70% of Earth’s forests lie within one kilometer of an edge

It’s not news that concrete is carving up the world into individual sections, forests becoming individually wrapped nature treats, each with its own micro-communities. The ability of animals to travel from one forest patch to another is severely hampered by cars and trucks. Wildlife underpasses and overpasses are good solutions to the problem, but only for some species, at some places, at some times. The world really is just a collection of islands; where they’re not separated by water, they’re divided by pavement.

One of the most obvious examples of habitat degradation is the loss of forest cover. It’s also perhaps the easiest study using satellite imagery. The world has lost nearly a third of all forest cover worldwide over the past few centuries, and of what’s left, some 70% is within just one kilometer of a forest’s edge. Around one in five trees is within just one hundred meters of an edge.

The only real remaining large, expansive, contiguous forests are in the Amazon and Congo River basins (where they still face serious threats), and in some stretches of the boreal zones in North America and Asia. While a similar analysis of grasslands is not possible, 37% of the planet’s grasslands are classified as “highly fragmented” as well.

But is it fragmentation itself that’s to blame for global declines in biodiversity? It’s certain that those declines can be traced right back to human activity and to habitat degradation. But degradation comes in many forms, fragmentation is but one of them. In some sense, it’s a bit of a twisted question: we know we’re killing off all the wildlife, but just how are we doing it? The sad truth is we’re not going to revert our planet back to its natural state (whatever that means), so what is left is mitigation. And to effectively mitigate, we need to understand just what the problem is in the first place.

That’s where a massive group of researchers led by North Carolina State University biologist Nick M. Haddad comes in. “Ecologists agree that habitat destruction is detrimental to the maintenance of biodiversity, but they disagree—often strongly—on the extent to which fragmentation itself is to blame,” he wrote last week in the journal Science Advances. To understand the role of fragmentation itself – that is, roads and urban development – on biodiversity losses, the researchers synthesized information from 35 years worth of experiments, on a diversity of ecosystems across all continents except Antarctica.

Altogether, the researchers had data from 76 different studies. They divided their data into bins according the type of altered landscapes that were studied: reduced area (a forest simply becomes smaller), increased isolation (the distance between parts of a forest increase), or increased edge (the overall surface area of a forest doesn’t change, but the proportion of it near an edge does). They found, for example, that reduced area decreases the amount of animals resident within it. That’s intuitive. Increased isolation reduced the animals’ movements between fragments. That’s also intuitive. Birds, mammals, insects, and plants all suffered in these cases, even if individual species appeared to benefit, such as via mesopredator release. All three types of landscape alteration led to a reduction in seeds eaten and herbivory. When the proportion of a forest near an edge increased, birds in particular suffered due to a higher likelihood of fledglings lost to predation.

Taken together, the experiments examined by the researchers revealed that after two decades, on average, the variety of animals and plants living in forests declined by half.

Areas in blue are >1km from an edge. Credit: Joe Sexton and Danxia Song.

Areas in blue are >1km from an edge. Credit: Joe Sexton and Danxia Song.

These effects were seen from biome to biome, ecosystem to ecosystem. The particulars, in this case, didn’t matter: habitat fragmentation was bad news. “Fragmentation consistently degraded ecosystems,” concluded the researchers, “reducing species persistence, species richness, nutrient retention, trophic dynamics, and, in more isolated fragments, movement.”

While most of these results were predictable, Haddad and his colleagues write that they were still struck by just how persistent those detrimental effects were. In one experiment, extreme rainfall allowed plant communities to persist even in their altered landscapes for five years after fragmentation, but eventually the effects of that habitat alteration became evident. In another experiment, it took 12 years before any effects were seen. Thus the effects of fragmentation might only begin to become observable after years or decades, rather than weeks or months, depending on the community of plants or animals of interest.

Our species’ needs for food and shelter will only increase in coming decades, resulting in more urban development and more conversion of landscapes for agricultural purposes. “The effects of current fragmentation will continue to emerge for decades,” concludes Haddad. “Understanding the relationship between transient and long-term dynamics is a substantial challenge that ecologists must tackle, and fragmentation experiments will be central for relating observation to theory.”

Given how rapidly the earth’s ecosystems have become fragmented in recent decades, the long-term effects on biodiversity and ecosystem functioning have, in many cases, probably not become fully realized. The long-term fragmentation experiments for which Haddad and his colleagues call are important and necessary for allowing us to make informed decisions, but will we have time to learn from them by the time the results are in? – Jason G. Goldman | 25 March 2015

Source: Haddad N.M., et al. (2015). Habitat fragmentation and its lasting impact on Earth’s ecosystems, Science Advances, 1 (2) e1500052-e1500052. DOI: 10.1126/sciadv.1500052.

Header image: The site of a forest fragmentation experiment in South Carolina. Credit: Ellen Damschen, used with permission.

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