THE 1997/98 POPULATION CRASH

The 1998 count observed less than half (48%) of the number of squirrels seen during the previous year.  The reduction was across all sectors.  Although there had been a few changes in protocol, spot checks throughout October and November indicate that the reductions were real.  Such a dramatic change begs for an explanation.  The most common causes for sudden population decline are habitat destruction, pollution, and food shortage.  With no noticeable change in the first two factors,  I sought a posteriori information regarding the latter.  Fortunately, Cathryn Greenberg of the USDA Forest Service located at the nearby Bent Creek Experimental Forest has been carrying out intensive studies on acorn production in Southern Appalachia since 19931.  Her studies show that while the so-called "hard mast availability" varies considerably within a tree species (consistent with conventional wisdom) that in any given year one species or another is masting, compensating for those having unproductive years.   In other words, different tree species in this region are not as synchronized in fruit production as usually assumed.  This is good news for squirrels as it evens out the fluctuations associated with the individual species.  However, 1997 was an unusual year in that just about all oak species had a very unproductive year.  Figure 1 below shows the combined output of five species of oak (Black, Chestnut, Northern Red, Scarlet, and White) for 1993 through 1999 in terms of acorns produced per square foot of basal area (to obtain acorns per acre, one would have to multiply the basal area of each tree species per acre times their individual productivity's and sum).   Of course, not all acorns are the same size, well formed or free from infection/rot.  Katie also measures the green weight and dry weight of acorns for the same species.   The overall trend is the same but the smaller numbers are not as visually dramatic so I chose to stick with number of acorns.  No wonder 1998 showed a dramatic reduction.  With 3.1% of the mast available the previous year and only 5.4% of the preceding 5-year average, its a wonder that the 1998 squirrel abundance didn't decline more.  Much of this resilience (48% of previous year) probably is owing to the squirrels omnivorous habitats.  Once Spring arrived they could feast on buds, young tender leafs, difficult to catch insects.  During summer and early fall, they can add berries and mushrooms to their diet.  The tough part was making it through the harsh winter without much in the way of cached nuts?  Many didn't, some wandered in search of "browner" pastures, and many mature adults probably skipped a litter, thereby reducing the following years population.  But thanks to many of you who (deliberately or not) supplement their diets at your backyard feeders, many more were able to make it through than would have otherwise.   So for the sake of all of Brevard's squirrels, gray and white, keep filling those feeders (and you might add a little extra for the birds, too).   With the return of mast to normal in 1998 and 1999, the squirrel population rebounded in 1999 and 2000, with the latter representing a new record since the beginning of the Squirrel Count in 1997.

1 Greenberg, Cathryn H.  1999.  "Summary: Acorn Production by Southern Appalachian Oaks, 1993-1997."  USDA Forest Service, Bent Creek Experimental Forest, 1577 Brevard Rd, Asheville NC.  (828)667-5261 Extension 118.

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