International and PNG ecologists have conducted a study along Mt. Wilhelm on how plants have resorted to the use of chemicals as defense mechanisms. Here is a summary of their find.
Contrary to the idea that plants are immobile and therefore defenseless against animals and fungi that feed on them, they have long developed a vast arsenal of physical and chemical defense mechanisms to protect them wherever they grow.
In fact, it is well-known among ecologists that the different abilities of plants to protect themselves from their herbivores determines the numbers and types of these herbivores found on certain plants, thus affecting animal species diversity in a given area.
A recent study by both international and PNG ecologists along the Mt. Wilhelm Altitudinal Gradient Transect has shown that fig plants (ficus) have developed an increased chemical defense against their herbivore attackers as well as to protect them from the harsh environmental conditions that come with increasing altitude.
The study (link: https://link.springer.com/article/10.1007/s10886-020-01173-7 ) titled “Compound Specific Trends of Chemical Defences in Ficus Along an Elevational Gradient Reflect a Complex Selective Landscape”, was published in the international Journal of Chemical Ecology in late April this year.
“Figs possess multiple chemical compounds that defend them against various insects or mammals eating their leaves. Furthermore, some of these compounds also have strong antimicrobial and antifungal activities,” said Dr Martin Volf, a Czech-based ecologist and the study’s lead author.
Plants at higher altitudes, because of limited resources for growing and maintaining their leaves, invest in higher chemical defenses to compensate for the significant losses they may face if their leaves are damaged by feeding animals.
“Our results suggest that as environmental conditions become less favourable with increasing elevation, figs invest progressively more into their chemical defenses. This is because environmental conditions in the highlands make producing leaves more difficult, increasing their value to the plant. Highland Figs thus need to put off any potential consumer. They do it by increasing contents and diversities of various toxic chemical, such as alkaloids,” Dr Volf said.
The Mt. Wilhelm Altitudinal Gradient Transect, where the study was conducted, begins at 200m above sea level (asl) in the lowlands of Madang and continues up to the base of Mt. Wilhelm in Simbu at 3700m asl, where the tree-line stops.
This Gradient has established research sites at 500m elevational intervals and was setup in 2009 by the New Guinea Binatang Research Centre, a Madang-based biological research and conservation NGO.
Being one of the few continuously-forested Altitudinal Transects in the world, it is an important research site for scientists to study how plants and animals respond to changing climatic conditions as you ascend into higher elevations, and can be used to mimic the effects of climate change on different organisms.
“New Guinea belongs among the global hotspots of biodiversity. Although accounting for only a 0.5 percent of Earth's surface, it is estimated to harbour about 5-10 percent of all species occurring on our planet, many of them being endemic. One of the reasons for such high biological diversity in New Guinea is its complex topography.
“The island is covered with multiple mountain ranges where environmental conditions dramatically change from lowlands to highlands. There are relatively few species of organisms that can cope with such changes. In most organisms, elevational shifts in temperature, humidity, or UV irradiation support local adaptations and formation of distinct lowland and highland populations or species,” Dr Volf further said.
For the study, the researchers focused on nine species of Figs, an ecologically important plant group with a total of 157 species in New Guinea (PNG and West Papua) that supports a wide range of animals, including humans.
During the study period, the researchers and their trained local assistants gathered leaf samples for chemical analyses and collected insect herbivores on the trees from elevation sites beginning at 200m to 2700m asl.
They also trapped Fig-feeding mammals like cuscus and rodents, interviewed local hunters, and recorded hunted animal remains to quantify these animal species from the sites.
The leaves and DNA samples from the insects were then sent overseas for chemical analyses and insect identifications.
“Our study represents many years of hard work in the field at our Mt Wilhelm site, combined with follow up work in several international laboratories and museums. Our team includes one father and son combination as well as one husband and wife partnership, and is made up of team members from PNG, Finland, Czech Republic, New Zealand, USA and the UK,” said Dr Simon Segar, an ecologist and the UK-based senior author on the paper.
The father and son are Brus Isua and Mentap Sisol, two self-taught botanists from Madang specialized in Figs. Legi Sam, a PNG PhD student studying insect ecology in the Czech Republic and his Czech ecologist wife, Dr Katerina Sam, are also co-authors in the study.
Analyzing the data from their study, the researchers found that increasing chemical defenses in the plants were mainly due to changes in temperature and humidity as altitude increases.
Also, since plants at higher elevations are exposed to higher Ultra-violet (UV) radiation, the study has shown that plants at these elevations invest in special chemicals to guard against this.
“They produce larger amount of leaf pigments to protect themselves against detrimental factors of the highland environment, such as high UV irradiation,” Dr Volf said.
Furthermore, different species of Figs have developed different types of chemical defenses, thus preventing herbivores specialised on a certain species from jumping over and feeding on other similar nearby species.
“Our results show that the chemicals in Fig leaves vary: they vary between Fig species and with elevation within a species! The changes are not always linear and can be complex, with rare compound groups having big effects on what insects eat,” Dr Segar explained.
Due to these chemical complexities, only animals that are well adapted to feeding on these plants are then able to feed, allowing a few specialised herbivores for these plants.
For example, Asota, a highly specialised caterpillar found on Ficus plants, has adapted to feeding on and storing these toxic chemicals in its body, thus making the caterpillar and its adult moth poisonous to birds and other predators – which the insect advertises with its bright, warning colours.
“In the future we hope to study in detail how insects, like those in the genus Asota that preferentially feed on Figs, can use these chemicals to their advantage,” Dr Segar stated.
Overall, the study has shown that plants, far from being defenseless, actively invest in chemical protection to guard against unfavourable environmental conditions and herbivores, and also seem to ‘manage’ the kinds and numbers of animals that are able to feed on them.
Previously published research (link: https://onlinelibrary.wiley.com/doi/full/10.1111/ecog.00979) conducted at the Mt. Wilhelm Gradient by one of the co-authors of this current study, Dr Katerina Sam, has also shown that plants damaged by insect herbivores can ‘call out for help’ by emitting chemical signals, which are picked up by birds who then home in on the plant and attack the caterpillars.
Finally, and more importantly for PNG, such studies point out the importance of conserving globally unique sites like the Mt. Wilhelm Gradient, which the Binatang Centre along with the local landowners, have been working to achieve.
“The study highlights the value of New Guinea’s rainforests for the organisms that share it, the fascinating stories still to come and the marvel of nature’s complexity,” Dr Segar, the author of several published studies from the site, noted.
“We hope that our work will contribute to the conservation of this remarkable mountain biome and the services that it provides to its human inhabitants. Many of the species that we studied are only found in PNG, and losing these habitats and their diversity would also come with huge costs to basic research and our understanding of complex natural ecosystems.”
(Mentap Sisol sampling Ficus leaves)