Environmental Factors Drive Chalcid Body Size Increases with Altitudinal Gradients for Two Hyper-Diverse Taxa

Simple Summary The size of an organism is closely correlated with its physiological and ecological characteristics and strongly influences its fitness. Bergmann's rule, originally widely applied in homeotherms, states that individuals living in a colder environment are larger than those living in a warmer region. Similar geographical patterns are also found in various groups of poikilotherms, but are still controversial, especially in Hymenoptera. In this investigation, we found a significant upward trend in body size with increasing elevation in two tiny groups of Chalcids (Pteromalidae and Eulophidae). The temperature and precipitation play a crucial role in their size variation. This result casts light on the environmental adaptation of parasitoids. Body size is the most essential feature that significantly correlates with insects' longevity, fecundity, metabolic rate, and sex ratio. Numerous biogeographical rules have been proposed to illustrate the correlation between the body sizes of different taxa and corresponding geographical or environmental factors. Whether the minute and multifarious chalcids exhibit a similar geographical pattern is still little known. In this research, we analyzed morphological data from 2953 specimens worldwide, including the two most abundant and diverse taxa (Pteromalidae and Eulophidae), which are both composed of field-collected and BOLD system specimens. We examined forewing length as a surrogate of body size and analyzed the average size separately for males and females using two methods (species and assemblage-based method). To verify Bergmann's rule, we included temperature, precipitation, wind speed and solar radiation as explanatory variables in a generalized linear model to analyze the causes of the size variation. We found that there was an increasing trend in the body size of Pteromalidae and Eulophidae with altitude. The optimal Akaike information criterion (AIC) models showed that larger sizes are significantly negatively correlated with temperature and positively correlated with precipitation, and the possible reasons for this variation are discussed and analyzed.

Automated summary

The size of organisms is closely related to their characteristics and fitness. This study found that two groups of Chalcids showed an increasing trend in body size with increasing elevation, and temperature and precipitation played a crucial role in their size variation. Body size is significantly correlated with insects' longevity, fecundity, metabolic rate, and sex ratio. This research provides insights into the environmental adaptation of parasitoids and confirms the correlation between body size and environmental factors.