Wednesday, May 20, 2009

The Decline and Fall of the Swamp Phantoms: a hypothesis

In recent months I have begun to think more and more about one particular possible cause for the decline and (we hope) near-extinction of the Ivory-billed Woodpecker and Bachman's Warbler. I am suspecting that a fundamental driver might have been the elimination of beavers from eastern North America. Beavers were (and now once again are) active in the large river swamps, not just the small streams one might first think of. They particularly like to build dams on the downstream ends of oxbow lakes to lift water levels during low-water periods. Since beaver activity shifts around over the landscape over the years, this causes tree death and the creation of open shrubby swamps and shores within the bottomland forest landscape. I have seen first hand how beavers have converted closed canopy forest to boneyards of dead and dying trees along with sunny tangled thickets of cane, swamp privet, and willows. In the case of these two bird species, one dependent on large numbers of dead and dying trees, the other dependent on clearings in the forest, I'd hypothesize that beavers may have been a critical factor maintaining the landscape they needed. The historical timing seems right as well, as both of the phantom swamp birds species seem to have already been well on the way to ghost-bird status even before massive forest clearing was underway.


At 3:33 PM, Blogger Paul Krusling said...

I also have speculated that the temporary loss of the beaver, in Ohio, was one of the reasons for the current scarcity and widely disjunct dstributions of many species of wildlife, particularly reptiles and amphibians (My main area of interest, which is closely folowed by birding). The open canopy and open marshland habitats created by beavers allowed for the dispersal of such rare species as the spotted turtles, many pond breeding amphibians, and possibly in other states, extremely rare species such as the bog tortle in the mountains of the Southeast.

The elimination of the beavers and the resulting elimination of the wetlands created by them restricted the distribution of these herps to less common, but more permanent wetlands such as fens, bogs, and more recently, man made wetlands.


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