Insects and non-woody plants slow down tropical forest succession: a community-wide experiment in Papua New Guinea

Ecological succession is a complex community-wide process whose
theoretical principles require further development. Here, we test the
hypothesis that the succession of woody plants in tropical rainforests is
determined by bottom-up plant competition, rendering top-down control by
insect herbivores insignificant. Over 18 months of rainforest secondary
succession, we removed insects and non-woody plants from replicate 5×5 m
plots in a factorial experiment at 700 and 1700 m elevation in New Guinea.
At 700 m elevation, insect removal increased biomass, reduced diversity,
and altered the species composition of woody plants, while removal of
non-woody plants increased both biomass and diversity of woody plants and
altered their species composition. At 1700 m, the effect of insects on
woody plant biomass and species composition disappeared while the effect
of non-woody plants on woody plant biomass, diversity and species
composition became stronger. The removal of insects did not increase the
proportion of alien species in the woody vegetation as predicted by the
enemy-release hypothesis. The increased disturbance caused by removing
non-woody plants also did not promote alien plants. Synthesis. The
importance of top-down insect herbivory decreased, while the importance of
bottom-up plant competition with non-woody plant species increased with
elevation, representing a gradient of increasing environmental stress, in
the succession of woody plants.