Analysis of monitoring data where butterflies fly year‐round

Abstract

Butterfly Monitoring Schemes (BMSs) engage the public in conservation and provide data sets that cover broad geographical areas over long timescales. Most existing BMSs are in temperate climates; however, the Israeli Butterfly Monitoring Scheme (BMS-IL), established in 2009, is a notable exception as it encompasses a large climatic gradient from Euro-Siberian through Mediterranean to hyper-arid regions. Israel's climate poses challenges in analyzing data from year-round butterfly activity, as in other tropical or arid countries. The Regional Generalized Additive Model (Regional GAM) is a butterfly phenology and abundance model based on repeat visits throughout species' flight season. We tested the applicability of Regional GAM for species with complex flight seasonality (e.g., multivoltine) by comparing estimated abundance and seasonal indices for the full data set and rarefied subsets. We assessed the reliability of modeled flight seasons and compared abundance estimates per site resulting from biologically plausible and unreliable seasonal models. The reliability of Regional GAM rises with the number of observations, and the model tends to produce more biologically plausible models for species with simple phenologies (e.g., univoltine with a single peak in activity). Abundance estimates based on unreliable models produce values with interquartile ranges of 90%-153% compared with biologically plausible models, while peak time changes with an interquartile range of 0-22.5 d when comparing all rarefied models with the full data set. Regional GAM should be applied with great caution for rare species and those with a complex flight season, and the date of year start needs to be carefully chosen for species that are active year-round. We identified the key sources of error and propose an operational workflow to address them. With few adaptations, Regional GAM can support new BMSs in analyzing data where butterflies are active year-round, including tropical climates. We propose guidelines for analyzing BMS data for species or regions with long activity periods and complex phenologies.

Environmental controls on butterfly occurrence and species richness in Israel: The importance of temperature over rainfall

“…The uneven distribution of active transects generates a biased representation of biogeographical regions and habitats (Figure 1a , Table 1 ), with better coverage of the low‐lying Mediterranean region. This is the only region where data were sufficient to reliably assess both occurrence and abundance (Comay et al., 2020 ). Transects are rare in desert and semiarid parts of the country, together covering 58% of the study area.…”

“…Regional GAM assumes each species flight season is identical in all input transects within a region. Many butterfly species have complex seasonal activity cycles, including multivoltine, aestivating, and migratory species (Comay et al., 2020 ). To derive reliable abundance estimates, we determined a minimum threshold of 100 visits for univoltine species with a single activity peak, and at least 200 visits for species with complex phenologies (for justification see Comay et al., 2020 ).…”

“…BMS-IL requests that volunteers perform observations twice per month year-round, with an optional break in July-September (see Schmucki et al, 2016 for justification of visitation frequency). For a detailed description of BMS-IL, see Comay et al (2020).…”

“…The abundance index is the sum of actual and imputed (modeled) counts. For more details, see Comay et al (2020).…”

“…This was done in order to include as many transects as possible and to better represent climatic and habitat variability within the low-lying Mediterranean ecoregion. In addition, BMS-IL has grown rapidly in recent years (Comay et al, 2020); thus, 2019 represents both the most recent data and also the year with the best geographical representation of the country. Future data, especially after one or more drought years, could result in considerably different abundance data, such as fewer V. cardui butterflies, which dominated all transects in 2019 (Table 1).…”