FOOD BODY PRODUCTION IN MACARANGA-TRILOBA (EUPHORBIACEAE) - A PLANT INVESTMENT IN ANTI-HERBIVORE DEFENSE VIA SYMBIOTIC ANT PARTNERS

1 Quantifying the costs is the first step necessary for assessing the net value of any plant trait, and the costs of defence mechanisms in p articular are largely unknown. 2 Several species of the important sout h-east Asian pioneer tree genus Macaranga (Euphorbiaceae) possess holl ow stems that harbour ant colonies that act as a 'biotic' anti-herbivo re defence. In Macaranga triloba, ants are nourished by food bodies (F Bs) 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. 3 FB production of 36 different-sized plan ts was estimated in the field by comparing even-aged stipules with and without access for ants. 4 FB dry mass production amounted to about 5 % of daily above-ground biomass production in unbranched saplings. Whe n the chemical composition of FB and leaf tissue was taken into accoun t, this represented about 9% of the above-ground tissue construction c osts. In energetic terms, unbranched saplings invested 0.6-5% of their total assimilation in the FBs. 5 The relative investment in anti-herb ivore defence arising from FB production decreased hyperbolically with increasing plant size. However, a linear relationship was found betwe en FB production and plant size. Thus, in spite of the plants' decreas ing relative investment with increasing size, a continually increasing food supply was provided for the ant colonies. 6 A second study was c onducted to investigate whether FB production is influenced quantitati vely 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 a nt-free plants. FB production of a whole plant therefore seems to be r egulated to a high degree via stipule longevity. 7 Since the ants prot ect their host-plant very effectively, nourishing specialized mutualis tic ants by FBs must be considered a rather expensive, but nevertheles s highly beneficial, strategy of anti-herbivore defence, but regulatio n ensures that FB production is maintained at high rates only when ant s are present.