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.