The availability and quality of resources affect agonistic interactions among eusocial territorial groups. In the Acacia drepanolobium ant-plant system, worker density significantly impacts aggression, expansion success, and long term survivorship of Crematogaster mimosae. Colonies of this species decrease aggression and face higher risks of takeovers at lower worker densities, but little is known about the role of host quality in regulating ant reproductive investment, and mediating take-over risks by surrounding competitors. Understanding to which extent do demographic parameters between host and symbiont are coupled is important because C. mimosaedenser colonies are better defenders against large herbivores, but also require higher nectar investment from their host trees. How does host tree performance and investment into the mutualism then impacts ant colony fitness and the effectiveness of this defensive interaction is a core question that remains unanswered. We manipulated local environments to create: enriched host trees using watering and fertilizing (WF), or stressed trees by damaging their main stems (D) so mimicking damage done by stem borrowing beetles. We then examined how modifications to host quality affected ant colony performance and the outcomes of this mutualistic interaction.
After 18 months of treatment initiation WF trees exhibited on average 40% higher growth, 68% more swollen thorns, and 10% more extra floral nectaries than controls. In turn, C. mimosae colonies in WF trees had higher patrolling ant densities and aggression. Although D trees grew more and produced more swollen thorns than controls, this did not translate into more extra floral nectaries, and ant density and aggression remained lowest throughout the experiment. Contrary to our expectations, despite harboring larger and more aggressive ant colonies, WF trees suffered almost 3x more browsing by megaherbivores than D trees. Micronutrient analyses of leaf tissues showed WF trees to be of better quality for herbivores. Overall, our results suggest that host quality and resource availability are important drivers of the effectiveness of this mutualistic interaction. Despite being better defended, their higher nutrient quality made WF trees more attractive to herbivores. Colonies in D trees were 30% more likely to be taken-over by intraspecifics, possibly because lower nectar production negatively impacted activity, protein foraging, reproduction, and ultimately survivorship.