Host tree manipulations effects in colony growth, aggression, survivorship, and the general outcomes of a keystone defensive ant-plant mutualism from East Africa

Dr Juan Ruiz will present the Department of Biological Sciences seminar, with a talk entitled, "Host tree manipulations effects in colony growth, aggression, survivorship, and the general outcomes of a keystone defensive ant-plant mutualism from East Africa">

In the Acacia drepanolobium ant-plant system four mutually exclusive symbiotic ants compete aggressively for sole possession of host trees. Trees provide nesting ground and nectar in exchange for protection against herbivores, but because most mature trees are occupied, to expand territories colonies often engage in intra- and interspecific wars that result in very high worker mortality and take-overs. Our previous work on this mutualism shows that worker density can significantly impact colony aggression, expansion success, and the long term survivorship of colonies of the co-dominant species Crematogaster mimosae. In this obligated symbiotic ant, colonies reduce aggression and face higher mortality risks when experiencing a low density period. Despite multiple studies demonstrating that resource limitation is a critical modulator of agonistic interactions among eusocial territorial groups, little is known about how fluctuations in the investment that host trees place into the mutualism (host quality) can have in modulating competitive ability, and the take-over risks from surrounding competitors. Understanding the extent to which the demographic parameters of the host and its symbiont are coupled is important because C. mimosae denser colonies are better defenders against large herbivores, but larger colonies require higher nectar investments that are energetically taxing for the host. How does host tree performance and resource investment into the mutualism affect colony fitness and the effectiveness of this defensive interaction is a critical question that our current work aims to explore. We eliminated competition with grasses by using herbicides, and subsequently manipulated local environments either by: enriched host trees through watering and fertilizing (WF), or stressing trees by damaging main stems to mimic an infestation by cerambycid stem borrowing beetles (D). We then examined how our manipulations impacted host quality (investment into the mutualism), affected ant colony performance, and the general outcomes of the mutualistic interaction. Our manipulations and monitoring were conducted longitudinally, and after 24 months of treatment initiation WF trees exhibited on average 40% higher growth, 68% more swollen thorns, and 10% more extra floral nectaries than our control trees. In turn, C. mimosae colonies inhabiting WF trees had higher patrolling ant densities and were more aggressive towards intruders. Although trees in the damaged treatment grew more rapidly and produced more swollen thorns than the controls, this did not translate into a higher number of extra floral nectaries, and both ant density, and ant aggression remained lowest throughout the experiment. Contrary to our expectations, despite harbouring larger and more aggressive ant colonies, WF trees suffered almost 3 times more browsing by mega-herbivores than did trees in the D treatment. Chemical analyses of micro-nutrient contents from fresh leaf tissues showed that the foliage of WF trees appears to be of higher nutritional 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, the higher nutrient quality of their leaf tissues made WF trees more attractive to mega-herbivores. In turn, ants inhabiting D trees were 30% more likely to be taken-over by an intraspecific competitor. It is possible that lower nectar production in D trees negatively impacted ant activity, protein foraging and reproduction, and ultimately reduced competitive dominance and the survivorship of these colonies.


Juan is a postdoctoral fellow at the department of Evolution and Ecology at The University of California Davis. He has an MSc. in Applied Ecology and Conservation from The University of East Anglia in Norwich, and a PhD in Ecology and Evolution from The University of Edinburgh in Scotland. He is a passionate evolutionary ecologist that is greatly concern about the effects of biodiversity loss in ecosystem services. He has conducted extensive fieldwork in Africa, Asia, Europe and the Americas where he had the pleasure to study several different species in terrestrial and marine environments. Overall, his research addresses questions related to the ecology and evolution of species interactions, in particular about how mutualisms shape population and community dynamics. He has studied pollination interactions in African wild plants, examined community assemblages using network approaches, and studied how spatio-temporal variations in resource and species diversity impact the structure of plant communities in East African savannahs. His most current work examines how several ecological covariates interact to shape and mediate the functioning of a keystone ant-plant mutualistic system in East Africa, the whistling thorn tree Acacia drepanolobium. He has extensive experience developing, and using molecular markers to tackle evolutionary questions regarding gene flow, population structure, genetic diversity and and landscape genetics of animal and plant species in tropical habitats.

Wednesday, November 2, 2016 - 13:00

Lecture Theatre 1, HW Pearson Building, Upper Campus, UCT