Final answer:
The wide variation in the estimates of species vulnerable to extinction is due to the different levels of knowledge about species, a focus on vertebrates, complex variables like climate change, habitat loss, and methodological differences in assessments.
Step-by-step explanation:
Estimates of the proportion of species vulnerable to extinction vary widely due to several factors. One primary reason is the difference in the level of knowledge about various species groups. For example, in the US, nearly 100% of vertebrates have had their conservation status evaluated, whereas many other species remain unstudied. Bias towards vertebrates in conservation research leads to more reliable estimates for these groups, compared to less-studied organisms like invertebrates, plants, and fungi.
Furthermore, predictions about species threatened by extinction incorporate variables such as climate change, habitat destruction, and pollution, which are complex and vary geographically. Estimates also differ depending on the inclusion of species not yet described by science; for plants alone, including these species could raise the proportion of those at risk to 37%.
Moreover, the methodology used in assessments can contribute to variance, ranging from IUCN Red List assessments to models predicting the impact of future climate scenarios on biodiversity. Each approach has its strengths and weaknesses, contributing to the range of estimates from 1% to 38% of species facing the threat of extinction.