Challenges in the Early Ontogeny of a Mutualistic Plant: Resource Availability and Plant Defense in Juvenile Cecropia Ant-Plants

Abstract

Mutualistic species often must survive periods of their development without their mutualist partner, but we lack a clear understanding of the ecological mechanisms that maintain mutualisms despite these gaps in partnership. In ant-plant protection mutualisms, plants house ant colonies that deter herbivores. Yet juvenile ant-plants often lack symbiotic ant colonies and must withstand herbivory pressure until they are colonized by ants. A sapling's ability to host ants or to employ alternative direct defenses, such as leaf secondary metabolites, may depend on access to key resources, like light, soil nutrients, and water. Alternatively, juvenile ant-plants may receive biotic protection at little resource cost from generalist predators, such as spiders. We examined whether juvenile Cecropia trees maintained leaf defenses, and whether sapling ant and chemical defenses were associated with the sapling's access to resources. We surveyed three species of naturally occurring juvenile Cecropia trees across a rainfall gradient in northwest Costa Rica. We found that both ant defense and chemical defense were regulated by the availability of light, soil fertility, and water in Cecropia saplings. Rather than trade off, larger saplings and saplings with more resources were more likely to invest in both defense strategies, whereas smaller saplings and saplings with fewer resources appeared to have little leaf defense. We also found that although spiders were common on such resource-poor, undefended saplings, spiders did not reduce herbivory. This study highlights the importance of resource availability in determining the performance of ant-plants during early ontogenetic stages.