"....The truth is out there."
-- Dr. Jerome Jackson, 2002 (... & Agent Fox Mulder)
“There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.”
"All truth passes through 3 stages: First it is ridiculed. Second, it is violently opposed. Third, it is accepted as self-evident."
-- Arthur Schopenhauer
Saturday, December 13, 2008
-- IBWO Population Dynamics --
Here an open-access published paper using population dynamics/stochastics to argue for how a small population of Ivory-bills or certain other large woodpeckers could persist for many years with few human encounters.
The variables in such an analysis will always be difficult to precisely define or measure (which goes for most ecology/field biology work) and such a study must be viewed with a grain of salt --- I'm sure skeptics will find much to quibble with, including certain assumptions made. And I'm certainly not competent to judge the specific technical mathematics involved, but I don't think there's anything overly profound in the general conclusions reached here, part of which read as follows:
"...based on our model, an initial population size of 5 females would have ensured likely persistence through modern times if annual demographic rates remained at least moderate, i.e., ≥ 1.1 recruited females/adult female and an adult survival rate ≥ 0.8, and their variances remained at most moderate, i.e., ≤ 0.04 for recruited females per adult female and ≤ 0.016 for adult survival rate, (Appendices 2–4). Second, if there were 30 or more females, then the population would likely have persisted despite a relatively strong Allee effect, as long as variance in survival was at most moderate and either survival or fecundity was high (i.e., 1.65 recruited females per adult female or an adult survival rate of 0.9; Appendices 2–4)."
Many years ago I did some simple common-sense paper-and-pencil calculations based on about 24 Ivory-bills starting in the 1940's (at the upper end of Tanner's estimate for the population then, but several folks think Tanner was too conservative), which convinced me that that number of IBWOs could easily have achieved population stasis for many decades (and still be with us), prior to either increasing in numbers or dying out.
In a similar vein, a related blog-post here 3 years back, started with an initial population of just 16 birds to make some points:
The simple point being, under a variety of circumstances, small populations of vertebrate creatures can hang on for a very long time (not growing in numbers substantially, but not dying out either) if certain minimal conditions are met.