The Earth is a remarkably diverse place. In a paper published in The Quarterly Review of Biology last year, researchers from the University of Arizona estimated that there are roughly 2 billion living species on Earth. It’s a huge number, but the threat to biodiversity is very real.

All species are interconnected. With less biodiversity, these connections weaken and sometimes are smashed, harming all the species in the ecosystem, National Geographic points out. In the last 100 years, biodiversity around the world has plummeted. Human activity has altered the natural processes of extinction and evolution. Scientists estimate that species are dying out at at least hundreds of times the natural rate, according to National Geographic, the New Scientist, the World Wildlife Fund and other sources.

Here are the biggest threats to biodiversity on our planet.