Determining Optimal Population Monitoring for Rare Butterflies

Abstract

Determining population viability of rare insects depends on precise, unbiased estimates of population size and other demographic parameters. We used data on the endangered St. Francis' satyr butterfly (Neonympha mitchellii francisci) to evaluate 2 approaches (mark-recapture and transect counts) for population analysis of rare butterflies. Mark-recapture analysis provided by far the greatest amount of demographic information, including estimates (and standard errors) of population size, detection, survival, and recruitment probabilities. Mark-recapture analysis can also be used to estimate dispersal and temporal variation in rates, although we did not do this here. Models of seasonal flight phenologies derived from transect counts (Insect Count Analyzer) provided an index of population size and estimates of survival and statistical uncertainty. Pollard-Yates population indices derived from transect counts did not provide estimates of demographic parameters. This index may be highly biased if detection and survival probabilities vary spatially and temporally. In terms of statistical performance, mark-recapture and Pollard-Yates indices were least variable. Mark-recapture estimates were less likely to fail than Insect Count Analyzer, but mark-recapture estimates became less precise as sampling intensity decreased. In general, count-based approaches are less costly and less likely to cause harm to rare insects than mark-recapture. The optimal monitoring approach must reconcile these trade-offs. Thus, mark-recapture should be favored when demographic estimates are needed, when financial resources enable frequent sampling, and when marking does not harm the insect populations. The optimal sampling strategy may use 2 sampling methods together in 1 overall sampling plan: limited mark-recapture sampling to estimate survival and detection probabilities and frequent but less expensive transect counts.

Extracting useful data from imperfect monitoring schemes: endangered butterflies at San Bruno Mountain, San Mateo County, California (1982–2000) and implications for habitat management

“…This approach adds a geographic dimension to the monitoring schemes for endangered butterfly species proposed by Haddad et al (2008) and Nowicki et al (2008). Haddad et al (2008) presented techniques to determine population parameters for the purpose of undertaking population viability analysis and concluded that a combination of transect and mark-recapture sampling would generate the most accurate results at least cost and harm to the butterflies.…”

“…Haddad et al (2008) presented techniques to determine population parameters for the purpose of undertaking population viability analysis and concluded that a combination of transect and mark-recapture sampling would generate the most accurate results at least cost and harm to the butterflies. Nowicki et al (2008) addressed efforts to define butterfly distribution and concluded that they must be improved by incorporation of statistical correctors for detectability (MacKenzie et al 2002(MacKenzie et al , 2003.…”

Standardised methods for the GMO monitoring of butterflies and moths: the whys and hows

“…in Leopold and Fartmann 2005). However, it is important to note that transect counts generate indices of relative abundance but not precise estimates of population densities, though transect count data can be correlated with population size, sometimes to a high degree (Haddad et al 2008, Nowicki et al 2008. Among others, differing detection probability by species, observer, region, year and season can bias the data recorded (e.g., Kery and Plattner 2007).…”

“…by distance sampling (see below); some freeware programs such as MARK or PRESENCE might also be helpful in this respect. However, mark-release-recapture (MRR) methods are most often used to calculate detection probabilities for species as well as estimates of population sizes and survival probabilities (e.g., Haddad et al 2008). Yet, MRR methods are quite laborious, may not be adequate for highly mobile species with low recapture rates, and are potentially harming the marked individuals or changing their behaviour (Sutherland 2006).…”

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“…Capture-mark-recapture methods such as those employed by 363 Calvert (2004), but replicated over years and locations, seem to offer the most promising means 364 of accomplishing this goal of improved overwinter density estimation. Capture-mark-recapture is 365 the most common method for estimating population size in butterflies (e.g., Gall, 1984; 366 Bergman, 2001;Baguette & Schtickzelle, 2003;Haddad et al, 2008), and its systematic use in 367 the high-elevation Oyamel fir forests of central Mexico would enable robust estimates of daily 368 and total overwintering population sizes, as well as survival and emigration probability 369 (Williams et al, 2002). However, capture-mark-recapture methods come at the considerable cost 370 of disturbing overwintering individuals, a practice that is currently disallowed and arguably not 371 prudent given the small population size and the negative impacts of disturbance.…”