Alice Walker: Indirect control of decomposition by an invertebrate predator

In this new post, Alice Walker, a new ecological researcher at the School of Environmental Sciences, University of Liverpool, UK, discusses her paper: Indirect control of decomposition by an invertebrate predator—recently shortlisted for the 2022 Haldane Prize for Early Career Researchers.

About the paper

Most people have probably heard of vicious the “battles” that go on between ants and termites, thanks to the 1998 DreamWorks film, Antz, in which an army of ants is ambushed by giant acid-spraying termite soldiers. Whilst the termites put up a good fight in this film, the ants are often the victors in real life. I once witnessed a harrowing scene in an African savanna, where a group of Opthalmopone ants (specialized termite predators) stormed the entrance to a termite colony which was being renovated by industrious termite workers. Their soft, slow-moving bodies provided no protection against the Opthalmopone sting: the ants captured and paralysed the termites one by one, each ant carrying in their mandibles up to 10(!) paralysed termite bodies back to their nest. The termites really stood no chance in this instance.

An Opthalmopone worker capturing a paralyzed termite during the raid (credit: Alice Walker)

This story is relevant because the predator-prey relationship between ants and termites is what inspired our paper. As part of my thesis, my supervisors and I were interested in finding out how ants influence decomposition. Ants themselves are not decomposers; however, they prey on many organisms that do act as decomposers (e.g. millipedes, beetles, termites). Therefore, we guessed that ants might have an indirect effect on decomposition via their effects on these invertebrates, but particularly via predation of termites, as termites play an exceptionally large role in decomposition in the tropics (including the African savanna where our study site was). Some previous studies have suggested that predation can have top-down effects on decomposition, but the findings of such studies have been mixed, and most studies were conducted within mesocosms, which can artificially inflate predation pressure. We wanted to see whether we could detect these effects on a large-scale in a fully natural system, and thought the ant-termite predator-prey model would be perfect as a model system.

A large Macrotermes mound on one of our experimental plots (credit: Alice Walker)

We were excited but surprised about what we found. We had expected to detect a top-down effect of ant predation on termite-mediated decomposition, but no one had expected the effect to be so large. When we excluded ants from our plots—i.e. reduced ant predation pressure—termite-mediated decomposition of wood doubled. This effect was far larger than that found in any previous study. This shows that ants exert extremely strong predation pressure on termites, which strongly limits decomposition rates. As termites are the major invertebrate decomposers in the tropics, this finding is important because it shows that predation can exert very strong pressure on decomposition rates, yet this is rarely considered in decomposition models. Broadly, this finding alludes to predation being a potential control on decomposition in other biomes too—macroinvertebrate decomposers and their predators exist in most biomes—although possibly not at the great abundances of tropical ants and termites. I would love to see large-scale predator exclusion studies happening in other places so that we can explore the role of predation in modulating decomposition on a global scale. I will probably not get the chance to conduct this research myself, unfortunately!

About the research

Decomposition bags containing wood, grass and elephant dung (credit: Katherine Bunney)

To do this project, we made use of a large-scale ant suppression experiment that I had been carrying out for my PhD at my field site (the Wits Rural Facility, next to the Kruger National Park in South Africa). In short, we suppressed ants in four 1-hectare size plots using a targeted poison bait (which has no negative effects on other invertebrates). We also had four control plots where ants were left at their natural abundances. The idea was that we would measure the processes (e.g. decomposition rate) occurring in the suppression and control plots, and the difference between the two plot types is the effect of ants on that process. To measure decomposition, we placed decomposition substrates (dried wood, grass and elephant dung) on the plots, enclosed within mesh decomposition bags (a standard method to measure decomposition). We punched holes in the bottom of half the bags to allow termites to get in, and left half the bags un-punched (similarly, the difference in decomposition rate between these two bag types is the decomposition performed by termites).

With the help of the lovely research assistants at Wits Rural, the bags were placed and collected from the plots and the remaining substrate was weighed. We wanted to use natural substrates so a co-author, Kath Bunney, had to make several visits to neighbouring game reserves to collect large quantities of elephant dung, probably to the bemusement of the gamekeepers. The termite soil within the bags was also weighed—termites move soil (sheeting) onto the surface of substrates they are eating for protection from desiccation and predators. We used this measure to determine the importance of termites as macroinvertebrate decomposers—higher levels of mass loss were associated with higher quantities of termite soil, indicating that they were major macroinvertebrate decomposers.

Termites building defensive tunnels to travel along the ground safely (credit: Alice Walker)

So, by quantifying decomposition due to termites in the suppression and control plots, and measuring the difference between them, we worked out the effect of ants on termite-mediated decomposition. I still sometimes get confused when thinking about this experimental design to this day—it’s safe to say the write-up of this paper was tumultuous! Wood decomposition was the most strongly affected (a 100% increase in in ant suppression plots), but the effect was consistent for grass (74%) and dung too (84%).

About the author

Me, looking a bit grumpy in 35 degree heat! (credit: Alice Walker)

I have always loved animals—growing up I was obsessed with going to the Natural History Museum in London, and watching David Attenborough (of course). Accordingly, my journey into ecology started in zoology (my BSc at Manchester University). I like to say I went into zoology for the polar bears but stayed for the invertebrates. I quickly learned that invertebrates were fascinating AND easy to study, and I was captivated by their unfathomable diversity. After taking a year’s hiatus from academia after my masters—due to not really knowing what I wanted to do with my life—I decided it could be fun to continue down the ant route. With this newfound direction, I applied for an ACCE DTP studentship with Kate Parr at Liverpool University looking at the functional roles of ants in a South African Savanna. I was lucky enough to spend many months in South Africa over the course of my PhD. I was basically living on a safari, albeit, whilst enduring 12-hour fieldwork days in 35-degree heat. I would 100% recommend this kind of experience to anyone—it was challenging but helped to build my confidence, independence, and resilience (both as a researcher and personally!). I met some wonderful people and experienced the most fantastic wildlife. During all this, I also did a three-month NERC-funded placement at Fauna and Flora International, working in the Agricultural team and on their pollinator initiatives. Again, I really recommend doing something like this, as this experience outside academia definitely helped me to make the subsequent steps in my career.

After my PhD, I really “had the bug” (for insects, that is). I didn’t really think I wanted to do a post-doc though, having been quite exhausted by my thesis write-up, but then the most perfect post-doc project popped up in my job searches and I had to apply. The project was at UCL, working with Carl Sayer and Jan Axmacher. I got to research how water availability and plant diversity influences pollinator communities at some beautiful farmland ponds in Norfolk. I learned loads of new field and ID skills, which were essential in helping me get my next job. After my post-doc finished, I felt like it was time to start making my way out of academia and into the “real world”. I didn’t see myself becoming a lecturer but I wanted to continue doing research. So earlier this year, I secured a role as an entomologist with Forest Research (the research branch of the Forestry Commission, a governmental department). I drew upon a really broad range of experiences throughout my career for the application and interview. This is why one main piece of advice I would give to PhD students and early career ecologists is to be open taking opportunities that you come across if you can, even if it seems daunting—you never know when something will be useful in the future.

I would say that, broadly, the best thing about being an ecologist is appreciating the natural world around you. It gives going for a walk a whole new meaning when you notice, enjoy (and maybe even understand!) the natural processes and interactions going on around us. For me, it’s humbling to be able to acknowledge that every species has its role to play within ecosystems, and makes me cherish the great diversity of species that share the earth with us.

Enjoyed the blogpost? Read the research here!

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