DOI: 10.5061/dryad.c481201
title: Data from: Phenotypic biomarkers of climatic impacts on declining insect populations: a key role for decadal drought, thermal buffering and amplification effects and host plant dynamics

description: 1. Widespread population declines have been reported for diverse
 Mediterranean butterflies over the last three decades, and have been
 significantly associated to increased global change impacts. The specific
 landscape and climatic drivers of these declines remain uncertain for most
 declining species. 2. Here we analyse whether plastic phenotypic traits of
 a model butterfly species (Pieris napi) perform as reliable biomarkers of
 vulnerability to extreme temperature impacts in natural populations,
 showing contrasting trends in thermally exposed and thermally buffered
 populations. 3. We also examine whether improved descriptions of thermal
 exposure of insect populations can be achieved by combining multiple
 information sources (i.e. integrating measurements of habitat thermal
 buffering, habitat thermal amplification, host plant transpiration, and
 experimental assessments of thermal death time (TDT), thermal avoidance
 behaviour (TAB) and thermally induced trait plasticity). These integrative
 analyses are conducted in two demographically declining and two
 non-declining populations of P. napi. 4. The results show that plastic
 phenotypic traits (butterfly body mass and wing size) are reliable
 biomarkers of population vulnerability to extreme thermal conditions.
 Butterfly wing size is strongly reduced only in thermally exposed
 populations during summer drought periods. Lab rearing of these
 populations documented reduced wing size due to significant negative
 effects of increased temperatures affecting larval growth. We conclude
 that these thermal biomarkers are indicative of the population
 vulnerability to increasing global warming impacts, showing contrasting
 trends in thermally exposed and buffered populations. 5. Thermal effects
 in host plant microsites significantly differ between populations, with
 stressful thermal conditions only effectively ameliorated in mid-elevation
 populations. In lowland populations we observe a six-fold reduction in
 vegetation thermal buffering effects, and larval growth occurs in these
 populations at significantly higher temperatures. Lowland populations show
 reduced host plant quality (C/N ratio), reduced leaf transpiration rates
 and complete aboveground plant senescence during the peak of summer
 drought. Amplified host plant temperatures are observed in open
 microsites, reaching thermal thresholds that can affect larval survival.
 6. Overall, our results suggest that butterfly population vulnerability to
 long-term drought periods is associated to multiple co-occurring and
 interrelated ecological factors, including limited vegetation thermal
 buffering effects at lowland sites, significant drought impacts on host
 plant transpiration and amplified leaf surface temperature, as well as
 reduced leaf quality linked to the seasonal advance of plant phenology.
 Our results also identify multi-annual summer droughts affecting larval
 growing periods as a key driver of the recently reported butterfly
 population declines in the Mediterranean biome.
description: carnicer_dryad