There is some (inconclusive) evidence that biodiversity conservation, replacing dirt flooring, and reducing environmental noise exposure might be much more cost effective areas for global health funding than we currently price in. These findings are, in my view, 'big if true' - with a big 'if'.
If you know of additional important research on these topics or are working on any of these areas, I'd be interested in your thoughts!
I have the opposite intuition for biodiversity. People have been studying ecosystem services for decades and higher biodiversity is associated with increased ecosystem services, such as clean water, air purification, and waste management. Higher biodiversity is also associated with reduce transmission of infectious diseases by creating more complex ecosystems limiting pathogen spread. Then we have the actual and possible discovery of medicinal compounds and links with biodiversity and mental health. These are high level examples of the benefits. The linked article gives the possibility of impact by considering two effects from bats and vultures. Multiply that effect by 1000+ other species, include all the other impacts previously mentioned and I can see how this could be high impact.