CAREER: Top-down and bottom-up controls on species coexistence in a variable world

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Part 1. The diversity of plants species in a given ecosystem can be shaped by resource availability (called a “bottom up process”), and herbivorous animals (called a “top down process”). Ecologists frequently ask how so many species can coexist in a community? Multiple limiting resources can increase species diversity; for example, if one plant species is more competitive for nitrogen and another is more competitive for water, limitation in both resources can help both species persist in the system without one outcompeting the other. Temporal variability in resources can also increase diversity; for example, if some plant species grow fast under wet conditions, but others better tolerate dry conditions, rainfall variability over time can help to maintain more plant species in a site. At present, the temporal dynamics of resources that limit plant growth are changing – the amount of available nitrogen has doubled due to atmospheric inputs from human-related sources, and the climate is shifting – with potentially profound consequences for plant biodiversity. Top-down processes like grazing animals or insect herbivores, may moderate the effect of these resource changes, however; for example, an increase in the growth of species favored by nitrogen enrichment may be offset by animals favoring those N loving plants. This CAREER project first experimentally disentangles the effect of shifting bottom-up and top-down processes in a highly diverse California grassland. Second, it assesses how these patterns shift along climate gradients by capitalizing on data from the a globally distributed experiment called NutNet, that has been manipulating nutrients and grazing animals in grasslands around the world for 10+ years. The overall goal of this project is to advance our understanding of the processes that maintain plant biodiversity and to further the capacity to predict how biodiversity will respond to global change. The project integrates research and educational aims throughout by involving undergraduate and graduate students in data collection, management, and synthesis.Part 2. Modern coexistence theory has emerged as a tool to partition the mechanisms of coexistence among species, providing a potential framework to predict how diversity will respond to on-going global environmental change. However, modern coexistence theory has been notoriously difficult to test empirically in field settings, reducing its applicability to natural systems. This CAREER project extends empirical tests of modern coexistence theory to quantify how bottom-up nutrient addition and top-down consumer pressure alter coexistence in environments varying in rainfall both across years, and over space. First, in an annual grassland system characterized by high climate variability, the project experimentally targets key resource-consumer-competitor interactions and interprets them using quantitative methods that partition coexistence mechanisms in an empirically tractable way. In addition, it parameterizes the range of conditions that support multi-species coexistence, better reflecting community dynamics than the pairwise species comparisons frequently employed in coexistence models. Second, the project assesses the generalizability of these patterns by synthesizing Nutrient Network data within a graduate seminar that trains students in team science, data analysis and data visualization. In addition, local Nutrient Network data is collected in conjunction with an undergraduate course sequence that pairs a summer field methods class with a data science class that uses the field-collected data. Finally, the project incorporates an independent undergraduate research program to support thesis students to test additional top-down and bottom-up dynamics at a local Nutrient Network site.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.