Why does community ecology need chemistry? Because keystone molecules can govern food webs

In an earlier editorial (Borges 2022), I had talked about the keystone concept formulated by Robert Paine. This important concept (Paine 1966) emphasised the idea that biodiversity is maintained by a top-down process in which the removal of a keystone predator would allow the unchecked proliferation of herbivores; during this process a dominant herbivore might outcompete another herbivore species, resulting in the removal of the lesser competitor from the community and a reduction in overall species richness (i.e., the number of species present). The keystone predator concept is well-established in community ecology. Are there keystone molecules whose presence can influence the diversity of communities? At an extreme, of course, one may say that oxygen is a most important molecule whose presence has fuelled the diversification of autotrophs such as diatoms and plants, and hence the flowering of life. A key feature of Paine's keystone concept is that the species must exert an impact that is disproportionate to its abundance (Power et al. 1996). By this definition, oxygen, although vital to all life, would fail to be recognised as a keystone chemical or molecule in species communities. Are there chemical compounds that can govern species diversity within trophic levels of an ecological community, as defined by Paine's keystone concept?