Final answer:
About 11.5 percent of Earth's land surface was estimated to be covered by preserves in 2003, as stated by the IUCN World Parks Congress, protecting portions of major biomes and some threatened species.
Step-by-step explanation:
In the context of conservation preserves and their role in biodiversity protection, we can look to authoritative estimates for guidance on the percentage of Earth's land that is set aside as a preserve. According to the IUCN World Parks Congress estimate in 2003, about 11.5 percent of the planet's land surface is covered by varied types of preserves. While this exceeds previous benchmarks, it's crucial to note that this coverage applies to only 9 out of the 14 recognized major biomes, and it encompasses only about 50 percent of threatened amphibian species, which indicates the need for possibly increasing both the percentage and quality of protected areas for effective biodiversity conservation.