• Enemy-free space;
  • escape from natural enemies;
  • herbivory;
  • host specificity;
  • invasive alien species;
  • Malesia;
  • niche saturation;
  • non-indigenous;
  • rainforest succession;
  • species diversity

Abstract.  1. Caterpillar assemblages feeding on two alien plants, Piper aduncum and P. umbellatum, were studied in lowland rainforest in Papua New Guinea and compared with assemblages from 69 species of native woody hosts, including congeneric P. macropiper.

2. Species richness of caterpillars feeding on P. aduncum (29 species per 1500 m2 of foliage) and P. umbellatum (36 species) was higher than the median richness for the 69 native hosts (23 species).

3. The probability that a caterpillar species colonised alien Piper increased with its host range from 3% for the species feeding on a single plant family to 92% for the species with host range >10 plant families.

4. The assemblage on P. aduncum was dominated by a single species (Herpetogramma sp. near licarsisalis, Crambidae), which represented 48% of individuals, and also had a high proportion (34%) of rare species, collected as single individuals. This community structure was indistinguishable from that of a typical native host. In contrast, the P. umbellatum assemblage was unusual as no species represented >10% of individuals.

5. The aggressive invasion by P. aduncum of early successional vegetation is not explained by a competitive advantage due to low herbivore load, as the abundance of caterpillars feeding on it was comparable to that of native pioneer plants.

6. The caterpillar assemblage on P. aduncum demonstrated that an assemblage indistinguishable from native assemblages in density, species richness, and dominance structure (but not in host specificity) can originate from the existing species pool in lowland rainforests on a recently established tree species in <50 years.