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Sand-verbena Moth (Copablepharon Fuscum)

Population Sizes and Trends

There is no quantitative information on population sizes and trends for Copablepharon fuscum. Its relatively recent description and the inherent difficulty in assessing population sizes, variability and trends in rare, nocturnal insects has greatly reduced the potential for detailed population information. Light-trap captures provide a biased estimate of relative population size and are inappropriate for characterizing population density within or between sample sites. In general, moth population densities vary widely between species and it is difficult to extrapolate total population sizes from published values: 0.1 adults per m2 is noted as an average density in habitat patches (Hanski et al., 1994); however, densities vary between 0.0001 and 10.0 adults per m2 (Nieminen, 1996). Based on hand-searching for mature larvae in sand beneath dense Abronia latifolia patches at two sites, population density of C. fuscum varied from about 0.2 moths per m2 to 6 moths per m2 (J. Troubridge, pers. comm., 2002). Total population size based on these density estimates is 350–10 500 adult moths in Canada.

Changes in the distribution and abundance of A. latifolia may be useful in inferring C. fuscum population trends. This relies on the assumption that C. fuscum population size is related to the quantity (m2) and quality (foliar or flower density) of A. latifolia. This assumption is supported by population sampling on moth species in other areas (Forare and Solbreck, 1997; Nieminen, 1996).

Based on a review of herbarium records and field assessment, the number of A. latifolia occurrences in Canada is stable. One population has been extirpated recently: A. latifolia was recorded in the Cheewhat dunes south of Nitinat Lake on the west coast of Vancouver Island in 1979 (Turner et al., 1983) but was not observed in 2001. Several additional sites in the Victoria area have also been extirpated; however, they were considered small transitory occurrences that do not provide habitat for C. fuscum populations.

In contrast, the size and health of A. latifolia populations in many Canadian sites has likely declined substantially in the past 50 years because of the vegetation stabilization trends described previously. A. latifolia appears to decline in vigour and flower density where competition from other vascular plants and bryophytes occurs. While vegetation stabilization occurs as a result of natural successional change in sand-dominated coastal sites, anthropogenic impacts have likely increased the rate of successional change.

In summary, both A. latifolia and C. fuscum populations in Canada appear to be declining because of habitat loss and change. The rate of decline is likely accelerating as open dunes are affected by vegetation stabilization, land development and recreation.