The impacts of climate and the extreme drought in 2018 on population growth in Swedish moth species

Insects are pivotal to ecosystem diversity and functionality, yet they face increasing threats from anthropogenic climate change impacts. A growing body of studies reports the effects of changing temperature and precipitation patterns, but relatively few studies focus on the consequences for insect populations of extreme weather events. Here, we examine population growth responses to temperature, precipitation, and the extreme summer drought in Sweden in 2018. For this purpose, we used longitudinal data for 54 moth species collected between 2005 and 2023 using light traps at three sites in southeastern Sweden. We found a positive relationship between temperature and population growth rates across all study sites, while precipitation showed a positive relationship at two sites and no effect at the third. The results indicated a negative time-lag effect on population growth of precipitation the previous year at two of the sites, while there were no significant effects of temperature the previous year. Despite the extreme drought in 2018, moth populations remained resilient, with no dramatic decline in population growth between 2018 and 2019. Our results contrast earlier studies reporting severe declines in population growth in response to extreme drought events. The discrepancy may reflect that selected species in this study predominantly consist of range-expanding and migratory species, better able to withstand adverse conditions due to a higher climatic tolerance and being habitat and food plant generalists. Our findings underscore the importance of an increased knowledge of site-specific responses and effects of extreme weather events such as droughts when outlining conservation efforts.

Methods
Statistical Analyses
Main Analysis:

Linear mixed models (LMM)

Random effects for species and year

Response variable: Population growth rate

Predictors: Climate variables, population size, year

Year Effects Analysis:

Reference years: 2018 (drought) and 2019 (post-drought)

Random effects for site and species

Data Collection Methods

Light trap sampling at three sites

Daily temperature measurements

May-August precipitation totals

Annual species abundance counts