Final answer:
The term used to describe animals and plants that no longer exist is 'extinct'. Many species are currently facing the risk of extinction due to factors such as habitat loss and climate change, and there are significant efforts to catalog and preserve these species before they disappear.
Step-by-step explanation:
The word used to describe animals and plants which do not exist anymore is extinct. Species become extinct when the last individual of that species dies, indicating there are no surviving members left to reproduce and continue the lineage. Extinction events have occurred throughout Earth's history, some naturally, while others are accelerated by human activities such as habitat destruction, overhunting, or overfishing. An alarming fact is that estimates suggest about 99% of all species that ever lived on Earth are now extinct.
Contemporary extinction rates are rising, and many plant and animal species are on the verge of extinction today, including a high number of species that haven't even been described by scientists yet. With ecosystems being in delicate balance, the loss of a single species can trigger a cascade of negative effects throughout the ecosystem. As many unknown species are threatened by environmental changes, efforts like preserving heirloom seeds and using DNA barcoding are vital to protect biodiversity.
The International Union for Conservation of Nature (IUCN) maintains the Red List to track *extinct* and endangered species, although it does not encompass all species. As of recent estimates, a substantial proportion of Earth's flora and fauna is at risk, with over 34,000 plant species facing potential extinction due to ongoing environmental pressures.