4.1. General composition of bat fauna
Summer assemblage of bats inhabiting forests of the WPN appeared to consist of 10 species, belonging to five genera and single family Vespertilionidae. These species represent five foraging guilds [
8]: open-air aerial foragers (
N. noctula,
N. leisleri), forest and clearing aerial foragers (
E. serotinus, P. pipistrellus, P. pygmaeus, P. nathusii), water-surface foragers (
M. daubentonii), foliage gleaners (
M. nattereri, P. auritus) and a ground gleaner (
M. myotis). The taxonomic and ecological composition of the assemblage is typical for the lowland Central or Northern Europe, similar to that recorded in southern Sweden or Germany [
8].
However, not all species, which geographical ranges cover the WPN (i.e. occur in the north-western Poland [
41]), nor even all species known to occur within the borders of the Park were recorded during our survey. Two species were netted in the WPN solely during autumn swarming at abandoned post-WW2 bunkers, these were Brandt’s bats
Myotis brandtii and western barbastelle
B. barbastellus [
42]. As both species are regional or facultative migrants, revealing moderate but regular movements between summer and winter areas [
43,
44], it is quite possible that they do not occur in summer on the island of Wolin at all. Previous acoustic survey in the WPN, based solely on Batcorder detectors, provided some scarce recordings of
B. barbastellus, however the authors used exclusively automatic identification and did not verify any IDs manually [
45]. Verifying recorded files manually, we provided evidence that batIdent software alone is not a reliable method for detection of
B. barbastellus during acoustic surveys due to widespread misidentification. The only published field test, comparing the accuracy of identification for software used in our study (batIdent and Kaleidoscope) unfortunately did not cover
B. barbastellus [
44].
Parti-colored bat
Vespertilio murinus was recorded more than 100 years ago on the neighboring island of Uznam, about 10 km from the study area, based on collected specimens [
46]. Its echolocation calls were identified by batIdent software both during our and earlier Batcorder surveys [
45], however relying exclusively on auto ID is even less reliable than in case of western barbastelle.
V. murinus is a species widely known from extreme call variation [
47], revealing almost complete overlap of all call parameters with four other sympatric species, i.e.
N. noctula,
N. leisleri,
E. serotinus and
E. nilssonii [
48]. Performance of both batIdent and Kaleidoscope in recognition of
V. murinus is generally low (66.7% and 16.7% of correct IDs, respectively; [
44]), thus we refrained from including that species in our database. As
V. murinus is a long-distance migrant [
44], regularly using coastal and insular areas of southern Baltic sea during its spring and autumn movements [
49,
50,
51], it is highly probable that it visits the WPN as well but we cannot provide any reliable evidence that it contributes to the summer bat assemblage of that area. Relatively large amount of capture data (third largest netting sample from any lowland forest in Poland –
Table S1), with not even a single
V. murinus caught, makes its occurrence in the WPN during summer unlikely. Three other species,
Myotis bechsteinii,
M. dasycneme and
E. nilssonii do also occur in north-western Poland [
41], known from sites located 50-100 km south of the WPN [
52,
53,
54]. Their absence within the border of the Park is also notable.
4.2. Species composition revealed by different methods
Both mist nets and ultrasound detectors can yield biased samples of bat assemblages, resulting in different proportions of detected species revealed by either method [
55,
56], thus both methods are often considered complementary [
57]. Mist nets tend to underrepresent species with extremely high flight altitude, high maneuverability and/or high sonar resolution, while ultrasound detectors tend to underrepresent species with low intensity sonar [
56]. Some of that features may seem mutually exclusive, e.g. fast-flying species with low maneuverability, like
N. noctula, are also hunting at high altitude [
11], usually above mist nets, but they can be effectively netted at drinking sites with low clutter [
28,
58], where they need to decrease their distance from the ground level. Among the highly maneuverable species, water-surface foragers, like
M. daubentonii, were considered overrepresented, compared to aerial-hawking pipistrelles [
28], probably due to significantly higher flight altitude [
59]. Some of that differences, revealed for mist netting at water bodies, hardly refer to mist netting on commuting routes, like forest roads, however. Increased popularity of monofilament (ultra-thin) mist nets, replacing thicker polyester or nylon nets, predominant in earlier studies, may partially reduce bias in capture data and allow capture even those insectivorous bats with highest sonar resolution [
60]. In our study, application of monofilament nets allowed to effectively sample even
M. nattereri, the species adapted to the smallest target range resolution among the representatives of genus
Myotis and, in consequence, to effectively navigate even few centimeters from the obstacles [
61].
Bias in acoustic survey is much easier to quantify, as it is linked almost exclusively to call amplitude and resulting sonar range, at least if the species is recognizable based on recordings alone [
56]. It explains much higher proportion of
P. auritus, emitting low-intensity calls and often relying on passive listening, in our netted sample and much higher proportion of
N. noctula, emitting the loudest calls among bats of the WPN, in parallelly collected acoustic samples. That bias might be adjusted for detectability using species-specific coefficients [
33], as we did in our study. Applications of that coefficients, although reduced biased towards the loudest species, did not change the overall picture of bat assemblage, with predominance of
P. pygmaeus in all three methods. No similar coefficients can be designed for detectability of species with mist netting at present. Still, a significant portion of species cannot be recognized based on echolocation calls, due to strong overlap in call parameters and in some groups (e.g. genus
Myotis) that problem cannot be entirely resolved, even with application of discriminant function analysis [
62] or artificial neural networks [
63]. It applies also to our survey, as our automatic classifiers worked based on similar principals and we had to confirm presence of the most numerous representative of genus
Myotis (
M. daubentonii) with capture data. Similarly, as mist nets made of mesh of different thickness, also various models of bat detectors may differ strongly in sensitivity and that differences are not even among call frequencies [
64,
65].
4.2. Uniformization of bat assemblages accros habitats – unique feature of the Wolin National Park?
Available acoustic data on forest bat assemblages in lowland Poland are scarce, restricted to just few woodland areas [
13,
17,
66,
67,
68], making broadscale comparisons, including cluster analysis, virtually impossible. Thus, we restricted our clustering test solely to capture data, as they are available to numerous locations from all geographical regions, usually collected as a base of local bat surveys (
Table S1). Usually, local bat assemblages clustered based on habitat affiliation. Both running and large stagnant waters share predominance of
M. daubentonii, a species being a specialist water-surface forager [
11]. Thus, only a large surface of open water can support a nursery colony in its vicinity, such function might be played not only by a lake or a group of fishponds but also a river, even small, as on the longer section it may provide large area of water as well [
28,
69]. Locally,
N. noctula and/or
N. leisleri, also appear as indicator of forest rivers but it is hardly universal feature of Polish bat assemblages. Valleys of rivers, where noctules predominate, are covered mostly by open meadows [
58], contrary to many riverine sites, where only
M. daubentonii predominate and river channels are covered by canopy of riparian woodlands [
28]. Meanwhile, noctules – hawking prey highly above the ground, and presumably flying down to the water only to drink – require open space, as their fast, unmaneuverable flight preclude them from flying close to obstacles, including vegetation [
11].
Divergence of bat assemblages visiting small forest ponds, compared to other water bodies, probably result from their specific function – not foraging but drinking sites. This is why they attract a number of species that do not hunt over water at all, especially gleaners (
Plecotus spp.,
M. myotis, M. bechsteinii, M. nattereri – [
11,
31]), but in some regions also large aerial hawkers that may hunt over waters only opportunistically (genera
Nyctalus and
Eptesicus [
67]) but primarily visit them to drink. As forest roads act both as commuting routes and foraging sites [
24], offering spaces with reduced clutter, they appeared also to be used by members of various foraging guilds. The latter is probably the reason why assemblages of bats using forest roads often cluster together with small forest ponds. Despite different functions, they aggregate a set of various species, revealing not only diverse hunting strategies but also diverse roost preferences – the indicator species of that habitat include obligatory house-dwellers (
E. serotinus), old-forest specialists (
B. barbastella) and generalists (
P. auritus) [
31]. However, there are some regions of Poland, where
N. noctula can also predominate in samples netted on forest roads; it might be explained by proximity of daily roosts, as that species is also a tree dweller, selecting shelters in sites with reduced clutter [
22].
Contrary to the general pattern of clustering similar habitats first, netted samples from the WPN formed a close cluster based on geographic location, a unique position among all analyzed data from lowland Poland, resulting from domination of a single species,
P. pygmaeus. Some significant, secondary differences between habitats were revealed and they partially followed the same pattern, as revealed for the remaining Polish forests (higher abundance of
P. auritus on small pools, or
M. daubentonii over streams and larger waters). These differences were not strong enough to overcome the effect of pipistrelles predominating in every habitat. High abundance and frequency of occurrence of bats from genus
Pipistrellus is specific for northern Poland, especially lakelands and Baltic Sea coast, compared to the southern part of the country [
41]. That differences are suggested to result from reduced competition with
Myotis mystacinus species complex, extremely rare in the north but numerous in the south [
70]. It does not explain, however, domination of a single species in all sampled habitats, despite application of various methods and does not explain, why among all Central European pipistrelles, the species occupying the position of dominant is
P. pygmaeus.
However, it is impossible to check, if unusual species composition of bats in the WPN is a result of recent changes or it was a feature of that region for a longer time. The only other systematic bat survey of the Park, which quantitative data have ever been published, was restricted to one method (acoustic survey with Batcorders) and one class of habitats (forest roads and interior of tree stands). Yet, it revealed predominance of
P. pygmaeus at all sampled sites, but that data were collected only 4-5 years before our study [
45].
4.3. Factors behind hyperabundance of Pipistrellus pygmaeus and scarcity of forest specialists
In our knowledge, the only other forest in Central European nemoral zone, where
P. pygmaeus was recorded to dominate in all sampled habitats, was Włoszczakowice Forest District in western Poland.
P. pygmaeus accounted for 42-48% of echolocation call sequences there, depending on forest type [
17]. It might, however, predominate in some extremely depauperated bat assemblages, like in the north-east Scotland [
71]. Both areas were sampled using acoustic surveys alone and in no other region,
P. pygmaeus predominated among netted bats or in all studied habitats. In Białowieża Primeval Forest, it was the most numerous bat recorded in the strict protection zone of the national park but it was outnumbered by
B. barbastellus among recordings from the part of that forest subjected to logging and among bats netted on forest roads [
72]. In Gdańsk Pomerania region, also located in northern Poland but further east,
P. pygmaeus appeared to be the rarest among three species of pipistrelles, outnumbered not only by
P. pipistrellus and
P. nathusii but also by
N. noctula,
E. serotinus and
Myotis sp. (69 sites, N=4063 call sequences, [
67]).
Pipistrelle bats are considered generalists, representing
r selection model on
r-K strategy spectrum among chiropteran life histories, due to relatively small body size, short life, high fecundity and dispersal [
73]. They can thrive in variable or unpredictable environments and outcompete more specialized via exploitation, which was already suggested as mechanism explaining decline of threatened lesser horseshoe bat
Rhinolophus hipposideros, following the expansion of
P. pipistrellus in Switzerland, due to complete overlap in spring diet [
74]. Thus, may one expect that their anthropogenically-driven expansion may even lead to depauperation of local bat fauna. Their ecological plasticity allows them to predominate in urban and agricultural landscapes, as revealed by acoustic surveys [
67,
75,
76], although, as revealed by our study, they appeared never to predominate among netted bats in forests (
Table S1). However, within
Pipistrellus pipistrellus species complex,
P. pygmaeus is always considered a relative specialist, regarding its habitat and landscape affiliation. It occupies much narrower habitat niche, requiring presence of wetlands, developed riparian zone and broadleaved forests, while its sibling taxon,
P. pipistrellus, appears to be eurytopic, widely utilizing an agricultural landscape beyond water [
67,
77,
78,
79].
Nonetheless, classification of
P. pygmaeus as ‘specialist’ relies solely on habitats used as foraging sites. The remaining aspects of its biology reveal its enormous ecological plasticity and resistance to anthropopressure. The species uses an unusual variety of nursery roosts, from buildings [
80], through hollow trees [
81,
82], spaces under exfoliating bark [
82], bat boxes [
81] and bridges [
83], while its house-dwelling behavior reveals no selection towards building age, material or structure [
84]. It also forms nursery colonies much larger than
P. pipistrellus [
85], furtherly increasing its competitive potential. Moreover, due to flexible foraging strategies,
P. pygmaeus is adapted to exploit abundant but ephemeral prey, like small midges, forming large swarms around even-aged, intensively managed plantations of non-native conifers [
86]. At least when foraging in riparian and wetland habitats,
P. pygmaeus hunts mostly small dipterans, belonging to family Chironomidae, i.e. one of the most abundant group of aquatic insects, often increasing their density with anthropogenic eutrophication of waters [
87]. Finally, the species appears not to be affected by exclusion from roosts, easily switching to alternative daily shelters and resume the former activities and their patterns [
80].
Therefore, roosting and foraging opportunities in forests of the WPN appear to form a set of conditions able to maintain unusually high abundance of
P. pygmaeus to the point that it numerically predominates in all bat assemblages. First, native beech forests of the park, due to consistent strict protection, undergo constant increase in stand age, abundance of dead and dying trees, providing loose bark and crevices, usually preferred by
P. pygmaeus [
82], but also appearance of canopy gaps. On the other hand, stand conversion conducted in forest communities with anthropogenically altered structure, based on selective cutting of formerly planted Scotch pine, led to increase in abundance of coarse woody debris, appearance of numerous pine snags with vertical splits and loose bark, which were left for natural decay. It also resulted in thinning of the whole stand and appearance of several, small clearings. Both processes – natural and anthropogenic one – can be considered a broad-scale disturbance in forest ecosystems of the WPN, and such disturbances are known to increase the abundance of
P. pygmaeus, as it was exemplified by outbreak of bark beetle
Ips typographus in Białowieża Primeval Forest [
68]. Processes that lead to appearance of numerous snags and deadwood should, however, also favorize old-forest specialists, like
B. barbastellus [
88], also known to select roosts under loose bark [
89] or
N. leisleri selecting rot cavities [
22]. This is not the case of the WPN, however, where
B. barbastellus seem not to occur in summer at all, despite large areas of old beech forests, its optimal habitat [
89], while
N. leisleri seem to be scarce and local species. The possible explanation for such high dominance indices of
P. pygmaeus can be cross-boundary subsidy in both surplus roosts and prey which act in synergy with processes affecting forests within the WPN. First, chaotic urbanization of areas located at the very borders of the Park, as well as unorganized expansion of recreational development in more rural parts of the WPN’s neighborhood may provide abundant new daily shelters. As
P. pygmaeus can easily inhabit even new buildings [
84], able to enter even very narrow crevices, can benefit rapidly from ongoing urbanization [
90]. However, the crucial subsidy that may contribute to the hyperabundance of
P. pygmaeus results from location of the WPN at shores of Szczecin Lagoon, subjected to increasing input of nutrients from polluted Odra River, leading to development of hypereutrophic conditions [
91]. Such conditions presumably led to exceptional abundance of aquatic nematoceran flies, especially chironomids [
87], i.e. predominant prey of
P. pygmaeus.
We presume that coincidence of spontaneous and active renaturation with provision of surplus anthropogenic roosts and abundance of prey due to contact with waters subjected to eutrophication might create optimal habitat for
P. pygmaeus. It might also allow
P. pygmaeus to outcompete from tree roosts other bat species that would, otherwise, benefit from renaturation of the Park’s forests but occupy much narrower ecological niches, like
B. barbastellus, being a rare example of food specialist among Central European bats [
92]. We cannot exclude that success of
P. pygmaeus affects even much more common and more generalist tree-dwelling bats that use different hunting tactics (
N. noctula), reducing their relative abundance in the quantitative structure of the assemblages. If the affected species are characteristic for particular habitat (e. g.
M. daubentonii for water courses and lakes), it increases the similarity of assemblages across habitats and might lead to general depauperation of bat fauna. If the proposed scenario is an accurate model, predominance of
P. pygmaeus in all studied forest habitats of the WPN should be treated as an example of hyperabundant native species, i.e. taxon that was not introduced by humans but benefited from their activities, negatively affecting other, less successful native species [
93].