Food body production in Macaranga triloba (Euphorbiaceae): a plant investment in anti-herbivore defence via symbiotic ant partners

Quantifying the costs is the first step necessary for assessing the net value of any plant trait, and the costs of defence mechanisms in particular are largely unknown. Several species of the important south-east Asian pioneer tree genus Macaranga (Euphorbiaceae) possess hollow stems that harbour ant colonies that act as a 'biotic' anti-herbivore defence. In Macaranga triloba, ants are nourished by food bodies (FBs) that are produced on the abaxial surfaces of recurved stipules. We estimated the costs arising from this kind of anti-herbivore defence by following FB production. FB production of 36 different-sized plants was estimated in the field by comparing even-aged stipules with and without access for ants. FB dry mass production amounted to about 5% of daily above-ground biomass production in unbranched saplings. When the chemical composition of FB and leaf tissue was taken into account, this represented about 9% of the above-ground tissue construction costs. In energetic terms, unbranched saplings invested 0.6-5% of their total assimilation in the FBs. The relative investment in anti-herbivore defence arising from FB production decreased hyperbolically with increasing plant size. However, a linear relationship was found between FB production and plant size. Thus, in spite of the plants' decreasing relative investment with increasing size, a continually increasing food supply was provided for the ant colonies. A second study was conducted to investigate whether FB production is influenced quantitatively by the presence of symbiotic ants. Ant-inhabited plants produced up to 35 times (mean 8 times) more FBs than similar-sized but ant-free ones. This difference resulted mainly from lower stipule numbers in ant-free plants. FB production of a whole plant therefore seems to be regulated to a high degree via stipule longevity. Since the ants protect their host-plant very effectively, nourishing specialized mutualistic ants by FBs must be considered a rather expensive, but nevertheless highly beneficial, strategy of anti-herbivore defence, but regulation ensures that FB production is maintained at high rates only when ants are present.