Flowers are essential to maintain high beetle diversity (Coleoptera) in a Neotropical rainforest canopy

Abstract

ABSTRACT Many hypotheses have been proposed to explain the megadiversity of angiosperms and insects. Flowers and their pollinators represent the most common terrestrial mutualistic interaction today and this is thought to have driven the evolution of angiosperms and their visitors. Within the framework of that interaction, this paper develops and tests two new hypotheses: 1) megadiversity of canopy beetles in tropical rainforests is largely based on flower resources, and 2) the majority of adult canopy beetles are adapted to visit flowers. To test hypothesis 1, the beetle fauna associated with 23 canopy tree species (13 families, 45 individuals) in a 1.4 ha canopy plot of pristine lowland tropical rainforest in southern Venezuela was studied over one year. In total, 6698 adult beetles were collected and identified to 859 species in 44 families. Of these 859 species, 647 species (75.3%) were found with at least one individual on flowering trees, and 527 species (61.4%) were associated exclusively with flowering trees. The proportion of beetle species visiting exclusively small white flowers of the morphological generalist syndrome amounts to 36.6% of the entire canopy beetle community. Based on these findings, the second hypothesis was formulated that canopy beetles are specially adapted flower visitors. To test hypothesis 2, a sub-set study was conducted of the beetles visiting flowers of one tree species in the canopy plot, Hymenopus heteromorphus (Benth.) Sothers and Prance (Chrysobalanaceae). This is a mass-flowering tree with white flowers of a generalised morphology. A total of 440 adult beetles were collected in 20 days and identified to 138 species in 54 genera in 23 families. This assemblage was restricted to the flowering season of this tree. Data collected for H. heteromorphus and other mass-flowering tree species in the same plot indicates that a particular combination of flowering characteristics – massively flowering, white colour, and generalised morphology – attracts the highest diversity of beetles. These findings together suggest that the enormous diversity of canopy beetles in Amazonian tropical rainforests is due mainly to flowers and that mass-flowering trees with small white flowers of the generalist syndrome play a particularly crucial role in assembly and nourishment of this megadiverse beetle community.