Hypothesised life cycle adaptation of stonefly (Plecoptera) in response to increased water temperatures

Water temperature is the most important and critical factor for freshwater invertebrates and it fluctuates with an increase in air temperature. In this study, the effect of water temperature on egg development was clarified in Stavsolus japonicus, and the response to climate change in stoneflies with long egg periods were considered. Water temperatures prior to 43 days before hatching likely do not affect egg development in Stavsolus japonicus. Instead, they use egg diapause as an adaptive strategy to survive hot summer conditions. Increased water temperatures may cause migration to higher elevations for stoneflies that have lower adaptability in their egg development period, with populations eventually becoming stranded where no higher elevation or cooler habitat is available. Species extinction is expected to increase with increasing temperatures, leading to reduced biodiversity in many ecosystems. Indirect effects of water warming on maturation and reproduction may lead to substantial reductions in benthic invertebrate populations.

Positive genetic covariance and limited thermal tolerance constrain tropical insect responses to global warming

Tropical ectotherms are particularly vulnerable to global warming because their physiologies are assumed to be adapted to narrow temperature ranges. This study explores three mechanisms potentially constraining thermal adaptation to global warming in tropical insects: (a) Trade-offs in genotypic performance at different temperatures (the jack-of-all-trades hypothesis), (b) positive genetic covariance in performance, with some genotypes performing better than others at viable temperatures (the 'winner' and 'loser' genotypes hypothesis), or (c) limited genetic variation as the potential result of relaxed selection and the loss of genes associated with responses to extreme temperatures (the gene decay hypothesis). We estimated changes in growth and survival rates at multiple temperatures for three tropical rain forest insect herbivores (Cephaloleia rolled-leaf beetles, Chrysomelidae). We reared 2,746 individuals in a full sibling experimental design, at temperatures known to be experienced by this genus of beetles in nature (i.e. 10-35°C). Significant genetic covariance was positive for 16 traits, supporting the 'winner' and 'loser' genotypes hypothesis. Only two traits displayed negative cross-temperature performance correlations. We detected a substantial contribution of genetic variance in traits associated with size and mass (0%-44%), but low heritability in plastic traits such as development time (0%-6%) or survival (0%-4%). Lowland insect populations will most likely decline if current temperatures increase between 2 and 5°C. It is concerning that local adaption is already lagging behind current temperatures. The consequences of maintaining the current global warming trajectory would be devastating for tropical insects. However, if humans can limit or slow warming, many tropical ectotherms might persist in their current locations and potentially adapt to warmer temperatures.