From a purely anthropological, 'selfish' point of view, human beings depend on the biodiversity of ecosystems on many fronts:
Environment. The biosphere helps make the planet habitable by regulating the surface temperature and cleaning the atmosphere.
Resources. We get all kinds of essential raw materials.
Knowledge. Biological diversity is the source of many of our pharmaceuticals, and other species help us understand ourselves.
Economics. Much of human activity rests on natural or naturalized environments, including cities.
Protection. Preserving ecosystems minimizes the risk of zoonosis and pandemics.
Health. It has been shown how environments with less biodiversity negatively affect health.