Periodic Reporting for period 4 - BABE (Why is the world green: testing top-down control of plant-herbivore food webs by experiments with birds, bats and ants)
Berichtszeitraum: 2023-06-01 bis 2024-07-31
Our goal was to test novel aspects of the “green world hypothesis” and focus on the importance of top-down control. Specifically, we estimated on how forest diversity and productivity along a latitudinal and vertical forest gradient affect the strength of trophic cascades. In detail, we succeeded to calculate which and how strongly the key predators (birds, bats and ants) contribute to the top-down effects individually and in synergy. For the very first time, we managed to exclude predators from forest canopies, using six canopy cranes between 51o N and 34o S. Against to our initial hypothesis, we did not find stronger top-down control of insect in canopies, and we confirmed this results by robust experiments conducted at several sites. We described spatially and temporally variable antagonistic and synergic interactions between different groups of predators. Thus, we were for the first time able to calculate, how would a loss of insectivorous predators affect functioning of lower trophic levels. Against to our original hypothesis, we found very strong effect of bats at some places, and we stress out their tremendous impact on pest control. In live with our hypotheses, we found ants to be unable to control lower trophic levels. Yet, we described novel interactions with spiders, which were able to take the role of major predator after the ant removal. Our fully factorial experimental design enabled us to disentangle the relative contributions of bats and while isolating the primary effects of birds. Last, but nor least, we quantified herbivory damage and accumulation of herbivory damage in time on 42 plant species spanning the latitudinal gradient, and we described their mechanical and chemical traits. This research is crucial for understanding the ecological roles of predators in maintaining ecosystem balance. By unravelling the mechanisms behind top-down control in forests, the findings can inform conservation strategies to protect biodiversity and ecosystem services, such as pest control. For example, bats' significant impact on pest suppression highlights their value in reducing reliance on chemical pesticides, which benefits agriculture, human health, and environmental sustainability. Similarly, understanding the interactions among predators can guide forest management practices, ensuring that ecosystems remain resilient in the face of species loss or environmental change. Moreover, the study provides insights into the effects of predator loss on lower trophic levels, offering a predictive framework to mitigate risks associated with biodiversity decline. These results have direct applications in addressing global challenges like food security, climate change adaptation, and ecosystem restoration.
Our project consisted of a standardized experiment which we replicated at 6 sites, in Germany, Japan, China, Papua New Guinea, and two study sites in Australia. The work at each of the study sites consisted of the manipulative experiment during which we prevent access of predators to saplings in forest understory and to branches in canopy. We worked with 42 focal plant species, preselected based on their traits, phylogeny and abundance. We marked 280 saplings in understory (35 individual per each of the 8 plant species) and assigned them to the 7 treatments (i.e. 5 individuals per treatment – CN1, BIRD, BAT; CN2, ALL, VER, ANT). Similarly, we select 96 large branches in canopy and assigned them to 4 treatments (CN2, ALL, VER, ANT). Exclosures of vertebrates (VER), birds (BIRD) and bats (BAT) were in form of large cage exclosures, which were placed at sites permanently or opened daily in the morning and in the afternoon to prevent access only of the respective predator. Exclosure of ants (ANT) were in form of a sticky insect barrier applied to the branches or trunk. ALL treatment then combined both treatments – cages and sticky barrier. Once we set the treatments, we marked permanently and count all leaves within the enclosed saplings or branches, photographed their herbivory damage, and collect all arthropods to start the experiment. After that, we managed the smaller bird & bat experiment (BIR, BAT, CN1 treatments) for a month, and surveyed the effect of the treatment after that. It means, we sampled newly established arthropod communities and re-scanned all leaves, keeping the leaves for chemical analyses. In the meantime, we kept surveying communities of predators we manipulated. Birds and bats are surveyed by point-count method and ants were collected from baits. After 5 months, we terminated the experiment the same way, as the short one-month experiment. About 6 months later, we replicate the whole experiment once again at the same site. Thus we obtained 2 full replicates for each of the 6 study sites. At each of the study site, we also surveyed predation rata by exposing dummy caterpillars. This work is a supportive to the main experiment. We found very interesting results contradicting all previous patterns found in forest understories, and we explained them by the abundance of predators, which is rarely measured in similar studies. Specifically, at different latitudes, we recorded between 95 and 845 insectivorous birds per 4 hours long survey. Considering the biomass of birds and their feeding specialization we obtained critically important data which we will be very important for global understanding of the importance of predators. We found also striking patterns in the abundance and richness of ants, for which we confirmed various feeding strategies by stable isotopes, supporting thus their weak effect on lower trophic levels.
As the first project ever, we managed to replicate standardized manipulative experiment aiming to exclude various predators individually and in combinations. We studied their synergic and individual effect on trophic cascades, at several sites globally in forest canopies, which are highly inaccessible. This is a new aspect of the ecological studies, as we will be the first who will be able to make some robust and combined conclusions about how predators and trophic cascades function in canopies of forests. Precision and scale of our methods are novel. It appears that by fine identification of all aspects of the trophic levels, and thus work of specialists on the predators as well as lower trophic levels and plants, novel factors affecting the trophic cascades appear. With respect to our work in canopies, our results go directly against the theoretical expectations. In most of the study sites, we did not find what was predicted for functioning of canopies. Thanks to the state of art work in several canopies by standardized methodology, we got the data which are very unique. Similarly, with identification of predators to species and counting their abundances in standardized manner, we were able to define their feeding guild, biomass which provides much stronger explanatory power than typically used indexes of diversity.