Final answer:
The increase in the number of evaluated species has led to higher estimates of the proportion of species threatened with extinction. The discovery of new species also contributes to these estimates. Climate change is an additional factor that poses a threat to many species.
Step-by-step explanation:
The increase in the number of evaluated species has led to a better understanding of the proportion of species threatened with extinction. Studies have shown that as more species are assessed, the estimates of the proportion of threatened species have increased. For example, one estimate suggests that 37% of all plants are threatened with extinction.
Furthermore, the discovery of new species has also contributed to the increase in estimates of threatened species. Even for well-studied birds, there is a steady trickle of new species each year, and a significant proportion of these new species are threatened or already extinct.
Additionally, climate change is a significant factor in the extinction of species. Estimates suggest that climate change threatens 15-37% of species within the next 50 years.