Mutualism, herbivory, and invasive ants as seasonally dependent drivers of root surface area in a foundational savanna ant-plant.

Abstract

Many plants are defended from herbivory by costly insect mutualists. Understanding positive associations between plants and mutualists requires a whole-plant perspective including roots. We hypothesized that root surface area increases with mutualist activity (to a saturation threshold) and recent rainfall but that this relationship shifts when herbivores are excluded. We also hypothesized that invasive ants limit root surface area and that mutualism breakdown driven by invaders blunts root responses to rainfall and herbivore exclusion. Using minirhizotrons (est. 2021), we surveyed root surface area of ant-acacias during a dry (2022) and then a wet (2023) season. Study plots either excluded or permitted vertebrate browsers, within a natural experiment comparing mutualist-defended ant-acacias to those invaded by a mutualism-disrupting ant. Root area increased with mutualist activity to a threshold, but this positive association was less apparent during rainy periods. Megabrowser exclusion increased overall root area but reduced the threshold for a positive association with mutualist activity and reduced the steepness of the root area-rainfall correlation. Ant-invaded acacias had smaller root areas that correlated less steeply with rainfall. Positive associations between insect defense and root area were thus contingent on rainfall, herbivory, and biotic invasion, drivers that are shifting under global change.