Forest corridors can act as pollination superhighways

Nearly three quarters of the earth’s forested lands lie less than one kilometer from the edge of the forest. Forests are becoming increasingly fragmented, with small pockets of natural landscape (or something approaching natural) surrounded by farms, towns, and roads. That fragmentation means several things, one being more extinctions, especially near the edge. For another, it makes it harder for animals to provide their ecosystem services, like pollination.

More than nine out of every ten tropical plants requires “animal mediated pollination.” They rely on critters like bees, bats, or butterflies to successfully reproduce. And those animal pollinators have a tough time pollinating in a fragmented forest. There are fewer pollen donors, there are smaller, less diverse pools of pollinators, and those pollinators have a tougher time moving around. In that environment, species that do not rely on animal pollinators could eventually come to dominate the scraps of forests that remain.

But agroecologist Urs Kormann of the University of Göttingen in Germany thinks that corridors – narrow lines of trees such as those bordering crop fields, for example – that connect larger swaths of forest could help pollinators do their job. It’s known that these living fencerows can themselves sustain substantial biodiversity, but it’s not yet known whether they can also act as pollination highways. A recent study shows that corridors can in fact be effective, at least when it comes to hummingbirds.

Kormann and his colleagues set out for an area in southern Costa Rica with just 30% original forest, its tattered remains scattered among pastures and coffee plantations. They chose to focus on hummingbirds. Compared to other pollinators, hummingbirds can cover a lot of ground. And some 18-34% of plants in the Costa Rican jungle are adapted for hummingbird pollination.

Kormann wanted to see whether the hummingbirds there could use fencerows composed of remnant trees from the original forest along with newly planted ones to move between larger forest patches. To find out, he placed an array of artificial flowers filled with dyed pollen particles within the forest, the hedgerows, and the surrounding fields. By using different colored dyes at different flowers, the researchers could trace how the hummingbirds were moving pollen around the landscape.

Most of the hummingbirds stuck to flowers in forests and along corridors, avoiding those placed in the exposed pastures. Two species—those with more generalist habitat preferences—were more willing to venture out into the fields. However, as the artificial flowers got further from the forest’s edge, even the more generalist species were less likely to visit the flowers.

While the pasture might as well have been a no-fly zone, both corridors and forest fragments were equally likely to serve as pollen conduits. Particularly at short distances, corridors were effective bandages applied to the gaping wounds of deforestation, but at longer distances, they simply couldn’t quell the flow of blood.

Large scale efforts, like the recovery of degraded wetlands or the formal protection of parks, will unquestionably remain critical for conservation. Meanwhile, smaller-scale efforts like maintaining living fencerows alongside pastures and plantations can still result in large gains at pleasingly low costs, a winning formula for most landowners.  – Jason G. Goldman | 27 January 2016

Source: Kormann U, Scherber C, Tscharntke T, Klein N, Larbig M, Valente JJ, Hadley AS, Betts MG. (2016). Corridors restore animal-mediated pollination in fragmented tropical forest landscapes. Proc R. Soc. B. DOI: 10.1098/rspb.2015.2347.

Header image: Green Hermit hummingbird (Phaethornis guy) via Eric Chan/Wikimedia Commons.



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