Examining the role of predators in ecosystems using pitcher plants

A Nepenthosyrphus hoverfly’s pupa attached to the wall of a dissected Nepenthes rafflesiana pitcher (view from below)

Predators are key players in many ecosystems. However, they are also often highly susceptible to habitat loss or human disturbances. The loss of predators can result in alterations to food webs and changes in the efficiency of major ecosystem processes like nutrient cycling.

The nutrient cycle can be described as nature's recycling system, and is the movement and exchange of organic and inorganic matter back into the production of matter.

A team led by Associate Professor Hugh Tan from NUS Biological Sciences investigated how the presence or absence of a predatory hoverfly species and a scavenging phorid species affects nutrient-cycling efficiency (NCE) within a particular ecosystem.

The researchers’ results were published in the journal Biology Letters on 2 November 2019.  

The Raffles’ pitcher plant (Nepenthes rafflesiana) is a carnivorous plant which captures insect prey using cup-shaped modified leaves. As these fluid-filled leaves are also homes to specialised insects which live directly or indirectly off the prey carcasses in them, they form their own aquatic ecosystems. Such ecosystems are useful ecological models because of their small size, manipulability and the relative simplicity of the communities that inhabit them. The principles concerning the NCE of ecosystems like this may be extrapolated to larger, more complex ecosystems.

Nepenthosyrphus is a poorly studied genus of hoverflies whose predatory larvae are found only in the fluid-filled pitfall traps of tropical pitcher plants in Southeast Asia. They are known to feed on other pitcher inhabitants, such as phorid larvae. 

Phorid larvae are known to increase the NCE of pitchers through their feeding activities, but the effect of predators such as Nepenthosyrphus has never been studied. The team from NUS Biological Sciences therefore explored how Nepenthosyrphus larva can alter the NCE in Raffles’ pitcher plants.

Since Nepenthosyrphus reduces phorid populations, it may be supposed that it would have a net negative impact on pitcher NCE. However, by using dynamic models fitted with experimental data, the team discovered that pitchers containing Nepenthosyrphus almost always had higher NCEs than in which they were absent. This is likely because the processing of nutrients through multiple different consumers and pathways is much more efficient than that which occurs through a single consumer.

While it was already known that phorid larvae increase the NCE of pitchers by feeding on carcasses, until now it was unclear how a hoverfly predator that eats these phorid larvae would alter the dynamics of such a system. In addition, although it would seem that the predator would cause a decrease in the NCE of the system since it would reduce phorid numbers, the NUS researchers found otherwise.

Dr Lam Weng Ngai, the first author of the paper, commented on these surprising results, “Counterintuitively, from the pitcher plant’s perspective, the enemy of my friend is still my friend,” he said. 

The next steps in this research will be to study this predatory hoverfly species as the NUS team suspects that it may be a species new to science.