Estimating global arthropod species richness: refining probabilistic models using probability bounds analysis

Abstract

A key challenge in the estimation of tropical arthropod species richness is the appropriate management of the large uncertainties associated with any model. Such uncertainties had largely been ignored until recently, when we attempted to account for uncertainty associated with model variables, using Monte Carlo analysis. This model is restricted by various assumptions. Here, we use a technique known as probability bounds analysis to assess the influence of assumptions about (1) distributional form and (2) dependencies between variables, and to construct probability bounds around the original model prediction distribution. The original Monte Carlo model yielded a median estimate of 6.1 million species, with a 90 % confidence interval of [3.6, 11.4]. Here we found that the probability bounds (p-bounds) surrounding this cumulative distribution were very broad, owing to uncertainties in distributional form and dependencies between variables. Replacing the implicit assumption of pure statistical independence between variables in the model with no dependency assumptions resulted in lower and upper p-bounds at 0.5 cumulative probability (i.e., at the median estimate) of 2.9-12.7 million. From here, replacing probability distributions with probability boxes, which represent classes of distributions, led to even wider bounds (2.4-20.0 million at 0.5 cumulative probability). Even the 100th percentile of the uppermost bound produced (i.e., the absolutely most conservative scenario) did not encompass the well-known hyper-estimate of 30 million species of tropical arthropods. This supports the lower estimates made by several authors over the last two decades.

New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods

“…Method 1: The host specificity of insects to trees (8,17,18) has been recently reexamined (19,20) and here, using the same modeling, we provide estimates for beetles alone. Method 2: The ratio of butterflies to other insects in the British fauna scaled up using estimates for all butterflies in the world (21) has been updated here using new estimates of the British insects (22) to give estimates for beetles and insects of the world.…”

How many species of arthropods visit flowers?

“…Overall, I estimate that about 352,000 of the estimated 1,214,255 described species of extant arthropods (*29.7 %) regularly visit flowers for food, mates, or other resources. When extrapolated to the estimated total number of living arthropod species (*6 million; Hamilton et al 2013), there are likely to be [1 million flower-visiting species of arthropods. These are very high numbers of species, which emphasises the importance of flowers for generating and maintaining invertebrate biodiversity in terrestrial systems.…”

Oak canopy arthropod communities: which factors shape its structure?

“…In contrast, efforts towards the conservation of arthropods have been limited, probably because many species have not been described or because their life history or their role within their communities (Redak 2000) is unknown. However, recent estimates suggest that this group contains the greatest diversity of species on earth (Hamilton et al 2013), in addition to representing a group of vital organisms for the maintenance of a wide variety of processes at the level of communities and ecosystems, acting as pollinators, herbivores, or predators and participating in decomposition, nutrient cycling, energy transfer, or soil formation (McIntyre et al 2001). This suggests that the loss of biodiversity of arthropods could have catastrophic consequences in ecological terms.…”

“…The results obtained by the works mentioned above indicate that the arthropod fauna associated with the canopy of temperate and tropical forests constitutes a very significant proportion of the global biodiversity (Erwin 1982;Novotny et al 2002;Novotny and Basset 2005;Basset et al 2007); the most recent estimate suggests 6.1 million species (Hamilton et al 2013). In general, the diversity of arthropod species associated with the canopy is influenced by a latitudinal gradient (i.e., mites, collembola, beetles, and ants.…”