Design Implications for Surveys to Monitor Monarch Butterfly Population Trends

Abstract

The Iowa Multiple Species Inventory and Monitoring (MSIM) Program includes a protocol for monitoring butterfly density on conservation lands using transects. Most data are collected from sites chosen randomly; additional sites are chosen non-randomly for other reasons. We analyzed a 12-year dataset for monarchs to address how density (per 50 m 2 transect section) responded to site selection (random vs. non-random), latitude, and measures of the amount of milkweed and canopy cover on survey transects. Between 2006 and 2017, we conducted 2,328 surveys on 420 sites and detected a total of 2,757 adult monarchs. Monarch densities peaked in 2010 for random sites and 2012 for non-random sites, but densities were lowest in 2013 for both site types. The density of monarchs at non-random transects (0.047, 95% CI = 0.031, 0.062) was higher than that at random transects (0.029, 95% CI = 0.019, 0.044) and the temporal trends of density for random and non-random sites were significantly different. Monarch density was positively correlated with UTM northing, suggesting that monarch density increases from south to north in Iowa. The percent of plots containing milkweed was positively correlated with monarch density whereas percent tree canopy cover was negatively correlated with monarch density. Our results show that non-random transects had more monarchs, which may be a concern when interpreting findings from some citizen science efforts that used non-probabilistic sampling designs. Collectively, the MSIM program data provide a comprehensive assessment of monarch densities statewide as well as the first empirically-derived density estimates for monarchs on the breeding grounds and may prove helpful when refining future monitoring efforts.

Rapid Assessment of Roadsides as Potential Habitat for Monarchs and Other Pollinators

“…The high milkweed density documented in this study in Minnesota (2,052 plants/ha by IMMP (834 plants/ac); 1,527 plants/ha (620 plants/ac) by Rapid Assessment) confirm that roadside rights-of-way can provide significant amounts of breeding habitat for monarchs (Kasten et al, 2016). And, adult monarch numbers are associated with percent cover of milkweed (Kinkead et al, 2019) and milkweed abundance has been associated with adult monarch abundance (e.g., Zalucki and Lammers, 2010;Pleasants and Oberhauser, 2013). Converted into linear miles of interest to road managers, for the average rightof-way width we surveyed (9.43 m), this is 2,316-2,641 milkweed plants/mile (using the range of IMMP and RA estimates).…”

Monarch Habitat in Conservation Grasslands

“…Previous studies have identified the importance of nectar plants as a component of monarch habitat (Stenoien et al, 2016;Thogmartin et al, 2017a,b;Kinkead et al, 2019), and have proposed that a loss of nectar plant resources could be a significant contributor to increased mortality during migration (Agrawal and Inamine, 2018). The general lack of data on monarch habitat as a whole (milkweed plus nectar plants) makes studies like ours even more necessary.…”

No broad decline of breeding monarch butterflies in North America: implications for conservation efforts

“…We are aware that a prior argument against using evidence from non-random, citizen science monitoring programs such as NABA, is that they provide misleading trends, because people tend to monitor where they know butterflies would be present (Pleasants et al 2015, Pleasants et al 2017, Kinkead et al 2019). It has also been argued that losses of milkweed in the agricultural fields of the Midwest has "shifted" adults away from these areas and into the more pristine monitoring sites (Pleasants et al 2015, Pleasants et al 2017, which could artificially inflate abundance at these sites.…”