Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications, without additional investigation, in an attempt to accentuate the importance and ubiquity of nematodes in the global ecosystem (rather than as a sign of agreement with the estimated taxonomic figure). Many other publications have since vigorously refuted this claim on the grounds that it is unsupported by fact, and is the result of speculation and sensationalism. More recent, fact-based estimates have placed the true figure closer to 40,000 species worldwide.