ECR feature: Cavity-nesting pollinators and their antagonists with Antonia Mayr

Antonia V. Mayr is a Postdoc at the University of Würzburg department of Animal Ecology and Tropical Biology. Her research is based in tropical mountain ecology and focuses on questions about how climate and land use change affect species, and how functional and phylogenetic diversity relate to changes in ecosystem services. Antonia provides background information on her recent work, which examines cavity-nesting pollinators and their natural antagonists on Mt. Kilimanjaro, Tanzania, where a large elevational gradient gives rise to different climates and ecosystems on a relatively small spatial scale.

(left) Antonia while doing her favourite work – fieldwork! She was also called mama nyuki (mother of the bees in Kiswahili) by her Tanzanian field assistants. (right) Antonia’s fieldwork was much nicer with often very interested friends. Here, they are checking occupied nests to see if they already hatched. If yes, Antonia took them to the field station, if no, she placed them back in the bucket.

Links: Institutional webpage | Research Gate

Institution: University of Würzburg, Department of Animal Ecology and Tropical Biology

Current academic life stage: Postdoc

Research interests: My main research theme is tropical biology, especially tropical mountain ecology. Specifically, I am working on questions about how climate and land use changes affect species, but also functional and phylogenetic diversity and how this relates to changes in ecosystem services. I am fascinated by trophic interactions between plants, pollinators, predators and parasitoids and am very much interested in how to translate these findings into practical recommendations for conservation biology.

Current study system: Currently, I am studying cavity-nesting pollinators and their natural antagonists on Mt. Kilimanjaro, Tanzania. This is a very cool combination of study systems because the large elevational gradient of Mt. Kilimanjaro offers the possibility to study completely different climates and ecosystems on a relatively small spatial scale. Within few days, you can walk from the hot savanna through different mountain rainforests up to alpine ecosystems. Furthermore, the study of so-called trap nests enables us to investigate insects of different trophic levels, to collect data about functional and life-history traits and to study host-antagonist and food networks – which would be hardly possible to collect on this scale using only observational data.

A view of the Kilimanjaro summit from the field station in Nkweseko.

Recent paper in Journal of Biogeography: Mayr AV, Peters MK, Eardley CD, Renner ME, Röder J, Steffan-Dewenter I. 2020. Climate and food resources shape species richness and trophic interactions of cavity-nesting Hymenoptera. Journal of Biogeography 47: 854-865.

Motivation for the paper: Biologists have investigated the drivers of species richness for centuries. For insects, temperature, resource availability and top-down regulation as well as the impact of land use are considered to be important factors determining diversity. However, the relative importance of each of these factors is unknown. The steep climatic gradient and different land-use regimes at Mt. Kilimanjaro, together with the trap-nest system enabled us to simultaneously investigate the effects of temperature, resources, top-down control and land use on species richness of different trophic levels. With this paper, we hope to contribute to the discussion on drivers of insect species richness.

Key methodologies: We used trap nests, which are good bio-indicators for habitat quality and environmental changes, to collect and monitor cavity-nesting bees, wasps and their natural antagonists. To the best of our knowledge, this is the first trap-nest study in East Africa. We continuously monitored trap nests for 15 months, checking monthly for new nests and recently emerged insects. If you include the pre-experiment phase – in which we placed trap nests up to 4,240 m a.s.l and adapted the methodology – and the closing phase, our monitoring lasted 26 months in total. It is important to note that we hatched every nest on its respective study site. Normally, trap nests are collected, cut open and reared in the lab (in temperate regions during winter months) and/or brought back into nature after identification. In contrast, we did not take the nests to the research station before they hatched. The reason was that by working along an elevational gradient we would have changed the climatic conditions during the development of the inhabitants. This extra effort enabled us to collect year-round data about the occurrence of species, their development time and natural mortality, and link it to climate data.

(top left) A pair of trap nests with the first version of the roofs, field assistant Jumanne, and Antonia. (top middle) An extra bucket attached to wooden poles with occupied nests waiting for hatching. Antonia closed the nests with metal nets instead of plastic nets because some bee species were able to cut their way out of it with their strong mandibles. (top right) An edited version of our roofs – the double-folded metal shields allowed the air to pass through and the nests underneath did not heat up anymore in a way that many larvae died. (bottom) Trap nests serve as a model system to investigate cavity-nesting communities and trophic interactions in different trophic levels. The picture shows the three different functional host groups which are subject to this study: bees (Apidae, Colletidae and Megachilidae), caterpillar-hunting wasps (Eumeninae) and spider-hunting wasps (Pompilidae, Sphecidae and Crabronidae), their respective natural antagonists, food resources and type of top-down effects. The trophic level affiliation is indicated by the colours and the colourful circles in green, blue, violet and red show the groups for which we analysed species richness patterns in this study.

Unexpected challenges: The biggest unexpected challenge was that a well-established method does not work well in super-hot and dry ecosystems, like the savannah during dry season. We used reed internodes (which are the sections of a reed stem) to artificially imitate natural cavities for cavity-nesting Hymenoptera. Bees in particular willingly nested inside the provided reed internodes. However, in our pre-experiment, many bee larvae died because they were either grilled by the heat and aridity or they drowned in their melted food made out of pollen and nectar. Our solution was to use double-folded metal sheets instead of plastic as roofs, allowing the air to pass through.

It was also surprising that there was a clear line at the border of the national park, above which no reed internodes inside the trap nests were occupied. Apparently, the relative humidity in the lower montane rainforest was too high and the temperatures too low to enable the occurrence of cavity-nesting Hymenoptera. Another lesson to be learned was that basically every material is valuable. It seems trivial, but it was very helpful to use torx screws to attach the roofs and trap nests to the wooden poles, as torx screwdrivers are still uncommon in Tanzania. This saved our materials from being taken away.

(left) A cut open brood cell. Here, the host, a megachilid bee did not emerge, but died for an unknown reason. (right) Pinning of bees and wasps for later identification. (bottom) Nest in which the megachilid bee mum used duct tape instead of leaf discs to construct the broodcells of her offspring.

Major result and contribution to the field: With 38 bee and 43 stem-nesting wasp morphospecies and 49 morphospecies of natural antagonists, we observed a very high diversity in roughly 4,050 nests. Our data suggest that temperature is a major driving factor for species richness patterns in bees, wasps and their natural antagonists. Furthermore, we found more trophic interactions in the warmer climates of the mountain, i.e. lowland ecosystems. By systematically analysing different trophic levels, we found that the importance of food resources increased for natural antagonists at higher trophic levels. Thereby, our study contributes to the discussion about the drivers of biodiversity along elevational gradients and provides novel insights into the relative importance of temperature, resources, trophic level and biotic interactions for bees, wasps and their natural antagonists.

What are the next steps? The next steps with this trap-nest dataset will be first, to understand seasonal changes in occurrence of species of different trophic levels. This is important because insect communities vary seasonally with related changes in climate and resource availability, strength of competition or pressure by natural antagonists, but are not well investigated in tropical mountain ecosystems. The second step is to investigate microclimatic effects on survival rates in savannah ecosystems, because upper thermal limits will first be reached in the lowland savannahs. Hence, it is useful to investigate how species cope with higher temperatures in partially warmer microhabitats to forecast effects of climate change. The third step is to investigate if traits are affected by land use, because traits may respond earlier to land use changes than species. Therefore, changes in traits might forecast changes in species compositions with increasing land use. The final step is to investigate if temperature and land use have an effect on host-parasitoid networks, which might be even more sensitive to environmental changes than species-richness patterns because they depend on very specialized species interactions. 

If you could study any organism on Earth, what would it be and why? I was never totally restricted to one organism/group of organisms (before, I worked with ants). The better you get to know a group of organisms, the more fascinating they become. But I like diversity and enjoy not having only one model species. That is why I am very happy at the moment with bees and wasps, which show a high variety of forms and behaviour, e.g. in sociality and nesting. In addition, they inhabit very different ecosystems which not only makes fieldwork manifold, but offers the possibility to investigate adaptations to different environments, too.

Any other little gems you would like to share? Really impressive for me was the high diversity of nest-building behaviour. I was able to distinguish around 35 different types of nest closures – and this is only a study about cavity-nesting bees and wasps! What was outstanding for me, however, was that over time I discovered nests of leaf-cutter bees that used duct tape instead of leaves to build their broodcells and nest closures, with which I fixed broken trap nests. In the heat of the savannah, the duct tape stuck to my fingers so much that it was difficult for me to handle. How the bees cut the artificial pieces of leaves out of it without sticking to it is a complete mystery to me. Maybe bees are way more flexible and creative than we think they are…

(top) Diversity of nest closures, built out of soils, resins, plant leaves or fibres or secrets. (bottom) Antonia and her Tanzanian field assistants, who not only helped her to carry heavy luggage up the mountain, shoot trap nests high up the trees with a slingshot and check the trap nests, but they also contributed many creative and practical field solutions.

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