To establish the biogeographic affinities of the caddisfly fauna of Mongolia, published records and results of our faunistic studies were analyzed. This study captured more than 47,000 adults collected from 386 locations beside lakes, ponds, streams/rivers, and springs in ten sub-basins of Mongolia using Malaise traps, aerial sweeping, and ultraviolet lights. In total, 201 species have been recorded, and approximately 269 species may occur in Mongolia according to our estimation. In a comparison of species richness for the family level, the Limnephilidae and Leptoceridae were the richest in species. The families Brachycentridae, Glossosomatidae, and Psychomyiidae had low species richness, but they included the most dominant species in terms of abundance and/or the percentage of occurrence in the samples from multiple sub-basins. Comparing the sub-basins, the Selenge had the highest Shannon diversity (H’ = 3.3) and the Gobi sub-basin had the lowest (H’ = 1.5). According to the Jaccard index of similarity, caddisfly species assemblages of Mongolia’s ten sub-basins were divided into two main groups: One group includes the Selenge, Shishkhed, Bulgan, Tes, and Depression of Great Lakes sub-basins; the other group includes the Kherlen, Onon, Khalkh Gol, Valley of Lakes, and Gobi sub-basins. The majority of Mongolian species were composed of East Palearctic taxa, with a small percentage of West Palearctic and Nearctic representatives and an even smaller percentage from the Oriental region, suggesting that the Mongolian Gobi Desert is, and has been, a significant barrier to the distribution of caddisfly species between China and Mongolia.

East Palearctic, habitats, regional affinities, river sub-basin, species abundance, species diversity, species richness

Mongolia is a large, land-locked country located in the southeastern East Palearctic Region (Morse 2021) for which knowledge of the freshwater fauna was poorly known. An understanding of a regional and local fauna is important for assessing ecosystem services and informing conservation management, especially for large areas with little faunistic research such as Mongolia. Survey efforts provide basic knowledge of faunal diversity within regional or local scales (Gelhaus et al. 2008; Heino 2009; Morse 2016) with cumulative diversity increasing as spatial and temporal scales of studies increase (Dodds 2002). Our long-term series of surveys for aquatic invertebrate diversity in Mongolia confirms these observations and expands the faunal and biogeographical knowledge of the country. Our four surveys occurred during 2002–2005 as the Hovsgol_GEF (Dynamics of biodiversity loss and permafrost melt in Lake Hovsgol, National Park, Mongolia), during 2003–2006 as the SRP (Selenge River Basin Project), during 2008–2011 as the MAIS (Mongolian Aquatic Insect Survey funded by US-NSF), and during 2016–2019 as the MACRO (Macroecological Riverine Synthesis funded by US-NSF) projects conducted as expeditions to study the aquatic insects in Mongolia.

Caddisflies (Trichoptera) constitute one of the major aquatic insect groups (Morse et al. 2019a). They are found in both lotic (streams and springs) and lentic (lakes, ponds, pools, and marshes) habitats (Wiggins 1996) and are great contributors to ecosystem functioning as shredding consumers of leaf litter (e.g., Limnephilidae), scrapers of periphyton (e.g., Apataniidae, Glossosomatidae, Psychomyiidae), filterers of suspended organic particles and tiny prey (e.g., Brachycentridae), and predators (e.g., Leptoceridae, Rhyacophilidae). In turn, they are an important component of the diet for fish and other invertebrates (Dodds 2002; Morse et al. 2019a, 2019b). Immature stages of caddisflies are well-studied and generally intolerant of environmental pollution, and thus, they are used as bioindicators in freshwater biomonitoring (Barbour et al. 1999; Dodds 2002).

The order Trichoptera includes more than 16,775 species belonging to 52 families in two monophyletic suborders, Integripalpia and Annulipalpia (Thomas et al. 2020; unpublished data). Trichoptera constitute the seventh most species-rich order of insects (Thomas et al. 2020). The fauna of the East Palearctic Biogeographic Region includes at least 1,244 species of caddisflies (Morse 2016; unpublished data).

The Trichoptera of Mongolia have been studied from the early 19^th century and were extensively investigated by foreign and Mongolian researchers through many expedition surveys, especially in the past 20 years (Chuluunbat et al. 2016). According to their investigations, 198 species have been recorded. For our checklist, we have reviewed 64 taxonomic publications which reported Mongolian caddisfly species and their distribution. We also include specimens collected and identified from our expeditions throughout the northern and western parts of the country from 2003 through 2011. The spatial distribution of species is reported and compared by provinces (or “aimags” in Mongolian), which is an administrative subdivision for the country and commonly reported and interpreted in previous publications (e.g., Chuluunbat et al. 2016).

In this study, we characterize caddisfly biogeographical distribution in ten major river basins and provide a revised and annotated checklist for the Trichoptera fauna in Mongolia. We assess the species richness and diversity of caddisflies in ten sub-basins (biogeographical regions), hypothesizing that they will be conspicuously different, and compare the similarities of species among the sub-basins and with the adjacent regions of neighboring countries.

Based on the results of our data mining (species data from previously published literature) and our survey investigations, we found 201 caddisfly species representing 72 genera and 16 families in Mongolia (Appendix). Families with the most diverse genera and species were Limnephilidae (23 genera, 62 species) and Leptoceridae (7, 32); families with the least diverse genera and species were Psychomyiidae (1, 3), Goeridae (2, 3), Thremmatidae (1, 1), and Stenopsychidae (1, 1) (Fig. 2). The genera with the highest number of species were Limnephilus (25 species), Ceraclea (11), Rhyacophila (10), Hydropsyche (9), Apatania (8), Agrypnia (7), and Glossosoma (6) (Fig. 2, Appendix). In terms of abundance, families Brachycentridae and Psychomyiidae were most abundant (Table 2).

Figure 2. 

Number of genera and species of caddisfly families in Mongolia.

Species diversity (H’) was highest in the Selenge River sub-basin (3.33) and the Depression of Great Lakes sub-basin (2.57), while evenness (J’) was highest in the Khalkh Gol sub-basin (0.79) and the Gobi sub-basin (0.78) (Table 2).

Species in families Brachycentridae, Glossosomatidae, and Psychomyiidae were most abundant (Table 2, Appendix). Brachycentrus americanus (Banks, 1899) was the most dominant species in the Tes (31%) and Valley of Lakes (48%) sub-basins, with both of these sub-basins belonging to the CAIB. Padunia bikinensis Martynov, 1934, was the dominant species in the Kherlen and Onon sub-basins (38% and 54%, respectively), both belonging to the POB; and Psychomyia flavida Hagen 1861 was the dominant species in the Depression of Great Lakes sub-basin (28%) and all of Mongolia (16%). Other species were dominant in the other five sub-basins (Table 2).

Abundance-based species accumulation analysis estimated that species of caddisflies occurring in Mongolia is 269 (Table 2). Among the 201 currently recorded species, 53 were represented by a single specimen at some sites (S1) and 16 were represented by two specimens at some sites (S2). In our study, 69 species occurred uniquely at a single site in Mongolia (SU) (Table 2).

Caddisfly species richness varied greatly among the four different habitat types of water bodies. From 386 sampling sites, the highest species numbers (178 species) were from the various types of rivers. The next most-diverse habitat was lakes with 106 species. Springs and ponds were inhabited by 47 and 40 species, respectively (Fig. 3).

Figure 3. 

Caddisfly species richness collected from different numbers of sites from four different types of water bodies.

The regional distribution and richness of caddisflies in Mongolia varied in the ten sub-basins, ranging from 7 to 157 species. The highest numbers of species and genera of caddisflies occur in the Selenge River sub-basin (157 species, 54 genera), followed by the Depression of Great Lakes (88 species, 41 genera), the Tes and Shishkhed River sub-basins (50 and 49 species, respectively, in 26 genera), the Bulgan River sub-basin (39 species in 20 genera), Kherlen River sub-basin (38 species in 21 genera), the Onon River sub-basin (34 species in 24 genera), Khalkh Gol (17 species in 13 genera), the Valley of Lakes sub-basin (15 species in 10 genera), and the sub-basin with the lowest species richness was the Gobi sub-basin (7 species in 4 genera) (Table 2, Fig. 4).

Figure 4. 

Richness of genera and species of caddisflies distributed in ten sub-basins of Mongolia. Abbreviations: AOB = Arctic Ocean Basin, CAIB = Central Asian Internal Basin, POB = Pacific Ocean Basin.

Based on the distribution of 201 species of caddisflies in the ten sub-basins of Mongolia, similarities of caddisfly assemblages among sub-basins are shown in Fig. 5. The Onon, Kherlen, and Khalkh Gol sub-basins (POB) were more similar to the CAIB’s Valley of Lakes and Gobi sub-basins as one cluster, whereas the Shishked, Selenge, and Bulgan river sub-basins (AOB) were similar to the CAIB’s Tes and Depression of Great Lakes sub-basins in another group. The interesting results of this clustering showed that the CAIB’s sub-basins were divided into two clusters. The Valley of Lakes and Gobi sub-basins were more similar to those of the POB, the Tes and Depression of Great Lakes sub-basins were more similar to those of the AOB. That is, the caddisfly assemblages in the AOB and the POB were most dissimilar geographically, with the CAIB partially similar to each of them (Fig. 5).

Figure 5. 

Similarities of caddisfly assemblages among ten sub-basins. Abbreviations: AOB = Arctic Ocean Basin, CAIB = Central Asian Internal Basin, POB = Pacific Ocean Basin.

China, Kazakhstan, and Russia are large countries bordering Mongolia on the south, west, and north, respectively. To assess similarities with these surrounding countries, we selected their closest regions. Species assemblages for the neighboring regions were clustered into three groups. The first group was composed of Chinese Gansu and Inner Mongolia. The second group was composed of Russian Chita region and Kazakhstan’s Balkhash-Alakol and Chinese Xinjiang region. Finally, Russian Pribaikalie, Altay, and Sayan Mountains, Kazakhstan’s Irtysh basin, and Mongolia were clustered into one group (Fig. 6). Mongolian caddisfly species were most similar to those of the Russian and Kazakhstan faunas and least similar to the Chinese fauna (Fig. 6).

Figure 6. 

Caddisfly fauna similarities among Mongolia and nine adjacent regions of China (C), Kazakhstan (K), and Russia (R).

Most caddisfly species of Mongolia also inhabit other parts of the East Palearctic Biogeographic Region (98%). Among those, 31% occur also in Europe and northern Africa (WP). Another 20% of the Mongolian species are Holarctic, occurring also in the Nearctic and West Palearctic Regions. Six percent of the Mongolian-East Palearctic species occur also in the Oriental Region, 4% occur also in the Nearctic, and 1% occur in all three of these latter regions (Fig. 7). Caddisfly endemism is very rare for Mongolia (Fig. 7). Endemic species (**) and new country reports (*) are highlighted in the species list (Appendix). The list includes the following five new records for the country:

Figure 7. 

Mongolian caddisfly species composition according to world biogeographic regions (Morse 2021): East Palearctic (EP), Nearctic (NA), Oriental (OL), and West Palearctic (WP).

1. Asynarchus sachalinensis Martynov, 1914: Tov Province, Erdene Soum, Upper Tuul River, 47.94981°N, 107.45511°E, elev. 1593 m, 2004.vi.29. coll. MAIS team, 1 male, det. S. Chuluunbat.
2. Cheumatopsyche infascia Martynov, 1934: Dornod Province, Khalkh Gol, 31 km east of from Khalkh Gol Soum, 47.52556°N, 118.98538°E, elev. 736 m, 2006.vi.22, coll. J. Puntsagdulam, D. Altanchimeg, 2 males, 11 females, det. S. Chuluunbat.
3. Oecetis nigropunctata Ulmer, 1908: Khentii Province, Batnorov Soum, Bayanbulag, 47.91561°N, 111.50480°E, elev. 1328 m, 2020.vi.28, coll. B. Bayartogtokh, light trap, 1 male, 4 females, det. S. Chuluunbat.
4. Drusus sp.: Khovd Province, Duut Soum, Tsagaan Burgas Gol, 47.55936°N, 91.76095°E, elev. 1865 m, 2008.vii.15, coll. MAIS team, 7 males, 2 females, det. S. Chuluunbat.
5. Nyctiophylax sp.: Uvs Province, Zuungobi Soum, Nariin Gol, 50.05245°N, 94.15410°E, elev. 923m, 2009.vii.22, coll. MAIS team, 1 male, det. S. Chuluunbat.

Survey investigations contribute to knowledge of a regional fauna. Up to the late 1900s and early 2000s, Mongolian Trichoptera were investigated mostly by foreign scientists who recorded 129 species (Morse et al. 2006). Starting in early 2000, regional scientific expeditions surveyed various parts of Mongolia intensively and expanded the caddisfly species list up to 198 (Chuluunbat et al. 2016). We are currently adding five new country records to this list. Two subspecies (Limnephilus extricatus sibiricus Mey, 1991, is a subspecies of Limnephilus extricatus McLachlan, 1865; Phryganea grandis rotundata Ulmer, 1905, is a subspecies of Phryganea grandis Linnaeus, 1758) are not shown as distinct taxa in the Mongolian species list (Appendix) and remain uncounted as species by Morse (2021). These findings result in a list of 201 species in Mongolia. This total number of documented species relative to the estimated 269 species indicates that (a) more survey work is needed in Mongolia to fully document the country’s caddisfly fauna and (b) the estimated number of species may have been overestimated due to lack of precise abundance data for some species as reported in previous literature.

A new synonym and changes in two species names are also reflected and updated from the list of Chuluunbat et al. (2016). Stenopsyche marmorata Navás, 1920, is a synonym of Stenopsyche griseipennis McLachlan, 1866, according to Kuranishi & Tanida (2016). Micropterna sequax McLachlan, 1875, was reported as Stenophylax sequax (McLachlan, 1875); Synagapetus inaequispinosus (Schmid, 1970) was reported as Agapetus inaequispinosus (Schmid, 1970) by Chuluunbat et al. (2016).

Different numbers of endemic species from Mongolia have been reported. By 2006, a single endemic species was reported in Lake Hovsgol, Limnephilus hovsgolicus Morse, 1999 (Morse et al. 2006; Chuluunbat et al. 2016). However, Puntsagdulam et al. (2017) reported 7 endemic species of caddisflies for Mongolia: Agapetus inaequispinosus (Schmid, 1970); Neureclipsis mongolica Schmid, 1968; Rhyacophila egijnica Schmid, 1968; Triaenodes kaszabi Schmid, 1968; Hydroptila pectinifera Schmid, 1970; Apataniana impexa Schmid, 1968; with Limnephilus hovsgolicus. However, Agapetus inaequispinosus (Schmid, 1970) is now acknowledged as Synagapetus inaequispinosus (Schmid, 1970) and has been reported from Russia (Ivanov 2011) and Japan (Kuranishi & Tanida 2016). Neureclipsis mongolica Schmid, 1968, is a synonym of Neucentropus mandjuricus (Martynov, 1907) and has been reported also from China (Yang et al. 2016). Rhyacophila egijnica Schmid, 1968, has been reported from Russia (Ivanov 2011). Triaenodes kaszabi Schmid, 1968, is a synonym of Triaenodes jakutanus Martynov, 1910, and reported from Russia (Ivanov 2011) and North America (Manuel 2010). Hydroptila pectinifera Schmid, 1970, has not been reported from any other country yet and the type locality is the Delgermurun River, Burenkhaan Soum (current administrative name, Burentogtokh Soum), Hovsgol Province in Mongolia (Schmid 1970). Apataniana impexa Schmid, 1968, has been reported from Russia (Ivanov 2011) and China (Yang et al. 2016). Limnephilus hovsgolicus Morse, 1999, is endemic to Lake Hovsgol (Morse 2021). Also, Agrypnia hayfordae Morse & Chuluunbat, 2007, has not yet been reported from other countries; it inhabits lakes and the type locality is Nuuriin Khooloi Lake, Thenkher Soum, Arkhangai Province in Mongolia. This species was collected also from Lake Terkhiin Tsagaan, Tariat Soum, Arkhangai Province, by colleagues in 2018. In conclusion, only three of the above species, i.e., Hydroptila pectinifera; Limnephilus hovsgolicus; and Agrypnia hayfordae, are the known caddisfly endemics for Mongolia (Appendix).

Distribution and diversity of Mongolian caddisflies are usually reported in the literature for the three main basins and administrative provinces or rivers rather than the ten sub-basins discussed here. Higher richness of caddisfly species was observed in the Arctic Ocean Basin (AOB) than in the other two basins by Dulmaa and Nansalmaa (1970). Our results reflect the same observation due to the fact that the same literature sources were used and higher sampling efforts (sites) by our own surveys occurred in the AOB than in the CAIB and POB. Strangely, higher species richness was observed in areas with a high density of water networks of rivers and lakes. The AOB and CAIB have a greater density of surface water networks than the POB, and a higher percentage (over 40%) of Mongolia’s geological formations including mountainous areas (Yembuu 2020) and isolated drainages. The AOB has higher stream connectivity than the CAIB, and the connectivity allows it to share similar species in the connected waterways, which tends to make the AOB to have lower species richness than the CAIB (Maasri et al. 2018). Despite this trend, the greater sampling effort of our surveys in the AOB has resulted in a higher species richness in the AOB than in the CAIB.

The highest species number was found in the family Limnephilidae, especially the genus Limnephilus. The genus Limnephilus is one of the largest genera with at least 185 species (unpublished data), inhabiting primarily cold water in northern latitudes and often found at higher altitudes (Ruiter 1995). These case-making caddisflies are known to be highly diverse and occur throughout the Holarctic Region (Morse 2016). Indeed, most of the species observed in Mongolia are case-making caddisflies (Apataniidae, Brachycentridae, Glossosomatidae, Goeridae, Hydroptilidae, Limnephilidae, Lepidostomatidae, Leptoceridae, Molannidae, and Phryganeidae). The elevated landscape of central and western Mongolian is especially suitable habitat for case-making and cold-water-dwelling caddisflies.

Among all types of habitats that were sampled, most of the species were observed in streams/rivers. The immature stages of most caddisflies can inhabit many available substrates in running water and are generally most diverse in streams and rivers (Resh and Rosenberg 1984); however, they are rare in springs (Thorp and Rogers 2011). The number of species occurring in lakes is relatively higher than those in ponds and springs; the 108 species observed from 48 lake sites indicate that lake-inhabiting Trichoptera have been investigated well in Mongolia.

The Mongolian Great Gobi Desert appears to represent an enormous barrier to distribution of caddisflies to and from the south. This pattern suggests a reason for the higher species richness observed in northern sub-basins (Selenge, Shishkhed, Bulgan, Tes, Depression of Great Lakes, Valley of Lakes, Kherlen, Onon and Khalkh Gol) than the Gobi sub-basin. Also, this might be the reason that caddisfly assemblages of Mongolia are more dissimilar to those in Chinese regions than to those in the Russian and Kazakh regions selected for comparison in this study. These results corroborate research indicating that more Mongolian caddisfly species are shared with Russia (Ivanov 2011) than with China (Yang et al. 2016). The composition of Mongolian caddisfly species and the low level of endemism we report here appear to be explained by similar biogeographic and meteorologic conditions in these and neighboring eastern, northern and western regions and the relatively dry, mostly inhospitable Gobi in southern Mongolia and northern China, resulting in a formidable isolation for aquatic insects (and possibly all freshwater biota) from the south for over 70 million years (Dashzeveg et al. 2005).

The similarity in the caddisfly species composition among the three main basins was not as different as we expected. The caddisfly assemblages in sub-basins of AOB and POB were different, but the CAIB was divided into two sub-basins more similar to either AOB or POB. This is probably due to the fact that the CAIB covers a large area from west to east in Mongolia. The faunas of the Depression of Great Lakes and Tes sub-basins of the CAIB in the northwest are more similar to those of the AOB, while the faunas of the Valley of Lakes and Gobi sub-basins of the CAIB in the south are more similar to those of the POB in eastern Mongolia, suggesting that Mongolian caddisfly species might be distributed differently than the faunas that were the basis of the current basin classification. In conclusion, the caddisfly fauna of Mongolia was investigated thoroughly, from the view of the distribution of species in different spatial scales with documented and estimated richness. Most of the species distributed in Mongolia are characteristic of the Palearctic Region. The caddisfly fauna of Mongolia was similar to Russia’s closest bordering regions of Altay and Sayan Mountains, Pribaikalie, and Kazakhstan’s Irtysh Basin, but different from that of China’s bordering regions due to the lack of connections of the surface water network and the presence of the Mongolian Gobi Desert. Sampling effort results in higher richness; thus, further sampling in the sub-basins especially in the Gobi may yield more species. Knowing the species richness in the basins, and sub-basins allow us to manage and protect aquatic systems better and provide necessary knowledge for future freshwater biomonitoring.

We thank all the members of “Hovsgol_GEF” (sponsored by World Bank GEF-MSP, TF028988), “Selenge River Basin” (sponsored by US National Science Foundation DEB-0206674), “Mongolian Aquatic Insect Survey” (sponsored by the US National Science Foundation INT-9630131, INT-9504867, BSI-0743732), and “MACRO” (supported by the US National Science Foundation Macrosystems Biology Grant #1442595) projects for their help in collecting caddisfly specimens. We are grateful also to Prof. B. Bayartogtokh and Dr J. Puntsagdulam for sharing information from their collections. Dr Jon Gelhaus and an anonymous reviewer provided especially useful comments and recommendations that helped us improve the manuscript, although any remaining errors are completely our own.