Taxonomic scale dependency of Bergmann’s patterns: a cross-scale comparison of hawkmoths and birds along a tropical elevational gradient

Abstract

Bergmann’s rule predicts a larger body size for endothermic organisms in colder environments. The contrasting results from previous studies may be due to the differences in taxonomic (intraspecific, interspecific and community) and spatial (latitudinal vs elevational) scales. We compared Bergmann’s patterns for endotherms (Aves) and ectotherms (Lepidoptera: Sphingidae) along the same 2.6 km elevational transect in the eastern Himalayas. Using a large data spanning 3,302 hawkmoths (76 morpho-species) and 15,746 birds (245 species), we compared the patterns at the intraspecific (hawkmoths only), interspecific and community scales. Hawkmoths exhibited a positive Bergmann’s pattern at the intraspecific and abundance-weighted community scale. Contrary to this, birds exhibited a strong converse Bergmann’s pattern at interspecific and community scales, both with and without abundance. Overall, our results indicate that incorporation of information on intraspecific variation and/or species relative abundances influences the results to a large extent. The multiplicity of patterns at a single location provides the opportunity to disentangle the relative contribution of individual- and species-level processes by integrating data across multiple nested taxonomic scales for the same taxa. We suggest that future studies of Bergmann’s patterns should explicitly address taxonomic and spatial scale dependency, with species relative abundance and intraspecific trait variation as essential ingredients especially at short elevational scales.

Larger insects in a colder environment? Elevational and seasonal intraspecific differences in tropical moth sizes on Mount Cameroon

“…Our results are consistent with results affirming Bergmann's Clines in moths from other biogeographic areas (Miller 1974(Miller , 1991Sullivan & Miller 2007;Brehm et al 2019;Fiedler & Brehm 2021;Mungee et al 2021), as well as for some other insects (Shelomi 2012). Although some adult moths, such as sphingids, can increase their temperature by vibrating their wing muscles (Heinrich 2013), we do not expect the relationship between body surface area and thermal metabolism as predicted by Bergmann (1847).…”

“…Most of these uncommon, intraspecific datasets mostly showed Bergmann's Clines (Shelomi 2012), including four tortricids in North America (Miller 1974(Miller , 1991, four geometrids and one noctuid in North Carolina (Sullivan & Miller 2007), and one geometrid moth in Costa Rica (Sullivan & Miller 2007). Brehm et al (2019) found intraspecific Bergmann's clines among most of 84 Costa Rican geometrid and arctiine moths they examined, and Mungee et al (2021) found significant but weak Bergmann's clines in 24 sphingid species in the eastern Himalayas.…”

“…However, no significant relationship between elevation and geometrid body size was found in the Ecuadorian Andes (Brehm & Fiedler 2004). Although temperature and other mechanisms are thought to be responsible for these interspecific trends (e.g., Shelomi 2012, Beck et al 2016, the phylogenetic complexity of multispecies communities makes the results difficult to interpret (Mungee et al 2021). Intraspecific changes in insect body size along elevation have rarely been studied (16% of 676 datasets included in Shelomi 2012), but such studies are instrumental for understanding the responsible mechanisms.…”

Tropical montane gradients elucidate the contributions of functional traits to competitive and environmental fitness

“…Birds were recorded visually or aurally along 48 transects of 100 m length, separated from their neighbours by 0.5-2 km of distance and 50 m in elevation, during April-July from 2011 to 2014. A detailed description of the study region and primary data can be obtained from Mungee et al (2021).…”

“…We concluded that a functional trait is: (i) an alpha-trait if the dispersion in the community trait distribution decreases with elevation, and (ii) a beta-trait if its community-mean value exhibits directional variation with elevation Our study transect is characterized by a steep decline in temperature, resource availability and habitat complexity -all of which may impose environmental stress on ecological communities. The transect also hosts an exceptionally high regional and local bird diversity, and high rates of species turnover (Mungee et al 2021), providing an ideal system to detect signatures of competitive overdispersion. Further, our fine-scale primary data (50 m elevational resolution) provides a fair picture of 'local' communities of co-occurring, and likely interacting species.…”