Decline in biodiversity is besides climate change one of the main topics when discussing about the effects of humankind on the planet. The human factor is substantial and all discourses revolve around what man has done. Therefore, I will not go too heavily into the influence of humans but give a short overview of biodiversity and pinpoint a few interesting phenomena.
Ecosystem diversity, species diversity and genetic diversity are the three features of biodiversity. They more intertwined these features are, the more resilient the biodiversity becomes. Species that lack genetic diversity due to isolation for example  and are vulnerable to climate change, diseases and habitat destruction. (The Amazon rainforest for example has a massive ecosystem with a variety of species, where the extinction of one species might not severely impact the biodiversity. Madagascar on the other hand is vulnerable due to its isolation and small size.)
An interesting note is the example of dolphins, mackerels and seagulls, which shows how intertwined life on earth is. Sahara dust, which contains algae spores, gets transported over the ocean. The algae that grow are one of the main food sources for mackarels, which swim together in big swarms. The mackerels are hunted by the dolphins, who drive them near the water surface, which in turn enables the seagulls to catch them. So every being has its place in our ecosystem and exists only because of the existence of another being.

This example also shows that not only the animals depend on each other, but that the atmosphere also plays a role. When Earth used to be a inhabitable fireball with an CO2 atmosphere, species in the ocean transformed the CO2 in the water into materials for their shells. These millions and millions of shells that sank on the ground formed the sedimentary rock, where the CO2 is now "trapped". Algae, which still produce over 70% of our O2, filled the atmosphere with O2 and paved the way for life outside the oceans.
Reforestation shows how little we understand about our ecosystem. It is not enough to plant "some" trees, the interaction between the beings and the ecosystem have to be considered. Willie Smits noticed when reforesting parts of Borneo, that you also have to introduce specific fungi and bacteria to the forest.
Another interesting note are the efforts of Jason Clay, who pursues a top-down approach. He noticed, that the main resources like cotton, palm oil, soy are produced by a handful of companies. Cargill for example controls 25% of the worldwide palm oil trade and 50% of the Chinese palm oil import. So by sitting together with Cargill and working out a plan for sustainable palm oil production, more is achieved than trying to convince hundreds of Chinese companies to overthink their palm oil usage.