Habitat Fragmentation Edge Effects: When Having an Edge is Not a Good Thing (Part 1 of 2)

We usually think it’s great when we have an edge, but that’s certainly not the case when we’re talking about habitat fragmentation edge effects and their impact on biodiversity and wildlife.  In this post we’ll explore the topic of edge effects and how it relates to business and biology. Note that I originally published a shorter version of this article on Greenbiz.

Habitat destruction is the #1 issue that impacts wildlife and biodiversity today.  This fact shouldn’t come as much of a surprise.  When we think of all the roads, power lines, buildings, clearcutting, and other development activities taking place all over the world, we can quickly get a sense of the widespread reality of this issue.

The World Wildlife Fund estimates that forests cover about 31% of the land area on Earth and, for a variety of reasons, we’re losing about 46,000 to 58,000 square miles of forest each year – roughly equivalent to losing 48 football fields every minute.  In the Amazon alone, we’ve lost about 17% of the forest over the past 50 years, mostly due to forest conversion for cattle ranching.  Habitat destruction is clearly a big issue, and it won’t be going away anytime soon.

The term “habitat destruction” can refer to the complete destruction of a habitat or, more commonly, habitat fragmentation, where a large, continuous area of a habitat is divided into two or more fragments.  The primary culprit behind habitat loss, degradation, and fragmentation is a change in land use, usually in the form of agriculture, logging, mining, and urban or residential development.  

There are three important conditions that characterize habitat fragmentation: smaller habitat, increased edge effects, and increased isolation.  Today, we’ll focus on the second characteristic – edge effects, which refers to the effect of an abrupt transition between two different, adjoining ecological communities.

We can see examples of edge effects occurring naturally all over the place.  These natural edges, such as the forest and meadow pictured below, can lead to greater biodiversity in the area.

http://www.profitableconservation.com/wp-content/uploads/2018/07/Forest-and-meadow-smaller-475x300.jpeg

However, the edge effects that I’m focusing on in this post are man-made edges that are created in the middle of an existing natural habitat.  From a business perspective, habitats are commonly fragmented by the construction of roads, power lines, and buildings, or the clearing of land for agriculture and forestry.

In the context of habitat fragmentation, edge effects increase the proportion of habitat edges in relation to the total area.  In other words, any given point within the fragment of land is, on average, closer to an edge.  Why does that matter?  Edges matter because they create changes in the species composition for a given chunk of land.  These species-composition changes found at edges are caused by the following conditions:

  • Edges of a forest have microclimatic changes that impact the types of vegetation that can grow there.  These microclimatic changes include more direct sunlight, higher soil temperatures, differences in humidity and depth of humus, and increased wind exposure and snow loads compared with the interior of a forest.  The seeds of some plant species are sensitive to drying out with increased sun and wind, leading to significant differences in the types of vegetation found at a forest edge compared with the forest interior.  To make matters worse, these species alterations extend into the forest interior.  In some tropical rain forests, vegetation changes have been detected as far as nearly 1,500 feet from the edge.  In the scenario where we have a small fragment of a natural habitat or a narrow corridor of land, the microclimatic changes associated with the edges can permeate throughout the entire piece of a habitat.  The result may be a decrease in the presence of rare and sensitive species, while weedy species and generalist predators may thrive.
  • Edges are suitable for some species but unsuitable for others.  If we build a road through a forest, some plant species will thrive with the extra sunlight, and some bird species will enjoy perches next to these open areas where they can pounce on exposed prey.  “Edge species” such as deer and elk like forest edges because they can find food in open areas and take cover in the forest.  Other species of animals will actively shy away from areas of increased sunlight and exposure, moving further into the interior habitat where the characteristics of land remain unchanged.  For example, spotted owls (pictured below) prefer old-growth, mature forests with a lot of canopy and few edges.  When we push these species into the now-smaller interior habitat, we are likely to see increased competition for limited resources.
  • Edge-tolerant species are often generalist predators and exotic species that outcompete native species and habitat specialists.  Examples of edge-loving species include brown-headed cowbirds, crows, raccoons, and opossums.  These species thrive in an edge habitat and act as nest predators and cavity competitors of interior species, which can decrease the populations of forest songbirds, ground-nesting birds, reptiles, and amphibians in the remaining habitat fragments.
  • Edges become areas with increased noise, light, pollution, human recreation, and roadkill.  The increased noise, light, and human activity may cause some species to move further inland, away from habitat edges. Traffic on adjacent roads can cause pollution in the form of nitrogen deposition, and the increase in noise and light can deter or disorient animals.  Roadkill continues to be a significant source of wildlife mortality with several million collisions per year reported worldwide.  In one study in Saguaro National Park on the United States–Mexico border, an estimated 30,000 animals were killed by vehicles annually.  This included a variety of reptiles, amphibians, birds, and mammals, such as the mountain lion pictured below at Saguaro National Park.

Well that was a depressing note to close on for this week, but hopefully this gives you a better understanding of edge effects and how they can impact biodiversity and wildlife. Next week we’ll switch gears and look at some strategies that businesses can implement to help manage edge effects.

Thanks for reading!

Mark

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Mark Aspelin is the Founder of Corporations for Biodiversity and author of the highly rated book “Profitable Conservation: Business Strategies That Boost Your Bottom Line, Protect Wildlife, and Conserve Biodiversity.”

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