The most important single species? I don’t think there is one. Pretty much any one species disappearing would have its niche filled by another. There are a number of fragile ecosystems where that wouldn’t happen and there would be a local collapse, but those would not have large-scale impacts.

If all environmental protections were removed, we’d see broad destruction of the environment across many species. Think broad deforestation, mining, waterway contamination, air pollution. It wouldn’t be species, it’d be whole ecosystems killed.