DOI: 10.25338/b8zh1q
title: Different factors limit early- and late-season windows of opportunity for monarch development

description: Seasonal windows of opportunity are intervals within a year that provide
 improved prospects for growth, survival, or reproduction. However, few
 studies have sufficient temporal resolution to examine how multiple
 factors combine to constrain the seasonal timing and extent of
 developmental opportunities. Here, we document seasonal changes in
 milkweed (Ascelpias fascicularis) – monarch (Danaus plexippus)
 interactions with high-resolution throughout the last three breeding
 seasons prior to a precipitous single-year decline in the western monarch
 population. Our results show early- and late-season windows of opportunity
 for monarch recruitment that were constrained by different combinations of
 factors. Early-season windows of opportunity were characterized by high
 egg densities and low survival on a select subset of host plants,
 consistent with the hypothesis that early-spring migrant female monarchs
 select earlier-emerging plants to balance a seasonal trade-off between
 increasing host plant quantity and decreasing host plant quality.
 Late-season windows of opportunity were coincident with the initiation of
 host plant senescence and caterpillar success was negatively correlated
 with heatwave exposure, consistent with the hypothesis that late-season
 windows were constrained by plant defense traits and thermal stress.
 Throughout this study, climatic and microclimatic variation played a
 foundational role in the timing and success of monarch developmental
 windows by affecting bottom-up, top-down, and abiotic limitations. More
 exposed microclimates were associated with higher developmental success
 during cooler conditions and more shaded microclimates were associated
 with higher developmental success during warmer conditions, suggesting
 that habitat heterogeneity could buffer the effects of climatic variation.
 Together, these findings show an important dimension of seasonal change in
 milkweed-monarch interactions and illustrate how different biotic and
 abiotic factors can limit the developmental success of monarchs across the
 breeding season. These results also suggest the potential for seasonal
 sequences of favorable or unfavorable conditions across the breeding range
 to strongly affect monarch population dynamics.
description: We collected data at approximately weekly intervals throughout
 each growing season in 2015, 2016, and 2017. For each observation on each
 milkweed, we used a standardized protocol that included assessments of
 plant status (presence of emergent stems, percentage of non-senescent
 tissue, percentage of leaf area removed by herbivores), measurements of
 plant size (number of stems &gt;5 cm, mean stem length, mean stem
 diameter), counts of milkweed reproduction (number of open floral umbels,
 number of non-senescent seed pods longer than 1 cm), and measurements of
 any monarch eggs or caterpillars present (number of monarchs eggs, number
 of monarchs of each larval instar, larval length). Percentages of
 non-senescent tissue and leaf area removed were estimated visually,
 measurements of stem length were taken with meter sticks to the nearest
 cm, and measurements of stem diameter and larval length were measured
 using dial calipers to the nearest 0.1 mm. Finally, participants collected
 additional notes, including observations of the surrounding predator and
 herbivore community. 
description: Please see the associated publication for method details. Please
 see the included Yang_et_al_MMMILC_README.txt for meta-data.