Impacts of a millennium drought on butterfly faunal dynamics

Abstract

Background: Climate change is challenging plants and animals not only with increasing temperatures, but also with shortened intervals between extreme weather events. Relatively little is known about diverse assemblages of organisms responding to extreme weather, and even less is known about landscape and life history properties that might mitigate effects of extreme weather. Our aim was to address this knowledge gap using a multi-decadal dataset of 163 butterfly species that recently experienced a millennium-scale drought. To understand faunal dynamics in the context of the millennium drought, we investigated the behavior of phenology (including date of first flight), species richness and diversity indices through time at 10 study sites spanning an elevational gradient. Linear models were developed to understand the differential sensitivity of butterflies to climate at low and high elevations. Results: Dates of first flight advanced across the elevational gradient during the drought, leading to an overall expansion of the flight window at low elevations and a compression of the flight window in the mountains. The number of species observed per year increased at lower elevations but decreased at higher elevations, apparently as a consequence of extreme sensitivity to hot and dry conditions.

Insects and recent climate change

“…2) have a negative association through time with minimum temperatures, which is consistent with a previous analysis, focused on species richness (27), that hypothesized rising minimum temperatures as a driver of declining montane butterflies. The association with precipitation sensitivity suggests that a successful subset of the montane fauna not only persists with warming nights but is able to take advantage of the highly variable precipitation of the region (27).…”

“…The multifaceted nature of climate change is illustrated by the fact that nighttime temperatures are warming faster than daytime conditions (26), and the consequences for insects are poorly-understood but potentially serious, including reduced time for recovery from daytime heat stress, and indirect effects through plants (26). In the mountains of California, rising average daily minimum temperatures had some of the most dramatic negative effects on insects, especially in combination with drier years (27). Rising minimum temperatures in particular seasons might impact insects through effects on critical overwintering stages.…”

“…While our understanding of biotic response to warming means and limits improves, the greater challenge of changing variance is now upon us: predictions for many parts of the world include an increased frequency of extreme weather, which can appear as more intense precipitation events separated by more prolonged dry periods (30). We have few studies on this topic from which to draw conclusions; only five studies in Table 1 explicitly investigated extreme weather events (27, 31–34). In the few cases where biotic response to extreme events has been examined, the results are as we might expect: extreme events are extreme population stressors.…”

“…Previous modeling work has highlighted the complexity of population response to weather in this diverse fauna (41), and has documented the array of factors impacting populations along the elevational and land use gradient. At lower elevations, the loss of open spaces, warming summers and pesticide application have been associated with widespread declines (42, 44), while the impact of climate change and an extreme drought have been more apparent at higher elevations (27). Here we revisit the question of climate change impacts in this system (with an additional three years of data), with an emphasis on understanding species-specific traits that predict persistence in the mountains.…”

Declines in insect abundance and diversity: We know enough to act now

“…Declines have been severe in areas highly impacted by human activity, such as industrialized agricultural landscapes, but ongoing insect declines are not restricted to farms or the footprints of suburban sprawl. Fewer butterflies have been observed per year at elevations in the Sierra Nevada Mountains of California high enough to be removed from the most direct effects of development (Forister et al, 2018), and repeated surveys in a protected forest in Costa Rica have found declines in entire genera of tropical moths (Salcido, Forister, Lopez, & Dyer, 2019).…”