Spiders newly observed in Czechia in recent years – overlooked or invasive species?

To learn whether the recent increase in the number of Central European spider species reflects a still-incomplete state of faunistic research or real temporal changes in the Central European fauna, we evaluated the records of 47 new species observed in 2008– 2020 in Czechia, one of the faunistically best researched regions in Europe. Because of the intensified transportation of materials, enabling the introduction of alien species, and perhaps also because of climatic changes that allow thermophilic species to expand northward, the spider fauna of this region is dynamic. Our analysis showed that only 15 spider species newly recorded in Czechia likely belong to the indigenous fauna. The remaining two-thirds likely appeared in this region recently. Half of these species are likely thermophilic species that expanded their distribution to the north, possibly due to global warming, and the second half are subtropical or tropical species introduced to heated buildings. Only three species were introduced to natural habitats, and only two of them, Mermessus trilobatus and Erigone autumnalis (Linyphiidae), can be considered true invasive species.


Introduction
The European fauna is subjected to accelerating changes due to global warming, changes in habitat structure, pollution and the spread of invasive species. Spiders are among the best-known invertebrate groups in Central Europe (Nentwig et al. 2021). Despite good knowledge, new species are found in this region every year. To document the dynamics and trends of the spider fauna in this region, we evaluated the species that have been found in Czechia over the last thirteen years. We aimed to distinguish the overlooked species from actual new colonizers. Among the new colonizers, we attempted to distinguish those that expanded their distribution due to global changes from those that were introduced by humans.
Czechia is one of the faunistically best researched regions in Europe. As of 2007, 852 species had been identified in this country (Buchar and Růžička 2002;Růžička and Buchar 2008). The Czech spider fauna lacks endemic species due to the glacial periods that have occurred. The oldest components of Czech spider fauna are glacial relics that survived in alpine and peat bog habitats and boreal relics that survived in steppe habitats. The Czech spider fauna is thus mainly composed of species with a Palaearctic distribution. Only a very small proportion of the species are endemic to Central Europe; in particular, few Carpathian endemics occur in the mountains in eastern Moravia, and few Pannonian endemics occur in the lowlands in southern Moravia. Many species of natural habitats are disappearing, and 58% of the Czech spider fauna are red listed (Řezáč et al. 2015).

Materials and methods
We collected faunistic records for the spider species that were newly found in Czechia over the last thirteen years (see Supplementary material Appendix 1), since Růžička and Buchar (2008) last evaluated the Czech spider fauna. Then, we evaluated the history, distribution and habitat of these species according to records from both Czechia and the literature. We classified the newly recorded species into eight categories representing different reasons for their new entry into the record in the region.
Species identity was based on Nentwig et al. (2021). The nomenclature follows the World Spider Catalog (2021).

Results
Since 2008, 47 new spider species have been observed in Czechia (Table 1). We classified these 47 species into eight categories:

Discussion
The evaluation of spider species newly observed in Czechia showed that only one-third of newly recorded species likely belong to the indigenous fauna (Table 2). Thus, the current knowledge of the indigenous Central European spider fauna is close to complete. The remaining two-thirds likely came to this region recently. Half of those species are likely thermophilic species that expanded their distribution to the north, possibly due to global warming, and the second half are subtropical or tropical species introduced to heated buildings. Only three species were introduced into natural habitats, and only two of them can be considered true invasive species.

Native species
The newly recorded native species have not been recorded before because of their rarity or because they were not distinguished from similar taxa. The rare species are usually restricted to small regions of Czechia. Examples include Palliduphantes ericaeus, Piniphantes pinicola and Tapinocyba pallens in westernmost Bohemia, Pardosa tenuipes in northern Moravia, and Zelotes mundus in southern Moravia. They may live in rare habitats, such as Caviphantes saxetorum on gravel banks, Erigone cristatopalpus in glacial corries, or Yllenus arenarius on sandy dunes. They may live in inaccessible habitats, such as Enoplognatha bryjai in the flooded littoral zone or Oreonetides quadridentatus, Iberina microphthalma and Centromerus piccolo in shallow subterranean habitats. Iberina candida was found on dry rocky steppe on limestone. It is possible that this species could be hidden in limestone crevices. Recent progress in research on shallow subterranean habitats was enabled by the application of pipe traps . Two species, Micaria micans and Porrhoclubiona leucaspis, were recently distinguished from M. pulicaria and P. genevensis, respectively, because of taxonomic progress (a similar case is Pholcus alticeps, which was recently distinguished from P. phalangioides, but both of these Pholcus species were introduced; they are not native to Czechia).

Species expanding their distribution
Several species are known to occur in regions south of Czechia and recently appeared in Czechia, mostly in south Moravia, the warmest region of Czechia. Their expansion to the north was possibly enabled by recent warming of the climate. We reject the possibility that these species are native to Czechia because they were found in some araneologically very well researched areas/sites, and the probability that they had previously been overlooked there is very low. Several of these species were found only (Emblyna brevidens, Theridion cinereum, Theridion hannoniae) or predominantly (Pardosa nebulosa, Gnaphosa modestior) in xerothermic habitats that developed in abandoned mining areas, such as quarries and sand and clay pits. Not only is the macroclimate of these habitats suitable for such thermophilic species, but these habitats are also colonized de novo; thus, the expanding species do not compete with established populations of native species. Philodromus buxi was found on trees in parks within large cities. Two thermophilic araneid spiders, Larinia elegans and Neoscona adianta, have long been expected to appear in south Moravia-the warmest area of Czechia-as they are known to occur close to the border. Sintula spiniger was found on the edges of fields and forests in southern Moravia. Collinsia inerrans was observed in crop fields; it was the only species from this category that also colonized the western part of Czechia, Central Bohemia in particular.

Species introduced by humans to natural habitats
Only three spider species have been recently introduced to natural habitats in Czechia. Zodarion ohridense was introduced from the Balkan Peninsula to one abandoned quarry in Central Bohemia (Krejčí et al. 2017), but it does not tend to expand in Czechia. Species of the genus Zodarion are well preadapted for passive human transport due to their tolerance of dry environments and especially their ability to attach silken shelters masked by soil particles to solid objects on the ground. Spiders are thus transported inside these objects, and if there are suitable conditions at their final destination, the spiders can establish new populations far from their continuous distributional area (Pekár 2002). However, because these spiders are not able to balloon (passive transport by means of wind and silk fibres), they do not tend to spontaneously expand further.
In contrast, Erigone autumnalis and Mermessus trilobatus, both native to North America, are good ballooners. In North America, they inhabit both natural and human-modified habitats (Millidge 1987). The ability to tolerate a wide range of environmental conditions, together with a strategy of aerial dispersal, allows the species to spread quickly and easily on the new continent (Hirna 2017). Mermessus trilobatus was able to colonize a wide range of habitats, mainly open habitats, across the whole Czech territory. It is currently the most invasive spider in Europe (Narimanov et al. 2020). It was first detected in 1981 near Karlsruhe in southwest Germany (Dumpert and Platen 1985). The concentric expansion through Europe, mainly eastwards (the prevailing wind direction), is briefly described by Hirna (2017).
Mermessus trilobatus is able to integrate into even the most relict natural communities: peat bogs, heathlands, relic steppe grasslands and alpine grasslands. Thus, it presumably causes changes to the structure of such communities of conservation interest and outcompetes endangered species. For example, we studied the alpine spider community near Vyšné Wahllenbergovo pleso lake (altitude 2157 m) in the High Tatra Mountains in Slovakia one year after the first record of Mermessus trilobatus in this mountain range (July 2014-September 2015, collected by K. Tajovský and P. Čuchta, identified by V. Růžička, unpublished In open habitats within agricultural landscapes in Germany, M. trilobatus can be among the most abundant spider species (Schmidt et al. 2008). According to our experience from Czechia, it does not tend to be dominant in the community (as well as another European invasive linyphiid spider, Ostearius melanopygius). Quick invasion is obviously facilitated by a remarkable ability and tendency to disperse by ballooning. Ballooning might also explain why we captured this otherwise ground-dwelling species high above the ground in oak canopies.
The speed of Mermessus trilobatus invasion eastward through Europe can be calculated from the first record near Karlsruhe, Germany, in 1981 and the easternmost records in the High Tatra Mountains in Poland in 2013 (Rozwałka et al. 2016) (840 km/32 years = 26 km/year) or the Carpathians in Ukraine in 2016 (Hirna 2017) (1170 km/35 years = 33 km/year). Interestingly, a very similar speed (30 km/year) was recorded during the invasion of another linyphiid spider, Ostearius melanopygius (O. Pickard-Cambridge, 1879), eastward through Europe (Růžička 1995). Together with Ostearius melanopygius, Mermessus trilobatus is the best example of an invasive spider species in Central Europe.

Species introduced by humans to heated buildings
Several subtropical and tropical species have been recently recorded in heated buildings in Czechia. Most of them were found only at a few unique sites, such as in greenhouses in zoos and botanical gardens (Modisimus culicinus, Nesticella mogera, Coleosoma floridanum) or insect breeding colonies in laboratories (Thanatus vulgaris, Physocyclus globosus). These species do not tend to expand to more common types of heated buildings (Hänggi and Straub 2016). The only exceptions are Pholcus alticeps, Psilochorus simoni, Holocnemus pluchei, Steatoda nobilis, Steatoda paykulliana and Paratrachelas maculatus, which were able to colonize private houses. Pholcus alticeps was possibly misidentified as Pholcus phalangioides and thus overlooked for some time.
Tegenaria parietina and Loxosceles rufescens were found only once; thus, they were likely once introduced to the heated pavilions in zoos and did not establish populations.
Cortestina thaleri is a special case because it was obviously introduced to Europe, but its area of origin is not known ). In Czechia, it was found in synanthropic habitats in cities outside buildings. The ability to survive outside heated buildings might be evidence of a subtropical rather than tropical origin.

Conclusion
In the future, we can expect further changes in the species composition of the European spider fauna. Climatic changes will allow more thermophilic species to expand to the north. Additionally, an increased amount of transported material will increase the cases of further introduction of alien species (Nentwig 2015;Kobelt and Nentwig 2008). Because of the limited number of niches, such enrichment will inevitably cause the suppression of some native species.