Species are the fundamental unit of ecosystems. As species become rare, their roles in the functioning of the ecosystem diminishes and disappears altogether if they become locally extinct  . Loss of species and functional groups reduces the ability of an ecosystem to provide services, and lowers its resilience to climate change   . For example, among crop systems, a key factor to succesful pollination is the phylogenetic diversity of bee species available, more than total abundances . Because many species have obligate interactions with, or are resources for, other species , the loss of one species affects the risk to another species, and, ultimately, ecosystem functioning 
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report