First Swarth Fogel Undergraduate Research Scholar studies vernal pool grazing

UC Merced undergraduate Mark Twomey became the first recipient of the Swarth Fogel NRS Undergraduate Research Scholarship in 2022. The grant enabled Mark to explore the short-term effects of cattle exclusion on vernal pools. Image: Mark Twomey.

By Joy Baccei, Reserve Director, Merced Vernal Pools and Grassland Reserve

Field research can transform the academic and professional careers of undergraduates, and help increase retention rates in college students majoring in the natural sciences. UC Merced has made it a high priority to enable more students to become involved in hands-on field experiences. The UC Merced Swarth Fogel Undergraduate Research Scholarship is a prime example. Offered annually, the scholarship encourages students to run their own research projects by providing a $1,000 stipend and encouraging recipients to develop relationships with a faculty mentor. In doing so, the scholarship invites undergraduates to think about how to answer scientific and management questions related to the Earth and its natural systems. The goal is to increase student excitement for, and self-confidence in, field research experiences.

The first recipient of the scholarship was Mark Twomey, an Environmental Systems Science major. His 2022 scholarship project explores the connections between cattle grazing and floristic diversity in the vernal pools. Mark describes his research experience below.

Twomey examined the effect of excluding grazing dairy cattle on the species diversity of vegetation found in vernal pools. Image: Joy Baccei

By Mark Twomey, UC Merced

What got me inspired to apply to the Swarth Fogel Scholarship was the Flora of California class led by Professor Jay Sexton. It was in our lab class tours and projects where my interest in the vernal pools was ignited. I learned that centuries ago, tule elk and bison once grazed the grasslands of the San Joaquin Valley, including the vernal pools adjacent to UC Merced. Although hunting and modern settlement patterns have driven both species out of the area, I learned that the grazing services they once provided is believed to be replicated by cattle. While earlier studies have shown that cattle grazing improves the diversity of native vernal pool plants in the Central Valley, no similar studies had yet been done on the NRS’s Merced Vernal Pools and Grassland Reserve. With the support of the scholarship, I investigated the theory that grazing conserves local native plant biodiversity in reserve vernal pools.

Professor Sexton and his graduate student, Daniel Toews, served as my primary project mentors. We decided I would examine vegetation inside and outside the cattle exclusion research plots that Daniel had established at the Merced Vernal Pools and Grassland Reserve. The reserve protects more than 6,500 acres of seasonal wetlands that pond water in winter and support a unique suite of animals and plants. This would enable me to evaluate and quantify the short-term impact of excluding cattle grazing on vernal pool biodiversity. Specifically, I was interested in how grazing affected floristic beta diversity–the variety of plant species found between vernal pools.

Over two months this spring, I and my classmate Christian Nunez carried out a series of vegetation surveys within and outside of the cattle exclusion plots. We set out quadrats (square sampling frames), clipped the vegetation that fell within its edges to measure plant biomass, and identified the plant species. I then analyzed the samples in the Sexton laboratory.

As they dry toward the end of spring, the vernal pools of the Merced Vernal Pools and Grassland Reserve bloom with a wide array of native wildflowers. Image: Mo Kolster

What the data show

The biomass samples we collected showed that average plant productivity increased inside pools, suggesting there was a difference between grazed and enclosed sites. However, these differences were not statistically significant, which could have been due to lack of sampling or because the plots had been fenced for an insufficient amount of time for differences to emerge.

Though the project focused mostly on diversity between pools, I also conducted an analysis of diversity within pools. Each vernal pool can be conceptualized as a gradient with three zones: a pool bottom, edge, and upland. Our data showed that each pool had a statistically distinct pool bottom community and upland zone community with a non-distinct edge zone. This is consistent with previous research.

Student researchers Christian Nunez (left) and Mark Twomey head out to their remote field site in the reserve’s 4WD vehicle. Christian was Mark’s field partner on his cattle exclusion research. Image: Joy Baccei

I also conducted a richness and diversity analysis. I considered both richness—the raw measure of the number of species present, and diversity—the number of species plus their percent composition. The results were unexpectedly mixed. There was much more variation among pool communities than expected, with some pools showing a trend but others apparently outliers. There seemed to be no major differences in the mean richness of species present in grazed or ungrazed pools, but a wider spread of species in grazed pools. While this could mean cattle grazing is a stabilizing force on volatility in the ecosystem, indirectly benefiting conservation, strictly speaking this is beyond the predictive capability of the data.

The native vs non-native species analysis was also a bit unexpected. There were almost no differences in native vs non-native species in terms of richness, and small differences in terms of diversity. But again, this analysis suffers from a variance problem and is not statistically significant.

Tidy tips (Layia fremontii, endemic to California, are often seen in spring on the Merced Vernal Pools and Grassland Reserve. Image: Mo Kolster

Lessons for management

This study did not ultimately turn up results robust enough to suggest any changes to current management practices. However, it does raise concerns about the validity of the current theory. This could be because the scale of the project was too small to capture this herbivory-biodiversity relationship. Biodiversity among sets of pools might be related to grazing at a scale that is outside this project design’s ability to comprehend. Future studies should incorporate study design that can account for diversity among individual pools, between individual pools, and between sets of. This would include more sites at a wider breadth of locations, to capture more interactions between grazing and diversity. Another area for future analysis is to investigate whether grazing has a  stabilizing effect on diversity.


I felt that this project was the highlight of my undergraduate degree, provided invaluable hands-on experience, and gave me an incredible opportunity to be mentored. In addition to my direct project mentors, I thank the late Marylin Fogel and Chris Swarth (scholarship founders), Reserve Director Joy Baccei, and UC Merced NRS Director Jessica Malisch for their support. In my field work I enjoyed getting just out of eyeline of the city and stopping for a moment to realize, that all around me, is untamed—natural land.

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