Our results indicate a mainly positive impact of the management of traditional fruit orchards on the studied insect taxa. We also found that orchards planted in xeric environments tended to have higher species richness.
Traditional fruit orchards might be depicted as grasslands with scattered trees (Plieninger et al. 2015) or forests with very sparse canopies (Horak 2014a). This depends on the density of planted trees and the stamina of the fruit tree species (Janeček et al. 2019). Fruit orchards provide suitable conditions for species associated with open landscapes that were formerly pastured (Mosquera-Losada et al. 2012). The abandonment of pastured grasslands and forests, from a long-term perspective, usually leads to a decline in biological diversity (Benes et al. 2006; Queiroz et al. 2014; Uchida and Ushimaru 2014). Therefore, from this perspective, it is not surprising that any type of orchard management has a positive effect. On the other hand, intensification of mowing regimes is also decreasing biodiversity (Uchida and Ushimaru 2014). Forest management has changed to more intensive approaches using high forests with dense canopies (Benes et al. 2006). Thus, orchards might serve as a habitat for many species that originally lived in traditionally hayed meadows (Cizek et al. 2012) and those associated with open forests (Horak 2014a).
Managed orchard grasslands with xerothermic biotopes can serve as refugia for many species (Čejka et al. 2018; Šantrůčková et al. 2020). The area of xerothermic grasslands is declining due to land abandonment, afforestation, building activities or even conversion due to atmospheric nitrogen input or fertilization (Tscharntke et al. 2005; Janišová et al. 2011). This has negative consequences for their biodiversity (Buscardo et al. 2008; Janišová et al. 2011). In addition, orchards appear to be very similar to dry traditionally managed woodlands (Hédl et al. 2010), where canopy openness is much higher than in managed forests. This is mainly caused by competition among trees and was even multiplied by wooded pasture (Konvicka et al. 2008) or coppicing in the past (Altman et al. 2013). These forests, typically with xerothermic vegetation, have recently declined (Hédl et al. 2010). Traditional fruit orchards might supplement these natural habitats and serve as habitats for species that originally lived in natural and seminatural habitats. This may lead to higher biodiversity in xerothermic orchards. In addition, such orchards can also serve as transitional habitats that facilitate migration between natural habitats (Steffan-Dewenter 2003).
Butterflies
The species richness of butterflies in our study was most promoted by the management of orchards. Furthermore, dry orchards were more species-rich than mesic orchards.
Abandoned orchards are often akin to shrubland or forests having undergone natural succession for some years after clear cutting (Prach 1994; Balmer and Erhardt 2000). The remaining grafted fruit trees are overgrown, and their crowns are withering due to competition (Horák et al. 2018). The majority of day-active butterflies in central Europe prefer open or at least semiopen habitats (Beneš et al. 2002). There are known examples of gradual declines in butterfly fauna with increasing densities of trees and shrubs (Erhardt 1985; Fartmann et al. 2013). Young tree forests are especially species poor (Balmer and Erhardt 2000). This is probably one of the most important reasons for the preference of butterflies for actively managed sites. This was also confirmed by the preferences of open landscape specialists for managed orchards. Succession toward forests was not profitable even for forest-related species. The main reason is that forest-associated species mainly prefer sparse forest canopies (Beneš et al. 2002, 2006).
We predicted that moderate management would offer more heterogeneous biotope conditions, which usually lead to higher species diversity (Noordijk et al. 2009; Cizek et al. 2012). More heterogeneously managed sites should have more nectar sources and provide a more diverse understory of vascular plants (Erhardt 1985; Steffan-Dewenter and Leschke 2003). In addition, research has shown that butterfly species richness mainly declines with intensive understory cultivation (Erhard 1985; Ekroos et al. 2010) and that butterflies prefer rather patchy mowing (Varah et al. 2013). Surprisingly, there was no significant difference in the effects of moderate and intensive management on the number of butterfly species. This nonsignificant difference can be explained by the complex 3D structure of managed orchards. This means that, even after mowing, this habitat type still offers flowers and shelter in tree crowns (Herzog 1998). In addition, some species can also benefit from tree sap, honeydew or rotting fruits (Shreeve 1984; Ômura and Honda 2003). Nevertheless, this observed issue remains relatively unclear – as abandoned orchards also often offer the abovementioned complementary resources. One of the possible reasons could be the temporal emigration of butterflies to neighboring habitats followed by regression when the condition of the intensively mowed orchard improves (Baum et al. 2004; Ouin et al. 2004). The most surprising finding was that red-listed species were not influenced by management. However, we observed one endangered species, two vulnerable species and eight near-threatened species, which illustrated the importance of orchards in a landscape context. These species might have such a strong biotope preference that variation in local management has little effect (Fattorini 2010).
Xeric orchards contained more butterfly species and of red-listed species. Surprisingly, forest specialists also preferred xeric orchards. Xeric biotopes, including orchards, are often less accessible than other biotopes. This is a possible reason why they have stayed relatively untouched by suburbanization (Ouředníček 2007). Therefore, xeric orchards and similar dry habitats can be suitable for many species – even those not specialized for dry habitats (Dostálek and Frantík 2008; Kadlec et al. 2008). This is confirmed by the finding that some urban areas (such as railway verges or brownfields) could mimic natural xeric habitats (Konvicka and Kadlec 2011). These orchards are still rather marginal. However, they can facilitate connectivity or mimic natural xeric steppes. In addition, they might also serve as transitional habitats or stepping stones (Horak 2014a) in fragmented areas of cities (Horák 2016). This is probably the reason why orchards in xerothermic biotopes contained more species of both butterflies and red-listed butterflies.
Forest specialist species richness was higher in xerothermic biotopes than in mesic biotopes. Xerothermic orchards might imitate and substitute natural xeric forests due to their similarly high canopy openness (Chytrý 1997; Hédl et al. 2010). Such openness is needed for forest specialist butterflies to find suitable basking sites, while forest cover serves as an important food source and shelter (Shreeve 1984; Dennis and Sparks 2006). In addition, xerothermic forests are currently less common in central Europe because modern forestry practices shifted them to mesic forests (Hédl et al. 2010). Thus, butterflies originally associated with dry forests had to find supplementary habitats with similar living conditions, and traditional fruit orchards provided such conditions.
Surprisingly, the species composition of butterflies did not differ among management and biotope types. For example, we observed some forest-related species (e.g., Argynnis paphia and Celastrina argiolus), but they were present in all types of orchards. Nevertheless, these species were not as common as other species in the orchards, suggesting that orchards are simply transitional habitats for these species in the landscape context. On the other hand, transitional habitats might be as ecologically important as natural habitats (Schmitt and Rákosy 2007), underlining the importance of traditional fruit orchards for insect biodiversity.
Hymenopterans
Hymenopterans benefitted from management in orchards, and they were unaffected by the type of biotope. The only exceptions were open-landscape specialists, which preferred xerothermic sites. This might be because ground cover in xerothermic biotopes is often bare soil, which is ideal for nesting in many hymenopteran species (Heneberg et al. 2014; Fortel et al. 2014; Bogusch et al. 2020). In addition, active management such as mowing might multiply this effect since disturbed habitat is often preferred (Fortel et al. 2014). Orchards with old veteran fruit trees also host a high diversity of species that nest in wood cavities, especially smaller ones (Horák et al. 2013b; Bogusch and Horák 2018).
Many hymenopterans prefer habitats in early successional stages (Heneberg et al. 2013; Taki et al. 2013; Heneberg and Bogusch 2020). Such stages could be maintained in orchards by intensive management. A more heterogeneous understory is at a surplus in managed orchards since intermediate disturbance reduces the dominance of competitive species and increases plant species richness (Curry 1994). This offers more available food sources (Steffan-Dewenter and Leschke 2003). In this context, it is not surprising that we observed one critically endangered species (Andrena similis), three vulnerable species and four near-threatened species. Their incidence is another indication that traditional fruit orchards could provide suitable habitats for insect biodiversity.
Management practices were clearly preferred, and there were fewer species in abandoned orchards. Nevertheless, some of the species – for example, Myrmica rubra and Andrena carantonica – preferred abandonment. An explanation in the case of M. rubra might be that this species prefers nesting in leaf litter or within woody debris (Groden et al. 2005), which is more common in abandoned orchards. In the case of A. carantonica, the only reasonable explanation might be that this species does not require as high a temperature for pollination as the European honeybee (Apis mellifera) (Chansigaud 1975) and collects pollen and nectar, usually from tree flowers (Macek et al. 2010; Westrich 2018). Thus, it might use shrub overgrowth as an inhibitor of high temperatures in the understory (Breshears et al. 1998), providing a competition benefit.
Other species found in the abandoned orchards, such as Temnothorax crassispinus and Lasius fuliginosus, usually occurred in forest biotopes. Therefore, their preference was rather natural (Macek et al. 2010). Moderate management was preferred in the case of species such as Lasioglossum lativentre and Formica pratensis whose natural biotopes are forest steppes. Thus, moderately managed orchards with patchy mowing and the presence of old-growth trees might serve as artificial supplements. Nevertheless, these orchards were also preferred by habitat generalists such as Andrena polita and Bombus pascuorum (Macek et al. 2010), which might instead primarily benefit from the continuous presence of nectar (Croxton et al. 2002). Intensively managed orchards were inhabited by species such as Andrena ovatula, A. floricola and A. strohmella, which corresponded to their preference for open biotopes and floral specialization (Macek et al. 2010; Westrich 2018; Bogusch et al. 2020).
Beetles
Click beetles are known to prefer veteran trees exposed to sunlight (Horák and Rébl 2013). Therefore, managed orchards appeared to be ideal habitats. Nevertheless, beetles were indifferent to the management and type of habitat in our study. Species richness was higher in moderately managed orchards than in abandoned orchards, but this difference was not significant. From this trend, we might conclude that moderate management could have a positive impact on beetle diversity.
We trapped species displaying all types of biotope preferences, from a preference for crop fields (Athous haemorrhoidalis and Agrypnus murinus) to a preference for high forests (Melanotus villosus and Dalopius marginatus). This leads to the conclusion that traditional fruit orchards can host a very wide range of species that can benefit from meadow, trees or both within one habitat. This was also confirmed by the presence of red-listed species – we trapped one vulnerable (Brachygonus megerlei) and one near-threatened species (Ampedus rufipennis). Both of these species are saproxylic (Zaharia 2006; Brunet and Isacsson 2009). Thus, old trees present within habitats are essential for them, even in agricultural habitats.
Species composition differed among the studied orchard categories. Abandoned orchards were preferred by Agriotes ustulatus. This species is usually found in places with lower vegetation cover (Mertlik 2016), which appears to contradict our findings. However, this species also prefers soil with a higher humus level (Čačija et al. 2018), which is found mostly in abandoned orchards due to decaying plant residuals (Horák et al. 2018). Moderately managed orchards were inhabited by Ampedus glycereus and A. pomorum, which are mostly known from forests, and they clearly benefitted from the presence of old trees (Laibner 2000). Our studied sites were typical in terms of the presence of old fruit trees, which could mimic the forest environment for these species. Intensive management of orchards was preferred by Selatosomus gravidus. This species is usually present in steppe biotopes, and intensively managed orchards are probably also suitable (Laibner 2000), as in the case of some hymenopterans.
Our results were in some cases opposite to the findings of current research regarding species composition between biotopes. In xerothermic biotopes, the species composition included Agriotes pilosellus, which is supposed to prefer mesic biotopes, and Melanotus villosus, which is usually described as a generalist (Laibner 2000). Nevertheless, in mesic biotopes, we trapped species such as Selatosomus gravidus, usually mentioned as having a preference for xerothermic biotopes, and Agrypnus murinus, which is thought to be a generalist or even a pest species (Laibner 2000). We can provide context for this information with the finding that Selatosomus gravidus could also be found at mesic sites (Bulgakova and Pyatina 2019). Moreover, this species prevailed in intensive orchards, where intensive mowing can cause even mesic biotopes to be drier than in the case of using less intensive practices (Kobayashi et al. 1997). Thus, all of the abovementioned reasons might explain this preference for mesic sites.