Biotic homogenization is the process by which the species composition of different ecological communities becomes more similar over time. This phenomenon is comparable to how chain stores make different cities feel the same by replacing unique local businesses. In nature, a region’s distinct collection of plants and animals is gradually replaced by a uniform set of species found worldwide, reducing the biological uniqueness of different locations.
Primary Drivers of Homogenization
The increase in similarity between ecosystems is driven by human activities promoting two interconnected processes. The first is the widespread introduction of non-native “generalist” species. These are adaptable organisms that thrive in a wide variety of environmental conditions, allowing them to establish populations far from their native habitats. Their success is facilitated by human-made changes, such as urbanization and agriculture, which create disturbed habitats where these hardy newcomers can outcompete native life.
The second driver is the decline and extinction of native “specialist” species. These species evolved to fit specific ecological niches, relying on particular food sources or climate conditions. When their habitats are altered or they face new competition from invasive generalists, these specialists are often unable to adapt and their populations decline.
Forms of Biotic Homogenization
The most direct form is taxonomic homogenization, which occurs when the species lists of two or more locations become more alike. This happens as widespread, non-native species replace unique, native ones, leading to a measurable increase in shared species between formerly distinct communities.
Functional homogenization alters the ecological roles performed within different ecosystems. Every species has a function, such as pollination, predation, or decomposition. This process occurs when the variety of these roles shrinks and becomes more similar across locations. For example, diverse communities of native pollinators might be replaced by a few generalist pollinators, like the European honeybee, which then dominate ecosystems worldwide.
Genetic homogenization occurs at the level of DNA, where the genetic makeup within a species becomes more uniform across its range. This reduces the genetic diversity that allows species to adapt to local conditions. This can happen when dominant populations, often associated with human introduction, spread and interbreed with genetically distinct native populations. This process erodes the unique genetic adaptations that made local populations resilient.
Consequences for Ecosystems
The increasing uniformity of ecosystems has consequences for their stability and health. One outcome is the loss of beta diversity, a term scientists use to describe the degree of difference between two ecosystems. High beta diversity means that traveling from one location to another involves encountering a very different set of species. As homogenization proceeds, this uniqueness fades and ecosystems begin to mirror one another, diminishing the planet’s overall biodiversity.
This simplification of ecosystems also reduces their resilience. A diverse ecosystem with many unique species is like a well-diversified investment portfolio and is better able to withstand shocks. If one species is affected by a disturbance, such as a new disease or a change in climate, others may be able to compensate. In a homogenized ecosystem, a single threat can have a greater impact because there are no alternative species to fill the void.
The loss of specialized species can also disrupt ecosystem services that humans rely on. These services include processes like water purification, soil retention, and crop pollination, which are often performed by native species with unique traits. When these organisms are replaced by a handful of generalist species, the efficiency and stability of these services may be compromised, as the new arrivals may not perform the same functions as the species they replaced.
Global and Local Scale Examples
Freshwater ecosystems provide a clear global example of biotic homogenization. Rivers and lakes on different continents are increasingly dominated by a small number of widely introduced fish species. Species like the common carp, tilapia, and rainbow trout have been transported by humans for aquaculture and recreational fishing and have established themselves globally. Their success often comes at the direct expense of native fish, which are outcompeted or preyed upon by the newcomers.
On a local scale, the phenomenon is evident in the plants found in urban and suburban areas. The lawns, parks, and roadsides of cities from North America to Europe feature the same handful of hardy, non-native ornamental plants, grasses, and weeds. Species like dandelions, clover, and specific types of turfgrass are ubiquitous in these human-modified landscapes. They have replaced the native meadows, forests, and grasslands that once characterized these regions, creating a nearly identical suburban flora across geographically separate areas.