Ant-plant-homopteran mutualism: how the third partner affects the interaction between a plant-specialist ant and its myrmecophyte host.

摘要

By estimating relative costs and benefits, the role of the homopteran partner in the protection mutualism between the myrmecophyte Leonardoxa africana T3, the ant Aphomomyrmex afer, and sap-sucking homopterans tended by ants in the trees swollen hollow twigs was studied in Cameroon. The ants obtained nest sites and food from their host plant (food was obtained either directly by extrafloral nectar or indirectly via homopterans). Aphomomyrmex workers patrolled young leaves of L. africana T3 and protected them against phytophagous insects. Because ants tended, either solely or primarily, coccids in some trees and pseudococcids in others, experiments were conducted to determine whether the nature of the interaction was dependent on the identity of the third partner. Firstly, the type of homopteran affected the benefits to the tree of maintaining a large ant colony. Larger colony size (relative to tree size) conferred a greater protection against herbivory; this relationship was more pronounced for trees whose ants tended pseudococcids than for those in which ants tended coccids. Secondly, for trees (and associated ant colonies) of comparable size, homopteran biomass was much larger in trees harbouring coccids than in trees with pseudococcids. Thus,the cost to the tree of maintaining ants may be greater when ants are associated with coccids. The net benefits to the plant of maintaining ants appeared to be much greater with pseudococcids as the third partner. To explore how the type of homopteran affects functioning of the system, resources (nest sites, extrafloral nectar, and homopterans) likely to limit ant colony size were determined. In trees where ants tended coccids, ant colony biomass was strongly dependent on the number of extrafloral nectaries. In contrast, in trees whose ants tended only pseudococcids, colony biomass was not related to the number of nectaries, and was most strongly determined by the volume of available nest sites. A hypotheses to explain how the type of homopteran affects functioning of this symbiosis are presented and the implications of this study for the evolutionary ecology of ant-plant-homopteran relationships are discussed.