# Taxonomy

Among the many challenges faced by taxonomy is the difficulty of determining the size of the task it faces. Estimates of the number of species on Earth are uncertain and inconsistent, and show no signs of converging \cite{Caley_2014}. Some estimates based on models of taxonomic effort suggest that two-thirds of all species have already been described \cite{Costello_2011}. Analyses that use the number of authors per species description as a proxy for effort \cite{Joppa_2011} ignore the global trend for an increasing number of authors per paper \cite{Aboukhalil_2014}, and assume that the effort required per species description has remained constant over time. An alternative interpretation is that the quality of taxonomic description is increasing over time \cite{Sangster_2014}, reflecting both increased thoroughness and the availability of new technologies \cite{Stoev_2013,Akkari_2015}.