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Life on the Edge: 3 Important Ways that Habitat Edges Affect Forests

“Edges exist in nature, but what happens when humans continue to fragment habitat?"

Sometimes, while flying to some far-off destination, I look down at the patchwork of roads, forests, cities, and farms that make up the New England landscape. Staring out the window, I wonder how humans, the ultimate “ecosystem engineers”, have altered the natural way that plants, animals, and the landscape interact with each other. In 2002, a paper was published in the journal Conservation Biology that summarized the findings of a 22-year-long study on just this question [1]. When I first read “Ecosystem Decay of Amazonian Forest Fragments: A 22-Year Investigation” by William Laurance and colleagues, I was struck by both the massive scale of the study and the results they found.

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Fig. 1 The study area for the Biological Dynamics of Forest Fragments Project, one of the largest, longest running, and most expensive ecological experiments in history. [1] Reproduced with permission from Conservation Biology. Copyright 2002 John Wiley & Sons, Ltd.

The famed Dr. Thomas Lovejoy, along with many other researchers of the Biological Dynamics of Forest Fragments Project (BDFFP) experimentally manipulated a 1,000-square kilometer forest in the Amazon by working with ranchers to strategically clear pastures for cattle grazing. The patches of forest surrounded by clearings were measured in hectares, and 1 hectare is about the size of a soccer field. They wound up with forest patches that were either 1 hectare, 10 hectares, or 100 hectares (see Fig 1). They then selected plots of the same sizes within intact forests that didn’t have any fragmentation to monitor and compare with the isolated forest patches. In addition to the 1, 10, and 100 hectare sizes, they also had 3 intact forest plots that were 1,000 hectares. These forests were intensely monitored for 22-years to see how cutting off a forest patch from the rest of the forest can affect habitat and wildlife over time.

Fig. 2 This Amazonian study area is host to incredible biodiversity, including this Pigmy Antwren. Source: Wikimedia Commons

Here are three key findings from the study:

1 – Fewer species, even in an area of the same size

When the forest area was fragmented, scientists found lower species richness - a simple count of number of species - than in a forest area of the same size that wasn’t fragmented from the rest of the forest. What this means is you might have 100 species of birds in a soccer field-sized forest surrounded by farms, but a soccer field-sized area of forest inside of a much larger forest would have 200 species of birds.

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Fig. 3 Even when surveying the same amount of forest, the number of species in a given area is far greater when it is within an intact forest than when it is fragmented[1] Reproduced with permission from Conservation Biology. Copyright 2002 John Wiley & Sons, Ltd.

2 – The edges experience different weather conditions

Life in the forest interior of the Amazon has evolved to cope with a specific set of conditions – wind, rain, temperature, humidity – and these can all change dramatically when there is a large opening in the forest. In the fragmented forest patches, the effects of increased windiness affected flora up to 400 meters from the edge of the forest. Scientists also found a reduction of tree canopy height up to 100 meters from the edge and reduced soil moisture 75 meters in. These may not seem like dramatic effects, but in a forest with so many species, many species have evolved to take advantage of specific locations and conditions within the forest. If these conditions change, we may no longer see this species in a forest patch.

Slash and burn agriculture in the Amazon

Fig. 4 Practices such as slash and burn agriculture leave flora and fauna that evolved to persist in deep interiors of forests exposed to unfamiliar conditions. Source: Matt Zimmerman

3 – Once species leave a patch, they often can’t recolonize that patch

In most ecosystems, you end up having some usable habitat patches that are occupied by a species, while other usable habitat patches aren’t occupied. Over time, species in occupied patches go “extinct” within the patch, but then the patch is recolonized by members of a patch nearby. The trick is that species in a patch need to be able to *get *to the unoccupied patch next door. And as someone who has seen his fair share of roadkill, this is easier said than done. In the Amazonian fragmentation study, the scientists found that clearing just a 15-meter-wide strip of forest is enough to pose a serious challenge for dung beetles to cross. And we need them to save the world! An unpaved logging road, or a paved highway can hinder the movement of understory birds, bats, small mammals, tree and plant seeds. All this isolation works to keep new individuals from entering new patches. Especially for species with small populations to begin with, this can threaten the species’ survival in the long term.

Fig. 5 Fragmentation, such as these logging sites in the Amazon, isolates populations of species within forest patches. The light green areas are roads and logging sites cleared by loggers. Source: Wikimedia Commons

What else can we learn from this remarkable study?

Massive undertakings like this Amazonian fragmentation study are rare and difficult to achieve. They are also incredibly valuable in increasing our understanding of how fragmenting forests affect flora and fauna in the long term. What I took away from this study is that there are more obvious consequences to fragmentation, such as isolating populations of species, but there are also less obvious effects, such as a change in wind disturbance up to a half kilometer into the forest interior. So, the next time you are on a flight and fly over a large reserve of intact forest, raise your glass of complimentary carbonated beverage to the women and men that dedicate their lives to protecting and understanding these ecosystems.

References:

  1. Laurance, William F., Thomas E. Lovejoy, Heraldo L. Vasconcelos, Emilio M. Bruna, Raphael K. Didham, Philip C. Stouffer, Claude Gascon, Richard O. Bierregaard, Susan G. Laurance, and Erica Sampaio. “Ecosystem decay of Amazonian forest fragments: a 22‐year investigation.” *Conservation Biology *16, no. 3 (2002): 605-618. Accessed August 01, 2017. doi: 10.1046/j.1523-1739.2002.01025.x

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