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As noted in the last section, biodiversity consists of both the number of species present in a community (species richness) and the relative abundance of each species (evenness) in an ecosystem. Interactions between the environment and random or preexisting variation in populations are the foundations of biodiversity. Keystone species, producers, and other abiotic and biotic factors all play a role in maintaining the diversity of an ecosystem. Ecosystems with lower biodiversity and fewer components are often less resilient to changes in the environment. This is similar to how in some cases, there are advantages to being heterozygous. When an organism has a diversity of alleles within its genotype, it can have a higher relative fitness when compared to homozygous individuals. This is called heterozygous advantage.
Keystone Species
Communities are complex networks of interactions between organisms. When a piece of this web is removed, it starts to unravel. The diversity of species within an ecosystem is a key component of the organization of an ecosystem. Removal of any component of an ecosystem will affect its short-term and long-term structure. There are some species that have an inordinately large role in maintaining the ecosystem’s balance. These are called keystone species. This term is a metaphor for the structure of an arch built with stones. The stone at the apex of the curve is called the keystone. Remove this stone, and the rest of the arch crumbles and falls down. In a similar way, if a keystone species is removed from an ecosystem, it can have detrimental effects. Usually, the keystone species plays a unique role in the ecosystem that is not filled by other species. Therefore, even if the relative abundance of the keystone species decreases slightly, the effects can be noticed across the ecosystem. Figure 8.03 provides an example of this with sea otters being a keystone species in a kelp forest community.
Figure 8.03: Keystone Species
