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Pasoh Pigs as Agents of Forest Diversity: A "Silver Swining”

A new Proceedings of the Royal Society B publication, “Wildlife disturbances as a source of conspecific negative density-dependent mortality in tropical trees,”  identifies the role that wild pigs play in maintaining the hyperdiverse structure of tropical forests, challenging the one-dimensional characterization of pigs as destructive enemies of the ecosystem. 

Photo caption below.
A pig nest at the Pasoh, Malaysia ForestGEO forest dynamics plot (FDP). 

When pigs are preparing to give birth, they create a nest using 200-300 tree saplings. In 1996 and 1998, Kalan Ickes (a coauthor on the paper) excavated pig nests at the Pasoh ForestGEO forest dynamics plot (FDP) in Malaysia and recorded all tree census tags he found. FRIM (Forest Research Institute Malaysia) has managed and led the Pasoh plot in partnership with ForestGEO since 1985, and field crews have conducted a total of seven censuses there.  By using ForestGEO census data alongside the pig nest surveys, they were able to determine the size, species, and original location of 1,672 stems that pigs used in the construction of their nests. They found that pigs typically nested in flat, dry sites and that this habitat preference determined which tree species were killed. The interesting twist was that common tree species habitat associations also tended to favor the flat, dry sites where pigs prefer to give birth. As a result, pigs increased the mortality of common species and consequently increased species evenness at the stand-scale. 

Although the pigs are native to Malaysia, oil palm plantations on the boundaries of Pasoh provide a consistent food source, and the pig population is now unusually large.  As such, their role in maintaining diversity through nesting may be undercut by sheer volume of these impacts.  “While pigs may contribute to diversity, these findings must be viewed in context,” said Stuart Davies, director of the ForestGEO program and a co-author of the study. “One has to remember, the hyper-abundance of wild boar in a number of Asian forests is dramatically reducing tree regeneration, while supporting lianas, and this is likely altering the functional composition of these forests. This may have long-term deleterious consequences for Asian rainforests.”

Highlighting the importance of better understanding pigs’ habits, Luskin said, “Pigs have become the most common large wild mammals on earth, so any new behaviours or impacts may have immense repercussions in Asia and globally.”

Matthew is a former postdoc with ForestGEO and the Asian School of the Environment at Nanyang Technological University in Singapore. ASE-NTU funded Matthew’s joint appointment as part of the collaboration between ForestGEO and the university, a partnership that was formalized through a Memorandum of Understanding signed in 2015. Matthew is now continuing his work in wildlife ecology in Australia at the University of Queensland’s School of Biological Sciences

In November of 2020, Matthew Luskin presented on this work as part of ForestGEO’s Virtual Seminar Series.  If you would like to request access to a recording of his talk, please reach out to Caly McCarthy, ForestGEO Administrative Assistant: