Variations in abiotic defense within myrmecophytic and non-myrmecophytic species of Macaranga in a Bornean dipterocarp forest

We examined the interspecific variations in intensity of total abiotic (che mical and physical) defenses in five sympatric Macaranga (Euphorbiaceae) sp ecies, including three myrmecophytic species. The intensity of the total ab iotic defense for each Macaranga species was estimated by measuring inhibit ing effects on the growth performance of the common cutworm, Spodoptera lit ura (Fabricius) (Lepidoptera: Noctuidae) when the cutworm larvae were fed f resh leaves of each Macaranga species. Indices of the growth performance, n umber of dead larvae, pupal weight and length of larval period were obtaine d. We found that the intensities of total abiotic defense of the two non-my rmecophytic species were significantly stronger than those of the three myr mecophytic species, and that there was a significant difference in intensit y even within the three myrmecophytic species. The former result supports t he hypothesis that, unlike non-myrmecophytic species, myrmecophytic species cannot invest so many metabolic resources in abiotic defense, because they have to allocate nutrients to biotic defense (toward biotic defense agents ). Moreover, the latter result suggests the possibility that the three symp atric myrmecophytes have different defense strategies, with a trade-off bet ween abiotic and biotic defense, and/or with a trade-off between defense an d other life-history traits such as growth and reproduction. Abiotic defens e can be roughly separated into physical and chemical mechanisms. To assess the intensity of the physical defense of Macaranga leaves, we measured the leaf toughness of each species. In addition, to assess the intensity of th e plant's general chemical defense, cutworm larvae were reared on an artifi cial diet containing dry leaf powder of each Macaranga species, and their g rowth performances were compared. The estimated orders of intensity of both leaf toughness and general chemical defense coincided with that of the tot al abiotic factors measured by the growth performance of cutworm on fresh l eaves. This suggests the presence of both physical defenses, represented by leaf toughness, and a general chemical defense affecting the intensity of the total abiotic defense in similar ways.