Challenges in the early ontogeny of a mutualistic plant: Resource availability and plant defense in juvenile Cecropia ant-plants

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 less 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.