Edge Effects: Habitat Biodiversity and Human Interference

Edge effects are the changes in biodiversity that occur inside the space surrounding the shared edge of two or more distinct ecosystems. This transitional zone rich in biodiversity is known as the ecotone; examples are between woodlands and plains, forests and mountains, and land and water. Informally known as the edge, the ecotone affects the plants and animals living there in a way that is unique from the connecting habitats.

In larger habitats, there is a smaller percentage of area affected by the edge. This allows flora and fauna to thrive in both ecosystems and along the edge. But in smaller habitats, it is more likely for conditions from the edge to threaten the stability of individual biomes—making it difficult, if not impossible, for many plants and animals to survive. Here, we review some examples of positive and negative edge effects.

Positive Edge Effects

When two adjacent habitats have enough individual space to allow for an ample gradient edge, the ecotone is uniquely positioned to provide habitable conditions for certain plants and animals. Thriving edges house the greatest variety of natural structures, ranging from small to tall, and they often boast wildlife populations exceeding any bordering habitats. 

Changes to the landscape, including geographic features, soil types, temperatures, and humidity levels, are called inherent edges.

Unlike the interiors of most ecosystems, edges receive more sunlight, experience less humidity, face more wind, and experience higher temperatures. These environmental differences enable a more hospitable environment for high light, drought-tolerant flora. Consequently, more herbivorous insects, birds, and other animals can make their homes inside the ecotone.

Some fauna, including rabbits, deer, and elk, require multiple environments and rely on the edge as a space for foraging and nesting. (Or, in the case of the dung beetle, waste removal). Humans, as animals inhabiting the natural world, benefit from the edge effect because a healthy, wide ecotone physically separates people from predators.

Negative Edge Effects

When people infringe upon the natural world, ecological edges sharpen, and the biodiversity of the ecotone diminishes. Narrow, human-induced edges can increase the risk for infectious diseases, degrade soil quality, and decrease humidity levels.

Urbanization, lumber harvesting, and food cultivation all result in induced edges. They can also have biological or climate origins: Floods, fires, winds, disease, and insect infestations can all create edges. Once these negative edge effects take hold, the climate along the edge can spread deeper into the environment, threatening habitat destruction for a number of species that can only survive in the original biomes. 

Consider how different an edge defined by commercial deforestation looks compared to the gentle transition from woodland to clearing. Higher winds along these anthropogenic edges often feed and exacerbate wildfires, causing further damage. Sufficient destruction to the forest edge can cause fragmentation, which creates more edges around increasingly smaller ecosystems.

Predators, pushed out of their habitats, venture out into the edge and beyond, affecting bird and, by extension, insect populations. Since the size of the edge remains constant, the already more plentiful edge plants and animals can grow in population, creating a subsequent environmental problem: invasive species.

Agriculture

From a planetary perspective, humans dominate the landscape. Agriculture alone takes up nearly 40% of all land on Earth. The negative edge effects associated with field clearing for crop growth matter not just because of deforestation concerns. Moreover, herbicides, fertilizers, and other chemicals used in food and animal production can leach out of these narrow boundaries and contaminate the surrounding natural habitats.

As the human population continues to grow, experts believe that global demands for dairy and meat will drive future land use changes, requiring even more space to grow food for humans and animals. Without sustainable natural habitats inside the edge, ranching can also force livestock to compete with native species for food supplies. 

Urbanization

Like agriculture, urbanization also threatens vital ecosystems around the world. More than half of humanity currently lives in an urban area, with over 70% of the global population predicted to live in cities by 2050. Urbanization can also create competition for natural resources and introduce new predators like domestic cats that can wreak havoc on wild bird populations.

The current state of the Los Angeles River provides a clear example of how urbanization can destroy an ecotone and injure the surrounding biological communities. What once provided a rich biome for plants, animals, and one of the largest groups of Native Americans in North America is now a 51-mile long concrete wash, with just 5% to 10% of its natural habitat still intact.

Not only have plants and animals disappeared from the river and along the anthropogenically-induced edge, but communities of color living nearby suffer some of the highest levels of poverty and pollution in all of Southern California—the consequences of environmental racism.

To create a just and sustainable future for all life on the planet, lawmakers need to work with scientists to develop specific and practical techniques to restore endangered ecosystems and maintain flourishing, biodiverse edges that connect humanity to nature.