4.2 Biogeography
The level of taxonomic diversity present in an environment can be quantified by either enumerating numbers of species (e.g. Simpson’s diversity) or estimating evolutionary divergences among species in which genetic divergences have been calculated (Webb, 2000). Moreover, besides the number of individuals sampled, the size of the local species pool, the evenness of species abundances in the community, size and environmental heterogeneity of the area, and the status of taxonomic understanding of the taxa investigated are parameters essential to the accuracy of estimates of taxonomic diversity (Antonelli et al., 2018). Although most measures of alpha and beta diversity rely on species numbers, DNA sequence data may provide an evolutionary framework to diversity estimates (Hebert et al., 2016). In this sense, genetic measures may also be used to evaluate species boundaries when compared with species richness in the same communities. Additionally, DNA barcodes can be used for species delimitation, assisting in documenting new species, and identifying targeted habitats for conservation (Faith, 1992; 2008). In geographic regions especially known for their unique lineages of organisms, biological diversity determined with DNA barcode sequence data can be essential for comparing diversity and establishing protected areas across the landscape (Shapcott et al., 2015, Hobner, 2021).
Numerical species delimitation methods require species to be sufficiently sampled (Dopheide et al., 2019) across geographical ranges to improve their ability to correctly delimit species (Parslow et al., 2021). In practice, this is a challenging task when it comes toPolypedilum due to its known worldwide diversity of ca. 440 described species and the expected number of undescribed species. Although recent taxonomic studies of regional fauna have been conducted (Song et al., 2016; 2018), particularly in East Asia, there are several regions that need modern taxonomic treatments, for example, Australia, Africa and South America. Therefore, it is difficult to determine the degree of sampling completeness of Polypedilum caused by the potentially large number of undescribed species. In the current study, many of the biogeographical differences in recorded species numbers can be ascribed to different sampling efforts and methods between regions. Usually, knowledge of species distributions and diversity patterns are strongly concentrated toward areas which are more easily accessible by roads, rivers, and research stations (Antonelli et al., 2018). This fact is evident in our investigation, as though we included all publicly available COI sequences for Polypedilum in BOLD, there was a bias towards Nearctic (33.2%) and Sino-Japanese (23.2%) taxa, with a reduced representation of Afrotropical (1.6%), Australasian (1.6%) and Panamanian (1.3%) species, regions known for receive less investment for research in Chironomidae.
Much of what we need to comprehend about biodiversity can be undertaken as a matrix of the presence or abundance of multiple species across time and space (Hobner, 2021). That said, plotting species accumulation curves permit researchers to measure and compare diversity across populations or to assess the benefits of further sampling (Deng et al., 2015). In our study, the rarefaction curve analysis suggests that even when randomization sampling methods are considered there are regional differences in species richness. Noticeably, the most species-rich regions were the Nearctic and Sino-Japanese regions. This came as a little surprise, since we expected the Palearctic region also to be among the most specious biogeographical areas, due to the high number ofPolypedilum sequences available in BOLD and the numerous studies performed on the family Chironomidae in this area. Although we used species accumulation curves to indicate the pattern of sequence accumulation within the current study, they are not expected to represent the accurate diversity of each region, as they are not based on actual random sampling (Schwarzfeld & Sperling 2015).
The Palearctic fauna overlaps partially with that of the Nearctic, especially in the north. This is similar to what is found in other studies (Ekrem et al., 2018; Marusik & Koponen, 2005) where distinct communities in the two regions share several species. This can be the result of numerous faunal interchanges that took place across the Bering land bridge (135 000 – 70 000 YBP). However, these migrations were mostly limited to large, cold-tolerant species (Rodríguez et al., 2006), and it is mainly these species which are found throughout the Holarctic realm today. Chironomids have also been observed as aerial plankton (Hardy & Milne, 1938; Gressitt et al., 1960; Cotoras & Zumbad, 2020) and one cannot rule out long distance dispersal as an explanation for trans-Atlantic distribution patterns in Polypedilum (Ekrem et al., 2018). Species overlap was also recorded between Palearctic and Oriental fauna, despite the Himalayas forming an altitudinal barrier between these realms, a pattern also previously recorded for butterflies (Larsen, 1984). Inasmuch as the majority of species (54.9%) were only recorded at a single location and only 3.8% of species were recorded at five or more locations, it is no surprise that a small number of wider distributed species are driving the regional and larger scale biogeographical patterns. The high number of species recorded only once is a typical result for understudied taxa (Velasco-Castrillón et al., 2014; Zhang et al., 2018).
The Neotropical region as one of the lesser studied regions with 71 species recorded from 37 localities, exhibited a higher species richness than that of the Palearctic and Oriental realms. Moreover, despite the Neotropical fauna being closely linked with that of the adjacent Nearctic fauna, from which it has received some, especial boreal components (e.g. Polypedilum beckae and Paralauterborniella nigrohalteralis , Silva et al., 2015), the results in the current study corroborate our hypothesis that there are significant differences in community structure between the Polypedilum fauna in South America, and the neighboring regions. Only a single unidentified species spanned from the Neotropics to the Nearctic region, recorded in Argentina and Mexico, which confirms our expectations of high levels of endemism and richness of Polypedilum species in the Neotropical region. The outstanding biodiversity there, when compared to other major biotic realms (Lundberg et al., 2000; Antonelli & Sanmartín, 2011) can be attributed to a complex process in which palaeo-geographical and palaeoclimatic forces have been constantly interacting and new species have originated continuously in that area since the late Eocene/early Oligocene (Rull, 2008). As such, the Neotropics is paramount for research on the origin of biological diversity. Finally, some neotropical areas are under manifest danger of biodiversity loss (Antonelli, 2021). Our study shows that DNA-based species delimitation approaches can be used in rapid biodiversity estimates of poorly known taxonomic groups so these can be utilized as basis for biodiversity conservation strategies, and to unravel biogeographical patterns at both local and global scales.