Reproductive Biology in Physaria globosa

Physaria globosa (Short’s Bladderpod) is a federally endangered species in the mustard family (Brassicaceae), occurring only in Tennessee, Kentucky, and Indiana. Unlike most Physaria species, which are concentrated in the arid western U.S., P. globosa represents one of the few eastern members of the genus and occupies specialized limestone habitats along major river systems including the Cumberland and Kentucky Rivers.

The species was listed as Federally Endangered in 2014 due to threats from habitat destruction, overstory shading, competition with invasive species, soil erosion, small population sizes, and altered hydrology. Currently, approximately 30–33 populations are known, most of which are small, isolated, and vulnerable to stochastic environmental changes.

Research Questions

Our lab is actively working to advance the understanding of Physaria globosa reproductive biology, habitat ecology, and conservation needs. Major research questions include:

• What insect species serve as effective pollinators for P. globosa?

• How does reproductive output vary across environmental gradients and populations?

• What abiotic factors influence growth, fecundity, and population stability?

• What is the life history of this species, and what indicators can be identified that help predict population size from year to year?

• Is reproduction limited by self-incompatibility, pollen limitation, or low pollinator visitation?

• How can emerging methods (e.g., eDNA metabarcoding) help inform conservation management?
  

Field Studies and Key Findings

Pollination Ecology

Initial field studies conducted at one of the largest Tennessee populations (Hartsville, Trousdale County) identified a diverse assemblage of floral visitors. Effective pollinators included two syrphid flies (Nemotelus bruesii, Toxomerus geminatus) and four halictid bees (Lasioglossum illinoense, L. versatus, Halictus ligatus, Augochlorella striata) (Thacker et al. 2019). This diversity contrasts with western Physaria species, where ground-nesting bees are often the dominant pollinators.

Reproductive Dynamics and Fecundity

Subsequent work at the same Tennessee site examined patterns of fruit production, flowering stem number, seed set, and resource allocation (Krosnick et al. 2022). Reproductive output was strongly correlated with basal stem diameter and flowering stem number, while environmental factors like light levels and soil depth had weaker effects. Many individuals flower over multiple years, and robust plants are typically found in open-canopy limestone outcrops with shallow soils.

Pollen Limitation Across the Range

Expanded studies across Tennessee, Kentucky, and Indiana documented varying levels of pollen limitation among populations (Powell & Krosnick, 2022). Populations in Kentucky and Indiana showed evidence of pollen limitation, while Tennessee populations did not. These findings emphasize the importance of pollinator availability for some populations.

Range-Wide Pollinator Communities

Waters et al. (2025) documented floral visitors across multiple populations to better characterize pollinator communities range-wide. Early results suggest that while flies were prominent in earlier studies, polylectic bees (e.g., Lasioglossum, Andrena, Ceratina) dominate in many populations. These findings help clarify both local and regional variation in pollination biology for P. globosa.

Emerging Methods & Metabarcoding Insights

In addition to field-based pollination studies, our lab is developing new molecular tools to better monitor plant–pollinator interactions over time. This study utilized environmental DNA (eDNA) extracted from herbarium floral material collected across the range of P. globosa. By targeting COI and 16S markers, researchers recovered over 1.5 million reads spanning 29 distinct taxonomic lineages, including Hymenoptera, Lepidoptera, Coleoptera, birds, and mammals. Remarkably, viable eDNA was detectable even from specimens over 60 years old, demonstrating that herbarium specimens can serve as long-term archives of pollinator interactions for rare plants like P. globosa.

This novel approach opens new avenues for monitoring pollinator communities over historical time periods, which may be particularly valuable for species at risk due to pollinator declines.

Figure 1 from Waters et al. 2005, available here.

Conservation Implications

Collectively, our work provides essential data for federal and state conservation agencies working to manage and recover Physaria globosa populations. These studies:

• Identify effective pollinators necessary for reproduction
• Clarify how habitat characteristics influence plant fecundity
• Document variation in pollen limitation across the species’ range
• Introduce novel eDNA methods for long-term monitoring
• Inform habitat restoration and management strategies for recovery planning

Our research supports the ongoing implementation of the U.S. Fish & Wildlife Service Species Status Assessment and recovery efforts in Tennessee, Kentucky, and Indiana.

Sample Publications

• Thacker, J.H., Krosnick, S.E., Maynord, S.C., Call, G.P., & Perkin, J.S. (2019). Pollination biology and reproductive phenology of the federally endangered endemic Physaria globosa (Brassicaceae) in Tennessee. Journal of the Torrey Botanical Society. 10.3159/TORREY-D-17-00039.1
• Krosnick, S.E., Thacker, J.H., Mattingly, H.T., Call, G.P., Maynord, S.C., Adams, D.S., & Wheeler, K. (2022). Ecological correlates of reproductive output in a Tennessee population of Short’s Bladderpod, Physaria globosa (Brassicaceae). Castanea, 87(1): 20–38. 10.2179/0008-7475.87.1.20
• Waters, C. G., Hurt, C. R., and S. E. Krosnick (2025). Looking to the past to inform the future: what eDNA from herbarium specimens can tell us about plant-animal interactions. Applications in Plant Sciences. 13:e11633. https://doi.org/10.1002/aps3.11633