Anthropogenic environmental change and global dispersal of a wide variety of species outside their native ranges has expanded the range of “cosmopolitan,” non-native species and shrunk the range of regional and endemic species. “This replacement of specific native forms by generalist non-natives in space and time has mixed the taxonomic composition of once disparate biotas, an occurrence termed ‘biotic homogenization’” [Olden 2004: 18].
The authors explore the effect of this “global erosion of regional distinctiveness” [Olden 2004: 18] at three levels: Genetic homogenization reduces genetic variability within species or among populations of species, while taxonomic homogenization reduces distinctiveness among communities. Functional homogenization refers to a reduction of functional traits within an ecosystem. The identity of species making up a community, along with their respective functional traits, determines “ecosystem functions (such as nutrient retention or energy flow)” [Olden 2004: 20], so that narrowing species compositions risks diminishing ecosystem function.
A decrease in functional diversity might reduce overall community and ecosystem functioning, stability and resistance to environmental change by simply narrowing the available range of species-specific responses. Consider a severe drought that strongly affects a subset of species in a community that has (or lacks) a particular suite of functional traits. Historical communities, with much greater breadth in functional space, should exhibit higher resistance or resilience when compared with homogenized communities [Olden 2004: 20].
Genetic homogenization occurs when two distinct locally adapted populations of the same species interbreed. It also occurs when a single variety (such as captive fish bred in a central location) are released in many places to replenish dwindling native stocks. While such mixing has the potential to increase species diversity, this outcome is not assured.
Intraspecific hybridization can homogenize the unique characteristics of geographically distinct populations, as well as compromise the fitness of individuals by disrupting local adaptations [Olden 2004: 19].
Olden, Julien D., N. LeRoy Poff, Marlis R. Douglas, et al., 2004, Ecological and evolutionary consequences of biotic homogenization, Trends in Ecology and Evolution 19(1), https://doi.org/10.1016/j.tree.2003.09.010