Calcareous fens are one of the most species-rich habitats of the temperate zone of the Northern Hemisphere. In spite of this species richness, however, calcareous fens are still rather poorly investigated. Consequently, the data of the fen-associated spider fauna are also largely lacking. The aim of the research was to study the spider fauna of the calcareous fens of Latvia and to draw conclusions about what kind of spider species and ecological groups typically inhabit calcareous fen habitats. Spiders were sampled in the summer months of 2010, 2011, and 2012 at nine different calcareous fens of the coastal lowland of Latvia. The spider collection was performed by pitfall traps and a sweep net. The examined material comprised 6631 adult spider individuals representing 21 families and 149 species. The main spider ecological groups that dominated in the studied calcareous fens were hygrophilous and photophilous species which largely reflect the main properties of our studied habitats, all of which were wet, open mire habitats. Nevertheless, the fen arachnofauna consisted also of spider groups which are less typical for moist, sun-exposed, and alkaline environments, like xerophilous, sciophilous, and sphagnophilous species, respectively. Finally, several spider species collected in this study have not been previously reported for the spider fauna of Latvia, and many more might still be undiscovered in these unique and poorly investigated habitats. Therefore, it is suggested that calcareous fens deserve special attention and they should definitely be investigated further.

Araneae , ecological groups, Latvia, mire habitats, spider fauna

Mire habitats (fens and bogs) are among the most important wetland ecosystems of Europe. They are characterised by specialized flora and fauna and the presence of specially protected species (Bambe et al. 2008; Auniņš et al. 2013). In contrast to bogs, fen habitats are rather poorly investigated, the same being applied to their arachnofauna. There are only very few studies in Europe regarding the spider fauna of fens – we could find only a single study from Latvia (Cera et al. 2010), as well as one study from Estonia (Vilbaste 1980) and one from Poland (Kajak et al. 2000). Bogs are much more popular habitats for arachnological studies – there are several studies from Latvia (Šternbergs 1991; Spuņģis 2008), as well as from Estonia (Vilbaste 1980), Lithuania (Rėlys and Dapkus 2002b; Rėlys et al. 2002; Biteniekyté and Rélys 2006, 2008), Poland (Kupryjanowicz et al. 1998), Finland (Koponen 2002a,b, 2003, 2004), Norway (Pommeresche 2002), Denmark (Bruun and Toft 2004), Germany (Buchholz 2016), Russia (Oliger 2004), Romania (Urák and Samu 2008; Samu and Urák 2014) and other countries (Štambuk and Erben 2002; Scott et al. 2006).

Although both fens and bogs are mire habitats, there are several fundamental differences between them: (1) fens are mires that receive water and nutrients from groundwater and/or surface water, as well as from rainfall, while bogs depend solely on precipitation (McBride et al. 2011); (2) fens are mineral-rich type of mires which are usually characterized by basic or circumneutral conditions, while bogs are nutrient poor mires which have strongly acidic (pH < 5.0) soil conditions (Kellner 2003; Spitzer and Danks 2006; Horsák et al. 2011); (3) fens are dominated by brown mosses and sedges (e.g., Carex, Cladium, Schoenus), while bogs – by peat mosses (Sphagnum spp.) (Rydin and Jeglum 2006; Gałka et al. 2016); and (4) fens are rich in a floristic sense, while bogs have a low species diversity (Kellner 2003; McBride et al. 2011). Because of these differences between fens and bogs, and because of the fact that fens are much less studied than bogs, it is clear that more studies are needed in fen habitats.

Calcareous fens are one of those fen types which are especially worth studying because they belong to the most species-rich ecosystems of the temperate zone of the Northern Hemisphere (Joosten and Clarke 2002). In addition, there are some plant and animal species that occur almost exclusively in this habitat type. For example, Scorpidium cossonii, Schoenus ferrugineus, Carex davalliana, Ophrys insectifera, Saussurea esthonica and Juncus subnodulosus are plant species that can be found only within calcareous fens (Auniņš et al. 2013). Also, calcareous fens is a very important habitat for specially protected snail species, such as Vertigo genesii and V. geyeri (Cameron et al. 2003; Auniņš et al. 2013). Overall, calcareous fens are very rare in most of the countries in the European Union (Stanová et al. 2008), and they are considered priority habitats in Annex I of the EU Habitat Directive (EC 1992). Thus, because of the rarity of the calcareous fens and because of the presence of unique species within these habitats, it would be important and worthwhile to assess the quality of the calcareous fens, as well as to investigate their flora and fauna.

Spiders have been shown to be very good bioindicators (e.g., Marc et al. 1999; Pearce and Venier 2006), and thereby they are proposed as a group of organisms that are potentially useful tools for assessing the conservation value of rare and threatened habitats. Unfortunately, arachnids from calcareous fens are very poorly studied. In Latvia, spiders within calcareous fens have been investigated in some of our previous studies (Štokmane et al. 2013; Štokmane and Spuņģis 2014, 2016). These previous investigations had examined the influence of vegetation structure on spider diversity, while little attention was paid on the faunistic aspects of the fen spider communities. Thereby, the main purpose of the present study was to investigate the spider fauna of calcareous fens in greater detail and to analyse what kind of spider species and ecological groups are more typical for this habitat type.

The present study is a compilation and an overview of our three previous studies made in the summers of the following years: 2010, 2011, and 2012. These studies were carried out in nine different calcareous fens of the coastal lowland of Latvia: (1) Kaņieris; (2) Apšuciems; (3) Engure-1; (4) Engure-2; (5) Slītere; (6) Platene; (7) Vītiņi; (8) Ječi; and (9) Ķirba (Figure 1). All the studied fens belong to the EU Natura 2000 network.

Figure 1. 

Map showing the calcareous fens studied (marked with circles). All fens are located in the coastal lowland of Latvia.

A short overview of the used sampling methods is given in Table 1. As it can be seen, the spider capture methods as well as the time of sampling differed in each of the three study years. This was done in order to access more spider species and enlarge the species list, since it is well known that, firstly, each collecting method targets different spider species (e.g., Churchill and Arthur 1999) and, secondly, spiders exhibit seasonal variation in their occurrence (e.g., Marc et al. 1999). The sampling period of the present study, however, was limited to the summer months only (June/July/August). We decided that summer will be the optimal time for collecting spiders since it is the warmest season in Latvia (LEGMC 2014) when vegetation biomass is at its prime and food resources are plentiful, and thus we assumed that spider diversity as well as the number of species and individuals will be much greater at this period of time. Higher abundance and diversity of spiders during summer is also consistent with the findings of other researchers (e.g., Hatley and Macmahon 1980; Reddy and Venkataiah 1986).

A binocular microscope at 45× magnification was used to identify the spiders to species level. The unidentified adult species were recorded as morphospecies. Since juveniles could be identified only to family level, they were excluded from the study. The full species list of calcareous fen spiders as well as the number of collected individuals in each fen can be found in Appendix 1. The nomenclature of spider species followed the World Spider Catalog (2018).

In order to understand what kind of spider species inhabit calcareous fens, we carried out a literature survey and prepared a short description on the habitat preferences for each of our collected spider species. The habitat affinities of the spider species were derived from many different literature sources, but mainly from Locket and Millidge (1951, 1953), Roberts (1996), Harvey et al. (2002a,b), Almquist (2005, 2006), Matveinen-Huju et al. (2006), Oxbrough et al. (2006), Nentwig et al. (2012)and Arachnologische Gesellschaft (2018). Based on the literature analysis, all the collected spider species were sorted into a number of ecological groups. These groups were distinguished mainly by taking into account the spider requirements for moisture and light, since these two abiotic factors are among the most important determinants characterizing the habitats of spiders (Entling et al. 2007). When taking into account the moisture preferences, the species were classified as either being hygrophilous (water-loving) or xerophilous (drought-loving), but when taking into account the light preferences, the species were classified as either being photophilous (sun-loving) or sciophilous (shade-loving). Species with a wide ecological amplitude (= found in many different habitat types) were classified as being habitat generalists.

In addition, we have summarized the information whether our detected spider species have been found within other European mires. We have chosen to include in our summary those mire studies in which the full spider species list has been published. Thus, we included the following studies: Cera et al. (2010) (calcareous fens of Latvia), Šternbergs (1991) (Baži bog of Latvia), Kajak et al. (2000) (fens of Poland), Kupryjanowicz et al. (1998) (bogs of Poland), Vilbaste (1980) (fens and bogs of Estonia), Koponen (2002a,b) (bogs of Northern Europe, including Sweden, Finland and northern Norway), Rėlys and Dapkus (2002b) (bogs of Lithuania), and Rėlys et al. (2002) (bogs of Lithuania and Finland). This information is presented as presence data in Appendix 2.

All our collected spider specimens are stored in 70% ethanol, labelled, and deposited in the Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga.

Overall, in the three study years a total number of 8,967 spider individuals (6631 adults and 2336 juveniles) were collected, representing 134 species and 15 morphospecies in 21 families. Most of the species (87 spp.) were collected only in a single year, while only five species were detected in all three study years (Dolomedes fimbriatus, Evarcha arcuata, Tibellus maritimus, Xysticus ulmi and Kaestneria pullata). Altogether eight spider species found during this investigation were registered as new species for the araneofauna of Latvia – Cheiracanthium punctorium (Eutichuridae), Gnaphosa lapponum (Gnaphosidae), G. nigerrima (Gnaphosidae), Bathyphantes parvulus (Linyphiidae), Centromerus semiater (Linyphiidae), Microlinyphia impigra (Linyphiidae), Pirata tenuitarsis (Lycosidae), and Leptorchestes berolinensis (Salticidae). The dominant spider species in each year and in each fen are given in Table 2. One of the most abundant and most frequently recorded species was Dolomedes fimbriatus, which occurred in the vast majority of the studied calcareous fens. Overall, however, there were rather large differences in spider species composition between fens, between study years, as well as between pitfall samples and the sweep-net samples.

By using relevant information from the literature (see the method section), we have prepared a short description of each of the collected spider species (Table 3). Also, all the collected spider species were sorted into ecological groups according to their habitat requirements. This classification was based mainly on spider requirements for moisture (hygrophilous/xerophilous species) and light (photophilous/sciophilous species). For some of the spider species we distinguished also sub-groups. In some cases, however, it was difficult to classify a spider species into a particular ecological group(-s), because the habitat preferences of some spiders are rather poorly defined (Eyre and Woodward 1996), and the information in the literature is sometimes contradictory (personal observation).

The spider ecological group composition in the studied calcareous fens and the number of spider species and individuals within each group is given in Figure 2. The most species-rich and the most abundant ecological group was hygrophilous species – more than a half of all spider species and individuals collected in the present study could be classified as hygrophilous (if including also hygrophilous-photophilous and hygrophilous-sciophilouspecies). Photophilous species (including photophilous-hygrophilous and photophilous-xerophilous) was another large group in the studied fens – overall, 46 of our collected spider species (34% of all spiders) and 3088 individuals (48%) could be classified as photophilous species. The rest of the ecological groups, xerophilous, sciophilous, and habitat generalists, were represented by a rather low number of species and individuals.

Figure 2. 

The proportional spider ecological group composition in the calcareous fens of Latvia by the number of species (dark grey columns) and by the number of individuals (light grey columns).

In the present study we investigated the spider fauna of the calcareous fens of the coastal lowland of Latvia. The full species list of the calcareous fen spiders is given in Appendix 1. The main purpose of the present study was to clarify the habitat preferences of the spider species collected during our investigations in the calcareous fens. The habitat preferences of each spider species are described in Table 3.

The arachnofauna of the studied calcareous fens consisted of a wide spectrum of different spider ecological groups. The vast majority of the spider species and individuals found in the fens were hygrophilous or photophilous or hygrophilous-photophilous. The dominance of these groups in the studied habitats is quite logical since all our studied fens were moist, sun-exposed habitats. Nevertheless, the fen arachnofauna consisted also of different other spider ecological groups, including even some groups which do not normally inhabit wet and alkaline environments, i.e., xerophilous and sphagnophilous species. The reason of the presence of such species within calcareous fens might be related with the fact that fens encompass a much broader range of microhabitat variation than other mire types. Fen surface often has a variable microrelief that consists of hummocks, hollows and pools, and since the tops of the hummocks are much drier than their lower part, they may serve as suitable habitat patches for the xerophilous species. Other researchers have also observed that drought-loving spider species can occasionally be found on raised, dry patches of vegetation within otherwise wet and marshy sites (Roberts 1996; Cattin et al. 2003). Similarly, the presence of sphagnophilous spider species within the studied mires might also be explained by the availability of hummocks. Usually these fen hummocks are dominated by acid-loving plant species (especially Sphagnum mosses) which are raised above the water level and thus protected from the influence of the alkaline groundwater (Rydin and Jeglum 2006). Consequently, the sphagnophilous spider species, which normally live in acid environments, especially bogs, and are related with Sphagnum mosses (e.g., Gnaphosa nigerrima, Pardosa sphagnicola, Pirata piscatorius), might also be supported in calcareous fens, since the Sphagnum-dominated hummocks may serve as discrete habitat patches for them. These findings are supported by several other researchers who have also discovered that spiders can persist in very small microhabitats (Wing 1984; Foelix 2011; Cobbold and MacMahon 2012).

Our study also showed that not only the within-habitat diversity but also the features of the landscape (i.e., the composition of the surrounding habitats) might be very important determinants of the spider species composition of the focal habitat. For example, in the studied fens we have collected several forest species, some of which were detected in fens in a rather great abundance. The occurrence of these forest-related species in our fen habitats could be associated with the fact that all our studied fens were surrounded by forested habitats. Similarly, the presence of coastal and halophilous species within our studied fens might also be largely explained by the proximity of appropriate habitats, since all the studied fens lie in the coastal lowland. Other researchers have also suggested that spider composition of a particular habitat is influenced by the quality of adjacent habitats (e.g., Uetz et al. 1999; Cobbold and MacMahon 2012). In addition, the quantity of nearby habitats also seems to be important: Gallé et al. (2011), for example, discovered that the number of forest specialist species increases in open habitats with increasing proportion of surrounding forests. Thereby, we must emphasise that different spatial scales should always be taken into account in the arachnological investigations, since not only local but also landscape variables could affect the spider fauna of the studied habitat.

Spiders in the present study were sampled by two different techniques, pitfall trapping and sweep netting. Both these methods are among the most popular techniques in spider surveys while pitfall traps have been used extensively for studies on surface-dwelling spiders (e.g., Rėlys et al. 2002; Koponen 2003; Seyfulina 2005; Fetykó 2008), the sweep-net is perhaps the most widely used piece of equipment for sampling spiders from vegetation (e.g., Turnbull 1960; Seyfulina 2005; Fetykó 2008; Horváth et al. 2009). It has been shown that pitfall trapping and sweep netting target different species (Samu and Sarospataki 1995). This was also true in our study: overall, quite different spider species (and even families) were collected with each of these methods (see Appendix 1). We need to emphasize, however, that it is quite hard to compare the obtained data, since using various methods in different sites may influence the results. Other studies have shown that the efficiency of pitfall trapping and sweep netting varies greatly with the structure of the surrounding vegetation (Henderson 2003; Sutherland 2006; Samways et al. 2010). Greenslade (1964), for example, has found that taller vegetation in the vicinity of the pitfall traps hinders invertebrate movement. The sweep netting possess some problems as well – although this method can be used on most vegetation, it is ineffective in some vegetation types, e.g. tall reeds, very short vegetation or flattened vegetation (Sutherland 2006; Henderson 2003). Also, sweep netting is relatively ineffective on sparsely vegetated ground (Sutherland 2006). Thus, we must conclude that it is very difficult to compare catches between different calcareous fens, since our studied fens differed quite greatly from each other in terms of the plant species composition and vegetation height (Štokmane et al. 2013; Štokmane and Spuņģis 2014, 2016). Furthermore, pitfall trapping and sweep netting tend to collect spider species that exhibit different foraging strategies. For example, pitfall traps collect mostly surface-living spiders with an active hunter lifestyle, e.g., many lycosids (Topping and Sunderland 1992; Mallis and Hurd 2005), however, some ground web builders such as those from the family Linyphiidae, can also be adequately sampled (Coyle 1981; Standen 2000). Pitfall traps will not efficiently sample spiders which inhabit the upper vegetation layers (Standen 2000). Sweep netting, on the contrary, is used to catch spiders which occur on the top of the vegetation (Southwood and Henderson 2000). This method is appropriate in low vegetation (Sutherland 2006) and it only collects those spiders that do not fall off on the approach of the collector (Henderson 2003). The sweep net captures primarily aerial web builders (e.g., Araneidae), however aerial hunters could also be collected (Coyle 1981). Overall, it can be concluded that pitfall trapping and sweep netting are methods that successfully complement each other.

In the present study we have also sampled several new spider species for the fauna of Latvia. Recording new species could mainly be explained by insufficient studies in calcareous fens, therefore we suggest that these habitats should be investigated further. In the future studies it would be worthwhile to use a combination of different other spider collection methods (e.g., hand collecting, beating, sieving, suction sampling, etc.) so that a greater variety of microhabitats is accessed. Also, it would be desirable to extend the sampling season throughout the spring, summer and autumn as well as to include both day and night collection, since it is known that spiders exhibit different seasonal and diel activity patterns (Coddington et al. 1996; Marc et al. 1999). Ideally, if the researchers could follow a standardized and optimized sampling protocol when collecting spiders (such as already-existing methodology prepared by Cardoso (2009)), because this could allow future studies in cooperation between different research teams.

The main conclusion from the present study is that calcareous fens are very diverse habitats not only structurally and floristically but also from the arachnofaunistic point of view. Our study showed that calcareous fens contain a very wide range of different spider species, including not only those that need wet and open habitats but also those that prefer other types of microhabitats (e.g., dry, shady, acid, salty, etc.). Besides, we found that along with the terrestrial spider ecological groups, calcareous fens can harbour also semi-aquatic and even aquatic spider species. Overall, however, calcareous fens are poorly investigated habitats, and therefore many spider species might still be undiscovered in this unique mire habitat type. Thereby, in order to get a more accurate picture of the spider fauna of the calcareous fens, these habitats should definitely be investigated further.

The authors would like to thank the people who provided invaluable help with the fieldwork: Voldemārs Spuņģis, Andris Ziemelis, and Agnese Žukova. Also, we thank the subject editor Pedro Cardoso and two reviewers Siddharth Kulkarni and Anna Šestáková who provided many valuable comments and suggestions that greatly improved the manuscript.