10.6.15

Edges

I was back in Georgia a few times this Spring, and made daily visits to one of my favorite sites - Johnson' Ferry, a ribbon of riverine old growth forest and bottomland swamp along the west bank of the Chattahoochee river.

It's not much more than a few hectares, barely outide metropolitan Atlanta, but it always amazes me how biodiverse it is. In an hour or two in early October here I had Philadelphia vireo, yellow-throated vireo, rose-breasted grosbeaks in the same tree, 7 warbler species and 6 species of woodpecker within the following 10 minutes. I was surprised, looking back at eBird, to see that I'd racked up 70 spp. here within 5 or 6 visits.

And it got me thinking about edge habitats.

Here's the acre in question (with a confiding - yes that's a cellphone shot - red-shouldered hawk in it):




There's a little scrub, some riparian willows along the creek, and a bit of successional woodland and tall grasses along an old clear-cut for a gas pipeline, which the sparrows love in winter.

But walk out of this scrubland and into the surrounding forest, and the diversity drops off a cliff.

Given that northern Georgia was entirely forested until less than a century ago, why is there such diversity in these (formerly scarce) edge habitats compared to the forest interior?

Some of these are migrants, clearly, but the same pattern plays out across the year.

Various studies have tried to untangle this question, some of which suggest that many species thought to be 'edge' specialists are in fact shrubland specialists. I was interested to see that the idea of an 'edge specialist' may be largely a misnomer, with a few possible exceptions:

If shrubland birds are not edge species, then what exactly is an edge species, and do such species exist? Imbeau et al. (2003) suggested that merely inhabiting an ecotone does not define an edge species. Rather, an edge species must make use of both of the adjacent habitats (see also Ries et al. 2004). By this definition, few birds included in our meta-analysis would actually be considered edge species. One exception may be Indigo Buntings, which nest in scrubby areas, but appear to prefer territories with tall trees, used as song posts (Taber and Johnston 1968). Still, buntings were edge averse in our meta-analysis, suggesting that their usage of edges and tall trees may be opportunistic rather than obligate.
Read More: http://www.esajournals.org/doi/full/10.1890/08-0020.1

But shrubland specialisms should develop where there are good shrubland niches available, and as the study above shows, these shrubland species tend to be edge-averse within shrubland habitat anyway!

Moreover, shrubland is rare in northern Georgia, which in evolutionary terms has been continuously forested until very recently, and retains more contiguous forest than most of the lower 48.

So, is the high diversity in places like Johnson's Ferry an adaptation to the (relative) continuing abundance of forest in the region, concentrating birds who avoid tall trees (blue-winged warblers and such like) and can't forage effectively in deeply shaded forest?

If so, it's almost the reverse of the scenario that plays out in urbanised and deforested suburban areas, which concentrate migrants in refugial forested tracts. Just go to Central Park's Ramble to see this effect at its most heightened.

Understanding how and why both residents and migrant use these kind of ecotones will only become more important as habitats of all kinds fragment further.







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NYC, Buxton, Buenos Aires
I work in NYC and own a wildlife and wilderness agency specializing in the southern cone of South America. I still do some guiding down there, especially looking for Fuegian and Patagonian avifauna. I'm particularly interested in the wintering ecology of neotropical migrants, and in avian biogeography in general. You can follow me at - @domhall And find me at - AventuraArgentina.com