Finding the habitat of an invisible plant

Rafflesia arnoldii is a paradoxical plant. Its flower is the biggest single flower produced by a plant, but it’s almost impossible to see when it’s not flowering. R. arnoldii is an endo-holoparasitic plant. Holoparasitic means that it doesn’t have any chlorophyll, and it’s dependent on its host for all its food. Endo- tells you where the plant lives, inside another plant. In R. arnoldii’s case, it’s inside Tetrastigma curtisii, T. pedunculare or T. leucostaphylum, lianas that climb in the rain forests of Sumatra and Borneo. Precisely where the lianas are, and the parasites inside them are, is unknown as Sumatra and Borneo’s forests are difficult to survey. If the habitat is unknown, then it’s challenging to protect. Elga Renjana and colleagues have worked out another way to assess the landscape as a suitable habitat. 

R. arnoldii has some demanding requirements of a habitat. First, it needs a suitable host. It lives inside its host for most of its life in a very reduced form as a thread just a cell wide. Here it gets all the energy, water and nutrients it needs. Tetrastigma is a good host as it’s a vine and so pulls water up in abundance. When it’s time for the plant to reproduce, it can push this water to help build its gigantic flower. Because R. arnoldii is dependent on the vines for everything, it also needs its hosts to have a suitable habitat.

Tetrastigma leucostaphylum has a much wider range than R. arnoldii. You can find it from Assam in India across to Vietnam and south in parts of western Indonesia. T. curtisii doesn’t have as wide a range but is found in Sulawesi, while T. pedunculare is found on Borneo as well as elsewhere in southeast Asia. There are plenty of places where the vines grow that R. arnoldii does not. It’s not just a matter of infecting a vine, it has to be a vine growing in the right conditions.

While the exact habitat requirements for R. arnoldii are unknown, it’s likely that a lot of its habitat is being lost. Renjana and colleagues report that Sumatra has lost half of its natural forest in the past forty years. A survey of the remaining habitat is not possible due to a lack of time and money, so the scientists have had to be inventive. They have turned to Species Distribution Modeling using Maximum Entropy (MaxEnt).

Maxent is a tool for making predictions from incomplete information. In the case of R. arnoldii, we know where some plants are. We cannot be sure plants are missing from other locations because if they’re not flowering, they won’t be visible. With Maxent, you can take measurements of the places where the plants are found, and maxent calculates “the probability distribution of maximum entropy.” In plainer English, it looks for the most straightforward set of rules that describes the location of a species.

The most crucial variable Renjana and colleagues detected was mean annual temperature. R. arnoldii likes to live between 25 and 29 °C. It also likes to live on a moderate slope. The botanists say this is so that it has reliable water flow from uphill. Elevation was less of a factor. The team also modelled the factors contributing to the distribution of host species and here found that soil type was important. Yet the authors also acknowledge there are limitations to the model.

In their article, Renjana and colleagues write: “The habitat suitability map of R. arnoldii and its host plants resulting from this study may not precisely predict the locations where those species currently occur because it depends only on biophysical variables including topography, soils, climate and land cover. Factors such as human management and site history could also be important.”

The authors hope their study could help the government establish protected areas to preserve Indonesia’s national flowers. Currently, most of the suitable habitat is predicted to lie outside conservation areas.

ORIGINAL RESEARCH

Renjana, E., Astuti, I.P., Munawaroh, E., Mursidawati, S., Witono, J.R., Yuzammi, Fijridiyanto, I.A., Raharjo, P.D., Solihah, S.M., Robiansyah, I., Cropper, W.P., Jr and Yudaputra, A. (2022) “Assessing potential habitat suitability of parasitic plant: A case study of Rafflesia arnoldii and its host plants,” Global Ecology and Conservation, https://doi.org/10.1016/j.gecco.2022.e02063