Assessing the change in saproxylic insect community across Singapore’s forest regeneration gradient

Hi this is Calvin Leung, a PhD student who studies the diversity and ecosystem functions of saproxylic insects in tropical rainforest. What are saproxylic insects though? Saproxylic insects refers to any insects with certain parts of their life cycle relying on dying or dead wood and/or the wood’s associated products like fungi, so by this definition, saproxylic insects range from the actual wood feeders like stag beetles (Family Lucanidae) and longhorn beetles (Family Cerambycidae), specialized predators such as ground beetles in the genus Orthogonius that specialized on termites, all the way to fungivorous insects like handsome fungus beetles (Family Endomychidae). While a piece of log itself symbolizes the death of a tree, this gigantic dead body supports many lives by providing various resources like food and shelter to many organisms in the forest. You might think what saproxylic insects do are just wood decomposition and nutrient cycling, but actually they can perform all sorts of functions. For example, tumbling beetles (Family Mordellidae) and many longhorn beetles from the subfamily Cerambycinae are important pollinators of beetle-pollinated trees like stone oaks (Lithocarpus spp.). Specialized ants that build nests in deadwood can also exert population control on their preys. Given the high diversity and importance of saproxylic insects, it is crucial to maintain a healthy community of saproxylic insects to sustain the ecosystem functions of the forest.

Figure 1. Examples of saproxylic insects: top left: Stag beetle Calcodes striatus in which the larvae feeds on deadwood and their symbiotic fungi (photo taken in Brunei); top right: Longhorn beetle Aeolesthes induta which prefer certain weakened tree rather than fully dead wood; bottom left: Ground beetle Orthogonius sp. that are specialized predator of termites; bottom right: ants Myopopone castanea specialized in hunting for large saproxylic beetle larvae.

However, saproxylic insect communities undergo substantial changes when land use change kicks in. Since the 19th century, most of Singapore’s pristine rainforests have been converted into agricultural land. Since the original rainforest habitat and the host trees are gone, populations of many saproxylic insects are expected to either decline sharply or go locally extinct. So after a few decades of forest regeneration, who’s left in our degraded landscape? And how well are our saproxylic community recovering both diversity wise and functional wise? To answer these questions, Shawn Ang, a former undergraduate student under our lab, and I are surveying saproxylic insects across forests of different ages in Bukit Timah and the Central Catchment Nature Reserve.

To do this, Shawn and I have been setting up aerial flight interception traps (FIT) to capture flying insects. In order to attract more saproxylic insects, we added a few drops of ethanol as the attractant. Not that they want to depress from work like us, but instead the insects associate alcohol with the fermented tree saps flowing out from a dying tree. Mistaken the FIT as a fermenting deadwood, the saproxylic insects therefore fly towards the panel boards of the trap, hit onto the board and fell into the collection jar situated at the bottom of the jar.

Figure 2. Shawn Ang and his hand-made aerial flight interception trap (FIT).

To obtain a finer resolution of understanding on the saproxylic food web structure across the forest regeneration gradient, segments of logs lying on the forest floor were dissected to obtain as many saproxylic insects as possible. However, quite a portion of the insects I caught from these logs are in their larval stages, and it is almost impossible to identify them to finer taxonomic level given the poor reference database for tropical insects. Therefore, I’m trying my best to rear some of these larvae to adulthood in order to obtain their adult morphology for identification.

Figure 3. Me carefully dissecting a heavily decomposed log in search for saproxylic insects. Each log takes anything between one to seven days to finish the search. Photo credit © Chung-Wing Ko.

Figure 4. Example of a saproxylic larva successfully reared to adulthood. Echinoaesalus sp. larva (left) and adult (adult). Echinoaesalus is a tiny stag beetle belonging to the primitive lineage Aesalinae. It looks nothing like the typical stag beetle with charismatic mandibles, right?

Another difficulty for collecting saproxylic insects from the logs is the small size of the insects. Many saproxylic insects are of similar size as this dot “.”, and obviously some of them will be missed by my naked eyes. To collect these tiny fellers, I placed the saw dust of the dissected log into Winkler extractors, in which the saw dust and the insects within were hung midair in mesh bags to air dry them in an apparatus. Since most of the tropical insects do not like dryness, they tend to escape downwards to look for wetter substrate on the forest floor, ending up falling into the collection jar at the bottom of the Winkler extractor.

Figure 5. Winkler extractors used to extract small saproxylic insects out of the dissected log.

By combining the biodiversity data of flying (collected through FIT) and less mobile saproxylic insects (collected through wood dissection), our results will provide a checklist of saproxylic insects currently living in the forest of Singapore, and also provide insights in changes in the saproxylic food web structure across the forest regeneration gradient. In this way, we can potentially identify any bottle necks encountered along the current forest succession process and provide respective suggestions to elevate the diversity and functionality of the missing functional groups.

Written by Calvin Leung