Effects of weather on S. adiaste transect counts

2011-2012

 

Comparison of weekly counts of adult S. adiaste in 2011 and 2012 (above) highlights several differences in population dynamics between the respective flight seasons:


  1. 1. Emergence of flying adults began one week earlier in 2012.

  2. 2. Adults in 2012 reached peak flight at least two weeks earlier than 2011.

  3. 3. The adult flight season ended about three weeks earlier in 2012.

  4. 4. Numbers counted during transects declined by 42% from 2011 (n=282) to 2012 (n=164).


These differences are perhaps best explained by differences between 2011 and 2012 in rainfall and temperature (see charts below).  The winter of 2011 featured average rainfall (21.4 in. at Pinnacles NM) followed by a cool spring.  Winter rainfall in 2012 was severely low (9.7 in. at Pinnacles), with only a few moderately wet March storms, followed by normal spring temperatures. 


One likely effect of the low winter rainfall of 2012 is its impact on vegetation, in particular growth retardation of the S. adiaste host plant, Viola purpurea.  No quantitative plant data were collected, but observations during transect counts noted fewer Viola plants (as well as Viola plants with smaller leaves) in habitat patches surveyed the previous year, 2011.  Dessication of plants would occur earlier, and doubtless not all larvae reached maturity.  As a consequence, the warmer, drier, winter-spring-summer conditions in 2012 would promote both an earlier start and a shorter duration of the adult flight season.


Despite the dry conditions of summer 2012, mark/recapture sampling at Chew’s Ridge marked 325 adult butterflies, suggesting a colony population size of over 500 adults for that season (see Chew’s Ridge monitoring 2012). 


















































 

2011

2012

n = 282

n = 164

2011

2012

21.4 in.

9.7 in.

2011

2012