In the Nature Recovery Project we emphasise biodiversity , because nature is all joined up - it is an ecosystem, so conserving and restoring the whole ecosystem is at least as important as saving individual species, or embarking on specific efforts, such as tree planting, to mitigate global warming.
The Convention on Biological Diversity defines biodiversity as:
"The variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems." (https://www.cbd.int/convention/articles/?a=cbd-02)
An important step forward in comparative measurement is the Biodiversity Intactness Index (BII) devised by Scholes and Biggs (2005) to provide a relatively simple and unbiased estimate of biodiversity loss. The BII expresses the abundance of a large and diverse set of organisms in a given geographical area as a fraction of a reference population in a minimally disturbed habitat. The comparison is thus not across historical time, but across comparable places - one disturbed by human intervention, the other not. On the BII, the UK came 189 out of the 218 countries for which the data were available (see https://www.predicts.org.uk/).
These measures show we are in a very precarious place, yet our current headline news is not on the loss of biodiversity, but about climate change. Concerns about the dramatic increase in atmospheric carbon dioxide now has led to an overemphasis on the role of trees, which form just one element of a large number of different ecosystems. Of course, trees have massive and measureable benefits for countryside and cities, and for earth's climate, but life exists in every nook and cranny, so to focus on trees in isolation is to neglect the ecosystems of which they form a part. Indeed, life on earth has evolved multidinous forms to exploit - and even create - the many ecological nooks and crannies that now exist.
In a world where the environment is degrading, the stresses due to pollution, loss and fragmentation of habitat, and global warming means that many species will inevitably go extinct simply because they do not have the resilience to adapt to the rapid degradation of their ecological niches. As Darwin's tells us, they are no longer fit enough for survival. When species go extinct, we lose biodiversity. This is happening in front of our eyes and the projections are sobering: e.g. it is estimated that one fifth of all mammalian species in the UK are at risk of extinction. http://publications.naturalengland.org.uk/publication/5636785878597632
As the definition of biodiversity indicates, variation is a key element - and variation is what Darwin told us is essential if a species is to adapt to changes in its environment. The Galapagos finches are the most famous example of this adaptation. What Darwin didn't know was that the mechanism that best endows a species with resilience to rapid changes is variation in the genes. Biodiversity and genetic variation are thus two sides of the same coin, and by increasing biodiversity, we increase genetic variation, and along with it comes increased resilience.