Final answer:
A keystone species is one that has a disproportionate effect on its community relative to its abundance. These species can influence many other species and ecosystem functions, with significant impacts even if they are rare. Their ecological roles can be severely affected by human activities such as overharvesting and habitat modification.
Step-by-step explanation:
A keystone species affects other members of the community in ways that are disproportionate to its abundance or biomass. This term refers to a species that plays an especially important role in its community so that major changes in its numbers affect the populations of many other species. Although it might seem that the most abundant or largest species would be most influential, in some cases, even rare species can have significant ecosystem impacts.
Major community effects are most often observed where species richness is low, and piscivores are more likely to alter invaded communities, whereas omnivores and detritivores are least likely to do so. This concept is an example of a density-dependent factor, as the dramatic effects a keystone species can have on its environment become more pronounced as its population density increases.
Various activities like overharvesting, logging, and grazing can cause pervasive trophic cascades and alter the quality of habitats, affecting not only the primary target species but also other community members through changes in food webs. Even when a species does not go extinct, overexploitation may erode its ecological role to a point where it no longer interacts significantly with other species, leading to ecological extinction.