Biotic homogenization is the process by which two or more spatially distributed ecological communities become increasingly similar over time. This process may be genetic, taxonomic, or functional, and it leads to a loss of beta (β) diversity. While the term is sometimes used interchangeably with "taxonomic homogenization", "functional homogenization", and "genetic homogenization", biotic homogenization is actually an overarching concept that encompasses the other three. This phenomenon stems primarily from two sources: extinctions of native and invasions of nonnative species. While this process pre-dates human civilization, as evidenced by the fossil record, and still occurs due to natural impacts, it has recently been accelerated due anthropogenic pressures. Biotic homogenization has become recognized as a significant component of the biodiversity crisis, and as such has become of increasing importance to conservation ecologists.
Overview
Homogenization versus differentiation
Homogenization is the process of assemblages becoming increasingly similar: the reverse is the process of assemblages becoming increasingly different over time, a process known as "biotic differentiation". Just as biotic homogenization has genetic, taxonomic, and functional components, differentiation can occur at any of these levels of organization.
Alpha and beta diversity
Understanding homogenization requires an understanding of the difference between alpha (α) and beta (β) diversity. Alpha diversity refers to diversity within a community: it addresses how many species are present. A community with high α diversity has many species present. Beta diversity compares multiple communities. For there to be high β diversity, two communities would have to have high α diversity but have different, unique species compositions.
Species introduction, extinction, and richness
When organisms are introduced to a habitat, be it naturally or artificially, overall species richness increases (assuming no other species are simultaneously lost). Similarly, when species become extinct, species richness decreases, once again assuming no other alterations to the assemblage. As such, when there is net increase in species richness, a common misconception is to assume that differentiation has occurred. This, however, may or may not be the case. While an increase in species richness does indicate an increase in α diversity, homogenization and differentiation specifically address β diversity.
Positive relationships with richness
While it may seem counterintuitive, there are times when increased species richness (α diversity) also leads to increased homogenization. If we imagine an example of two communities: community one contains four species (A, B, C, and D). Community two contains three species (C, D, and E). While there is overlap between these two communities, they are certainly different. However, if community two undergoes drastic change where E becomes extinct while A and B are simultaneously introduced, it now demonstrates higher species richness (greater α diversity), because there are now four species present instead of three. Yet, at the same time, communities one and two have become identical, removing any β diversity: they have homogenized. This particular trend is frequently observed in studies of biotic homogenization.
Sometimes decreased species richness can lead to greater β diversity and differentiation. If, in the example above, community one had lost species D and community two had lost species C, both communities would have lower α diversity because each would have one less species. However, the two communities would have no species in common, which would dramatically increase the β diversity, leading to differentiation.
Negative relationships with richness
In some cases, increased α diversity could theoretically lead to increased β diversity and differentiation. When we return to the previous example, community one still contains four species (A, B, C, and D) and community two contains three (C, D, and E). This time, C goes extinct in community two, but F and G are introduced at the same time. Community two now has greater richness and therefore greater α diversity. It also only now has one species in common with community one instead of two species. The two communities are now more different from each other than they were initially, indicating greater β diversity and therefore biotic differentiation.