By Elina Mäntylä
This blog post is based on an oral presentation* I had at the European Conference of Tropical Ecology in Paris, France on 28th March 2018.
Several meta-analyses and reviews (Van Bael et al. 2008, Mooney et al. 2010, Mäntylä et al. 2011) have shown that insectivorous birds and bats help plants by removing herbivorous arthropods. So, they cause trophic cascades where plants benefit of the presence of predators. Trophic cascades are thought to be common only in simple ecosystems but they are possible also in the tropics with a rich biodiversity.
Part of my PhD thesis was a meta-analysis review article titled “Birds help plants: a meta-analysis of top-down trophic cascades caused by avian predators”. While I was searching for original research articles to include in the meta-analysis, I found out that there are so many different plant responses** you can measure in these predator exclosure studies (Image 1). But I also found surprisingly many exclosure studies where the researchers hadn’t measured any plant responses at all. To encourage every researcher to measure at least some plant response (also in other studies than predator exclosure experiments), I will introduce some options here.
Predator exclosures around the plants are needed to study trophic cascades. Then you can compare the effects with and without predators. Conducting a predator exclosure experiment requires usually a lot work; starting from building the exclosures and maintaining those. Therefore, researchers usually want to measure everything possible happening to the predator, arthropod and plant communities during the study. Counting the arthropods, and observing birds or bats is usually rather straightforward. But what should be measured of the plants inside the exclosures and their uncaged controls?
The easiest measure is often how much leaf material herbivorous arthropods have eaten. This can be called as “leaf damage”, “leaf area removed (LAR)”, “missing leaf area” or “herbivory”. This is also usually the strongest plant response to predator exclusion, especially in experiments with a short duration. There is even a mobile phone app called BioLeaf for measuring the leaf damage (Image 1).
Other rather easy measurements are specific leaf area (SLA; the ratio of leaf area to dry mass), water percentage of the leaves [(fresh weight – dry weight)/fresh weight × 100], stem width (or circumference), plant height, biomass or mortality. Especially when studying smaller plants the biomass of the whole plant, including underground roots, can be interesting and possible to measure. If you want to compare the whole plant biomass in the beginning of the experiment and in the end, you will need to have a separate group of plant individuals that will be collected in the beginning, and therefore will not have any treatment.
A big portion of the exclosure studies in tropics have been done in agricultural environments, such as coffee or cacao plantations. There it is understandably interesting to measure the amount of the final product – so how many kilograms there are kales, coffee beans or cacao pods (Image 2). It is worth to measure the flower, fruit or seed production of also non-agricultural plants, especially in experiments of longer duration.
There are much less measurements done showing of what is happening inside the plant. For example, are the plants without insectivorous predators forced to emit more herbivore-induced plant volatiles (HIPVs), produce defensive chemicals inside the leaves, invest more in trichomes covering the leaves or lower their photosynthetic rate? Some of these can be tricky to measure in remote locations but not impossible. There is a way to collect HIPVs passively with polydimethylsiloxane (PDMS) tubes (i.e. without pumps requiring electricity; Kallenbach et al. 2014). Several chemical analyses are possible even when you cannot freeze or dry the leaf samples immediately.
There is need for more plant measurements in predator exclosure studies, especially in the natural tropical forests. I have collected research articles of predator exclosure studies where they have had birds (and bats) as predators. There are only 23 studies done in tropics (or subtropics), and of these only 8 are done with non-agricultural plants. The agricultural plants used are coffee (5), cacao (4), kale (2), oil palm (2), broccoli (1), and rice (1). I hope to see in the near future more published predator exclosure studies with also measured plant responses.
* I wrote this blog post text first, and then did the slides for my talk. It was so easy to do the slides when I had the text ready. I can recommend everyone to try this.
** Just a reminder that for a study to be included in a meta-analysis there needs to be mean, some error measure (SE, SD, CI) and sample size provided of the studied variable.
Author biography: My research has been mainly about tritrophic interactions between insectivorous birds, herbivorous insects and plants. This includes vision and olfaction of birds, and plant chemistry. But I have also research of territory choice and migration of birds. I’m now postdoctoral researcher at Biology Centre of the Czech Academy of Sciences in Ceske Budejovice, Czech Republic. Previously I have studied and worked in University of Turku, Finland and Freie Universität Berlin, Germany. Twitter: @elinamantyla
Image caption and credit:
Image 1– Example how to calculate the amount of leaf damage (Image by BioLeaf)
Image 2– Cacao pods (Image by Luisovalles via wikimedia.org)