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COSEWIC Assessment and Status Report on the White Flower Moth (Schinia bimatris) in Canada

Appendix 3

Methodology used to calculate population numbers.

Population size is very difficult to estimate with data from light-trap catches, particularly with the very limited data available. The estimate provided is based on the amount of suitable habitat available, and the author’s experience with catch efficiency of light traps. The most important variables that come into play are the radius over which a light ‘pulls in’ moths, the amount and directionality of moth dispersal (which is weather-dependent), and the seasonality of the flight period. To make a rough approximation, the following assumptions were made:

  • an attracting radius of light trap of between 3 m and 10 m (Baker, R.R. and Y. Sadovy. 1978. The distance and nature of the light trap response of moths. Nature 276: 818-821.);
  • an average of 0.5 moths/trap based on Troubridge and Lafontaine’s trapping results from 2003 (11 moths for 24 trap-nights);
  • the 2003 sampling occurred during the peak flight time, and about ½ of the total population was available to sampling during the peak (with the remainder having either already completed their flight or not yet emerged); and
  • suitable habitat occurs only within the 5 km² of open dunes, i.e., the area of occupancy.

Since the attraction radius to light is relatively small, and moths are flying/dispersing, the area sampled by light traps is effectively much larger than just the “area of attraction” based on the 3 to 10 m estimate; this can be thought of as the effective sampling area and has to be estimated. Setting the effective trap radius at 50 m yields a total population estimate of about 600, and a radius of 20 m yields an estimate of about 4000 with the above assumptions. If one accepts the radii as reasonable and assumes 2003 was a year with average population size (we know 2004 was likely a very low year), year to year fluctuations could easily be as low as 100 at the low end and 5,000 or more at the upper end.

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