Collaborative Research: STAR: Resilience of a foundational mutualism following the removal of a global "Top 100" invader

This project will test how savanna ecosystems respond to the removal of invasive species. Invasive species are introduced from outside their natural range that rapidly expand across areas where they were introduced. Invasive species negatively affect the economy and can change ecosystems in undesirable ways. Because of these negative effects, many restoration efforts involve removing invasive species. The project will test if ecosystems are resilient and can return to their original state after invasive species are removed. This project will also examine how long it takes for ecosystems to return to their original state once invasive species are removed. Both questions will be answered by experiments done in central Kenya, where an invasive ant has displaced native ants that defend Acacia trees against elephants and other browsing mammals. In invaded areas, browsing on trees is common, transforming savanna woodlands into open landscapes with few trees. Through targeted removal of the invasive ant, the project will discover whether invaded areas can be returned to their original state. Removal of invasive ants may restore the partnership between trees and native ants and reduce browsing by elephants. Across much of East Africa, Acacia trees are critical to the bioeconomy because they provide food for black rhinoceros, giraffe, and other animals. These trees are also used for fuel by humans. This project provides an opportunity to answer important questions about ecosystem resilience. Research in this system will address conservation issues that are relevant for land managers and restoration planning. This project will test the hypothesis that a foundational ant-Acacia mutualism responsible for giving rise to near monocultures of the whistling-thorn tree is resilient following the removal of the invasive big-headed ant. Invasion fronts occur along a rainfall gradient, providing an opportunity to quantify whether and under what contexts the ant-acacia mutualism is resilient to removal of big-headed ants. Specifically, this project will answer two questions: (1) following big-headed at removal, how faithfully do stability-promoting feedbacks return to approximate those from uninvaded areas (i.e., is the foundational ant-acacia mutualism resilient)? (2) does resilience hinge on time since invasion, rainfall, or both? To answer these questions, the project will employ large-scale removal of big-headed ants, assays of photosynthesis, and demographic modeling to quantify the restoration of feedback loops involving symbiotic ant activity, elephant browsing, and whole tree photosynthesis. 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.