The Overlooked Biodiversity of Flower-Visiting Invertebrates

Abstract

Estimates suggest that perhaps 40% of all invertebrate species are found in tropical rainforest canopies. Extrapolations of total diversity and food web analyses have been based almost exclusively on species inhabiting the foliage, under the assumption that foliage samples are representative of the entire canopy. We examined the validity of this assumption by comparing the density of invertebrates and the species richness of beetles across three canopy microhabitats (mature leaves, new leaves and flowers) on a one hectare plot in an Australian tropical rainforest. Specifically, we tested two hypotheses: 1) canopy invertebrate density and species richness are directly proportional to the amount of resource available; and 2) canopy microhabitats represent discrete resources that are utilised by their own specialised invertebrate communities. We show that flowers in the canopy support invertebrate densities that are ten to ten thousand times greater than on the nearby foliage when expressed on a per-unit resource biomass basis. Furthermore, species-level analyses of the beetle fauna revealed that flowers support a unique and remarkably rich fauna compared to foliage, with very little species overlap between microhabitats. We reject the hypothesis that the insect fauna on mature foliage is representative of the greater canopy community even though mature foliage comprises a very large proportion of canopy plant biomass. Although the significance of the evolutionary relationship between flowers and insects is well known with respect to plant reproduction, less is known about the importance of flowers as resources for tropical insects. Consequently, we suggest that this constitutes a more important piece of the ‘diversity jigsaw puzzle’ than has been previously recognised and could alter our understanding of the evolution of plant-herbivore interactions and food web dynamics, and provide a better foundation for accurately estimating global species richness.

How many species of arthropods visit flowers?

“…However, recent work has revealed that flowers in tropical rainforests attract a very high density and diversity of beetles (Kirmse et al 2003;Ødegaard and Frame 2007;Wardhaugh et al 2012Wardhaugh et al , 2013a. Moreover, Wardhaugh et al (2013bWardhaugh et al ( , 2015 showed that the host specificity of flower-visiting beetles could be relatively high, introducing the possibility of high regional and global diversity as well.…”

“…These rare pollinator-flower interactions may be less likely to be recorded in less intensive pollination studies. In contrast, studies that view the flower-invertebrate interaction from the invertebrate's perspective, where flowers are a resource to be exploited and all visitors are recorded, are rare (but see Gottsberger and Silberbauer-Gottsberger 2006;Wardhaugh et al 2012Wardhaugh et al , 2014.…”

Insects on flowers

“…As our study 11 is one of the few to examine the fauna attracted to flowers in tropical rainforests, it is impossible to know how they generalize to other locations. It seems, however, that there is prima facie reason to expect high levels of specialization rather than low levels.…”

“…The primary results of our paper 11 was the demonstration that (i) the invertebrate assemblage inhabiting the leaves is not representative of the wider canopy community, and (ii) each microhabitat supports a unique assemblage in terms of species composition, species richness, relative abundances, and density. In particular, flowers constitute a very important, but until now under-appreciated, microhabitat for species richness and abundance.…”

Lycaenid caterpillars (Lepidoptera, Lycaenidae) eating flowers of Dalea pennellii var. chilensis (Fabaceae) in the northern Chilean Andes

“…Despite their generally ephemeral life, flowers are important resources for many phytophagous insects (Held & Potter 2004;Wardhaugh et al 2012;Tsuji & Sota 2013). The feeding damage on any structure of developing floral buds or mature flowers before seed coat formation is known as florivory (McCall & Irwin 2006).…”