Periodic Reporting for period 3 - BABE (Why is the world green: testing top-down control of plant-herbivore food webs by experiments with birds, bats and ants)
Periodo di rendicontazione: 2021-12-01 al 2023-05-31
Our goal is to test novel aspects of the “green world hypothesis” and resolve the importance of top-down control and how varies with forest diversity and productivity along a latitudinal gradient, and how the key predators, birds, bats and ants, contribute to the top-down effects individually and in synergy. Unlike other studies, our project aims to resolve these questions in the forest canopies, the places where food-webs are predicted to be stronger that in forest understories, but which are difficult to access. It is important to understand these questions because while there is some evidence that predators reduce herbivore abundance, the importance of top-down control is poorly understood across a range of forests. The importance of key predatory groups, and their antagonistic and synergic interactions, have been rarely studied, despite their potential impact on ecosystem dynamics in changing world. Without knowing how much the individual types of predators contribute to functioning of the whole forest ecosystems, we can’t correctly calculate the impact of changes of diversity and abundance of predators on lower trophic levels. A similar study was not possible before, because factorial experimental exclusion of predatory groups replicated on a large spatial scale is logistically difficult and canopy access network along a latitudinal gradient has only recently become available.
Our project consists of a standardized experiment which we replicate at several study sites. Our aim was to do it at 6 sites, in Japan, China, Malaysia, Papua New Guinea, and two study sites in Australia. So far, we managed rather well and finished all field collections at 4 sites completely, while we continue field collection in Papua New Guinea and aim to finish it in January 2022. Unfortunately, we failed to conduct the field work in Malaysia and will replace this site by a work in Leipzig next year (2022), and we describe the issue in the section below.
Specifically, the work at each of the study sites consist of the manipulative experiment during which we prevent access of predators to saplings in forest understory and to branches in canopy. First, after the arrival to site, we search for the saplings and trees of our focal plant species, preselected based on their traits, phylogeny and abundance. We mark 280 saplings in understory (35 individual per each of the 8 plant species) and assign 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 assign them to 4 treatments (CN2, ALL, VER, ANT). Exclosures of vertebrates (VER), birds (BIRD) and bats (BAT) are conducted by large cage exclosures, which are either placed at site permanently or are opened daily in the morning and in the afternoon to prevent access only of the respective predator. Exclosure of ants (ANT) are conducted by tangle glue, a sticky insect barrier applied to the branches or trunk, to prevent access of ants. ALL treatment then combines both treatments – cages and sticky barrier. Once we set the treatments, we mark permanently and count all leaves within the enclosed saplings or branches, photograph their herbivory damage, and collect all arthropods to start the experiment. After that, we manage the smaller bird & bat experiment (BIR, BAT, CN1 treatments) for a month, and survey the effect of the treatment after that. It means, we collect again completely the newly established arthropod communities, all leaves, scan them for survey of change in herbivory damage, and keep leaves for chemical analyses of nutrients and plant defences. In the meantime, we keep surveying communities of predators we manipulate. Birds and bats are surveyed passively and ants are collected from sugar and tuna baits. The whole one experiment thus lasts ca. 5 months in the field and after that, the work continues in the laboratory where all collected arthropods are identified into morphospecies, measured. Leaves are analysed for herbivory damage and chemical analyses of traits and defences. About 6 months later, we replicate the whole experiment once again at the same site.
At each of the study site, we also surveyed predation rata by exposing dummy caterpillars. This work is a supportive to the main experiment, but as it was the least time demanding, the resulting manuscript is already prepared and submitted to Science. We found very interesting results there, contradicting partly all previous patterns found in forest understories, and explain them by the abundance of predators, which also has never been a part of similar studies.
Saying so, we are continuously analysing the diversities of abundances of focal predators. There, the abundances differ significantly between the forest understories and canopies and sites, ranging between 95 and 845 insectivorous birds per 4 hours long survey. Considering the biomass of birds and their feeding specialization we analyse; we are getting very interesting data which we believe wil be very important for global understanding of the importance of predators. Also, in the abundance and richness of ants, striking patterns are appearing as we so far analysed and identified 92,183 ants to species. Surprisingly, analyses of their feeding specialization reveal that not all can be considered as predators, which is in line with their effect in lower trophic levels we observe but against the expectations. In conclusion, the first years of the project were heavily packed with intense field work, work on research permits and import permits and we are slowly getting to the point where we are summarizing all accumulated data together.
Specifically, the work at each of the study sites consist of the manipulative experiment during which we prevent access of predators to saplings in forest understory and to branches in canopy. First, after the arrival to site, we search for the saplings and trees of our focal plant species, preselected based on their traits, phylogeny and abundance. We mark 280 saplings in understory (35 individual per each of the 8 plant species) and assign 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 assign them to 4 treatments (CN2, ALL, VER, ANT). Exclosures of vertebrates (VER), birds (BIRD) and bats (BAT) are conducted by large cage exclosures, which are either placed at site permanently or are opened daily in the morning and in the afternoon to prevent access only of the respective predator. Exclosure of ants (ANT) are conducted by tangle glue, a sticky insect barrier applied to the branches or trunk, to prevent access of ants. ALL treatment then combines both treatments – cages and sticky barrier. Once we set the treatments, we mark permanently and count all leaves within the enclosed saplings or branches, photograph their herbivory damage, and collect all arthropods to start the experiment. After that, we manage the smaller bird & bat experiment (BIR, BAT, CN1 treatments) for a month, and survey the effect of the treatment after that. It means, we collect again completely the newly established arthropod communities, all leaves, scan them for survey of change in herbivory damage, and keep leaves for chemical analyses of nutrients and plant defences. In the meantime, we keep surveying communities of predators we manipulate. Birds and bats are surveyed passively and ants are collected from sugar and tuna baits. The whole one experiment thus lasts ca. 5 months in the field and after that, the work continues in the laboratory where all collected arthropods are identified into morphospecies, measured. Leaves are analysed for herbivory damage and chemical analyses of traits and defences. About 6 months later, we replicate the whole experiment once again at the same site.
At each of the study site, we also surveyed predation rata by exposing dummy caterpillars. This work is a supportive to the main experiment, but as it was the least time demanding, the resulting manuscript is already prepared and submitted to Science. We found very interesting results there, contradicting partly all previous patterns found in forest understories, and explain them by the abundance of predators, which also has never been a part of similar studies.
Saying so, we are continuously analysing the diversities of abundances of focal predators. There, the abundances differ significantly between the forest understories and canopies and sites, ranging between 95 and 845 insectivorous birds per 4 hours long survey. Considering the biomass of birds and their feeding specialization we analyse; we are getting very interesting data which we believe wil be very important for global understanding of the importance of predators. Also, in the abundance and richness of ants, striking patterns are appearing as we so far analysed and identified 92,183 ants to species. Surprisingly, analyses of their feeding specialization reveal that not all can be considered as predators, which is in line with their effect in lower trophic levels we observe but against the expectations. In conclusion, the first years of the project were heavily packed with intense field work, work on research permits and import permits and we are slowly getting to the point where we are summarizing all accumulated data together.
As the first project ever, we managed to replicate standardized manipulative experiment aiming to exclude various predators individually and in combinations. We study their synergic and individual effect on trophic cascades, at several sites globally and mainly also 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. Despite our methods used are not all new, their precision and scale 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 so far go directly against the theoretical expectations. In most of the study sites, we did not find what was predicted for functioning of canopies based on the previous work (mostly originating from a single crane in Panama). Thanks to the work in several canopies by standardized and precise methodology, we are getting data which are very unique. Similarly, with identification of predators to species and counting their abundances in standardized manner, we can define their feeding guild, biomass which provides much stronger explanatory variables for the effect we observe, than typically used indexes of diversity. Our state of art thus lies on the scale and precision of the survey, which is rarely achieved.