200 years ago, French American zoologist Nicolas Hentz estimated global
spider species richness to be above 2,000. Current estimates
certainly closer to reality, but the wide range of estimates also shows
that we are still far from having a reliable estimate. For this
reason I started a along-term project over 10 years ago with the aim
producing a reasonable estimate of the actual pholcid global species
richness. The idea
was to use patterns of species collecting events across large
geographic areas on several continents to estimate total species
numbers from ratios of ‘new’
(i.e., not formally described) to known species.
I was very happy to have Anne Chao do the statistical part of this paper, so we were able to take into account also undetected species. This was done using the Chao2 estimator based on species incidence data; basically, we estimated undetected species by extrapolating from rare species, i.e. species that occurred at only one or only two sites.
The raw global cumulative percentage of new species was 75.1%, and was relatively constant across large biogeographic regions (see below, left column). The estimated percentage of new species based on the date by species matrices was 76.0% with an estimated standard error (s.e.) of 2.6%. This leads to an estimated global species richness of about 4,200 with a 95% confidence interval of (3,300, 5,000). The corresponding values based on locality by species matrices were 84.2% (s.e. 3.0%) and 6,300 with a 95% confidence interval of (4,000, 8,600).
Our results suggest that the currently known 1,700 species of Pholcidae may represent no more than about 27–40% (20–52% with a 95% confidence interval) of the total species richness.
Below on the left side are cumulative percentages of new species (y-axis) as a function of cumulative field days (x-axis); on the right are cumulative numbers of total (upper line) and new (lower line) species (y-axis) as a function of culumative field days (x-axis), for the three major tropical megatransects shown on the map. Each green dot on the map represents a sampling locality.