Changes in butterfly species richness (Lepidoptera: Papilionoidea) over two decades in the Koroška region, Northern Slovenia

. Changes in butterfly fauna in the Koroška region in northeastern Slovenia have been compared over a period of 22 years. Records from 12 sampling sites from late July 2016 are compared with the same sites from late July/early August 1994. On average, 30% of species richness was lost per site, with a maximum loss of 80% at one site with complete habitat degradation. Records from 35 sampling sites from July 2016 are presented, including records of several rare habitat specialist species, such as Lycaena dispar, Plebejus optilete, Boloria titania, and Phengaris arion, the latter being of particular importance for conservation. All in all, habitat degradation has been confirmed to be the main driver of the loss of butterfly richness in the Koroška region


Introduction
Historical records and scientific reports of local species diversity are extremely valuable because they provide baseline data that can be used for the evaluation and preservation of biodiversity, which is the main goal of the Biodiversity Convention (UNEP 1992).Only a few historic and recent scientific works have focused on the species diversity of the butterfly fauna in the Koroška region, therefore, every additional contribution is very important in providing information that can help to protect and preserve its butterfly fauna and the species' habitats.
The landscape of the Koroška region is characterized by predominantly extensively farmed meadows and a patchy landscape of forests, mountains, and valleys (Senegačnik 2012), which contribute to high butterfly diversity.The butterfly fauna of the region is relatively well studied given the data included in the Atlas of Butterflies of Slovenia and its supplement (Verovnik et al. 2012(Verovnik et al. , Čelik 2014).However, very little published information is available.The only overview covering the Koroška region dates back to 1983, when the butterfly fauna of the Podravje region was reviewed (Jež 1983).Important species distribution information is also available for the neighbouring part of Koroška in Austria (Thurner 1948, Hassler & Tschinder 1998).Two reports based on field work at the student research camps in 1994 and 1995 also added important new information on species distribution of the Koroška region (Verovnik 1995a, b).Most recently, a rigorous study of butterfly diversity of Mt Košenjak was published (Kadiš 2016).
Apart from general surveys of the butterfly fauna of the Koroška region, two cases of species extirpation in the region have been reported.The first concerns Coenonympha tullia, which was found in the Koroška region in 1962 (Lesar 1998) and at several additional sites in the 1990s (Verovnik et al. 2012).The species was last recorded in Koroška in 2001 and is considered locally extinct due to habitat degradation, including the overgrowing of wet meadows and drainage of mires (Čelik et al. 2005(Čelik et al. , Čelik 2012(Čelik et al. , Verovnik et al. 2012).The second case concerns Colias myrmidone (Esper, 1781), which was reported from several sites in the Koroška region up to 1989 (Predovnik & Verovnik 2004), but is now considered extinct in Slovenia (Verovnik et al. 2012).Both cases are examples of extreme effects of land use change on butterfly habitats.Koroška also includes several important areas designated as Natura 2000 sites aimed at conserving qualifying butterfly species (Ur.l.RS 2004).Four qualifying species of butterflies have been observed in the Koroška region, i.e.Colias myrmidone, Lycaena dispar, Euphydryas aurinia and E. maturna.Already at the time when these areas were designated, the problem of intensive overgrowing and habitat deterioration was recognized as the main threat to the habitats of Lycaena dispar and Euphydryas aurinia in the area of Gornji Dolič and Razbor due to changes in land use (Čelik et al. 2005).
The main goal of this study is to compare the change of species richness in the Koroška region over the past 22 years.The basis of our comparison is a field survey report on the species richness of butterflies in the Koroška region from 1994, when 14 localities were visited at the end of July and beginning of August (Verovnik 1995a).A total of 61 species were observed during the survey, including some rare species associated with wet grasslands like Lycaena dispar, Phengaris teleius, and P. nausithous.In order to implement a comparable survey of butterfly fauna in the region, we revisited 13 of the 14 previously surveyed localities in the second half of July 2016 during the Biology Students Society's (DŠB) research camp.By comparing 12 of the revisited sites, we wished to elucidate particularly the changes in butterfly richness, the loss of butterfly diversity in the region, and to provide comments on the causes of the decline.

Materials and methods
Butterflies were surveyed in the second half of July 2016.In total, 35 localities were visited; among them, 13 localities were revisited after 22 years (Fig. 1).The 22 new localities were selected based on known distribution records of rare and endangered butterfly species (Verovnik et al. 2012) and Google Earth satellite images where potentially extensive or partially overgrown grasslands, which could host a large number of butterfly species, were identified.Adult butterflies were captured using entomological nets and released after identification.For identification purposes, the field guide Butterflies of Britain and Europe (Tolman & Lewington 2008) was used.

Sampling sites
The list of localities contains the relevant toponyms, a short description of the habitat, altitude, coordinates (WGS 1984), and dates of the visits.The locations in bold represent those that were also surveyed in 1994 and are included in the comparison herewith.

Comparison of the two surveys
Twelve locations were sampled in both years, which enabled us to prepare a complete list of the species observed and to make their direct comparisons (excluding locations 8 and 36, which were only partially sampled in 1994).In 1994, 57 species had been observed at these locations and 48 in 2016.Therefore, in total, nine species fewer were observed in 2016, marking a 16% decline.Looking at the species, 18 of those observed in 1994 were not seen at these same locations in 2016.On the other hand, 10 new species were observed in 2016.
The mean value of the number of species per site was 18.3 in 1994 and 12.8 in 2016 (Tab.2).Therefore, the calculated mean number of the species detected per site in 2016 is by 30% lower than in 1994.The difference between the means is significant when tested with the Student's Paired t-test (t=2.088;p=0.017), at a confidence interval of 95% (Fig. 2).When looking at the locations, nine out of 12 of the compared locations had lower species richness in 2016 (Tab.2).

Discussion
At first glance, the total number of species observed in 2016 (70) surpasses the number of species in 1994 (61).However, not all of the locations from the surveys can be directly compared as the 2016 survey involved more than twice as many sites and a larger area.When only the sites surveyed in both years are compared, a 16% decline in richness is evident in 2016.Additionally, several species not recorded in 1994 are the result of a partial mismatch in the timing of both surveys and potential seasonal differences as several species typically flying in spring, such as Anthocharis cardamines, Brenthis ino, Boloria euphrosyne, and Erebia medusa, were observed only in 2016.The absence of species not recorded in 2016 could, however, not be interpreted by their phenology.According to the implemented statistical test, the negative change in species richness is significant for the analyzed sites.The loss of more than six species on average per site is alarming as it is equal to 30% fewer species observed (Fig. 2).The four sites with the most drastic declines (Tab.2) have been transformed from flower rich meadows to intensive grasslands, supporting a much lower number of butterfly species, which are mostly habitat generalists (Dolek & Geyer 1997, Van Swaay et al. 2016).
The loss of wet meadows is evident in the entire Koroška region (authors' pers.observ.).
None of the aforementioned wet grassland specialists, including Phengaris teleius, and P. nausithous were observed during the 2016 survey (Tab.1).P. alcon was not found at a historically known location (location 33) (Verovnik et. al. 2012).Even their larval host plants were missing from those sites.These wet grassland specialist butterfly species disappeared from the Koroška region before 2010 (Verovnik et al. 2012, Verovnik pers.observ.).Another species associated to wet grasslands was observed after a long absence: L. dispar was found again at a new site close to the border crossing Holmec at location 8 in 2016 (Tab.1).In remaining fragments of wet meadows and fens, we found only Melitaea diamina, Brenthis ino, and Boloria selene, which are generally not exclusively linked to wetland habitat types in Slovenia (Verovnik et al. 2012).All visited sites where the three mentioned species had been found in the past were in different stages of overgrowing with tall herbs, bushes, and trees, hence unsuitable for other specialist butterfly species.
The most encouraging finding of the surveys in 2016 is the observation of Phengaris arion at several sites on Mt Strojna and Mt Košenjak.The species is limited to extensively managed dry grasslands with an abundance of thyme (Thymus spp.) or origanum (Origanum vulgare) and is considered vulnerable (VU) in Slovenia (Ur.l.RS 2002).It has been historically recorded from only two locations in the Koroška region (Verovnik et al. 2012) until several colonies were discovered on Mt Košenjak in 2009 (Kadiš 2016).In 2016, we confirmed its presence on Mt Košenjak, and made new sightings on Mt Strojna.Only single specimens were observed at all sites, however, P. arion is generally known to occur in low densities (Mouquet et al 2005).Additional interesting records of specialists are: Boloria titania, which was observed on Mt Peca at 1440 m, within the main altitudinal belt of its distribution in Slovenia (Verovnik et al. 2012), Plebejus optilete, found again within its range in the western part of Pohorje Mts (Jež & Verovnik 2012), and Erebia stiria, which was found at Kozja peč in Bistra valley on screes and rocky slopes, a typical habitat of the species in Slovenia (Jutzeler et al. 2001).
The goal of the Natura 2000 sites, to preserve and, at best, improve the quality of the protected species' habitats (Ur.l.RS 2004), has not yet been met in the Koroška region.
Even more disappointingly, drastic negative habitat changes are evident in the last decades and are ongoing in many areas included in this study (authors' pers.observ.).Although there is sufficient legislative support for the conservation of threatened butterfly species and their habitats in Slovenia, the implementation is completely lacking and the negative processes of either abandonment or intensification continue.We fear that without the engagement of landowners, local communities, and authorities, coupled with regular monitoring, we will be faced with continuous erosion of butterfly species richness in the Koroška region.To halt this negative trend, the appropriate funding by the state and development of nature oriented tourism will be of great importance-the sooner the better!

Table 1 .
List of butterflies recorded during survey in the Koroška region in 2016 with a comparison to the 1994 survey.The nomenclature follows the European Red list of Butterflies (Van Swaay et al. 2010) with some modifications.The numbering of locations is the same as in the list of the locations.An »x« marks the species that were not found in the corresponding sampling session.Tabela 1. Seznam dnevnih metuljev, opaženih med terenskim delom na Koroškem v letu 2016, ter primerjava s popisom iz leta 1994.Poimenovanje je povzeto po European Red List of Butterflies (Van Swaay et al. 2010) z nekaterimi modifikacijami.Številčenje lokacij je enako kot v seznamu lokacij.»x« označuje vrste, ki v posameznem letu vzorčenja niso bile najdene.