. A wasp known as Dasyproctus agilis has begun making nests within the same tree stems.

“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 had never previously been recorded in this plant family. It was also the first evidence of wasps using these ant habitats.

Oil palm plantations favour wasps

The team compared trees across different habitats, examining logged forest and oil palm plantations.

Their findings revealed a distinct pattern: wasps occurred in just one tree in logged forest, whereas they occupied far more trees in plantations.

The results indicate that plantation landscapes may enable these wasps to spread.

Oil palm plantations are markedly different from natural forest. Their vegetation is less structurally complex, conditions are more uniform, and temperature and humidity are altered.

Such conditions appear to benefit generalist species such as this wasp. The insect can adjust to varied environments and is even known 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 identified a further important pattern: trees containing more wasps supported smaller ant colonies.

This points to competition for space between wasps and ants. Both depend on the tree’s limited internal cavities.

Exactly how this interaction occurs remains uncertain. The wasps might colonise stems before the ants, force ants out, or be affected alongside them by other environmental influences.

It might be assumed that reduced ant numbers would mean greater plant damage. Yet the study did not find a pronounced rise in herbivory.

The researchers suggest this could reflect the age of the sampled trees. Smaller, younger plants may not yet rely entirely on protection from ants.

The consequences may be greater in mature trees. As Macaranga trees grow, ants take on a more significant protective role, meaning 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 long-established biological partnerships.

As habitats are altered, incoming species can modify systems that already exist. These shifts may initially be difficult to detect, but their effects can accumulate.

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

Researchers also note potential evolutionary consequences. Should the trees cease gaining an advantage from accommodating ants, they may no longer invest in the structures that house 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 changing ecological balance

The findings illustrate how vulnerable ecological relationships can be. A partnership that has remained stable can change when new pressures emerge.

The arrival of one species, together with habitat change driven by people, can transform an entire system. An arrangement that functioned effectively for millions of years may now be confronted by new difficulties.

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