Tanner and Population Density

I recently gave a talk to the Rockland County Audubon Society, and a member raised what I think is the strongest question about our evidence and about the persistence of the Ivory-billed Woodpecker in general. “How could the species have survived in such low numbers and at such low densities?”

In other posts, we’ve pointed to evidence that Tanner missed a population in Mississippi and was somewhat cavalier in his approach to evaluating potential habitat, disregarding advice Herbert Stoddard gave him in 1936, “The area where they (Ivory-billed Woodpeckers) may occur at present is simply tremendous, not restricted as many believe.”

A recent study on Magellanic Woodpeckers points to another factor that raises even more doubt about Tanner’s estimated population of 22 in 1939.  The study was conducted in an old growth Lenga forest in Patagonia. According to this study: “Our results show that Magellanic Woodpecker family groups require a minimum of 100 ha in old-growth forest habitat; thus, forest patches in less favourable forest conditions (e.g., younger, managed, fragmented, mixed forests) should probably be much larger to support a resident pair or family.”

The specific criteria that Tanner used for estimating the 1939 population at approximately 22 are unclear, but he assumed a maximum carrying capacity of six square miles per pair. The Wikipedia entry on the IBWO is even worse and is generally rife with bad information; the editors there expand Tanner’s six square miles to “9.7“.  Elsewhere, Tanner suggested a minimum home range of two and a half to three square miles. These numbers are somewhere between six and 16 times the minimum for a large southern congener that, like the ivorybill, lives in more temperate habitat than others in the genus.

Thus, there is a strong possibility that Tanner severely underestimated ivorybill populations in Florida. If he was so badly wrong about home range, he’s more likely to have missed populations in areas that he rejected for being suboptimal and not expansive enough.

Double the minimum acreage required by the magellanic in optimal habitat and apply that number to the ivorybill, and Sherburne, a large Louisiana WMA, could theoretically be home to just under 90 family groups. (We don’t think this is the case.) Even at 450 hectares per family group, the carrying capacity in Sherburne would be nearly 40. Such numbers are improbable in the extreme, but 9 or 10 family groups in an area that size would be very hard to detect.

With significantly smaller home range requirements, a substantially larger population in 1939, and a recognition that Singer Tract-like conditions are not a requirement (as Tanner himself made clear), various survival scenarios become considerably more plausible, assumptions about low densities become more questionable, and the quantity of potential habitat is far greater than anyone has imagined.

Edited to add: Although the study referenced above is more recent, Noel Snyder made the same basic argument about magellanics and other congeners in The Travails of Two Woodpeckers (2009). Snyder posits that hunting pressure, not habitat loss, was the primary cause of the ivorybill’s decline.

Snyder (who to the best of my knowledge has little hope for the ivorybill) does not fully address how Tanner’s assumptions might have affected his population estimates and habitat evaluations. He also doesn’t consider how taking Tanner at face value has influenced both search protocols and the “credibility” of post-Singer Tract reports. Nevertheless, he does hint at what I suspect is the key to the species’ survival: “With food supplies degraded, not eliminated, a reasonable possibility appears to exist that many ivory-bill populations in logged regions might still have found enough food to persist and might have endured at modest densities, had they been free of shooting pressure. The long persistence of the ivory-bill in one quite thoroughly logged region in Cuba supports this possibility . . .”

In my view, Snyder goes a little too far in downplaying specialization as a factor, even if Tanner overplayed it. It’s pretty clear – from range, habitat, and morphology – that ivorybills are more specialized than pileateds. But if the IBWO did persist after World War II and Snyder is right that hunting was a major factor in the species’  decline (even in the Singer Tract), there may be even more room for optimism, since hunting practices changed considerably in the post-war era.