In this post Emily Waddell, a Phd candidate at the UK Centre of Ecology and Hydrology in Edinburgh, talks about her experience working in Tropical forest in South-East Asia, the importance of functional traits for community tolerance to invasive species and her future plants on ecological research in oil palm plantations.

Emily Waddell in Danum Valley Conservation Area in Sabah (Photo credit - Charlotte Gallagher).
Emily Waddell in Danum Valley Conservation Area in Sabah (Photo credit – Charlotte Gallagher).

About the paper

What’s your paper about?

Plant invasions in tropical forests in general are poorly understood, and particularly why some species manage to establish whilst others do not. I have been investigating the processes of plant invasion in Sabah, Malaysian Borneo where oil palm is widespread across the lowlands and remnant patches of rainforest are important refuges for biodiversity. In this paper I looked at how the exotic plant community changes across four habitats of increasing disturbance: oil palm, oil palm-forest edges, highly disturbed forest and more intact forest within rainforest remnants embedded in oil palm landscapes. I related the species occurrence data to information on four plant functional traits which could be especially important for successfully colonising new environments (e.g. dispersal and persistence), to test whether the exotic species that invade forest habitats share a particular set of traits.

What is the background behind your paper?

Oil palm plantation in the South-East of Sabah, showing the different stages of oil palm tree growth from first planting to mature trees. Embedded rainforest remnants can be seen on the horizon.
Oil palm plantation in the South-East of Sabah, showing the different stages of oil palm tree growth from first planting to mature trees. Embedded rainforest remnants can be seen on the horizon.

Agricultural areas within human-modified tropical landscapes are often heavily invaded by exotic plants, but my previous research in Sabah has shown that relatively few exotic species are able to colonise the remnants of rainforest that remain in these landscapes (Waddell et al. 2020 Landscape Ecology, 35(9)), even though they have been subject to previous disturbance – often an indicator of susceptibility to plant invasion. I was fascinated to understand further why certain species had successfully encroached the remnants, and others not; finding out why became the focus of my second field season, and the topic of this paper. Developing our understanding of trait-driven occurrence may help us predict species that could become problematic invasive species in the tropical forest biome in the future and the vulnerability of tropical forest remnants to plant invasion. The importance of these remnant patches for conservation of biodiversity means it is vital we improve our understanding of plant invasions because they could threat native forest regeneration.

What are the key messages of your article?

The most common exotic species, invasive shrub Clidemia hirta, found inside rainforest remnants in Sabah.
The most common exotic species, invasive shrub Clidemia hirta, found inside rainforest remnants in Sabah.

I found that invasion (both the number of exotic species and their occurrence) declined significantly and gradually along the disturbance gradient from heavily invaded oil palm areas to intact forest habitat where only one species, a neotropical shrub Clidemia hirta, was found. This result suggests that even fragmented and degraded forests seem to be quite robust to invasion if canopy cover remains high. Those exotic species that invaded the more disturbed forest of these remnants were more likely to have long-distance dispersal, especially dispersal by vertebrates, and be woody and taller than the exotic plant community found in oil palm. For each plant trait, the value for the forest-oil palm edge community was intermediate between oil palm and disturbed forest, suggesting that trait filtering occurs during the invasion of these forest remnants. I conclude that successful invasion of these forest remnants requires long-distance dispersal in order for plants to be able to reach forest areas from the oil palm, and that exotic plants which are more similar to native forest species (i.e. tall and woody) are more likely to be successful colonisers, in order to compete and persist in the forest environment.

How is your paper new or different from other work in this area?

Studies on exotic plant invasion in tropical rainforests are rare and few have considered the role of plant traits. My paper is the first to look at environmental trait filtering along a disturbance gradient from anthropogenic habitats (e.g. oil palm plantations) into more natural areas of remaining, albeit degraded, rainforest.

Does this article raise any new research questions?

It would be interesting to know whether other traits play an important role in shaping the exotic plant community. The traits I looked at (maximum height, woodiness, seed mass and dispersal syndrome) were from online databases, and I lacked some information, for example comparable foliar trait data. The inclusion of data on leaf functional traits (e.g. specific leaf area, foliar nutrients) could provide additional insight on the trait filtering mechanisms occurring during colonisation, because they provide information on another dimension of resource acquisition and allocation which may be particularly important along gradients of light availability. It would also be really interesting to know whether the trait-filtering I have observed can be seen in other fragmented landscapes that are becoming increasingly common throughout the tropics – do the same traits confer successful establishment everywhere?

Who should read your paper (people that work in a particular field, policy makers, etc.)?

Oil palm-dominated landscape in the North of Sabah with small embedded rainforest remnants.
Oil palm-dominated landscape in the North of Sabah with small embedded rainforest remnants.

I hope the study will be of broad interest for anyone working on plant community ecology, and especially those interested in invasion ecology, tropical forest ecology and the impacts of human-driven disturbance. The research is also of interest to land owners managing forest remnants for sustainability – my PhD project is affiliated to the Socially and Environmentally Sustainable Oil Palm Research (SEnSOR) programme (www.sensorproject.net) that is testing the impacts of the Roundtable on Sustainable palm Oil (RSPO; http://www.rspo.org): the major certification standard for sustainable palm oil. The results of this paper will be written up as a short report with management suggestions that will help to inform guidelines on improving sustainable oil palm cultivation (RSPO).

About the research

What is the broader impact of your paper (outside of your specific species/study system)

My paper contributes to the invasion literature in tropical forests, which is currently very sparse. It therefore helps to build up our understanding on patterns of invasions across ecosystems, and determines whether the processes underlying plant invasion in temperate and tropical ecosystems are similar or distinct.

Why is it important?

In South-East Asia, conversion of rainforest to oil palm plantations is the leading cause of deforestation and loss of biodiversity, and forest remnants embedded within these landscapes are crucial refuges for biodiversity and provide vital ecosystem services. It is important to know which traits of exotic species make them likely to invade these rainforests, providing opportunities for targeted removal of such species from the oil palm matrix. My finding that there is reduced invasion at sites with greater canopy cover (i.e. only one species in intact forest habitats) will contribute towards conservation management strategies for forest restoration to improve the habitat quality of these forests.

Did you have any problems setting up the experiment/gathering your data?

Working in remote tropical environments, you often encounter logistical issues when picking field sites, trialling fieldwork methods and navigating difficult terrains. Luckily I had the help of staff working in the oil palm plantations (owned by Wilmar International) who helped me find and safely access these forest remnants. I had also planned to collect leaf trait data (SLA) but a strong drought started just before I arrived (early 2019) and continued for half of my field season, resulting in the leaves of most exotic species becoming wilted and impossible to get meaningful leaf area measurements. Instead, I focused on just the most common exotic species, Clidemia hirta, as this species has tougher leaves than many of the other exotics I encountered – I did find evidence that SLA increases along the disturbance gradient into intact forest, and I am currently preparing these data for a publication focusing on herbivory in this system.

Where you surprised by anything when working on it?

Throughout my field seasons in Sabah, I was surprised to find C. hirta individuals deep in closed canopy intact forests in both large forest remnants in oil palm plantations and continuous old-growth forests in Danum Valley Conservation Area. Individuals were smaller in lower light conditions but the worry is that, as soon as there is a break in the canopy, these individuals may grow to form dense thickets like those I observed at the oil palm-forest edge and in disturbed forest, with detrimental impacts for native forest communities.

What does your work contribute to the field?

My study provides a good starting point in investigating tropical plant invasions, that can be followed up in more detail in oil palm-dominant landscapes and replicated across other tropical regions. From a systematic search of the literature, I found few studies looking at the drivers behind plant invasions in tropical rainforests, especially at the community level, with most studies focused on one invasive species, or a particular group such as one genus. During my literature search, I found no studies from either of the two largest remaining areas of tropical rainforest, the Amazon or Congo Basin, but invasions are most likely occurring in these regions, and so more research is needed.

What are the big questions still to answer? What is the next step in this field going to be?

An important question is whether or not these exotic species are detrimental for forest regeneration, and if so, which species and what is their impact on native species. In previous research I found an association between higher invasion and reduced native tree sapling and seedling diversity in these forest remnants. However, manipulative field experiments would be required to determine the direction of this relationship – whether exotic species reduce native tree sapling and seedling diversity or if invasion is just lower where young native tree diversity is higher (i.e. native biotic resistance to invasion).

What would you like to do next?

After my PhD I would love to continue in tropical rainforest ecology, ideally a research project that focuses on the anthropogenic impacts, either in plant ecology or herpetology. I think there is still so much to learn about this amazing ecosystem, and it is vital to determine how human activities are changing tropical rainforests and how we can mitigate some of the impacts to build up long-term resilience.

About The Author

How did you get involved in ecology?

I have always had an interest in nature and animals which led me to do an undergraduate degree in Zoology and a Masters of Research in Ecology, both at the University of Glasgow in Scotland. During my five years there I got involved with the Exploration Society, which is a student-led society that undertakes research expeditions all over the world. Through this I have travelled to Ecuador, Peru and Egypt, studying amphibians, reptiles, birds, mammals and coral reefs. These experiences progressed both my personal development and scientific research skills and solidified my interest in tropical ecology.

What’s your current position? What are you currently working on?

I am just coming to the end of my PhD based at the UK Centre of Ecology and Hydrology in Edinburgh, Scotland, through the University of York in England and the Royal Botanical Gardens in Edinburgh, Scotland. I am currently finalising my last data chapter of my thesis, which looks at herbivory rates in C. hirta, investigating whether there is more damage when in close proximity to native confamiliars, testing the invasion hypothesis that exotic species escape from natural enemies when invading new regions and so gain a competitive advantage.

What project/article are you most proud of?

I am probably most proud of this paper as I had developed a plan of research that I hoped to submit to Functional Ecology prior to starting the fieldwork. This meant it was a much smoother process than writing and submitting my first paper from my PhD, because I had learned from my mistakes and had a much clearer idea of what was required to publish a paper. I was very pleased it was accepted, after some improvements and constructive suggestions from reviewers.

What is the best thing about being an ecologist?

I love fieldwork, getting to work outside and seeing amazing ecosystems like tropical rainforests and coral reefs. I also really enjoy the planning aspect to fieldwork and figuring out how to overcome issues that inevitably happen when you are out in the field. I also love getting to travel and working with people from different cultural backgrounds.

What is the worst thing about being an ecologist?

Sometimes it can be a bit overwhelming collecting lots of field data and not necessarily knowing where to start with the data analyses and structuring the narrative of the paper. Through my PhD training I’ve learned to be really clear with the research questions that I wish to answer, and how these plans help guide the structure of the analyses and results. Following this process made my second paper much easier to write than the first!

What do you do in your spare time?

I got a dog (Teddy the Shih-poo) just over a year ago, so he takes up a lot of my spare time with walking, playing and training him to be less naughty! I enjoy upcycling furniture and growing my indoor plant collection, and my guilty pleasure to relax is watching reality TV.

Read the paper here!