Compare Population Fluctuations and Percent of the White Variant

Over eight years, the abundance of Eastern Gray Squirrels in the study area has varied tremendously.  Although the squirrel count, itself, underestimates true abundance (see Census page) the average of observed squirrels over the three yearly counts permits a relative comparison from one year to another.  As can be seen in Figure 1 below, the relative abundances vary by over a factor of two.   Needless, to say, other statistics verify that the population varies significantly over the years.  Yet as can bee seen in Figure 2 below, the percent white does not vary significantly over the same time period.

For whatever reason, the percent white is stable, i.e., it rose dramatically from almost zero in 1951 to approximately one out of four by the 1990's but has reached equilibrium.  The fact that two variants have come to reach some kind of non-trivial equilibrium suggest that there is a trade off.  That is, there must be both a cost and a benefit of being white that off set each other at the 25% level.  The cost would seem to be obvious: increased visibility to predators.  Other than human preference, the off setting benefits are yet to be determined.

For a discussion of why squirrel populations fluctuate so dramatically, visit the (out of date but still informative) hard mast page.