Which of the following best describes the concept of keystone species?

The concept of keystone species refers specifically to those species that have a disproportionately large impact on their ecosystem relative to their abundance or biomass. This means that even if these species are not the most numerous, their role in maintaining the structure and diversity of their ecological community is critical. For example, a keystone predator may control the population of herbivores, which in turn affects plant communities and the overall health of the ecosystem.

In contrast, the other descriptions do not capture the nuanced role of keystone species. Species that are the most abundant in an ecosystem may contribute to its dynamics, but they do not necessarily exert the same critical influence as keystone species do. Species that are easiest to observe during field studies might be significant for research but lack the specific ecological role that characterizes keystone species. Lastly, species that have no real impact on ecosystem dynamics would not qualify as keystone species since their defining characteristic is their essential role in ecosystem functioning. Thus, the definition that focuses on the significant and disproportionate effects of certain species on their environment aptly describes the concept of keystone species.