Peter S. Grimbacher

Quantifying Uncertainty in Estimation of Tropical Arthropod Species Richness

There is a bewildering range of estimates for the number of arthropods on Earth. Several measures are based on extrapolation from species specialized to tropical rain forest, each using specific assumptions and justifications. These approaches have not provided any sound measure of uncertainty associated with richness estimates. We present two models that account for parameter uncertainty by replacing point estimates with probability distributions. The models predict medians of 3.7 million and 2.5 million tropical arthropod species globally, with 90% confidence intervals of [2.0, 7.4] million and [1.1, 5.4] million, respectively. Estimates of 30 million or greater are predicted to have <0.00001 probability. Sensitivity analyses identified uncertainty in the proportion of canopy arthropod species that are beetles as the most influential parameter, although uncertainties associated with three other parameters were also important. Using the median estimates suggests that in spite of 250 years of taxonomy and around 855,000 species of arthropods already described, approximately 70% await description.

Beetle assemblages from an Australian tropical rainforest show that the canopy and the ground strata contribute equally to biodiversity

There remains great uncertainty about how much tropical forest canopies contribute to global species richness estimates and the relative specialization of insect species to vertical zones. To investigate these issues, we conducted a four-year sampling program in lowland tropical rainforest in North Queensland, Australia. Beetles were sampled using a trap that combines Malaise and flight interception trap (FIT) functions. Pairs of this trap, one on the ground and a second suspended 15–20 m above in the canopy were located at five sites, spaced 50 m or more apart. These traps produced 29 986 beetles of 1473 species and 77 families. There were similar numbers of individuals (canopy 14 473; ground 15 513) and species (canopy 1158; ground 895) in each stratum, but significantly more rare species in the canopy (canopy 509; ground 283). Seventy two percent of the species (excluding rare species) were found in both strata. Using IndVal, we found 24 and 27% of the abundant species (n≥20 individuals) to be specialized to the canopy and the ground strata, respectively, and equivalent analyses at the family level showed figures of 30 and 22%, respectively. These results show that the canopy and the ground strata both provide important contributions to rainforest biodiversity.

What determines whether a species of insect is described? Evidence from a study of tropical forest beetles

Abstract.  1 The rainforest canopy has been called ‘the last biological frontier’, and if this is true, there should be more undescribed species in this stratum than the ground stratum. 2 Here, we test this and other hypotheses regarding traits of described and undescribed species by a sub‐sample of 156 species into 96 described and 60 undescribed species from a beetle assemblage of 1473 species collected from the canopy and ground in an Australian lowland rainforest. 3 We show that described species are significantly more likely to be in the canopy, are more likely to be larger and, if they are large, are more likely to have been described earlier. 4 Undescribed species are just as likely to be found near the ground as in the canopy and are more likely to be smaller. 5 After the first year of sampling, ‘new’ described and undescribed species not previously encountered continued to appear in each of three further years of trapping. 6 These data show that the canopy fauna is in fact relatively ‘well known’, and that the undescribed species to be found in both strata are likely to be smaller than described species and are less likely to be plant feeders.

The overlooked biodiversity of flower-visiting invertebrates.

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.

Low host specificity of beetles associated with fruit falls in lowland tropical rainforest of north-east Australia

Most host‐specificity studies of tropical rainforest insects have focused on those species feeding on leaves. Apart from the fruit flies, the level of specialisation among fruit‐associated insects is poorly known. The relative contribution to local species richness made by insects feeding or associated with fallen fruits is also unknown. Beetles from fruit falls over a 5‐year period in lowland tropical rainforest at Cape Tribulation, Australia were sampled. A total of 5157 individual beetles of 73 species were sampled from the fruits of 18 different plant species. Only a few species were of frugivorous families, and most species are likely utilising resources associated with the breakdown of the fruits. The size of the fruit fall‐associated beetle assemblage (73 spp) was small compared with the number of species collected during a 4‐year sampling program conducted at the same site prior to the current study using 10 combined Malaise‐Flight Interception Traps (1473 spp, 77 families). The number of beetles and species collected from fruit that were very strongly correlated with the number of times fruit falls from a particular species of plant were sampled. The locally common palm, Normanbya normanbyi, produced fruit throughout the year and supported the largest number of fruit‐associated species. It is suggested that this might be a keystone resource for local fruit fall‐associated insect species. Although most beetle species showed a preference for the fruits of a particular plant species, overall host specificity for beetles was low. This is probably because fruit resources at this site are spatially and temporally patchy. Our results challenge the notion that most insects associated with fruit falls in tropical rainforests are highly host‐specific.

Temporal variation in abundance of leaf litter beetles and ants in an Australian lowland tropical rainforest is driven by climate and litter fall

Determining if the seasonality of leaf litter invertebrate populations in tropical rainforests is driven by climate or availability of litter, or both, is important to more accurately predict the vulnerability of litter invertebrates to climate change. Here we used two approaches to disentangle these effects. First, the influence of climatic seasonality was quantified by sampling a fixed volume of litter monthly over 4 years and counting extracted beetles and ants. Second, litter volume was experimentally manipulated (addition and exclusion) to test the influence of litter quantity independently of climatic variation. There were significant seasonal peaks for both beetle and ant abundance and these were positively correlated with rainfall, temperature and litter volume. As abundance was measured on a ‘per litter volume’ basis we conclude that there was a significant effect of climate on abundance. The litter manipulation experiment showed that beetle and ant abundance per litter volume were also influenced by litter volume, when it was low. We recognise that other factors such as litter structure or complexity may have affected temporal ant abundance. Beetle and ant abundance were depressed in litter exclusion plots but did not differ significantly between control and addition plots, suggesting a possible ceiling in the effect of litter volume on population sizes. We conclude that seasonality in climate and litter quantity are driving most temporal variation in insect abundance and that there may be some resilience among leaf litter insects to cope with higher temperatures. However, future responses by plants to increased climatic variability and higher CO2 concentrations may alter litter fall dynamics and thus temporal patterns in litter insect abundances.