Allergies are on the rise for many Californians as spring has sprung. But while our eyes may be itchy, the pollinators are loving it! Many species are closely tied to the sneeze-bringing flowers that you may have seen recently blooming. In today’s throwback to Rebecca Nelson’s Field Notes article from 2021, we take a dive back into plant-pollinator relationships.

At least 87.5% of flowering plant species rely on animal pollinators such as bees, birds, bats and butterflies for help with reproduction [1]. Plant-pollinator mutual relationships contribute to biodiversity, ecosystem stability, and promote food security through crop pollination [2]. Much of the food and medicine we use comes from plants that need pollinators to reproduce. For example, bees pollinate apples, coffee, blueberries, carrots and many other crops and vegetables. Pollinators and pollinator-dependent plants, however, are declining rapidly due to climate change, invasive species, and habitat loss [2-4]. As a graduate student in the Harrison Lab at UC Davis, I study how invasive species affect plant-pollinator relationships and what the implications are for grassland restoration. 

This spring, I am in the middle of my first season of fieldwork at the University of California McLaughlin Reserve in northern California. I’m very excited to be out here at the reserve! What drew me to McLaughlin is its diversity of soils. Different soils create different types of grasslands, some with and some without invasive flowers. For example, the reserve has unique, nutrient poor soil called serpentine soil that provides a refuge for native California wildflowers. The lack of nutrients on serpentine soils prevents most non-native plant species from growing. In contrast, the regular, non-serpentine soils at McLaughlin support a grassland of mostly non-native plant species. I am studying how invasive grassland flowers affect plant-pollinator interactions in the neighboring serpentine grassland. 

At McLaughlin, I am on a quest to find and study an invasive plant called hairy vetch. Hairy vetch is a pea-like plant that originated from Europe and Asia. It likely gets its common name from the hairy texture of its leaves. Vetch is often used in agriculture to help restore nitrogen to the soil because the vetch plant has special microbes in its roots that can add more nitrogen to the soil. At McLaughlin as well as throughout California, vetch has become one of many non-native species to establish in the grasslands. However, it can also attract a diversity of native pollinators including bumblebees [5]. 

Here’s what I do on typical morning of fieldwork! I walk through the grassland, carrying my phone and notebook. The hills shimmer with expansive light. Lichen-splattered boulders thrust up from the grass. A red-tailed hawk wheels above the pines and oaks. I take note of the location and abundance of any vetch I encounter, using the CalFlora Observer Pro app. You can download this app for free on your phone and use it to keep track of what types of plants you find throughout California. Wind ripples through the grassland, forming a sighing sea of green. A cloud of red-winged blackbirds whooshes up. 

I decide to focus on boundaries between the serpentine and regular grasslands. Vetch grows only in the regular grassland, while the serpentine grassland has lots of native plants. These boundary areas thus provide an opportunity to examine how the presence of vetch affects the pollination of nearby native plants. I will observe which pollinators visit which plants at varying distances away from the boundary. My hypothesis is that the vetch will have a greater negative effect on native flowers closer to the boundary by competing with them for pollinators. However, it’s also possible that the vetch could help the native plants by drawing in more pollinators to the boundary area. 

The main pollinators of these flowers are bees. Many different types of bees live at McLaughlin, ranging from tiny, shiny-looking bees that nest in the ground (e.g. the sweat bee Halictus ligatus) to large fuzzy bumblebees that live in social groups (e.g. the yellow-faced bumblebee Bombus vosnesenskii). Most of the bees found at McLaughlin are native to California, but honeybees (Apis mellifera), which originate from Europe, are also present. Flies, moths and butterflies further pollinate the flowers. Several species of fuzzy black and gold flies (bee flies Bombyliidae family) mimic bees in their appearance. To observe the pollinators, I will visit each boundary area for a consistent period of time and record the types of insects that visit each flower. From this data, I hope to create a network of plant-pollinator interactions. Similar to how looking at a social network can tell us who is an influencer, a plant-pollinator network can provide information on which plants are important to pollinators and vice versa. This knowledge can help us figure out which species to prioritize for conservation efforts. By looking at the shape of the network and the types of interactions present, I can infer if vetch is competing or helping native plants in this plant-pollinator community. 

To conduct my research, I am currently living at the field station of the McLaughlin Reserve which has dorm style beds, a cozy lounge, and a kitchen. The nearest town of Lower Lake is about 15 miles away. The reserve is on the site of a former mine, so the landscape is marked with quarries and piles of leftover mining waste. I have limited cell service where I work, so I carry a radio around with me. I enjoy the feeling of solitude that comes with wandering around the soft pastel meadows of wildflowers. 

These experiences are a lot different than my first visit to the reserve in October of 2020 after a fire had swept through the reserve. I felt like I was stepping into Mordor from Lord of the Rings. Fields of bare, black ash stretched on and on, punctuated by occasional oaks. Scorched red manzanita bark glinted. Fire can have positive or negative effects on the diversity of plant-pollinator interactions, depending on the frequency and strength of the fire [6]. In addition to the fire, a drought occurred during the winter leading up to my first field season. Droughts can reduce the amount of pollen and nectar available for pollinators, which can be bad for pollinator health [7]. By comparing data I obtain from this fire-drought season to future years of fieldwork, I hope to get a sense of how year to year variation in weather may affect the types of plant-pollinator relationships I observe. Monitoring these sites across years with differing amount of fire and rain can help inform how climate change affects plants and pollinators. 

Ultimately, I seek to investigate the extent to which native flowers compete with non-native vetch for pollinators. I further aim to understand how year to year variation in rainfall can affect these plant-pollinator relationships. The combined stressors of invasive species and climate change may jointly disrupt plant-pollinator relationships.  I hope these data can inform the restoration of grassland plant-pollinator interactions in a changing world. 

Rebecca Nelson is a PhD student in the Graduate Group in Ecology at UC Davis in Dr. Susan Harrison’s lab. She studies how invasive species and restoration strategies affect plant-pollinator interactions. 

Acknowledgements

I would like to thank my Major Professor Susan Harrison for her guidance as well as UC McLaughlin Reserve Directors Cathy Koehler and Paul Aigner. I would further like to thank Alexis Grana and Bita Rostami for their assistance in the field and the greenhouse. This research is sponsored by Hedgerow Farms and funded through a scholarship from the California Native Grasslands Association. All nature photos were taken by the author. The photo of the author in the field was taken by Dylan MacArthur-Waltz. 

I would like to acknowledge the land on which I conducted fieldwork. For thousands of years, this land has been the home of Patwin and Miwok peoples. The Patwin and Miwok peoples have remained committed to the stewardship of this land over many centuries. It has been cherished and protected, as elders have instructed the young through generations. I am honored and grateful to conduct my research on their traditional lands. 

References

[1] Ollerton, J., Winfree, R., & Tarrant, S. (2011). How many flowering plants are pollinated by animals? Oikos, 120(3), 321-326.

[2] Potts, S. G., Biesmeijer, J. C., Kremen, C., Neumann, P., Schweiger, O., & Kunin, W. E. (2010). Global pollinator declines: trends, impacts and drivers. Trends in ecology & evolution, 25(6), 345-353.

[3] Biesmeijer, J. C., Roberts, S. P., Reemer, M., Ohlemüller, R., Edwards, M., Peeters, T., & Kunin, W. E. (2006). Parallel declines in pollinators and insect-pollinated plants in Britain and the Netherlands. Science, 313(5785), 351-354.

[4] Vanbergen, A. J., & Initiative, T. I. P. (2013). Threats to an ecosystem service: pressures on pollinators. Frontiers in Ecology and the Environment, 11(5), 251-259.

[5] Harmon‐Threatt, A. N., de Valpine, P., & Kremen, C. (2017). Estimating resource preferences of a native bumblebee: the effects of availability and use–availability models on preference estimates. Oikos, 126(5), 633-641.

[6] Carbone, L. M., Tavella, J., Pausas, J. G., & Aguilar, R. (2019). A global synthesis of fire effects on pollinators. Global Ecology and Biogeography, 28(10), 1487-1498.

[7] Waser, N. M., & Price, M. V. (2016). Drought, pollen and nectar availability, and pollination success. Ecology, 97(6), 1400-1409.

[Edited by Josie Hubbard & Maggie Creamer, and re-formatted by Cassidy Cooper]