Patterns of Monarch Site Occupancy and Dynamics in Iowa

Abstract

The monarch butterfly (Danaus plexippus) is the focus of large-scale habitat restoration efforts because of recent population declines. From 2006-2017 we monitored monarchs at >400 sites throughout Iowa to link site occupancy and colonization/extinction dynamics to the presence of milkweed, site-specific habitat metrics, and landscape context at differing spatial scales. We used a robust design occupancy model in Program MARK and a hierarchical model selection approach to estimate site occupancy, extinction and colonization probabilities, and detection probability. Occupancy models revealed that monarchs responded differently to landscape features, environmental conditions, and local habitat conditions for site occupancy, extinction, and colonization probabilities. For site occupancy, the mean patch size of grassland at the 1-km spatial scale had a positive effect (β GrassPS1K = 0.94, SE = 0.54) while the percent of the landscape in woodland at the 200-m spatial scale had a negative effect (β WoodPL200 = −1.68, SE = 0.34). For extinction, there were additive effects of the percent of the landscape in woodland at the 100-m spatial scale (β WoodPLAND100 = 2.70, SE = 0.63), the interspersion of grassland at the 1-km spatial scale (β GrassIJI1K = −2.30, SE = 0.63), and litter depth (β Litter = 0.46, SE = 0.13). Finally, there were negative effects of the percent of the landscape in woodland at the 200-m spatial scale (β WoodPLAND200 = −4.67, SE = 1.37) and the interspersion of grassland at the 100-m spatial scale (β GrassPS1K = −2.02, SE = 0.70) on colonization probability. Detection probability was affected by the additive effects of canopy cover and monarch density; no other detection model was competitive. In the top model there was a positive effect of monarch density (β Density = 0.28, SE = 0.05) and a negative effect of canopy cover (β Canopy = −0.18, SE = 0.03) on detection probability. In Iowa, monarchs are widespread on conservation lands where they avoid sites with lots of canopy cover. Colonization and extinction processes are influenced by an interplay of landscape attributes across multiple spatial scales and site habitat attributes. Our study provides the first comprehensive insight into monarch use of conservation lands in Iowa, and predicted responses to important covariates may be useful for future conservation efforts.

Landscape- and local-level variables affect monarchs in Midwest grasslands

“…While the resource concentration hypothesis posits that specialist herbivore insects are more likely to nd and linger within larger habitat patches (Root 1973, Hill et al 1996, we found that patch size did not predict adult monarch density. Previous researchers reported a positive effect of patch size on monarchs (Davis et al 2007, Dinsmore et al 2019. Patch size assessment differed between our study and theirs, which may account for different results.…”

“…We used a digital raster-based land cover classi cation map with a grain size of 30 m × 30 m to determine, for a 10 km-radius circular buffer surrounding each survey site, the proportion of grassland habitat (National Land Cover Dataset 2016). The 10-km assessment was chosen to correspond with the estimated daily travel maximum for a female adult monarch (Zalucki 2016, see also Dinsmore et al 2019). Area-based landscape metrics, speci cally the proportion of suitable habitat in the landscape within a given buffer, are reliable metrics of patch isolation when compared to simple distance-based or omni-directional distance-based metrics (Bender et al 2003, Fahrig 2013.…”

“…Patch size can also in uence butter y movement based on individuals' ability to navigate an unfavorable landscape, but the directionality of this effect varies based on species. Established theory suggests larger patches are better at supporting specialist herbivore populations (Root 1973, Hill et al 1996, and monarch studies investigating the effect of patch size indicate that larger patches contain more monarch adults (Davis et al 2007, Dinsmore et al 2019. However, other studies suggest that small patches can have higher immigration rates than large patches for some butter y species (Shultz & Crone 2005), and that monarchs might bene t from small patches (Grant et al 2018).…”

Design Implications for Surveys to Monitor Monarch Butterfly Population Trends

“…Therefore, based on this previous work and our results, we emphasize caution when interpreting results from studies based on targeted sampling strategies and argue for randomization in future studies of long-term monarch population trends to ensure unbiased inference at appropriate spatial scales. It may be worth adding that we included non-random sites in a robust design occupancy analysis (Dinsmore et al, 2019) and argued that the inclusion of these sites was of less concern for presence/absence data than for density or count data (as in this study). This is due to the high occupancy rate of monarchs (often > 0.90), independent of actual counts.…”

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

“…Land-use land-cover (LULC) studies describe how land cover, and land use by humans, changes through time and the causes/consequences of those changes. LULC analyses have tracked the status of oyamel fir ( Abies religiosa ) forests supporting monarchs in Mexico 38 – 42 , identified restoration potential of monarch habitat in the US 43 – 45 , modelled monarch and milkweed distributions 7 , 46 , 47 , migratory connectivity 48 and threats during migration 49 . Analyses of how LULC may impact monarch habitat in Texas and Mexico, have not, to date, occurred, despite the important role this region plays in the eastern population’s life cycle.…”