Neha Mohanbabu: Large herbivore impact on plant biomass

Throughout April, we are featuring the articles shortlisted for the 2022 Harper Prize. The Harper Prize is an annual award for the best early career research paper published in Journal of Ecology. Neha Mohanbabu’s article ‘Large herbivore impact on plant biomass along multiple resource gradients in the Serengeti‘ is one of those shortlisted for the award.

👋 About me

My interest in ecology developed serendipitously. Growing up in Bengaluru, a large and densely populated metropolitan city in India, I had limited opportunities to spend time outdoors, and as a result was not inherently curious about nature. I was, however, always fascinated by the idea of doing research and moved to the Indian Institute of Science Education and Research (IISER-Pune) to pursue an integrated BS-MS degree. My first summer at IISER, I got the opportunity to work with Nature Conservation Foundation on a snow leopard camera trapping project at their field site in the trans-Himalayas. Disappointingly I never saw a snow leopard, but participating in the various ongoing projects gave me a chance to notice and appreciate patterns in nature. I spent the next few years of my undergraduate education exploring different sub-fields in ecology. Eventually, for my final year project, I worked with Dr. Deepak Barua and Dr. Mahesh Sankaran to study within species variation in plant traits for widely distributed tree species in western and eastern ghats (vast mountain ranges in peninsular India). These experiences further motivated me to pursue a PhD in ecology. Graduate education at Syracuse University exposed me to a completely different ecosystem – the grasslands and savannas of Eastern Africa. During my first visit to the Serengeti National Park in Tanzania, I was amazed at the diversity of plants and herbivores and was inspired to develop a dissertation project focused on plant-herbivore interactions. After a few rounds of discussion with my advisor, Dr. Mark Ritchie, I decided to study the role of multiple resource limitation of plant-herbivore interactions and their influence on plant defense traits using both theoretical and empirical approaches.

Following my PhD, I moved to the University of Minnesota to work as a postdoctoral researcher with Dr. Peter Reich, Dr. Forest Isbell, and Dr. Sarah Hobbie to study responses of grassland plants to multiple global change drivers. I am currently working on better understanding species- and functional group-specific responses to nitrogen, CO2, temperature, and drought manipulation in a 25-year long experiment. I am excited about leveraging this unique experimental setup to explore other consequences for plant communities.    

🔎 About the shortlisted paper

At the beginning of my PhD, my main interest was to study responses of plant defenses to large mammal herbivory in herbaceous plants. To achieve this, my advisor and I designed a study to measure chemical and physical defenses for several commonly occurring species across different resource gradients in the Serengeti. Although our initial plan was just to focus on rainfall and nitrogen [N], we decided to include phosphorus [P] as an afterthought, given the wide P gradient in the Serengeti. To my surprise, even preliminary patterns in some plant defense traits were showing strong associations with soil P. This unexpected finding inspired me to explore the influence of P on herbivory impact, a potentially important determinant of plant allocation to defenses in a system with intense herbivory.

Credit: Mohanbabu 2022, Journal of Ecology

We decided to use a simple consumer-resource framework to understand the potential role of different types of resources on plant-herbivore interactions. A growing body of evidence from different ecosystems had shown that plants can be simultaneously limited by multiple resources, implying that plant biomass (and other functions) can respond to the addition of more than one resource. However, whether and how this limitation by multiple resources may influence herbivory is less understood in terrestrial systems. Given this, we developed several hypotheses on how herbivore impact may change for different resource gradients including P gradients. We posited that 1) herbivore impact may indirectly respond to any resource that increases plant biomass as large mammalian herbivores can be limited by availability of plant biomass; 2) herbivore impact may respond to plant quality which may be lower at high rainfall sites and higher at nutrient-rich sites; and 3) herbivore impact may be low at resource-rich sites as these plants can grow quickly and compensate for any loss to herbivores. Instead of designing a new study, we decided to use data from an existing long-term grazing exclosure experiment in the Serengeti as it spans a wide range of rainfall and edaphic gradients, and data on plant biomass were available for multiple years spanning two decades.

We found that herbivore impact, measured as a ratio of plant biomass in fenced to biomass in unfenced plots, decreased with rainfall, increased with P availability, and did not change with N availability. This combination of patterns with different resources suggests that plant quality, in this case driven by plant P, was an important predictor of herbivore impact in the system.

To our knowledge, this is the first empirical study in a terrestrial system to show a strong positive association between large herbivore impact on plant biomass and plant P content. Our work highlights the importance of P to mammalian herbivores in terrestrial systems and emphasizes the need to consider the unique contributions of different resources while studying trophic interactions, especially given the rapidly changing precipitation patterns and biogeochemical cycles globally.

Find Neha on Twitter and GoogleScholar.

Read the full list of articles shortlisted for the 2022 Harper Prize here.

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