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3 June 2024

Changes in Population Densities and Species Richness of Pollinators in the Carpathian Basin during the Last 50 Years (Hymenoptera, Diptera, Lepidoptera)

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1
Rippl-Rónai Museum, Fő út 10, H-7400 Kaposvár, Hungary
2
Institute of Zoology, Slovak Academy of Sciences, Dúbravská cesta 9, SK-84506 Bratislava, Slovakia
3
Koppert s.r.o., Komárňanská cesta 13, SK-94001 Nové Zámky, Slovakia
4
Bakony Museum of Hungarian Natural History Museum, Rákóczi tér 1, H-8420 Zirc, Hungary
Diversity2024, 16(6), 328;https://doi.org/10.3390/d16060328
This article belongs to the Special Issue Emerging Effects of Pollinator Loss on Biodiversity
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Abstract

Temporal changes in population densities and species richness of three main pollinator groups—moths and butterflies (Lepidoptera); bees, wasps and sawflies (Hymenoptera); and hoverflies, horseflies, tachinids and bee flies (Diptera)—were investigated in the Carpathian Basin. Maintaining pollinator diversity is a crucial factor for preserving our biodiversity and ecosystems; furthermore, several pollinator species have a strong economic role in maintaining crop and fruit cultures. Our conclusions are based on our three and four decades of faunistic surveys in various regions of the Carpathian Basin. Analyzing and comparing our data with the historical data of the last 50 years, we concluded that densities of some pollinators declined during the past decade and a half (Symphyta, hoverflies), although populations of several species of Mediterranean origin grew (Aculeata) and new species even migrated from the warmer regions. In numerous cases, this decrease was dramatic: more than 90% decline of certain butterfly species were detected. On the other hand, the composition of pollinator fauna significantly changed due to the disappearance of some mountainous or mesophile species. The main reason for the decrease in pollinator communities is due partly to climatic change and partly to anthropogenic factors. Different groups of pollinators react differently: some groups like Syrphidae, Tachinidae, most of the butterfly families and bumblebees suffered a strong decline in the last two decades; other warm-loving groups like most of Aculeata and horseflies and bee flies showed a significant increase in population densities. Our conclusion: in our region, the pollinator crisis is present but moderate; however, there is a clear sign of the gradual transition of our pollinator fauna towards the Mediterranean type.

1. Introduction

“The apple trees were coming into bloom but no bees droned among the blossoms, so there was no pollination and there would be no fruit. The roadsides, once so attractive, were now lined with browned and withered vegetation as though swept by fire. These, too, were silent, deserted by all living things. Even the streams were now lifeless”. (Rachel Carson: Silent Spring).
In 1962, Carson predicted the future silent spring, when neither the noise of bumblebees nor the song of birds would disturb the peaceful growth of green vegetation. The very beginnings of social environmental movements are linked to this iconic work. Today, after 62 years, we have reached a point where the nightmare of a silent spring is within reach. The gradual disappearance of pollinators threatens to become an ecological catastrophe, since pollinators play a fundamental role in maintaining our ecosystem. The reproduction of the vast majority of cultivated and wild plants depends on the pollination activity of various animals (mainly insects). In recent years, we have seen a significant decline in various pollinator groups. Although the causes of this decline are not yet fully understood, the consequences are commonly referred to as a pollination crisis [1]. This pollination crisis goes far beyond the reproductive biology of plants, as Rhodes [2] writes: “The decline in the health and abundance of pollinators may significantly threaten the integrity of global biodiversity, the inclusion of food webs, and risks to the health of humans and other animals”. Ultimately, the pollination crisis may cause a global food crisis and a social crisis at the same time. It draws attention to the vulnerability of food chains. Here in our region, we can see this, particularly in the decline of several songbirds (e.g., swallows), since pollinators play a role not only in pollination but also as an essential food source for many bird species through their biomass [3]. According to the IPBES report [4], pollinator-dependent plants contribute to 35 percent of global crop production. According to Dicks et al. [5], the pollination crisis primarily affects the Global South. The risk of pollination crisis in our region is moderate, but at the same time, the decline in diversity among certain groups seems to be truly tragic, as our results show. Of the eight reasons analyzed, they cited changes in land use, climate change, pollution and the spread of invasive species. Other scientists [6,7] also emphasize habitat fragmentation and degradation, as well as the excessive and inappropriate use of pesticides and herbicides, as the main causes of the pollinator crisis. All other reasons are listed as “other factors”.
The 2021 EU directive on the protection of pollinators, entitled “Protecting Pollinators in the EU” [8], is a response to the current situation, which includes the development of action plans and monitoring programs, the provision of resources and the extension of the ban on certain pesticides such as imidacloprid and all neonicotinoid-based pesticides containing clothianidin and thiamethoxam.
In our work, we monitor the changes in population densities and species richness of different groups of wild pollinators in Hymenoptera, Diptera and Lepidoptera during the last half-century. We also study pollinators at the species level. In this study, only those species were selected that were once common or widespread but now are less common or are sporadic, or conversely, those species that are winners of the changes and whose populations are increasing due to recent climatic circumstances. We also compiled a list of sporadic species from several groups that we had not been able to collect in the last 20 years, even though they were not at all rare earlier. We also included species whose populations have increased in recent decades. These newly spreading species may have arisen due to the expansion of their range (as a result of global warming) or introduced by humans from distant regions of the Earth. During the circa-50-year study period between 1970 and 2022, the climatic conditions changed significantly in our region as well as everywhere in the world. A gradual Mediterranean transformation is taking place in the Carpathian Basin continuously. The first table (Table 1) and the first chart (Figure 1) show the changes in local climatic conditions (days with heat wave: number of days when the mean daily temperature reaches or exceeds 25 °C for at least 3 days).
Table 1. Climatic parameters in the Pannon biogeographic region in the first and last 5 years of the investigated period (source: Hungarian Central Statistical Office, Budapest).
Figure 1. National average annual temperatures between 1970 and 2023 in Hungary with a trend line (based on the data supply of the Hungarian Meteorological Service).

2. Materials and Methods

Our results are based on regular faunistic surveys carried out over the past 50 years in diverse areas of the Carpathian Basin (Pannonian Basin). This kind of tradition of faunistic investigation can be documented dating back to the time of Scopoli (“Observationes Zoologicae”). This time, we compare quantitative and qualitative faunistic data from the 1970s and 1980s with our own recent and subrecent data from the last 2–4 decades. Statistical analysis of these data determines various trends. These trends either confirm or reject our initial hypothesis regarding the pollination crisis, and as a final result, they allow us to figure out the direction of pollinator changes and point out those pollinator groups and species that suffer from the recent changes or benefit from them. Finally, we refer to some drivers of the experienced changes. Climatic change is discussed in detail; however, there are several other drivers. These and their detailed analysis for each group and their strengths are the subject of a separate paper.

2.1. Data Selection

Our present work has two main sources. The first comprises unpublished databases from diverse areas of the Carpathian Basin spanning 4 or 5 decades. Voucher specimens are deposited in various natural science collections in the region (Zoological Institute at Bratislava, Rippl-Rónai Museum at Kaposvár, Natural History Museums at Zirc and Budapest). The second source comprises published faunistic papers.
The majority of our data are original, the results of 40 to 50 years of continuous collection covering the Pannonian biogeographical region. These databases contain circa 100,000 Aculeata data, 45,000 Diptera data and 40,000 Lepidoptera data. These databases, wherever possible (Hymenoptera, Diptera), are the results of the work performed by one specialist per group over several decades (Dr. Sándor Tóth, Zsolt Józan, Dr. Ladislav Roller and Dr. Attila Haris), consistently using the same methods. The same person and the same applied method ensure the reliability and consistency of our data and analysis through the decades. To supplement these databases, we used the following publications, predominantly from the previous authors: Symphyta: Zombori [9,10,11,12,13], Haris [14,15,16,17,18,19,20,21,22,23], Haris et al. [24] and Roller [25,26]; Aculeata: Józan [27,28,29,30,31,32,33,34,35,36,37,38,39,40]; Diptera: Tóth [41,42,43,44]; Lepidoptera: Ábrahám [45,46,47,48], Ábrahám et al. [49], Ábrahám and Uherkovich [50], Uherkovich [51], Pillich [52], Sáfián [53], Ács et al. [54], Čanády [55], Dietzel [56], Sarvašová [57], Gergely [58], Gór [59], Schmidt [60], Németh [61], Szabóky et al., 2014 [62], Hudák [63], Varga et al. [64], Árnyas et al. [65] and Kovács [66].
We selected those data only (in Hymenoptera and Diptera) that were results of comparable and regular field recordings, 30–35 field days per year, and we omitted results of nonregular collections and scattered data, as these data cannot be processed statistically.
The analyzed species were selected according to the following criteria: those species that have an important role in pollination (i.e., rare species were excluded), and those species whose change proved to be the strongest. In the light-trap data series, we checked the beginning and end of the time series to see which species occurred in the largest number in the beginning and at the end of the studied 50-year period from 1970 to 2022. After this, we analyzed those species that provided the most significant changes over the past 50 years. Since the scope of our work does not allow the analysis of circa 2500 Hymenoptera, Diptera and Lepidoptera species, and due to a lack of space and opportunity, we restricted it to 15–40 species from each group, especially those species where the changes were outstanding, or those that we found characteristic of the total group or those that play an important role in pollination due to their population densities.

2.2. Changes in Methods and Their Statistical Balancing

In nocturnal lepidoptera, methodological changes had to be taken into account. Since 2014, a UV LED light trap has been used, which replaced the black light UV 20 W (between 1990 and 2010) and the Jermy-type light trap with a 125 W mercury vapor lamp (used between 1970 and 2010). These different light sources had different selectivity, according to Infusino et al. and Pan et al. [67,68]. This is the reason that we calculated trends from 1970 and a separate trend from 2014 to draw reliable conclusions. From light-trap data, we selected only those data series which span 6 months of intensive, daily collections, generally from April to October and, in a few cases, from May to November. Light traps worked continuously during these periods. Other shorter or incomplete light-trap data were excluded from the analysis. The number of sampling points was 24. Several of them (like Dráva Plain or Zselic Hills) were repeated after 40 years.
Regarding diurnal lepidoptera, we have minimal original data. Regular butterfly monitoring started only in the last decade in our region. Therefore, we have to rely on the processing of previously published datasets. We analyzed the earlier commonest pollinator species (but not all) where, according to our field experiences, we noticed the greatest changes.
In the absence of reliable quantitative data, we used the method followed by Ábrahám [47], as follows: 0: the species is not present or has disappeared; 1: rare; 2: sporadic; 3: occasionally frequent; 4: generally frequent; 5: common. Other methods, finally, had to be rejected: the database of the European Butterfly Monitoring Scheme (eBMS) contains different amounts of observations from various years. For example, we had approximately 600 observations from Transdanubia in 2018 and 2280 from the same region in 2023. We also attempted to compare the number of diurnal butterflies collected during an average collecting day. These data have a high standard deviation without showing any trend, e.g., Csombárd 2015 [60]: 330 individuals per day on average; Zvolen between 2009 and 2011: 143 individuals per day on average [57]; Tapolca between 1977 and 1990: approximately 28 individuals per field day [61]. In other words, these butterfly surveys proved to be completely useless. In this way, the first-mentioned approach, with its many subjective elements, remained the only possibility for us.

2.3. Data Processing

For Hymenoptera and Diptera, we formed our data into 5- or 6-year batches (providing even time intervals). After plotting these data groups on a bar graph, we added trend lines, including the trend equation and coefficient of determination (r2 value). For nocturnal Lepidoptera, instead of creating batches, we analyzed separate data series for each location (5000–15,000 specimens per location per year) throughout the various ecosystems of the Carpathian Basin. These data series date back 50 years, except for the 1990s (a period of change in the political system in this region), when the light-trap network was suspended and we have a 10-year gap this time. After arranging these data series in chronological order and creating the bar charts, trend lines were drawn (mostly linear trends) and the trend parameters were recorded. These parameters are the slope of the trend (linear x coefficient) and the plus or minus value of linear x coefficient indicates the direction of the trend (minus: decrease; plus: increase), and its value indicates the slope of the trend line, i.e., the intensity of change (for some Syrphidae species, this trend is exponential). The coefficient of determination indicates how strong the trend is and how our data fit to the trend line, i.e., in cases of a high proportion of continuous temporal changes (real trend: high r2 value), and a low r2 value when spatial or cyclical variables have a main role in the changes in populations or species richness.

2.4. Sampling Sites

Diptera and Hymenoptera (except Symphyta) were collected from approximately 1200 various locations in all parts of the Pannonian biogeographic region. These are smaller samples (1–50 specimens) from each location.
Lepidoptera and Symphyta specimens are from separate and significantly lower numbers of locations (26 locations were sampled in different years in Lepidoptera for instance), as listed below. These are larger samples, between 500 and 30,000 specimens from each location (Symphyta samples are smaller, between 500 and 4000 per location per year; Lepidoptera samples are larger, 5000 and 30,000 specimens per location per year).
Collecting sites for Symphyta were as follows: Nagykovácsi and its surroundings, Aggtelek National Park, Fertő-Hanság National Park, Zselic Hills, Keszthelyi Hills, Cserhát Hills, Vértes Hills, Southern Transdanubia at River Drava, North Somogy, Börzsöny Mountains, Szeged and its surroundings, South Somogy, Ivanka pri Dunaji, Javorina, Mošovce, Pernek, Devin, Hriňová, Stefanova, Horša, Bokroš, Tvrdošovce, Virt and Malacky regions. Applied methods were sweeping net and Malaise trap.
Collecting sites for Lepidoptera were as follows: Felsőtárkány, Tompa, Gilvánfa, Magyarszombat, Mike, Vásárosbéc, Almamallék, Palé, Lipótfa, Aggtelek, Tompa, Répáshuta, Sopron, Plain of River Dráva, Bakonynána, Boronka, Tapolca, Keszthelyi Hills, Aggtelek National Park, Biatorbágy, Pomáz, Székesfehérvár, Őrség, Csombárd, Sliač in Zvolenská kotlina and surroundings of Košice (Figure 2). Applied methods were light trap and sweeping net.
Figure 2. Sampling points of lepidoptera between 1970 and 2022.

3. Results

3.1. Symphyta

Of the 797 species living in the Carpathian Basin, 376 are rare, 277 are sporadic and 144 are frequent or common. The last three categories (sporadic, frequent and common) can be considered important pollinators. Similar to Aculeata species, we can observe an increase in species that are predominant in the Mediterranean region (Table 2 and Table 3). On the other hand, we found a significant decrease in most sawfly species, which is clearly evident from the Malaise trap data (Table 3 and Figure 3). In two countries, we measured changes in individual numbers and species richness over five and three decades, respectively, using two different methods. We have Malaise trap data from Slovakia and sweeping net data from Hungary, which are shown in Table 2 and Table 3 and in Figure 3. Although there are differences between the two areas and the two methods, the similarities are as follows: an increase in Mediterranean sawfly species and a gradual decline in the number of individuals of most sawfly species. Species that are tolerant to climatic changes are Tenthredo distinguenda (Stein, 1885), Arge nigripes (Retzius, 1783), Arge ochropus (Gmelin, 1790) and Arge cyanocrocea (Forster, 1771). These species mainly dominate the fauna of the Mediterranean region and Anatolia. The strongest decline was observed in moisture-loving species such as Tenthredo mesomela Linné, 1758. However, as suborder Symphyta reaches its maximum species richness and abundance in northern Europe, the negative trends appear realistic and are indications of the Mediterranean transformation of our pollinator fauna. As a matter of sporadic species (regularly collected but in low numbers of one to three specimens per year), we listed in Table 4 those species that have not been collected in the Carpathian Basin in the last 20 years. These species were not rare in the middle of the 20th century (until the 1970s), but despite our efforts, we have not succeeded in collecting them in the area of the Pannonian biogeographical region. It seems, these species have not disappeared, but their populations have fallen below the detection limit. Of the 41 species listed, 9 were not found in the entire Carpathian Basin, while 32 species were found in the high-altitude regions (alpine and subalpine) of Slovakia. The list shows that Tenthredo species and several Nematinae species (genera Pristiphora and Pteronidea) have become rare. The decline of the northern Nematinae subfamily is explained by global warming. The reasons for the rarity and disappearance of the Tenthredo species are still unclear. As for the Tenthredo species, similar observations were made by Goulet in Canada [69]. We can say that the sporadic species listed in Table 4 are now rare. So far, we have not detected any species spreading from south to north that have appeared in recent decades, although the occurrence of some southern species, such as Macrophya superba Tischbein, 1852, can be expected in the near future. In addition, the population densities of Tenthredo bifasciata ssp. bifasciata O. F. Müller, 1776, and Tenthredo costata Klug, 1817, are expected to increase. M. superba reaches its distribution area at the very southern border of the Carpathian Basin [70]. T. costata has the northernmost limit of its distribution range in our region; this is the reason that its populations are subject to fluctuation here. Several specimens of T. costata were captured in the 1970s, and after a long disappearance, they were captured again two years ago in Nagybajom, Southern Transdanubia. T. bifasciata ssp. bifasciata was recently recorded in Slovakia [70]. It reaches its highest density in the Anatolian region [71]; we can expect its increase in our region as well. In the fauna of the Carpathian Basin, an invasive species, Aproceros leucopoda Takeuchi, 1939, appeared in the early 2000s. Overall, based on our Malaise-trap data series, we observed a strong decline in sawflies over the last three decades, which is not surprising if one knows the ecological needs of this group (Table 3 and Figure 3).
Table 2. Number of individuals of sawflies between 1971 and 2022, net-sweeping method.
Table 3. Population densities of sawflies between 1991 and 2021, Malaise trap.
Figure 3. Changes in sawfly populations between 1991 and 2021 with a trend line and equation (based on Table 2).
Table 4. Earlier sporadic sawfly species not recorded in the Pannonian biogeographic region in the last 20 years.

3.2. Aculeata

Population densities of most Aculeata groups have intensively increased in the last 40 years (Figure 4). In Table 5, we listed those species that have changed the most in number of individuals. In wild bees, the most intense increase was observed at Nomiapis diversipes (Latreille, 1806), Nomada distinguenda Morawitz, 1874, and Halictus sexcinctus (Fabricius, 1775) (Table 6). Since the main distribution area of these species is in the Mediterranean biogeographic region, they are excellent indicators of global warming. In other groups of Aculeata, Priocnemis perturbator (Harris, 1780), Scolia hirta (Schrank, 1781) and Ancistrocerus gazella (Panzer, 1798) also produced outstanding growth (Table 6).
Figure 4. Changes in Aculeata populations between 1988 and 2023 with a trend line and equation (based on Table 5).
Table 5. Number of individuals of various Aculeata genera and families collected between 1988 and 2023.
Table 6. Number of individuals of various Aculeata species between 1988 and 2023.
In terms of proportions, the average population increase in these species compared to the first half of the 1980s is between 1.6 and 3.8×. In the Carpathian Basin, Nomiapis diversipes (Latreille, 1806), Nomada distinguenda Morawitz, 1874, Coelioxys conoidea (Illiger, 1806), Andrena symphyti Schmiedeknecht, 1883, Lasioglossum villosulum (Kirby, 1802), Melitta nigricans Alfken, 1905 and Stelis breviuscula (Nylander, 1848) were rare until the 1980s, but now they are sporadic or even frequent species.
In contrast, certain moisture-loving or mountainous species became particularly rare, or their populations decreased from frequent to sporadic, such as Anthophora plumipes (Pallas, 1772), Ceratina cyanea (Kirby, 1802), Dasypoda hirtipes (Fabricius, 1793) and Andrena limata Smith, 1853.
The decline in bumblebees is discussed separately in the next entry. In other nonbee groups of Aculeata (Crabronidae, Philanthidae, Scoliidae, Pompilidae, Chrysididae, etc.), this increase was between 1.5 and 3.4×. Some species of Crabronidae associated with the wet conditions of marshy meadows, such as Ectemnius continuus (Fabricius, 1804) or saprophytic Hymenoptera, which are associated with old trees and forests, like Xylocopa valga Gerstaecker 1872 or Crossocerus elongatulus (Vander Linden, 1829), became significantly rarer.
At a generic level, we have obviously experienced similar changes. The most striking increase (3–4×) was observed in the genera Priocnemis, Oxybelus, Gorytes, Cerceris, Sceliphron and Megascolia (Table 5). The decline in the moisture-loving and saproxylic species was observed only at the species level.
According to our observations, certain xerotolerant Mediterranean species are gradually spreading north. These species are Cerceris rubida (Jurine, 1807) (Philantidae), Chrysis taczanovskii Radoszkowski, 1876 (Chrysididae), Megascolia maculata (Drury, 1773) (Scoliidae), Colletes hederae Schmidt & Westrich, 1993 (Colletidae), Nomiapis bispinosa (Brullé, 1832) (Halictidae), Pasites maculatus Jurine, 1807, Scolia galbula (Pallas, 1771) and Scolia hirta (Schrank 1781) (Scoliidae) (Table 6). Additionally Scolia galbula (Pallas, 1771) was recently discovered in Slovakia. Until now, the northern border of its distribution was Hungary, inside the Carpathian Basin. Furthermore, two species with originally Mediterranean distribution, namely, Lasioglossum griseolum (Morawitz 1872) and Heriades rubicola Pérez, 1890, were recently captured in Slovakia [72,73,74] in 2014 and 2009.
Non-native species also enrich our pollinator fauna. This enrichment occurs in two ways: one is the expansion of the area of certain species due to climatic change, and the other is the introduction of species from distant regions by human activity. The following species reached the Carpathian Basin from the Mediterranean region (Table 7): Sceliphron madraspatanum (Fabricius, 1781) (Sphecidae), Diodontus brevilabris Beaumont, 1967 (Pemphedronidae) and Chelostoma styriacum M.Schwarz & Gusenleitner, 1999 (Megachilidae). On the other hand, Sceliphron curvatum (F. Smith, 1870), Sceliphron caementarium (Drury, 1773), Isodontia mexicana (Saussure, 1867) (Sphecidae), Megachile sculpturalis Smith, 1853 (Megachilidae) and Vespa velutina (Lepeletier, 1836) (Vespidae) were introduced from distant regions of the Earth (Table 7). Among these, Sceliphron curvatum (F. Smith, 1870), Sceliphron caementarium (Drury, 1773) and Isodontia mexicana (Saussure, 1867) were found to be frequent (Table 7). We managed to collect only four specimens of Megachile sculpturalis near Harta (Bács-Kiskun County). These invasive species expanded quickly in the southern and moderately quickly in the northern areas of the Pannonian biogeographic region (comparing Hungarian and Slovak data in Table 7). As for the rest, only the voucher specimens were captured.
Table 7. Invasive and recently appearing expansive Aculeata species between 1988 and 2023 (Hungarian data above, Slovak data below).

3.3. Bumblebees (Bombus spp.)

Trends in population densities are strikingly different from the majority of other Aculeata species; this is the reason that we discuss separately the true bumblebees and also their social parasites, the cuckoo bumblebees (Table 8). Their trends are opposite to other aculeate species. Almost all bumblebee species have experienced significant decline. According to our data, only Bombus argillaceus (Scopoli, 1763) went through a moderate and Bombus haematurus Kriechbaumer, 1870, an intensive increase in populations. Numerous bumblebee species such as Bombus confusus Schenck, 1859, Bombus subterraneus (Linné, 1758) and Bombus pomorum fall below the detection limit in the Pannonian biogeographic region. Particularly interesting are those species that were common in the middle of the last century but showed a considerable decrease in frequency in the last 20 years: Bombus lapidarius (Linné, 1758), Bombus pascuorum (Scopoli, 1763), Bombus hortorum (Linné, 1761) and Bombus ruderarius (Mueller, 1776) suffered the strongest decline. Data from the longer time window (1980–2023) provide a more realistic and even optimistic picture about the above-named bumblebees (Table 8, sum of historical and more recent data). Here, the trend in overall abundance increase is observable. As a conclusion, we may say that the population of these common and widespread species increased.
Table 8. Number of individuals of various bumblebee and cuckoo bumblebee species between 1980 and 2023.
Cuckoo bumblebees were never frequent, but for now, they have become even rarer than they were before. They did not disappear (since from time to time they are observed by various entomologists and their observation is reported on the internet), but they fall below our detection limit (Table 8).

3.4. Diptera

3.4.1. Syrphidae (Hoverflies)

Being a moisture-loving group of insects, it is not surprising that their number shows a decreasing trend (Table 9 and Table 10). The surprising thing is the intensity of this trend. At the family level, the decrease compared to the beginning of the 1980s is about 80%. Some species, namely, Sphaerophoria scripta (Linné, 1758), Cheilosia variabilis (Panzer, 1798) and Syrphus torvus (Osten Sacken, 1875), suffered a drastic decrease in numbers (96–97%) (Table 9). We did not find any species in the family whose population density had been positively affected by the climatic conditions of recent decades. No invasive species were detected in this group. The northern expansion of a Mediterranean species, Chalcosyrphus pannonicus (Ooldengberg, 1916), was detected in Poland and Slovakia in 2010 and 2011: Poland, Carpathians, Lower Beskid, Magura NP, Żydowskie, 530 ma.s.l., 24 July 2011, one male; Slovakia, Carpathians, Lower Beskids, Ondavskie Foothills (Slov. Ondavská vrchovina) district. Chalcosyrphus pannonicus (Ooldenberg, 1916) is a rare species; so far, it has been caught in Croatia, Romania, Bulgaria, Greece and the Caucasus [75]. According to our experience, the strong decline in hoverflies is independent of their ecological type, whether the insect is eurythermal or mesophytic. This decline also affects all hoverfly groups independent of their lifestyle: if the species develops in water, compost, plant parts, fungi or even as an aphidophage or nest parasite, all of them have suffered a serious decline in their population density. The decline in some species is so strong that it can be modeled by an logarithmic or exponential trend (ln x coeff. value) instead of a linear trend (Table 9). Fifty earlier regularly collected sporadic species became so rare that they fell below the detection limit in the last two decades (Table 11).
Table 9. Number of individuals of various hoverfly species between 1980 and 2010.
Table 10. Changes in frequency of various Diptera families between 1980 and 2019.
Table 11. Earlier sporadic Diptera species not collected in the last 20 years.

3.4.2. Tabanidae (Horseflies)

Certain xerotolerant, warm-loving species, namely, Therioplectes gigas (Herbst, 1787), Chrysops caecutiens (Linné, 1758), Haematopota italica Meigen, 1804, and Tabanus bovinus Linné, 1758, increased remarkably; taking the early 1980s as a base, this increase is 3–5× (Table 12). We have so far caught neither Mediterranean newcomers nor invasive species. The decline in Haematopota pluvialis (Linné, 1758) moorland and silvicol species and Atylotus rusticus (Linné, 1761) mesophile species shades the overall picture (Table 12). At the family level, the number of individuals shows a decrease of about 0.6× compared to the beginning of the 1980s (Table 10).
Table 12. Number of individuals of various horsefly species between 1980 and 2014.
Only the Mediterranean Pangonius pyritosus (Loew, 1859) can be assumed to have been able to expand to the north due to climate change. In 1991, it appeared for the first time in the Carpathian Basin near Homorúd [76], but its population density has not expanded so far. Until this time, invasive species have not been detected either.

3.4.3. Bombyliidae (Bee Flies)

We experienced an intensive increase in population densities in familiy (Table 10) and at the species level as well (Table 13). Taking the beginning of the 1980s as the base period, this increase is about 80%. It makes this group one of the winners of climate change. The following species produced outstanding growth: Bombylius discolor Mikan, 1796; Conophorus virescens (Fabricius, 1787); Bombylius fimbriatus Meigen, 1820; Bombylius cinerascens Mikan, 1796; Villa hottentotta (Linné, 1758); Bombylius canescens Mikan, 1796; Bombylius fulvescens Meigen & Wiedemann, 1820; Bombylius major Linné, 1758; Anthrax anthrax (Schrank, 1781); Anthrax leucogaster Meigen & Wiedemann, 1820; Bombylius pictus Panzer, 1794; and Hemipenthes morio (Linné, 1758) (Table 13). The extreme 10–18× increase in density of Exoprosopa jacchus (Fabricius, 1805), Lomatia sabaea (Fabricius, 1781) and Bombylius medius Linné, 1758, is associated with the intensive expansion of these species. The reasons for this expansion are unknown, but it is likely their hosts are warm-loving insects (for instance, antlions [77,78] and other xerotolerant groups like Acrididae, Tenebrionidae, Aculeata, etc. [79]). We have not detected any invasive species so far.
Table 13. Number of individuals of various bee fly species between 1980 and 2020.

3.4.4. Tachinidae (Tachinids)

They suffered a significant decline similar to that of Syrphidae (Table 10 and Table 14). However, this decrease is not a strong trend; the average r2 value is around 0.2. For hoverflies, it is 0.5. Probably, thanks to their endoparasitoid way of life, they are less exposed to external influences than moisture-loving Syrphidae. No species has seen population density increase; however, some previously common species, such as Phasia pusilla Meigen, 1824, or Gymnosoma dolycoridis Dupuis, 1961, have suffered such a strong decline in populations that their numbers have fallen below the detection limit. We have not detected any invasive species until this time. Twenty-seven earlier regularly collected sporadic species became so rare that they fell below the detection limit in the last two decades (Table 14).
Table 14. Number of individuals of various Tachinid species between 1980 and 2014.

3.5. Lepidoptera

3.5.1. Nocturnal Macrolepidoptera

Considering the beginning of the 1970s as the base period, the population decline of moths is relatively strong (Figure 5 and Table 15) but with a low determination coefficient, which means the long-term, tendentious changes (for example, climate change and habitat changes) are only partly the cause of their decline; other cyclical, temporal variables and their spatial distribution pattern are also significant and strongly influence their populations. According to our observations and our available data, the declining trend lasted until the 2010s. After this time, the trend reversed, and we may consider a certain increase in the number of individuals (linear x coefficient from −157 up to 147, Table 15 and Table 16, moths total). The low r2 value (0.04) indicates some influence of different methods of light trapping (see Section 2). Taking a closer look at the various groups, owlet moths, sphinx moths and drepanids suffered the most drastic changes. At the family level, the trend is continuously declining in these groups. Nolidae and Notodontidae species show an increasing trend in the number of individuals during the last decade (only UV LED portable light traps were applied in this decade). In terms of species richness, a negative trend is experienced. In each year, light traps catch fewer and fewer species, until 2014, when this trend stopped (Table 17 and Figure 6). In Geometridae, we observed the strongest decline in species diversity. This decrease in species richness in this group has not stopped to this day (Table 17), while in other families, this trend has stopped and turned into a slight increase (Table 17). During this phase, there was no change in methodology. In the last decade, we noticed some changes in the order of the 10 most frequent species: the relative proportion of Mythimna turca, Athetis furvula and Mythimna pallens declined, and the proportion of Eilema lurideola and Colocasia coryli increased. For details, see Table 15. Our macrolepidoptera fauna is enriched by two imported species (Antheraea yamamai (Guérin-Méneville, 1861) and Tarachidia candefacta (Hübner, 1831)), one accidentally introduced species (Hyphantria cunea (Drury, 1773)) and one expansive species (Helicoverpa armigera (Hübner, 1808)). Hyphantria cunea (Drury, 1773) is very likely not a pollinator, and this species has a strong tendency toward gradation, so its occasional eruptions (such as in the 1970s) make it difficult to determine its population trend. Our data show a declining trend, which can easily be overwritten by a population eruption (gradation) at any time. The trend of the other three species shows a slight increase (Table 18).
Figure 5. Changes in moth populations between 1970 and 2023 with two trend lines: from 1970 and from the 2014 break point (based on Table 16).
Table 15. Changes in frequency of various moths in the last 50 years.
Table 16. Changes in frequency of various moth families in the last 50 years.
Table 17. Changes in species richness of various moth families in the last 50 years.
Figure 6. Changes in moth species richness between 1970 and 2023 with a trend line and equation (based on Table 17).
Table 18. Invasive, expansive and introduced moths species in the last 50 years.

3.5.2. Butterflies (Rhopalocera)

A significant decline in the diversity and species richness of butterflies was observed. Based on expert estimations (verbal communication of Levente Ábrahám), this decline in population densities is approximately 60%, but in some groups it is even up to 90% (compared to the 1970s). The estimated relative changes in populations of some important butterflies are displayed in Table 19. Certain species, especially Aglais urticae (Linné, 1758), were once among the most common species, but for now, they have almost completely disappeared. Species of Pieridae, Lycaenidae, Nymphalidae and Hesperiidae show a strong decrease either way. Species of the family Paplionidae seem to be stable. Population densities of some species tend in the opposite direction. The increase in Iphiclides podalirius (Linné, 1758) is the strongest. Our data in Table 19 indicate the expansion of Euphydryas aurinia Rottemburg, 1775, and Libythea celtis (Laicharting, 1782).
Table 19. Relative frequency of some butterfly species in the last years.
Recent reinvestigation of the 40 years before or even earlier in researched areas was carried out in three regions: Dráva Plain (border region between Hungary and Croatia), Bátorliget Nature Reserve (NE Hungary, close to Ukraine) and the area around Simonfa town. The decline in species richness in these regions shows strong change. Compare the data in Table 20.
Table 20. Changes in butterfly species richness in various regions after 40, 50 and 100 years.

4. Discussion

4.1. Symphyta

We have only a small amount of the literature on the changes in population densities of sawflies. In the Pannonian biogeographic region, the decline in the diversity of certain localities and the number of individuals of the Nematinae group is logical, as this subfamily typically reaches its maximum diversity and number of individuals in Scandinavia and the north [70]. The decline in Tenthredo species is somewhat incomprehensible. Goulet [69] found the same trend in Canada. Goulet attributes the decline in Tenthredo species to the use of pesticides. However, the spread of beekeeping is also having an unfavorable effect on this group [80].
To date, the negative effects of climate change on the Symphyta group have been studied in Andalusia [81]. Four species, namely, Megalodontes bucephalus, Macrophya militaris, Strongylogaster multifasciata and Dolerus puncticollis, disappeared from the Andalusia region except at high altitudes, where they found habitats to survive. This “vertical shift” can be observed in the higher areas of the Carpathian Basin as well (Table 4). High altitudes, like the surrounding mountains of the Carpathian Basin, help us to save the diversity of the sawfly fauna for a while longer.

4.2. Aculeata

Scientific papers do not discuss the change in Aculeata in total; however, our results are supported by scientific publications on specific genera and species. These results are consistent with our experiences. Olszewski et al. [82] reported the increase in population density of Philanthus triangulum (Fabricius, 1775), and Eickermann et al. [83] wrote about the increasing population of Polistes spp. in Europe. Similar tendencies take place in North America, as indicated by the proliferation of several Sphex species [84]. Also, in South America, the northern expansion of Centris nigrescens Lepeletier, 1841, was reported in connection with global warming [85]. Zimmerman et al. [86] published their similar experiences about the gradual increase and expansion of wild bee populations in Eastern Austria: Ceratina nigrolabiata, Icteranthidium laterale, Lithurgus chrysurus, L. cornutus, Osmia bidentata, O. spinulosa, Pseudapis diversipes and its parasite Pasites maculatus, which have currently expanded their distribution from other warmer regions to Austria. According to Sánchez-Bayo and Wyckhuys [87], in the U.K., there were 139 species of wild bees between 1980 and 2013; the average decrease in occupancy was 25%. In the Netherlands, 207 bee species (48% of the total) have expanded their nationwide distribution by 74% since 1950. In the southern Paranà state of Brazil, nest abundance declined by 94% among 28 species of ground-nesting bees at 20 sites that were sampled in 1958 and then again in 2018. According to our experience, this decline in wild bees is restricted to moisture-loving and saproxylic species only, as listed in Table 6. However, most wild bees show an increasing trend. It clearly indicates two major problems: one is global warming causing the Mediterranean transformation of our fauna, and the other is intensive forestry and logging causing the decline of saproxylic Aculeata. As reported (from the Netherlands), this expansion of several species towards the north can be observable very well in our region as well.
The paper by Powney et al. [88] reveals that “while a third of wild pollinator (wild bees and hoverflies) species (33%) have decreased over this period, approximately a tenth have increased, with the remaining species showing no clear trend”. In contrast to this, in our region, most wild bees show an increasing tendency (since most of the local wild bee fauna is categorized as Mediterranean, holo-Mediterranean, north Mediterranean and Ponto-Caspian Mediterranean from a zoogeographic point of view). But for hoverflies, we experienced the same dramatic decline.
Data on the decline of Ectemnius and Crossocerus species are provided by Bogusch et al. and Pearce-Higgins et al. [72,89]. There are two reasons for their decline: one is the general decline of saprophytilic Hymenoptera due to the disappearance of old forests (the energy crisis), and the other is climatic change. Some Ectemnius species are mesophiles, and they are characteristic species of marshy meadows. Finally, these marshy meadows are threatened not only by global warming but also by the non-native goldenrod (Solidago spp.) expansion, which kills the original vegetation.

4.3. Bumblebees (Bombus spp.)

A long-term decrease in bumblebees was reported by Bartomeus et al. [90]: “Over a 140-year period, aggregate native species richness (of wild bees) weakly decreased, but richness declines were significant only for the genus Bombus. Namely, Bombus pensylvanicus, B. affinis, and B. ashtoni suffered serious decline”. This observation from the U.S. agrees very well with our own experiences and with European trends as well. Data from international scientific papers confirm what we experienced in the Pannonian Basin: a slighter or similar decrease in population densities and diversity was observed in many other European countries [91]. At the same time, in Western Europe, originally common species (e.g., Bombus lapidarius, B. pascuorum and B. terrestris) became rare [92]. In England, this decline was so strong in the 1980s that only 6 bumblebee species were collected in regions where 19 species had been captured before 1960 [93].
According to Plowright et al. [94], between 1977 and 1994, Bombus muscorum disappeared from many habitats in Northern England and was replaced by B. pascuorum. Our observation may confirm this conclusion, although in our region, the Mediterranean B. haematurus has a more significant role in replacing other bumblebee species. Regarding Bombus terrestis, B. hortorum, B. lapidarius, B. pascuorum, B. haematurus, B. ruderarius and B. argillaceus, our long-term data confirm the results of Slovak and Hungarian papers on changes of populations of bumblebees [95,96,97]. According Sárospataki et al. as well [96] “36% of the bumblebee fauna can be considered rare and 24% moderately rare, i.e., over half of the total number of species can be classified into these two categories. Almost half (47%) of the species are still living in the Pannonian biogeographic region of the Carpathian Basin, showing a decreasing trend starting in the 1950s and 1960s”. Our data from the period 2000–2003 are well in line with these results. Moreover, recently, a new species, Bombus semenoviellus Skorikov, 1910, appeared in our region [98]. Mountainous regions of the Western Carpathians situated north of Pannonicum (Carpaticum occidentale and Carpaticum orientale) could show different trends. Bumblebee communities in the Outer and Inner Western Carpathians are apparently still rich in diversity and relative abundance. Rare and infrequent species, such as Bombus distinguendus, B. subterraneus, B. pomorum, B. confusus, B. veteranus, B. quadricolor and B. norvegicus, are still present in this area [99,100,101]. Especially interesting species are Bombus haematurus Kriechbaumer, 1870, and Bombus argillaceus Scopoli, 1763. These two species were historically very sporadic [27,100,101,102,103]. Due to the changed climatic condition, they are spreading to newer territories situated northwest of their original area of distribution [95,104,105,106,107,108]. The center of distribution of these two species is the western Palearctic, Ponto-Mediterranean region. Their increase is a strong indication of ongoing climatic change.
According to projections by Rasmont et al. [109] (confirming our results), Bombus haematurus Kriechbaumer, 1870, and Bombus argillaceus Scopoli, 1763, can benefit from climate change and potentially enlarge their current distribution in Europe in the upcoming decades. For more precise knowledge on the status of the bumblebee fauna of the Pannonian Basin, we would strongly advise continuing systematic and frequent monitoring of all members of the bumblebee fauna in the entire area of interest.

4.4. Diptera

We have limited information available from the scientific papers; therefore, our research brings original and new results. The available literature that is published in the neighboring regions agrees that certain groups, especially hoverflies, have suffered a significant decline in the number of individuals, similar to our results [110]. According to an IUCN study [111], “hoverflies generally ensure better pollination than bees at higher altitudes, under Nordic climatic conditions, or in cool microclimate or weather situations”. Barendregt et al. [112] reported a 44% decline in hoverfly populations over 39 years, between 1982 and 2021 (Boeschoten, The Netherlands). This value in our region is 87%, which is logical since the deep Carpathian Basin is more exposed to Mediterranean effects than the Atlantic Netherlands. According to Sommagio et al. [113], in addition to the catastrophic decline in the number of hoverflies, mountainous regions are able to provide shelter for them, but this is not true for bumblebees. They observed that the two taxa showed different distribution patterns: hoverflies had a unimodal distribution (richness and abundance) with a peak at middle altitude (1500 m), while bees had a monotonic decline with increasing altitude.
The observed increase in populations of horseflies is explained by their need for warmth. Herczeg et al. [114] did not manage to collect even a single Tabanid specimen below 18 °C. At the same time, they note that variability in moisture requirements per species is high, which explains the often opposite trends in populations of various species, similar to our experiences. Also interesting is the opposite trend of two similar and closely related large horsefly species, namely, Tabanus bromius Linné, 1758, and Tabanus bovinus Linné, 1758 (Table 12). Probably there is niche competition between these two species. According to Dörge et al. [115], Tabanus bovinus and T. bromius have similarly large niches, which are mostly overlapping. In the case of tachinids, climatic factors have a strong but indirect and very diverse effect. The most important effect is the optimization of the synchronicity between the presence of the host and the parasitoid’s egg-laying time: an optimally developed host animal larva should be available at the time of reproduction. Climatic conditions can improve this, but they can also shift it in an unfavorable direction [116], which could also be the reason for the fluctuation shown in Table 12 and Table 13. For Bombyliidae, Boesi et al. [117] provide a good explanation for their increased reproduction: bee flies (Diptera: Bombyliidae) have a virtually cosmopolitan distribution and are commonly found in warm arid to semiarid habitats, where they can form a conspicuous part of the flower-visiting insect fauna [118].

4.5. Lepidoptera

4.5.1. Nocturnal Macrolepidoptera

Only three papers study and discuss the temporal changes of various moth species during the last three to four decades. The most important comparative study available is the PhD thesis of Fox [118]. Fox investigated changes in about 600 moth species between 1970 and 2010. Our trend for many species is opposite to the tendencies set in Great Britain by Fox. These are Phragmatobia fuliginosa (Linnaeus, 1758), Xestia c-nigrum (Linnaeus, 1758), Eilema lurideola (Zincken, 1817), Spilosoma lubricipeda (Linnaeus, 1758), Lacanobia oleracea (Linnaeus, 1758) and Mythimna turca (Linnaeus, 1761) (Table 15). Their decrease in the south and their expansion in the north may indicate that they find better living conditions in the northern and humid Atlantic areas due to the gradually warming climatic conditions in their original habitats. A major proportion of the declining or rare species from the south (including the Carpathian Basin) are usually mesophile and silvicol species. Their typical habitats are meadows, swamps, tall sedges, mesophile forests, groves and alder forests. The opposite movement can be observed at warm-loving species: their populations in England declined, while in the Carpathian Basin, according to our data, their individual density increased, for instance Paracolax tristalis (Fabricius, 1794). Other xerotolerant, warm-loving species, like Macdunnoughia confusa (Stephens, 1850), Drymonia obliterata (Esper, 1785), Athetis furvula (Hübner, 1808), Zanclognatha lunalis (Scopoli, 1763) and Earias vernana (Fabricius, 1787), are not on the British list since they are Mediterranean species. These species have expanded their territories and population densities in the Carpathian Basin. These are eurythermal and/or polyphagous species, adapted better to more extreme conditions. Those species that prefer cold and moist ecosystems are strongly declining, like Diachrysia and Abrostola species (Table 15).
The global trend is described by Dar and Jamal [119]: “The substantial decline of moths has been reported in various countries such as the U.K., U.S., Germany, Sweden, India, the Netherlands, Siberia, and New Zealand. 31%, 44%, 27%, and 71% of moths declined in Great Britain, Southern Britain, Sweden, and the Netherlands, respectively”. This was also experienced in our region. The initial decline was taken over by a slight increase in the last 10 years. However, if we consider the last 50 years between 1970 and 2022, comparing the first and last decade, in our region, the decline in the moth populations was 31% (similar to that which was experienced in the Netherlands). Locally, opposite experiences may occur: “In Hungary’s mountains, yearly light trapping in two oak and beech forests between 1962 and 2007 showed that overall species richness of moths did not change (6% increases at one site and 6% decreases at another), whereas overall abundance increased by 24–39%” [120].
Other papers, like Conrad et al. [121], treat 337 species from Britain, and Mikkola’s work [122] discusses 54 species from Finland. However, those species that are discussed in these monographs hardly overlap with the fauna of the Carpathian Basin; therefore, we do not discuss these works in detail.
The reasons behind the above-described trends can be traced back to many influencing factors that act in a very complex way: global warming, frequent extreme temperature maximums, changed temporal distribution of precipitation that includes droughts and torrential rains, improper forest management, degradation of habitats, large-scale clear-cutting that affects microclimate, intensive lawn management, fertilization of lawns, incorrect selection of lawn-mowing dates, groundwater depletion, underground piping, etc. Internal factors may also influence population densities. Just an example, according to Hill et al. [123], Xestia c-nigrum (Linné, 1758) can produce a so-called heat shock protein, Hsp70: “Another molecular marker that is likely to be important in the response to climate change is the heat shock protein (Hsp70). Hsp70 genes play a critical role in helping insects survive exposure to extreme temperatures by increasing heat tolerance”. In this aspect, the Xestia c-nigrum population shall increase; however, grazing and mowing/fertilization (in Germany) have opposite effects on the moth assemblages: some species declined under intense livestock grazing and were replaced by other species like Mythimna pallens and Xestia c-nigrum [124]. In our region, livestock grazing has declined, and the trend is opposite that in Germany. Finally, there is still an open discussion about which moths or even which insects are pollinators [125]. We may say pollinators are those moths (and even those animals) that have at least minimal parts of their life cycle that are temporarily or regularly connected to flowers, and in this way, they transport pollen, helping the fertilization of plants. These insects could be predators hunting for their prey on flowers; insects attracted to various colors and wavelengths emitted by flowers; insects attracted to various odors and pheromone-like chemicals of flowers or attracted to the special appearance of flowers; animals feeding on nectar or consuming various parts of flowers; or those that find temporary shelter or a place to warm themselves on the surfaces of flowers, etc.

4.5.2. Butterflies (Rhopalocera)

Mountainous regions provide an escape for butterflies. A published study by Habel et al. [126] based on the database of the collection of the Museum of Natural Sciences in Salzburg, Austria, shows a slight decline in the species richness of butterflies from 152 to 146 in the period between 1975 and 2019 and an increase (from 128 to 146) in the 30-year time span between 1986 and 2019. On European butterflies, 40 out of 84 species of butterflies and burnet moths (Zygaenidae) in German forests and grasslands experienced marked declines in the period of 1972–2001. Historical records for 1890–2017 reveal declines among 42 species (59% of the total), including 15 species that have become extinct in recent times in the Netherlands. Surveys throughout Ohio between 1997 and 2017 found that 40% of species of butterflies were decreasing. In Singapore, since 1854, 32% of the 413 recorded species of butterflies have been extirpated from the island. In Australia, in a small location in northern New South Wales (Murwillumbah), the overall abundance of 21 species declined by 57% [87]. According to Martin et al. [127], distribution trends of butterflies showed that their distributions began decreasing long ago, and between 1890 and 1940, distributions declined by 80%.
The population increase in Euphydryas aurinia is described by Dietzel, Ábrahám and Ács et al. [48,54,56]. Euphydryas aurinia has two ecotypes. The wet-meadow ecotype has drastically decreased, while the dry-meadow ecotype has been spreading since the 1990s. Other results confirm the expansion of Libythea celtis (Laicharting, 1782) [56,126]. Libythea celtis is a migratory species that reaches the Carpathian Basin from the south. Its population is increasing step by step as a result of global warming. Also, its food plant (Celtis occidentalis) is being planted in parks and also in forests. Bury et al. [128,129] observed the population increase in Iphiclides podalirius (Linné, 1758) in Poland, the same as we experienced in the Carpathian Basin (Table 19). Its gradual population increase was caused by the expansion of blackthorn (Prunus spinosa) in abandoned areas. Neptis hylas (Linné, 1758) was frequent in southern and western Transdanubia only. In recent years, it has appeared in areas where it has not bred before. Further, this species started to feed on locust trees (Robinia pseudoacacia). Till recently, only Lathyrus spp. were its host plants [56,126]. The decline of populations of other rare butterfly species like Nymphalis antiopa (Linné, 1758), Apatura and Maculinea spp. is rather a nature conservation problem.
In terms of population densities of butterflies, Hill et al. [123] came to a similar conclusion: oligo- or polyphagous species (diet generalists) have better adaptation ability, allowing them to easily colonize new habitats. By contrast, diet and habitat specialists typically have poor dispersal ability and may not be able to track environmental changes.

5. Conclusions

Our results support the conclusion of Dicks et al. [5] that pollinator decline (mean risk score = 19.6) belongs to the “no high risks” category in Europe.
Beyond the above-mentioned moderate pollinator decline, most important is the gradual transition of our continental-type pollinator fauna towards the Mediterranean type.
Population densities of warm-loving and drought-tolerant species and species groups are increasing, while those of northern, silvicol species are declining.
Butterflies (Rhopalocera), hoverflies (Syrphidae), tachinids (Tachinidae), Symphyta and bumblebees (Bombus spp.) have suffered decline in the last two decades. Meanwhile, population densities of Aculeata, Bombyliidae and most of the Tabanidae are increasing.
High altitudes may provide shelter and help to maintain diversity for some moisture-loving groups. We provided evidence for this with sawflies (Table 4). It is probably true for other pollinator groups that prefer moderate climatic and moisture conditions.
The decline in bumblebees started around 2015, Syrphidae around 2000, Tachinidae around 1995 and butterflies around 2000 (the latest is indicated by the disappearance of the hitherto common Aglais urticae).
The influx of Mediterranean species into the Carpathian Basin was the strongest for Aculeata. Introduced species also enriched local pollinator fauna, especially in Aculeata and Lepidoptera (in Symphyta, only one species has been introduced recently).
The Mediterranean transformation of our pollinator fauna is a response to the gradual aridification of the Carpathian Basin [130]. Considering the general global data from the 10 major insect taxonomic orders, which indicate that an average of 37% of species are declining in numbers while populations of 18% species are increasing globally [87], and some regional data for all pollinator groups describing a seasonal decline of 76% and a midsummer decline of 82% in flying insect biomass over the 27 years of study (in Germany) [131], our situation is significantly better since, of the investigated 33 families, in total, only 18 families show a declining trend in population density. These are Tenthredinidae, Syrphidae, Tachinidae, all families of moths (10 families) and 5 butterfly families. This decline is compensated for by the increase in warm-loving Mediterranean groups. It indicates that the major problem in our region is the Mediterranean transformation of our fauna, probably due to climatic change. Although the pollinator crisis is present, it is a moderate danger.

Author Contributions

Symphyta and Lepidoptera, A.H.; Symphyta, L.R.; Aculeata, Z.J.; Diptera, S.T.; bumblebees and partly invasive Aculeata, P.Š.; statistical analysis, A.H.; resources, all authors; data curation, all authors; writing—original draft preparation, all authors. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partly financed by the project No. 2/0070/23 of the Slovakian Funding Agency VEGA.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

Authors express their grateful thanks to Vladimír Smetana (Tekov Museum, Levice, Slovak Republic), Levente Ábrahám (Rippl-Rónai Museum, Kaposvár, Hungary), Ákos Uherkovich, (Janus Pannonius Museum Pécs, Hungary) and György Csóka and Anikó Hirka (University of Sopron, Forest Research Institute, Hungary) for their generous support, data provision and advice.

Conflicts of Interest

The authors declare no conflicts of interest.

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