On the edge: fragmented forests signal biodiversity crisis

The world’s most famous forest, Sherwood, was once so vast it was entirely believable that Robin Hood and his merry men could hide in it from the forces of the Sheriff of Nottingham. These days it barely covers 423 hectares. Bounded by farmland, housing developments and a recently abandoned coal mine, it is also bisected by roads and includes large areas where trees have been cleared, leaving just small, isolated wooded thickets.

What has happened to Sherwood typifies what has happened to forests the world over. Not only has total forest area been drastically reduced by human activity, but the forest remaining has been fragmented by incursions such as clearing patches for farms, logging coupes, mining operations and settlements, as well as roads, fire tracks and corridors for power lines. 

The “edge effects” of this ongoing subdivision are profoundly reshaping forest ecosystems, with dramatic effect on biodiversity within them. Some animals are winners, but at the expense of others, including a majority of forest-dwelling mammal species.

“There are few remaining areas where you don’t have intrusions into the forests, such as roads and other activities,” says Adam Hadley, of the Oregon State University, US, co-author of a study that quantifies the biodiversity effect of fragmenting large forests into many smaller ones.

An estimated 70% of the world’s remaining forest now within 1 kilometre of an edge, 50% within 500 metres, and 20% within 100 metres.

The way “edge effects” alter the amount of effective habitat area for forest-dwelling creatures is at least as important as habitat amount for understanding and preserving biodiversity and ecological balance.

The new study, involving an international team of 32 researchers led by statistical ecologist Marion Pfeifer of Newcastle University in northern England, highlights how even a simple fire track can alter the ecological balance of the surrounding habitat, increasing the abundance of animal species adapted to the forest-edge while reducing those dependent on the forest core.

Small amphibians, for example, might depend the high humidity of the forest core to avoid overheating and dehydration, maintain gas exchange, cultivate “bacterial symbionts with immune functions” and protect their eggs. Larger forest core reptiles are also likely to be more vulnerable to overheating through greater sun exposure at a forest edge.

Thus, while the researchers’ findings indicate that marginally more species were positively rather than negatively impacted by forest edge effects, they warn against a simplistic interpretation of these results. This is not apparent good news, but masks a deeper, more complex problem. 

“When simply counting the number of positive versus negative impacts and assuming that one cancels the other out, one disregards the more important fact that 85% of species are impacted and that the resultant community that now persists near the edges bears little resemblance to those communities in the forest interiors,” their paper cautions. 

“Maybe we should consider not putting roads into them,” Hadley says. “It may be valuable just to keep them as remote as possible.”

Their research, published in Nature, involved mapping and quantifying changes in the abundances 1,673 vertebrate species (103 amphibians, 146 reptiles, 1,158 birds and 266 mammals) using data from 22 fragmented forest landscapes distributed across seven major biogeographic regions.

The analysis was unable to determine edge responses for 369 species. Of the remaining 1,304 species, 46% seemed to have increased in abundance, 39% decreased in abundance, and 15% showed no change. 

Negatively affected animals include threatened species such as the Sunda pangolin (Manis javanica), the Bahia tapaculo (Eleoscytalopus psychopompus), the long-billed black cockatoo and and the Baird’s tapir (Tapirus bairdii). 

More generally, mammals were most threatened by the encroachment of forest edges (57% of species examined negatively affected), followed by amphibians (41%), reptiles (30%) and birds (11%), the study reports.