Local and cross‐seasonal associations of climate and land use with abundance of monarch butterflies Danaus plexippus

Abstract

Quantifying how climate and land use factors drive population dynamics at regional scales is complex because it depends on the extent of spatial and temporal synchrony among local populations, and the integration of population processes throughout a species’ annual cycle. We modeled weekly, site‐specific summer abundance (1994–2013) of monarch butterflies Danaus plexippus at sites across Illinois, USA to assess relative associations of monarch abundance with climate and land use variables during the winter, spring, and summer stages of their annual cycle. We developed negative binomial regression models to estimate monarch abundance during recruitment in Illinois as a function of local climate, site‐specific crop cover, and county‐level herbicide (glyphosate) application. We also incorporated cross‐seasonal covariates, including annual abundance of wintering monarchs in Mexico and climate conditions during spring migration and breeding in Texas, USA. We provide the first empirical evidence of a negative association between county‐level glyphosate application and local abundance of adult monarchs, particularly in areas of concentrated agriculture. However, this association was only evident during the initial years of the adoption of herbicide‐resistant crops (1994–2003). We also found that wetter and, to a lesser degree, cooler springs in Texas were associated with higher summer abundances in Illinois, as were relatively cool local summer temperatures in Illinois. Site‐specific abundance of monarchs averaged approximately one fewer per site from 2004–2013 than during the previous decade, suggesting a recent decline in local abundance of monarch butterflies on their summer breeding grounds in Illinois. Our results demonstrate that seasonal climate and land use are associated with trends in adult monarch abundance, and our approach highlights the value of considering fine‐resolution temporal fluctuations in population‐level responses to environmental conditions when inferring the dynamics of migratory species.

Disentangling data discrepancies with integrated population models

“…, Saunders et al. ) while others have failed to detect any significant decreases in adult counts (Ries et al. , Inamine et al.…”

“…Census data on monarchs at their overwintering grounds in Mexico indicate a 19-yr downward trend (Ries et al 2015). However, trend estimates from count data at summer Midwestern U.S. breeding sites have been inconsistent; some studies suggest declines in the northern extent of their breeding range (Stenoien et al 2015, Saunders et al 2017 while others have failed to detect any significant decreases in adult counts (Ries et al 2015, Inamine et al 2016. Issues of spatial scale mismatches may also lead to data set discrepancies, given that fine-scale indices may show contrasting trends compared to largescale indices.…”

Assessing Role of Major Drivers in Recent Decline of Monarch Butterfly Population in North America

“…They also found a negative association between glyphosate use and monarch summer population abundance during 1994-2003, however there was no significant relationship between these two variables for the second decade (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) (Saunders et al, 2017), suggesting a need of similar studies in other regions.…”

“…Quantifying the effects of climate and land use changes on monarch population dynamics between 1994 and 2013, Saunders et al (2017) determined that spring seasons with higher precipitation and cooler temperatures in Texas were associated with higher summer population abundances in Illinois. They also found a negative association between glyphosate use and monarch summer population abundance during 1994-2003, however there was no significant relationship between these two variables for the second decade (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013) (Saunders et al, 2017), suggesting a need of similar studies in other regions.…”

“…This suggests that monarchs are only affected by climatic conditions in certain points along their migratory path and only at specific times of the year. Similarly, Saunders et al (2017) (Taylor, 2009). Meanwhile, high temperatures in the spring of 2009 affected monarchs migrating north from overwintering (Brower et al, 2012).…”