Introduction 

The European hermit beetles belonging to Osmoderma eremita species complex (Coleoptera: Scarabaeidae, Cetoniinae) live in old hollow broad-leaved trees containing wood mould derived from the fungal decay of the dead heartwood and are obligate saproxylic species ([37]). They can be found in the remnants of natural forest, but also in other habitats such as urban parks, trees around agricultural fields and along streams, etc. ([37]). Although the taxonomy of this group has not been completely resolved, following Audisio et al. ([2], [3]), the Osmoderma eremita species complex consists of five species distributed in Europe: O. barnabita Motschulsky, 1845; O. cristinae Sparacio, 1994; O. eremita (Scopoli, 1763); O. italica Sparacio, 2000; and O. lassallei Baraud & Tauzin, 1991, with O. barnabita present in Croatia ([1]). Many studies conducted within European countries reported a strong decline in the distribution range of Osmoderma eremita species complex, and extinction in some areas, mainly due to habitat loss and fragmentation ([38], [7]). Therefore, they have been added to the IUCN Red List with Near Threatened status ([1], [32], [7]) and listed in the Annexes II and IV of the EU Habitat Directive ([14]). O. barnabita, eastern hermit beetle, is known from the Central and East Europe, and Western Russia ([2], [1]). However, the distribution range of O. barnabita in Croatia and western Balkan is poorly known ([1]), with very scarce records for Croatia.

The European red click beetle, Elater ferrugineus Linnaeus, 1758 (Coleoptera: Elateridae), is a rare species associated with old hollow trees, whose larvae prey on the eggs and larvae of Scarabaeid and Lucanid beetles ([37], [4]). It is distributed in Europe, Caucasia and Asia Minor ([44]), with deficient data on its distribution mainly in South-Eastern parts of Europe, including Croatia. As a consequence of scarce and old records, and lack of recent findings, E. ferrugineus has not been listed for Croatia in the latest relevant checklists ([8], [9]). The species can be found in old deciduous forests, parks, and wooded meadows ([4], [35]). The larvae of the species live in rotten wooden substrate inside of deciduous trees, e.g., oak and beech, which have been inhabited by primary consumers like the xylophagous larva of Osmoderma, Cetonia, Potosia (Cetoniinae) and Dorcus (Lucanidae) species ([44]). Since E. ferrugineus is also associated with old hollow trees which are usually scattered across the landscape at the very low densities, it is thus negatively affected by the loss of such trees caused by, e.g., forestry management practice and deforestation ([30]). Therefore, E. ferrugineus has been added to the IUCN Red List of European saproxylic beetles with the Near Threatened status ([32], [7]) and it is also included in the Red Lists of protected species in many countries ([4], [35]).

In this paper, we presented the recent findings of O. barnabita and E. ferrugineus during field mapping and monitoring of saproxylic beetles, listed at Annex II and IV of EU Habitat Directive (Council Directive 92/43/EEC), in three areas (Fig. 1) located in the Mediterranean, Continental and Alpine bio-geographical regions in Croatia aiming to extend the knowledge in their distribution. Data were used to model habitat suitability maps in order to direct future monitoring activities. Some of those data represents the first findings for certain parts of Croatia. Old records of these saproxylic species in Croatia, dating from the end of the 19^th and from the 20^th centuries (see Tab. S1 in Supplementary Material), were presented on the map as well, and together with nature protected areas were overlapped with habitat suitability map to suggest future studies and nature protection actions.

Fig. 1 - Map of the recent findings of Elater ferrugineus and Osmoderma barnabita in Croatia. Symbols of locations (see Tab. 1 and Tab. 2) at the Papuk Nature Park: (CV): Cešljakovačka visoravan; (J): Jankovac virgin forest; (K): Kozjak; (Ka): Kaptol; at the Plitvice Lakes National Park: (C): Corkova uvala virgin forest; (Kv): Kameni Vrh; (L): Liman-draga; (M): Medvedak; (S): Stubica; (Sa): Sartuk; (SD): Sužanjska Draga; (V): Vreline; at Mt. Učka: (G): Grbac. Sampling sites within each location are represented as symbols: squares - empty trap/s; triangle - traps with O. barnabita and E. ferrugineus recorded (Elater & Osmoderma); grey circle - traps with records of O. barnabita only (Osmoderma); black diamond - traps with records of E. ferrugineus only (Elater) (see the legend above).

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  Material and methods 

Study areas

The study has been conducted in mountain forests (Fig. 1) within all three biogeographical areas present in Croatia: (i) Mt. Učka in western Croatia, within Mediterranean biogeographical region; (ii) Papuk Nature Park at Mt. Papuk in eastern Croatia, in Continental region; and (iii) Plitvice Lakes National Park in Alpine region.

Mount Učka is situated in the north eastern part of the Istrian peninsula in Croatia, near the Adriatic Sea, and represents a part of Mediterranean karst with a strong Mediterranean influence on climate and floral elements. Two study sites were situated in the area called Grbac, in the southern part of Mt. Učka, close to the borders of Učka Nature Park, placed on moderate to steep slope, with south exposition, and the elevation of 300 m a.s.l. The ground is shallow to medium deep brown soil on flysch sediment with the partial outbreak of rocks and screes. The present forest community is Seslerio autumnalis-Ostryetum Horvat & Horvatić ex Horvat et al. ([19]) densely overgrown with bushes. Hop-hornbeam (Ostrya carpinifolia Scop.) and manna ash (Fraxinus ornus L.) grow alongside downy oak (Quercus pubescens Wild.). Pines (Pinus sp.) also occur. Shrub layer consisted of sprouts of above-listed tree species and juniper (Juniperus sp.), terebinth (Pistacia terebinthus L.), European smoke tree (Cotinus coggygria Scop.) and common dogwood (Cornus sanguinea L.). The study sites were situated in the state-owned coppice forests managed by Croatian Forest Ltd. National Company, positioned in the vicinity of Nature Park Učka, that is part of the Natura 2000 network.

The Papuk Nature Park is situated on the Mt. Papuk, in the Continental biogeographical region, in the Eastern Croatia, surrounded with lowland mainly used as an agricultural land. The Papuk is mostly covered by forest with the sessile oak (Quercus petraea [Mattuschka] Liebl.) dominant up to 350 m a.s.l. Beech trees (Fagus sylvatica L.) cover more than 50% of the forested area, whereas mixed beech-fir forests grow in areas higher than 700 m a.s.l. Although commercially managed by Croatian Forests Ltd., except “Jankovac” virgin forest area, all forests have a natural composition of tree species. The deciduous forests are managed as even-aged stands, while the beech-fir forests are managed following the selective logging system. Beech stand at Jankovac area represents secondary virgin forest, spread over 660 ha, that has been left unmanaged for more than 160 years. The Papuk Nature Park is a part of the Natura 2000 network, and based on one old literature record of hermit beetle ([22]) and accidental finding of one specimen in Jankovac forest on July 11^th, 2008 (Vlatka Dumbović Mazal private collection), this area has been designated as a site of community importance (SCI) in order to ensure the favourable conservation status of the species.

The Plitvice Lakes National Park has a unique landscape and has been added to the UNESCO World Heritage list. The Plitvice area is a part of the Lika County, belonging to the Alpine biogeographical region. It is situated in the southern parts of Mala Kapela Mountain chain, ranging from 450 to 1280 m a.s.l., with limestone and dolomite rocks. Around three-quarters of the park surface has been covered with the forests in their various stages from scrubs to the virgin forests. From 1994 until nowadays, according to the Nature Protection Act (The Official Gazette of the Republic of Croatia No. 30/94 and 72/94), forests within the Park have been left to develop naturally. Beech and beech-fir forests repres-ent climate-vegetation with predominant beech forest with dead nettle Lamio orvalae - Fagetum sylvaticae (Horvat, 1938), [6]. In the dependence to the relief, geology, soil depth and moisture, a range of other forests have been developed as well (e.g., vegetation with dominant willow, European alder, hop-hornbeam, spruce, silver pine, etc.). O. barnabita was mentioned for Plitvice area (Croatia: Plitvice, July 30^th, 2002) in the paper of Audisio et al. ([3]) without providing more precise location, and there was one accidental finding of dead insect in virgin forest in Corkova uvala area, reported to the National Park authorities in season 2006/07, without precise date (collected by Krešimir Culinović). Based on those data on O. barnabita, Plitvice NP has been designated as a site of community importance (SCI) in order to ensure the favourable conservation status of the species.

Sampling methods

Several sampling methods (counts on transects, ground and tree pitfall traps and flight intercept window traps) were used in studied areas as a part of three projects aiming for mapping and monitoring of saproxylic beetles listed at Annex II and IV of EU Habitat Directive (Council Directive 92/43/EEC): Lucanus cervus (Linnaeus, 1758) (Lucanidae), Morimus asper funereus Mulsant, 1863 (Cerambycidae), Osmoderma eremita species complex, Rhysodes sulcatus (Fabricius, 1787) (Rhysodidae) and Rosalia alpina (Linnaeus, 1758) (Cerambycidae), run by Croatian Ecological Society and Faculty of Science in Zagreb. Of those, tree pitfall traps and flight intercept cross-vein funnel traps with pheromone were efficient in sampling O. barnabita and E. ferrugineus, and will be presented in detail.

At Mt. Učka, field research was conducted according to the national monitoring program for stag beetles Lucanus cervus in the Mediterranean biogeographical region ([40], [21]) from June 12^th until July 05^th in 2014. Two transects, Grbac 1 (G1) and Grbac 2 (G2), each of 200 m in length, were chosen following the forest road. Stumps and dry trees were not numerous, but there were a greater number of old hollow trees: 24 hollow trees, 11 dry trees and three stumps were counted alongside the chosen transects. The tree pitfall traps were made of plastic bottles (2 L) with the upper part cut and placed inside the lower part making a plastic pot with a funnel. Overall, 24 tree traps were employed during three weeks period, twelve traps along each transect were attached to the oak tree trunks along the transect path, at 1.5 to 2 m of the tree height (Fig. 2), as recommended by Serić Jelaska ([40]) and Vrezec et al. ([48]). In three traps, the fruit (chopped peaches and bananas) was used as a bait, in three wine flavoured with vanilla extract and vanilla sugar, three traps had grated ginger and three traps without any bait were used as a reference ([18], [40], [48]).

Fig. 2 - Tree pitfall trap with wine and vanilla sugar and extract.

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At the Papuk Nature Park and Plitvice Lakes National Park, besides methods used for monitoring of the stag beetle following the national protocol ([40], [21]) similar to that conducted at two sites on Mt. Učka, flight intercept cross-vein funnel traps with pheromone ([47] - Fig. 3) were utilised aiming to record hermit beetle. Flight intercept cross-vein funnel traps (Fig. 3) were baited with the racemic (R)-(+)-γ-decalactone ([24]). The pheromone was applied to a small piece of cotton which was protruded from a small tube that was hanged in the middle of the plastic cross and was re-applied during visits. The traps were hooked at the height around 2 to 2.5 meters on tree branches, in a small forest openings (Fig. 3).

Fig. 3 - Flight intercept trap with pheromone applied on the cotton pad placed in the middle of the crossed plastic panels.

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Five flight intercept cross-vein funnel traps with the pheromone were placed at each of four locations at the Mt. Papuk: Cešljakovačka visoravan (CV) and Jankovac, a secondary virgin forest (J) in beech stands, and Kaptol (Ka) and Kozjak (K) in the sessile oak forests, with 20 traps overall (Tab. 1). The traps were exposed from June 19^th until 24^th and again from July 1^st until 6^th in 2015 (10 days in total) and visited every day.

In Plitvice Lakes National Park, overall twenty flight intercept cross-vein funnel traps with the same pheromone were employed at eight locations: Stubica (S), Medvedak (M) and Liman-draga (L) near Plitvički Ljeskovac village placed in a beech forest with dead nettle; and other five sites, Corkova uvala virgin forest (C), Kameni Vrh (Kv), Sartuk (Sa), Sužanjska Draga (SD) and Vreline (V) positioned within beech-fir mixed forests (Tab. 1). The traps were exposed between June 24^th and August 3^rd in 2016 and visited every three to ten days. The traps that were exposed for the longest period were checked seven times. Of the five traps placed in Corkova uvala virgin forest, one trap was left without pheromone and was exposed from June 25^th until July 14^th (for 19 days).

Identification of collected specimens was done based on the following publications: Freude et al. ([15]) and Schimmel & Tarnawski ([44]) for E. ferrugineus, and Sparacio ([46]), Han et al. ([17]) and Maurizi et al. ([28]) for Osmoderma barnabita. Pronotum and elytra of E. ferrugineus are variable in colour. Besides specimen with nominal variety (var. ferrugineus) with red pronotum and elytra, those with black pronotum and red elytra have been recorded at Plitvice sites. The external characters of the examined specimens from Plitvice with black pronotum and red elytra fit the description of E. ferrugineus by Schimmel & Tarnawski ([44]) belonging to var. occitanicus Villers, 1789 ([26], G. Platia, personal communication).

Assessment of distribution patterns

Given the lack of current data on the presence of O. barnabita and E. ferrugineus in Croatia, we used data from our study (Tab. 1) and available records published after 2002 (that were not listed in [37]): [44], [39], [5], [11] (Tab. 2), together with unpublished recent records of local entomologists and data from several collections (BIUS Association at the University of Zagreb, Biology Department at the University JJ Strossmayer in Osijek, G. Platia private collection - Tab. 2), in order to develop habitat suitability maps for all of Croatia using the software MaxEnt ([36]) and to estimate areas that could be potentially occupied by the species. As independent variables or descriptors, we used 11 environmental variables that represent the climatic conditions (amount of precipitation and average temperature for autumn, winter, spring and summer seasons) and topographic (slope and aspect). Aspect values because of their “circularity” (e.g., 1° and 359° representing an almost identical aspect), were transformed by calculating the sine and cosine of aspect values, forming two variables ranging from -1 to 1, representing an inclination from north (cosine) and from east (sine) as in Guisan et al. ([16]) and Jelaska et al. ([20]). All environmental variables were with 300 m spatial resolution (source: the Department of Biology, Faculty of Science, University of Zagreb). The resulting MaxEnt maps, with their logistic outputs (i.e., values ranging from 0 to 1) were reclassified to binary (0: non-suitable habitat; 1: suitable habitat) grids using the customized threshold values. Instead of using a case sensitive adjusted threshold values as in Katušić et al. ([21]), who dealt also with rare species with very scarce data, we used conservative value of 0.5 as in Sadeghi & Malekian ([42]). The smallest logistic output value for O. barnabita was 0.117 on locations used to construct the models, which seemed as it could yield too large overestimation of the model for O. barnabita. The obtained two reclassified MaxEnt models were spatially overlapped, and mutual and unique areas of suitable habitats for both beetles were calculated.

Historical data were assembled by searching: (i) literature available within national scientific and university libraries and via internet; (ii) collections deposited in national museums and universities, private collections and museum collections available online; (iii) in personal communication with other entomologists and curators from Croatia and neighbouring countries. Historical records have been listed in Tab. S1 (Supplementary material). Identification of specimens belonging to historical data deposited in museum collections were not revised within this study; therefore for historical data we used names Osmoderma eremita complex species and “hermit beetle”.

To test the habitat suitability models, we used geo-referenced historical data from (i) literature dating before 2002: Ranius et al. ([37]), with compilation of records of hermit beetle within European countries until 2002; Koča ([22]), Depoli ([10]), Strbac ([41]), Schimmel & Tarnawski ([44]) with the records of E. ferrugineus; (ii) entomology collections: Varaždin Town Museum (with record of hermit beetle within Košćec collection that were not included in [37]) and Natural History Museum in Zagreb (in Igalffy, Koča and Korlević collections), Faculty of Forestry in Zagreb (Hensch collection) and G. Platia private collection with data on E. ferrugineus (see Tab. S1 in Supplementary material). Majority of those data were around 100 years old, and all localities were described with rather general toponyms. Hence, instead of spatially overlapping of point localities assigned to each historical data with habitat suitability models, we have identified the 10 km² EEA reference grid cells ([13]) that contain historical data and overlapped those grid cells with the MaxEnt models to estimate the amount of their congruency.

  Results 

Field records of Osmoderma barnabita in study areas

Overall 29 individuals were recorded during a field survey in the Papuk Nature Park in 2015 and in the Plitvice Lakes National Park in 2016. There were no records of hermit beetle on Mt. Učka during the project in 2014 (Tab. 1, Fig. 1).

Four individual species were caught using flight intercept cross-vein funnel traps with pheromone placed in two sites on Mt. Papuk: in secondary virgin beech forest in Jankovac area and in oak forest mixed with beech trees in Kozjak area, the same day on July 6^th, 2015 (Tab. 1, Fig. 1).

In the Plitvice Lakes National Park all but one O. barnabita individuals were caught using flight intercept cross-vein funnel traps with pheromone. Observations of 24 individuals occurred between 5^th and 31^st of July, at seven out of eight locations (Tab. 1, Fig. 1, Fig. 4), three sites within beech forest with dead nettle (Medvedak, M; Stubica, S; Liman-draga, L) and four (Virgin forest Corkova uvala, C; Sartuk, Sa; Vreline, V; Kameni Vrh, Kv) within the beech-fir forest.

Fig. 4 - Eastern hermit beetle Osmoderma barnabita caught at the Plitvice Lakes National Park, Croatia.

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One individual was observed in the beech forest in Medvedak area at 723 m a.s.l. in Plitvice Lakes NP on July 13^th, 2016, when landed on a ground, around noon, during sunny day.

Field records of Elater ferrugineus in study areas

Overall 9 individuals of E. ferrugineus were recorded, one on Mt. Učka in 2014 and eight in the Plitvice Lakes National Park in 2016. There were no records of E. ferrugineus in traps used in Papuk area during the project in 2015 (Tab. 1, Fig. 1).

On Mt. Učka, on June 24^th, 2014 one individual was caught in tree pitfall trap baited with wine flavoured with vanilla extract and attached on downy oak tree Quercus pubescens with 77 cm in diameter (Fig. 2).

In the Plitvice Lakes National Park, eight individuals were caught (7 females and one male) using flight intercept cross-vein funnel traps with pheromone (Fig. 3), at four out of eight locations: Stubica (S), Medvedak (M), Liman-draga (L) near Plitvički Ljeskovac village south of Prošćansko Lake in beech forest with dead nettle and at Sartuk (Sa) in beech-fir forest. Individuals were caught during four visits: on July 12^th, 14^th, 21^st and 31^st in 2016 (Tab. 1, Fig. 1). In the same sites, O. barnabita was caught as well (Tab. 1).

Environmental response and habitat suitability

Obtained AUC on training data was 0.784 for E. ferrugineus, and 0.948 for O. barnabita. For both species, three environmental variables contributed to the MaxEnt models with more than 10%, with identical two environmental variables, namely: autumn precipitation (E. ferrugineus 41.3%; O. barnabita 10.4%); and the slope (E. ferrugineus 36.7%; O. barnabita 32.5%). Northness contributed further 20.3% to the model for E. ferrugineus, while spring temperature contributed 28.5% for O. barnabita. Based on the environmental variables response curves (Fig. 5), it seems that wetter autumns on steeper areas are favourable for the potential occurrence of E. ferrugineus, or at least its habitats. For O. barnabita response curves are not so straightforward, showing mostly a unimodal response.

Fig. 5 - Response curves of three environmental variables that contributed most to MaxEnt models for Elater ferrugineus and Osmoderma barnabita. (aprec): autumn precipitation; (sptemp): spring mean temperature; (slope): slope of the terrain; (northn): inclination from the north.

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Habitat suitability maps for E. ferrugineus and O. barnabita beetles are shown in Fig. 6. Suitable habitats are expected to be found on 15.125 km² for E. ferrugineus, and on 3.163 km² for O. barnabita, with those two overlapping on 2.424 km². Historical data (from 2002 and older) for hermit beetle (33 locations) and E. ferrugineus (8 locations - see Tab. S1 in Supplementary Material) were within 33 of the 10 km² grid cells, out of which 27 has overlapped with the combined MaxEnt map (Fig. 6) for both species.

Fig. 6 - MaxEnt habitat suitability derived maps based on variables representing climatic and relief condition. 10 km² grid cells containing historical data on hermit and the European red click beetles and protected areas are superimposed (for more details, see the Methods section).

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  Discussion 

Findings of hermit and the European red click beetle in Croatia

The fact that this was the first systematic survey conducted in Croatia focusing on hermit beetle may explain why data on this species have been scarce. Most of the previous findings are accidental records. Besides findings of O. barnabita from our research in Papuk and Plitvice Lakes areas, there were several recent records that were included in habitat suitability analyses from other locations: Mt. Svilaja in 2008 ([11]), on a willow tree close to the spring of Zrmanja river ([39]), Kvarner area and Northern Adriatic Islands (L. Katušić and F. Kostanjšek personal communications, G. Platia collection) belonging to Mediterranean biogeographical region; in the beech forest in Mt. Dinara and Velebit (BIUS Association and G. Platia collections) within Alpine biogeographical region. In Ranius et al. ([37]), the latest paper in which the distribution of the species complex across Europe were presented, 31 locations for Croatia were listed based on data from the Croatian’s museums, private collections and literature for period between 1982 and 2002. Since the authors in Ranius et al. ([37]) did not include data from Franjo Košćec collection deposited in Varaždin Town Museum, we added those records on the map within 10 × 10 km grid (Fig. 6) as historical data (Tab. S1 in Supplementary material).

The European red click beetle has been reported in the majority of European countries, including some countries neighbouring to Croatia ([8], [9]). Although Cate ([8], [9]) does not report the species for Croatia, there were few publications listing its presence in the country ([45], [10], [44]), and several data from the collections (G. Platia collections, Igalffy, Koča and Korlević collections from NHM in Zagreb and Košćec collection from Varaždin Town Museum - Tab. S1). In the past it had often been named in different generic combinations, within genus Ludius Berthold, 1827, Steatoderus Dejean, 1833, Parallelostethus Schwarz, 1906 and Orthostethus Lacordaire, 1857 ([44]). Of historical data Schlosser Klekovski ([45]) reported both, hermit and the European red click beetles but without specific date or location of the findings, only adding Croatia as a whole and Slavonian part of the country. Schimmel & Tarnawski ([44]) in the monography of the subtribe Elaterina reported several findings of E. ferrugineus in Croatia belonging to G. Platia collection, mainly from the Mediterranean biogeographical region (Kvarner region: Karlobag, Krk and Cres Islands - Tab. 2, Tab. S1 in Supplementary material). Doubtful is the record of Strbac ([41]). In his dissertation about Elateridae and Carabidae in arable land of Slavonija and Baranja regions he mentioned only L. ferrugineus larva near Beli Manastir in Baranja region (5 individuals, in autumn 1981). Since the finding occurred in agricultural field this could be misidentification, but also the species could have been present if old hollow trees had been preserved at the arable field to make the shadow for workers, as was common practice in the past with less intensive mechanisation in the field. Although listed in recent literature, the presence of E. ferrugineus in the north-east part of Croatia in Kopački rit Nature Park has not been added in mapping analyses neither as recent nor historical data, due to the fact that it was not possible to trace back the original reference based on which this occurrence was included in list compiled by Krčmar ([23]).

In Hensch Collection (Faculty of Agriculture, University of Zagreb) few specimens were collected in Gorica, but since there are few localities in Croatia and as well in Slovenia and in NE Italy, with this same name, Gorica as location has not been added on the map. Beside hermit species, Franjo Košćec Collection of Varaždin City Museum contains specimens of the European red click beetle (Linnaeus, 1758) collected outside of Croatia, in St. Paul in Carniolia. Other collections such as Karaman’s collection (Museum in Split), then Mikšić, Novak and Weingartner collections (in NHM in Zagreb) and collections in the Hungarian Natural History Museum in Budapest, according to museum curators and entomologists do not contain E. ferrugineus exemplars.

In our field studies, E. ferrugineus was lured mainly by pheromones of hermit beetle, though it was not a target species. The same pheromones and flight intercept traps were used in study of Batrnek et al. ([5]) in Osijek Town Park where seven individuals of E. ferrugineus were caught (Tab. 2). On the other hand, catch of E. ferrugineus at Mt. Učka using tree traps baited with wine flavoured with vanilla was most probably accidental, although P. Rapuzzi collected one specimen in Kvarner region, using wine-traps exposed in period June 8^th until July 8^th in 2008 (G. Platia, personal communication). However, we suggest pheromone traps for further research on the distribution of the species.

Male-produced sex pheromone of hermit beetle showed very good results as kairomone for females of E. ferrugineus, as well as female-produced sex pheromone of E. ferrugineus to attract conspecific males ([4], [25], [30], [50]).

Adults of hermit and the European red click beetle are active from the end of June to the end of August ([37], [4]) therefore observations in Mt. Učka and Mt. Papuk might represent the beginning of seasonal activity of adult beetles, since both surveys ended at the beginning of July, while the project at the Plitvice Lakes area started at the end of June and finished at the beginning of August. Although O. barnabita has been recorded at Papuk Nature Park, E. ferrugineus was not detected. The reason could be that male-produced sex pheromone of hermit beetles used in the research attracts both sexes of hermit beetle but only females of E. ferrugineus who emerge later in the season. Another possible explanation relies on the fact that E. ferrugineus females react to kairomone only after the mating ([50]) and possibly by that time the field sampling ended.

Habitat suitability map potential use and constraints

Since the purpose of this survey was primarily to establish baseline data of the occurrences of hermit beetles and subsequent monitoring activities at Natura 2000 sites, the data collected had some limitations as to the lack of unbiased random sampling and sample size, which are both quite frequent with data collected in the field, especially on rare and/or endangered species. However, analyses of the data collected, for O. barnabita and E. ferrugineus, provided interesting results. Osmoderma barnabita was the target species for this survey, and E. ferrugineus has been observed as a “bonus”. This resulted in a much larger number of input data for O. barnabita (19 localities) than for E. ferrugineus (12 localities). Interestingly, when using historical data, the hermit beetle again had much more data (33 localities) than the European red click beetle (8 localities). Contrary to a number of current data on its occurrence, the MaxEnt models resulted in much larger areas of habitats predicted for E. ferrugineus than for O. barnabita. Even testing with the historical data, resulted in that majority of historical hermit beetle data spatially coincide with current MaxEnt European red click beetle model. Having in mind the biology of both species, where E. ferrugineus can predate on other species besides the hermit beetle, could support obtained discrepancies, as E. ferrugineus with smaller input data set resulted in larger predicted suitable areas than those of the hermit beetle. Smaller sample size could affect predicted models in sense of larger suitable areas, but MaxEnt is rather robust in comparison to other algorithms ([49]). Oleksa et al. ([33], [34]) analyzed dispersal abilities of Osmoderma barnabita and Protaetia marmorata marmorata (Fabricius, 1792) (Cetoniinae), both connected to tree hollows, and their larval predator, E. ferrugineus. Their results imply that dispersal distance of more specialized species is more restricted than those of generalists. Dispersal differences could support our findings that habitats suitable for hermit beetle may also host the red click beetle, but all habitats occupied by the red click beetle may not fit for the hermit beetle.

Environmental response curves (Fig. 5) of both species are adherent with such trends, indicating that E. ferrugineus has much wider environmental envelopes in which it can occur. The latter could explain obtained results when comparing historical records with that of MaxEnt models obtained based on current data. Surrogacy is a well-known concept used in various ways ([43], [29]), and it seems that it can be used here, at least for O. barnabita as a surrogate for E. ferrugineus. Spatial overlap of predicted suitable habitats for two beetles conforms in part to the protected areas in Croatia (Fig. 6). In the National Parks commercial logging is not admitted, while in the Nature Parks (less strict category of protection) numerous conservation measures have been prescribed, e.g., leaving a certain amount of dead wood per hectare (which is a necessary habitat for the saproxylic beetles - [27], [31]). All these are pointing that MaxEnt habitat suitability models spatially coincide with areas in which management practice should promote sustainability of the microhabitats suitable for the saproxylic beetles. Furthermore, hermit beetle is “not an easy to record” species, which can use even non-woodland habitats ([12]). Hence, knowledge of its populations, in particular in forest stands, may provide information for appropriate management of those forests that should ensure hermit beetle survival.

  Conclusions 

The distribution models predicted significantly larger area potentially suitable to E. ferrugineus than to O. barnabita. This may imply that all habitats suitable for O. barnabita are also suitable for E. ferrugineus, but only part of habitats that fit to E. ferrugineus may host O. barnabita. Based on this, it seems that O. barnabita can be used as surrogate species for E. ferrugineus. Due to the fact that the pheromone-baited traps designed for the hermit beetle were used for the first time in Croatia, further observations can be expected by the application of pheromones directed to both species in future surveys within the EU Habitat Directive actions, as was the case in the other EU Member States. Given the substantial lack of systematic field studies of both species in this part of Europe, the data presented can contribute in directing further studies and extending general knowledge of the distribution and habitat preferences of these species.

  List of abbreviations 

The following abbreviations have been used throughout the paper:

1. a.s.l. - above sea level;
2. leg - legit;
3. coll - collection;

  Acknowledgments 

We would like to thank to: Croatian Ecological Society for their logistic support, colleagues A. Smoković, B. Špadina and M. Knjaz, M. Doboš , I. Špoljarić, K. Culinović and rangers from Plitvice Lakes National Park and Papuk Nature Park for their help during the field sampling; G. Platia for providing data from his collection and valuable suggestions on species distribution; S. Bowden (Oak-fortress Proofreading International) for the linguistic revision of the manuscript, G. Nardi and two reviewers whose comments and suggestions substantially improved the manuscript.

This project was financed by Croatian Agency of Environment and Nature (HAOP), Papuk NP and Plitvice Lakes NP.

Permission for performing field work was obtained from Nature and National Parks authorities and Croatian Ministry of environment and nature protection.

  Authors’ contribution 

LSJ, IR, FK and AP carried out the field measurements, SDJ performed the statistical analysis and habitat suitability models, LSJ conceived the study and together with coauthors wrote the manuscript.

  References

  Supplementary Material

Tab S1 - Historical records of hermit and the European red click beetles in Croatia dating before 2002, based on Croatian museums and literature data, added on the map in 10×10 km grid and overlapped with models using recent findings listed in Tab. 1.