Invisible Engineers

Myriad soil organisms beneath our feet play a vital role in supporting ecosystem services and sustaining our lives, but they often receive less attention from scientists and the public compared to the more visible ‘star’ species. A new study has revealed just how crucial invertebrates such as termites are, particularly in the world’s tropical and subtropical areas.

“Invertebrates are responsible for about a third of global forest leaf litter decomposition,” said PhD candidate Ms Zeng Xiaoyi from the School of Biological Sciences, who led the study supervised by Professor Louise A Ashton. “In particular, those found in tropical and subtropical forests have a more substantial impact on this process compared to their counterparts in forests outside of these regions. As significant ecosystem engineers in the tropics and subtropics, termites play a crucial role in these decomposition processes.”

The team conducted a comprehensive literature search on databases like Web of Science and ScienceDirect, and initially yielded 2,278 articles. They finally included a total of 60 articles, encompassing 476 observations across 93 study sites from all continents except for the polar regions. “Fortunately, the extensive work on the Global Termite Diversity Map by our collaborators – Dr Joel S Woon from the University of Liverpool and Dr Paul Eggleton from the Natural History Museum in London – provided us with an opportunity to analyse the relationship between termite diversity and the effect of invertebrates on forest litter decomposition,” said Ms Zeng.

The HKU team’s study showed that invertebrates contributed 31 per cent to global forest litter decomposition, and in tropical and subtropical forests their contribution was 1.4 times higher than in temperate and boreal forests. They also found that termites – often considered pests to humans – contributed to the greater decomposition in tropical and subtropical forests.

As well as termites, detritivore invertebrates such as earthworms, woodlice, and millipedes directly consume plant litter, and many invertebrates indirectly drive litter decomposition. “For instance, the activity of detritivores leads to litter fragmentation, thereby increasing the exposure of leaf litter to microbial decomposers,” said Ms Zeng. “Other invertebrates, including beetles, ants, mites and springtails, influence decomposition processes by mediating soil food webs and affecting soil physical and chemical properties, such as moisture and temperature.”

Sampling bias

Professor Ashton explained that one of the reasons the role of such invertebrates in the subtropics has often been overlooked is due to regional sampling bias. “There is a bias in ecology towards the temperate zone,” she said, adding that a recent study has also observed a significant taxonomic bias in biodiversity conservation research favouring vertebrates, which constitute 89 per cent of the most-studied species, while invertebrates remain markedly underrepresented.

“However, these types of taxonomic bias have become better recognised recently and there are efforts to better understand the ecology of the tropics,” said Professor Ashton.

It is hoped the new findings can be used to improve the accuracy of Earth System Models in future. “While existing biogeochemical models typically incorporate climate, vegetation and soil characteristics to predict carbon and nitrogen turnover in terrestrial ecosystems, we are now advocating the inclusion of invertebrates for both their direct contributions and their indirect effects stemming from their interactions with microbes, vertebrates and environmental variables,” said Ms Zeng.

“Secondly, we advocate for consideration of the regional variation that our study has revealed so as to enhance the precision of model forecasts. Finally, we suggest introducing invertebrates into Earth System Models by developing global invertebrate databases through combining traditional taxonomic and molecular approaches.”

Ecosystem services

The findings also highlight the importance of conserving invertebrate biodiversity and the valuable ecosystem services they provide. “The decline of many invertebrates, often considered as crop pests [for example, termites], is alarming due to the overuse of pesticides, further emphasising the need for their conservation,” said Ms Zeng.

“Policies and scientific initiatives for soil animal conservation need to be more widespread. We anticipate that our study will reveal to the public the immense power of these ‘invisible’ creatures. They are not only an integral part of nature but also a group that urgently requires our protection and conservation efforts.”

Moving their focus to the role of the decomposition process as the primary source of available nutrients for plants, the team will next investigate how invertebrates mediate plant growth through this pathway. They aim to employ stable isotope technology to track the nitrogen flux from leaf litter to living plant tissues so as to determine whether such knock-on effects vary across different climatic regions. Implementing this approach will enable them to identify the specific roles that invertebrates play in vegetation development and how these roles are influenced by climatic factors.