Ecological significance of discarded containers
This study revealed that discarded containers are frequently dwelled by various groups of spiders. Detailed discussion with previous results is hardly possible because there is almost no research that shows influence of discarded containers on arthropod populations or communities. However, in contrast to our recent findings on ants (Kolenda et al.
2020), the containers serve for spiders more as an artificial microhabitat than as a deadly trap. Indeed, the number of containers with dead specimens was very low (1.7% of all), especially in contrast to ants (10.3% of containers with dead workers, and only 4.4% used as a nest; for details see Kolenda et al.
2020). Moreover, Lavers et al. (
2020) found that discarded plastic bottles are a lethal trap for crabs on beaches, while Kolenda et al. (
2015) and Poeta et al. (
2015) noted mostly dead beetles and molluscs inside containers collected from suburban forests and sandy coastal dunes, respectively. Discarded containers can, however, act as suitable habitat for animals in aquatic conditions. In a lowland dam reservoir, macroinvertebrates diversity found in the bottles was as high as on phytolittoral bottom and higher than on other natural or artificial studied substrates (Czarnecka et al.
2009). Probably such items provided shelter against harsh environmental conditions of the near-shore zone of the reservoir (Czarnecka et al.
2009).
Some spider traits may explain why they did not die so often in the garbage traps but rather inhabited them, even if the containers were not dry. Many spiders readily walk on steep surfaces due to some adhesive structures (the so-called
scopulae in wandering spiders) and support themselves with security threads in case they fall down. Many of them walk quite easily on the surface of water using surface tension; they may, in fact, use numerous techniques to move on liquid surfaces (Stratton et al.
2004). Similarly, some Linyphiid species easily avoid being trapped and readily walk on steep walls of pitfall traps (Topping
1993) and even build webs within (so exploring artificial habitats similar to the containers we studied). As predators, the spiders seem also not to be lured into the traps by the water and decaying matter, as some beetles or small mammals are (Benedict and Billeter
2004; Kolenda et al.
2015).
We observed that spiders which dwell in discarded containers use them for at least three different purposes: hunting, hiding, and breeding. As a confirmation of the latter, we found adult couples and cocoons. The only cocoons we found belonged to
Ero spp., which are commonly found in European forests where they are attached to plants (Finch
2005a), e.g. to tree trunks. Their presence in containers can be additionally explained by the fact that
Ero are small predators of the web spiders, using aggressive mimicry for hunting spiders sitting in webs (Czajka
1963). Discarded containers are also a presumably convenient moulting place for several spider families, even those that we did not record from live specimens; for instance, Gnaphosidae. This shows that containers serve as hiding places for some wandering spiders. However, this role cannot be overestimated, as spiders can use any shelter places in the forest litter. They may also serve as a hunting site for some spiders. Many studies have shown that prey availability is often crucial for spiders locating their webs on a site (e.g. Samu et al.
1996; Harwood et al.
2003; see also Wise
1993). Nearly 1/3 of collected containers contained webs. In many randomly checked containers in the field, we found prey remains in these webs (Fig.
1b).
Despite the observation of various life stages and low mortality of spiders, as well as the fact that the containers are used by spiders for different purposes, we cannot conclude that they are a suitable habitat for these arthropods. Further studies comparing reproductive and hunting success, or species richness and specimen number in the containers and in their surroundings are necessary. This could explain which species of spiders avoid containers and which of them take any advantage of inhabiting the containers or fall into ecological trap.
The presence of more than one web in a single container does not necessarily mean that each of them was built by a single spider. As the example of
Tenuiphantes tenuis shows, one specimen may build several webs and one web may be used by several specimens consecutively (Samu et al.
1996). The web spinners are often characterised by a kind of ‘floating populations’ (as shown by a sheet-web spider
Linyphia triangularis; Toft
1998), due to frequent web takeover and the fact that empty webs are often colonised. We assume that this could be the case with some of the observed species. We cannot also exclude the occurrence of competition between spiders within the containers. In several cases, we have observed inter- and intraspecies pairs of spiders inhabiting one container. Some studies reported a high level of competition for web sites in the field (Samu et al.
1996; Heiling and Herberstein
1999; Riechert and Hall
2000; Hardwood et al.
2003). Related linyphiid spiders often have very similar niche preferences (e.g. a preference for web positioning; Toft
1987). Some observations showed that it is a common phenomenon that a heavier or larger spider overtakes the web (Samu et al.
1996; Eichenberger et al.
2009) and intraguild predation is also present among spiders (Finke and Denno
2002). Small linyphiid spiders may not be deterred from entering the site even in the presence of chemical clues from the larger spider predator (Wetter et al.
2012). However, some data shows that smaller spiders avoid places with the scent (the kairomons) of a larger spider predator (Persons and Rypstra
2001).
The open question is whether the containers provide conditions suitable for overwintering. Generally, in temperate zones, arthropods are prone to mortality due to low temperature; thus, they overwinter in habitats with stable conditions (Roume et al.
2011). Some, however, present adaptations to survive and are active even at temperatures below freezing—especially when prey is available (Aitchison
1984; Korenko et al.
2010; Lee
2012). Forests are important overwintering habitats for arthropods. Some species, including those found in our study (e.g. belonging to genera
Enoploghatha,
Neriene), overwinter in litter (Martyniuk and Wise
1985; Nähring
1991). Furthermore, whole season observations are required to assess if containers are left by spiders before winter or pose a year-round habitat.
Fauna overview—species filtering and biases
We recorded an array of typical litter- and undergrowth-dwelling species of the forest litter of the European temperate zone (Stańska et al.
2002; Finch
2005b; Milasowszky et al.
2015; Košulič et al.
2016), but only three of them dominated. The most dominant spider of these “garbage-assemblages” was
Tenuiphantes flavipes, a very common species from Linyphiidae, which predominantly lives on the ground surface in different woodlands (Hänggi et al.
1995) where it builds small sheet webs. The species has a slight preference for shade and a stronger one for moist habitats (Entling et al.
2007) but its niche preferences and the habitats it utilises are broad (Hänggi et al.
1995; Entling et al.
2007). We observed the dominance both of the juveniles (not identified with certainty, but many of them most probably belonged to this species) and of adult specimens. This species builds webs for hunting and mating; we, therefore, suppose that it explores the garbage for both purposes.
The second most common spider was
Enoplognatha spp., most probably
E. ovata, which is confirmed by habitat type (Oxford
1992; Barthel
1997) and the presence of an adult specimen. This spider builds webs in the understory of different habitats, including forests, and prefers dense vegetation. This refers, however, to the adult specimens and not necessarily to the juveniles—spiders may shift microhabitats with successive life stages. The juveniles, which predominated in our samples, are known to overwinter in litter and disperse in spring into higher strata of the understory (Nähring
1991). A similar rule most likely pertains to other relatively big species of Linyphiidae, such as
Neriene clathrata or
Linyphia hortensis, which are common in our samples. For instance,
Neriene clathrata generally weaves its sheet web in the lower strata of forest undergrowth (Wright and Coyle
2000).
When considering guild composition, there were hardly any ground or ambush hunters in the containers; however, we noticed some signs of their presence (the exuviae) and a few dead specimens belonging to these ecological groups. The wolf spiders (Lycosidae) are known to have a ‘sit-and-move’ strategy of exploring prey in their environment (Samu et al.
2003). Therefore, we suppose that they may be only the temporary visitors of containers, performing in this case a ‘come-and-go’ strategy. The predominance of sheet and space web weavers in our samples may result from the fact that these groups may easily adapt to structural properties of habitats and spend more time in discarded containers while waiting for their prey or while mating. Some of them even enlarge their webs with time by overbuilding the older parts of their snare (Benjamin and Zschokke
2004).
Without comparative study of species from the surroundings, we cannot precisely determine if the dominance of spider species in the containers results from their high abundance in the environment or from some species filtering towards the new microhabitat. Moreover, a total of 22 species and several further taxa that could not be recognised so accurately is a low number for typical temperate forests (Scharff et al.
2003; Stańska et al.
2016), which may reach about 80 for a sampling plot when sampled extensively (Scharff et al.
2003). However, the intensity of sampling and the used method was different in the studies, so our data are hardly comparable. The environmental factors, which influence richness and composition of forest spider assemblages, are the canopy cover, humidity, litter structure, and plant structure and density (Bultman and Uetz
1982; Samu et al.
1996; Pearce et al.
2004; Entling et al.
2007; Oxbrough et al.
2010); however, they were not considered in this study.
It should be also noted, that other factors can affect species richness. For instance, each sampling method used in arthropod studies is somehow biased (Topping and Sunderland
1992; Prasifka et al.
2007). Typical for the pitfall traps is a high dominance of spiders belonging to Lycosidae, the large ground hunters, which—along with some other families of wandering spiders—are very active and, as a result, dominate the yield of pitfall traps (Uetz and Unzicker
1976). On the contrary, this family was hardly present in our samples and recorded from dead specimens (or exuviae). The sampling period is also important. Spiders show a strong pattern in their phenology (e.g. Riecken
1999; Hsieh and Linsenmair
2012; Blandenier et al.
2013). Our study was conducted in September and the highest species richness is recorded in spring and early summer (Niemelä et al.
1994).
The other factor which we did not consider is spiders’ circadian activity (see Krumpálová and Tuf
2013 for data from central European forests). Some hunters which are active during the day time may have been absent in our samples as we sampled the containers during those hours. Thus, we do not claim that our list covers the whole diversity of spiders being entrapped or utilising the litter left over in the forests.
Preferences toward containers
Spiders are the generalist terrestrial predators which explore every possible space niche that is suitable for them to forage or breed. Our results indicate that there was no clear preference of spiders for any type of discarded containers. One of the exceptions was the low preference towards brown glass bottles and those containing beer or high-volume alcohol. This result should be treated with caution, as it is very hard to relate it to spider biology and the sample size was small in some subclasses. More data and further (meta)analysis could probably show clear preferences of spiders towards container types. We assume that there might be some preferences, based on studies analyzing efficiency of pitfall traps that are used in ecological studies. Methodological works on the construction of pitfall traps suggest that spiders catching rate may vary depending on their features. Namely, the effectiveness of these traps might be affected by the presence or absence of covers (Buchholz and Hanning
2009), the colour of traps (Buchholz et al.
2010), the entrance size (Work et al.
2002) or properties of fluid inside the container (Pekár
2002; Schmidt et al.
2006). Therefore, we had anticipated some bias in the number of spiders according to specific container categories.
The other factor that might have influenced spiders could be the shape of the containers, which we did not analyse, because web-building spiders are dependent on the availability of sites for spinning their snare. On the other hand, the linyphiids may also easily adapt the shape of their webs to the available spatial conditions (Samu et al.
1996; Rybak
2007). The position of the container opening could also be significant, because forest litter is not a single layer—it is a diversified three-dimensional space which creates several microhabitats for spiders (Wagner et al.
2003) and some spiders may even migrate into very deep soil layers (Laška et al.
2011). However, the lack of any explicit influence of container features on spiders, suggests that these animals—in contrast to many other arthropod groups—may not be attracted to them and simply exploit every available microhabitat of the forest litter.