Introduction

Renewable energies are a key tool to reduce greenhouse gas emissions and combat climate change (Gielen et al. 2019). Many countries around the world have set themselves ambitious expansion targets, meaning that the share of renewable energies in energy generation has risen sharply in recent years and will further increase in the future (REN21 2023). Among the renewables, solar energy plays a pivotal role and a strong expansion is expected in the coming years (Dhar et al. 2020; Hernandez et al. 2019). As only part of the solar energy can be generated by photovoltaic systems on buildings, a major part of the expansion will focus on solar parks. These are areas on which ground-mounted solar panels are installed in order to generate solar energy on a large scale and in a spatially concentrated manner (Hernandez et al. 2014).

The establishment of solar parks can lead to both environmental conflicts and synergies (Hernandez et al. 2014; Randle-Boggis et al. 2020). In addition to climate protection, other positive effects include contributions to soil and water protection, as solar parks (at least in temperate regions) are usually characterized by grassland vegetation without fertilizer usage, which protects the soil from erosion and reduces nutrient inputs into groundwater and surface water (e.g. compared to conventional arable land), thus improving water quality (Badelt et al. 2024; Hernandez et al. 2019; Uldrijan et al. 2021; Walston et al. 2021). At the same time, however, negative effects are also possible, e.g. on the aesthetic and recreational value of landscapes (Badelt et al. 2024; Scognamiglio 2016).

With regard to biodiversity, solar parks also offer both opportunities and risks, but there is still a considerable need for research (Agha et al. 2020; Moore‐O’Leary et al. 2017; Tölgyesi et al. 2023). For example, diverse and flower-rich grassland vegetation within solar parks may provide habitat for flower-visiting insects (Blaydes et al. 2022; Dolezal et al. 2021; Walston et al. 2024). On the other hand, solar parks can lead to the loss, degradation or fragmentation of habitats for certain bat species (Barré et al. 2023; Szabadi et al. 2023; Tinsley et al. 2023) or act as a barrier for larger mammals due to the fencing of the facilities (Moore‐O’Leary et al. 2017; Sawyer et al. 2022).

Basically, solar parks create novel habitats that differ markedly from other habitats in our landscape (Tölgyesi et al. 2023). Within solar parks, a small-scale mosaic of different (micro)habitats exists: The installation of the solar panels creates areas where various abiotic conditions are significantly altered compared to non-covered areas. Below solar panels, light availability is reduced (Lambert et al. 2022; Vaverková et al. 2022) and temperatures are less extreme and more balanced (Armstrong et al. 2016; Wu et al. 2014). The soil moisture under solar panels can be reduced as a result of the interception of precipitation (Armstrong et al. 2014, 2016), however, shading can also lead to increased soil moisture and decreased drying out in drought periods (Lambert et al. 2021; Uldrijan et al. 2021). By modifying abiotic conditions, the solar panels also affect the vegetation within solar parks. Previous studies have shown that plant species richness and vegetation cover under solar panels is usually lower compared to areas between panel rows, and that plant communities are more homogeneous and mostly characterized by shade tolerant species (Armstrong et al. 2016; Lambert et al. 2022, 2023; Uldrijan et al. 2021; Vaverková et al. 2022). Studies comparing different solar park locations (e.g. areas under the solar panels, gaps between panel rows, areas at the edge of solar parks) are almost exclusively available for vascular plants (Armstrong et al. 2016; Montag et al. 2016; Uldrijan et al. 2021; Vaverková et al. 2022). In contrast, the habitat function and quality of different locations (with different biotic and abiotic habitat characteristics) within solar parks for the fauna has rarely been investigated in detail so far (but see Graham et al. 2021 for pollinators, Lambert et al. 2023 for springtails and mites, and Herden et al. 2009 for grasshoppers).

With this work, we aim to increase the knowledge about the potential of solar parks for biodiversity conservation with a particular focus on the habitat function and quality of different locations within solar parks. We used carabid beetles (Coleoptera: Carabidae) which are excellent bioindicators for analyzing small-scale differences in habitat function and quality, as they respond very sensitively to small-scale differences in habitat conditions (Buchholz et al. 2013; Kotze et al. 2011). To our knowledge, no (peer-reviewed) work has yet been published on the importance of solar parks as habitat for carabid beetles, so this study also makes a further contribution to the comprehensive assessment of the biodiversity value of these novel ecosystems, considering various species groups. We aimed to determine which carabid beetles use solar parks as a habitat. More specifically, we asked how different locations within solar parks—areas under solar panels “UNDR”, gaps between panel rows “BTW” and areas at the edge of solar parks “EDGE”—differ (i) in habitat characteristics (temperature, vegetation structure), (ii) in species richness and activity densities of carabid beetles and (iii) in carabid beetle species composition and community weighted mean of traits.

Material and methods

Study area and study sites

We carried out our investigations in three solar parks in the district of Schaumburg, located in the federal state of Lower Saxony, west of the state capital Hannover (Fig. 1a, b). Schaumburg is located in the transition zone between the northern German lowlands in the northern part of the district and the central German low mountain ranges in the central and southern part of the district. The terrain in the district rises from north to south from 40 to 380 m above sea level. Agriculture is the dominant land use in the lowlands in the north and in the valleys and plains in the central and southern part of the district, while the mountain ranges are covered with forests. The climate in the district is sub-oceanic, with a long-term average annual temperature of 10.3 °C and an average annual precipitation of 698 mm for the period 1991–2020 (weather station Bückeburg, district of Schaumburg, 68 m a.s.l.; DWD 2023).

Fig. 1
figure 1

a Location of the district of Schaumburg (black dot) in Germany. b Location of the three solar parks (red dots) within the district of Schaumburg (dark grey). c Random distribution of pitfall traps within the different locations of the solar parks (EDGE edge area of the solar park, BTW area between the solar panels, UNDR area under the solar panels) using the example of the site in Rehren (Background map: Digital orthophoto DOP20, State Office for Geoinformation and Geodesy of Lower Saxony). d and e Investigated locations EDGE, BTW and UNDR within the solar parks. f Pitfall trap with plexiglass roof and data logger for temperature measurement (small red flag). (Color figure online)

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The solar parks are located in the villages of Bad Nenndorf, Sachsenhagen and Rehren in the northern and central part of the district (Fig. 1b). The sites are between 10 and 18 km apart. All three sites are located within or at the edge of industrial areas. The immediate surroundings of all parks consist of industrial areas with some residential buildings and associated traffic infrastructure. In addition, meadows, arable land and linear woody structures (Sachsenhagen), roadside verges and groves (Rehren) as well as fallow land and linear woody structures (Bad Nenndorf) border the respective sites (Table 1). The solar park Bad Nenndorf directly borders on a railway, the solar park in Rehren is located next to the motorway A2. The three sites represent typical locations for solar parks in Germany, which in recent years have frequently been built in or on the edge of industrial areas and, due to the subsidy regulations (increased feed-in payments), often next to railways or motorways (Niemann et al. 2017; Walz et al. 2022).

Table 1 Main characteristics of three investigated solar parks in the district of Schaumburg. Information on main soil types according to LBEG (2015)
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The main technical and management-related properties of the three solar parks are similar: the panels are mounted in rows on steel constructions facing south (inclination angle of approx. 35°) (Fig. 1d, e). The distance between the panels and the ground is 60–90 cm on the near-ground side and around 2–2.3 m on the far-ground side. The width of the area under the solar panels is 3 m, spacing between the panel rows is 3.5–4 m. A grass mixture was sown in all solar parks immediately after construction in order to achieve quick vegetation cover and prevent soil erosion. Subsequently, no further sowing took place. Thus, the vegetation currently consists of the sown grasses along with plant species that migrated from the surrounding or spontaneously established from the soil seed bank. The vegetation is mown 2–3 times a year between May and October to prevent shading of the solar panels and reduce fire hazards (cf. Vaverková et al. 2022). Mowing is carried out between the panel rows and at the edges of the solar parks (areas outside the panel rows but within the fencing of the solar parks). In addition, the area under the panels is mown with a boom, whereby a small part between the central steel constructions is not covered and is therefore mown manually at irregular intervals. After mowing, the cut material remains on the sites (mulch mowing). Beyond this, no additional fertilization takes place. In the year of our study (2023), the solar parks were mown twice: once at the end of May/beginning of June and the second time in mid-August. All three parks already existed for more than 10 years at the time of the study. The size of the sites was between 2 and 5 ha (Table 1) and all solar parks were completely fenced in.

Sampling and data preparation

Carabid beetles

We used pitfall traps to capture carabid beetles. The trapping vessels (transparent jars with a volume of 350 ml) were buried in the ground so that the opening was even with the soil surface. They had an opening diameter of 6.5 cm and were each filled with 150 ml ethylene glycol to which a drop of non-perfumed detergent was added to reduce the surface tension. The traps were protected from the rain with roofs (25 × 25 cm) made of transparent Plexiglas (Fig. 1f). We installed four traps in each of the following locations within each of the three solar parks: (Fig. 1c–e):

  • UNDR: Area in the center directly under the solar panels

  • BTW: Area between the panel rows (the traps were placed exactly in the middle of two rows)

  • EDGE: Area at the edge of the solar park, i.e. outside the area covered with solar panels (UNDR) and the gaps between them (BTW) but within the fencing of the solar park (control without the influence of the solar panels).

Thus, we used a total of 36 pitfall traps (4 × 3 × 3), i.e. 12 traps per solar park and 12 traps for the different locations (UNDR, BTW, EDGE) within the solar parks. The traps were randomly distributed (using the “Create Random Points function in ArcGIS, version 10.8.2; Esri (2021)). Our only requirement was that the traps had to be at least 10 m apart from each other in order to minimize any mutual influence and that the traps in the locations UNDR and BTW had to be set at least 20 m away from the edge of the solar parks. The trapping period lasted from 13 April to 17 October 2023 (set-up and set-down at midday, i.e. 187 trapping days per trap). During this period, the traps were constantly active and were emptied every 2 to 4 weeks, whereby the trapping fluid was also replaced.

After sampling, we transferred the carabid beetles to 70% ethanol until further analyses. We pooled all beetles per trap for the entire period (no differentiation between different sampling periods). We identified the carabid beetles to species level following Müller-Motzfeld (2004) and Lompe (2002). Nomenclature follows Schmidt et al. (2016). We were not able to identify some female individuals of the species Trechus obtusus or T. quadristriatus to species level. We classified them as Trechus spec. and only counted them as species when calculating species numbers if no clearly identifiable individuals of T. obtusus or T. quadristriatus were present in the respective trap, in the respective location or in the respective solar park. We did not use Trechus spec. in the multivariate data analyses.

For further analyses, we calculated species richness (number of species) and activity densities (number of individuals) for the three solar parks (Bad Nenndorf, Rehren, Sachsenhagen), for the three different locations (UNDR, BTW, EDGE) within the solar parks and for each pitfall trap. In addition, we checked the Red List status of the species for both Germany (Schmidt et al. 2016) and Lower Saxony (Assmann et al. 2003). Furthermore, we calculated the community weighted means for humidity and shading preference of carabids. For this purpose, we used the trait values for humidity and shading from Irmler and Gürlich (2004). Thereby, lower values indicate a preference for dry and open habitats, respectively, while higher values indicate a preference for humid and shady conditions. Additionally, we calculated the community weighted mean of the body size based on the mean values given in Müller-Motzfeld (2004).

Vegetation structure and temperature

We recorded the following variables in 1 m2 plots at each pitfall trap (n = 36) at the beginning of August 2023 (shortly before the second mowing date) to characterize the structural habitat properties: Vegetation cover (total and separately for cover of grasses and herbs), mean vegetation height (in cm), litter cover and bare ground cover. We estimated cover in 5% intervals, in the range between 1 and 10% in 1% intervals.

In addition, we measured temperature at ground level (2 cm above the ground) at each trap (n = 36) using Tinytag Plus 2 TGP-4500 data loggers (Fig. 1f). The temperature was recorded every hour. Data loggers were active from 15 April to 17 October 2023. During this period, each logger took 4,452 temperature measurements. Based on the recorded values, we calculated the following variables for each logger: Mean temperature, number of measurements with temperatures > 30 °C, > 40 °C and < 0 °C and the range (difference between the highest and lowest measured value). One data logger (solar park Rehren, trap UNDR#3) was destroyed during mowing. In this case, we calculated the mean value of the hourly temperature measurements from the three remaining data loggers from this location (UNDR) at this site (Rehren) and used it as the basis for calculating the above-mentioned variables.

Statistical analysis

We used R version 4.3.2 (R Core Team 2023) for all data analyses. Vegetation structure and temperature variables as well as community weighted means of traits (shading, humidity, body size) were related to the location within solar parks (EDGE, BTW, UNDR) using linear mixed models (command ‘lmer’ in R package lme4; Bates et al. 2015). Because of our nested design with multiple traps within one solar park, we included the solar park ID (Bad Nenndorf, Rehren, Sachsenhagen) as a random effect in our models. The species richness and activity densities of carabid beetles were related to the location using generalized linear models with a negative binomial error structure for overdispersed count data (command ‘glmer.nb’ in R package lme4). Again, solar park ID was used as a random effect. We used type-II analysis-of-variance tables based on Chi2 statistics as a significance test for the mixed models. Multiple comparisons of means were conducted using Tukey post-hoc tests (command ‘glht’ in R package multcomp; Hothorn et al. 2008).

The species composition of carabid beetles (excluding Trechus spec.) was analyzed using partial distance-based redundancy analysis based on the Morisita-Horn index as a similarity measure (Jost et al. 2011) using the command ‘capscale’ in R package vegan (Oksanen et al. 2022). In a first step, we included all potential explanatory variables (see Table 2) in a full model and then used a stepwise forward model selection based on the adjusted R2 as goodness of fit measure for model building (Borcard et al. 2018) using the command ordiR2step (R package vegan). Solar park ID was considered as a conditional effect. The best-fit model contained the four explanatory variables location (EDGE, BTW, UNDR), mean temperatures, grass cover and bare ground cover. Whether these explanatory variables significantly affected the carabid species composition was tested using permutation tests based on 999 permutations. Finally, we used a Venn diagram in order to investigate the number of shared and exclusively occurring species for the three locations within the solar parks (command ‘ggVennDiagram’ in R package ggVennDiagram, Gao 2023).

Table 2 Comparisons of vegetation structure and temperature variables (Mean ± SE, n = 12 samples per location) between the three solar park locations
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Results

Vegetation structure and temperature

The three locations within the solar parks strongly differed in terms of vegetation structure and temperature. Total vegetation cover and grass cover were both significantly lowest in UNDR and did not differ between EDGE and BTW (Table 2). Herb cover was also significantly different between the locations and lowest in BTW, while bare ground cover was highest in UNDR and did not differ between EDGE and BTW. In contrast, there were no significant differences in litter cover and vegetation height between the three locations.

Mean temperature differed significantly between the three locations and was highest in EDGE and lowest in UNDR, while BTW took an intermediate position between these two locations (Table 2). The variation in temperatures (range) was significantly higher in EDGE and BTW while it was remarkably lower in UNDR. This is also reflected by the number of measurements of extreme temperatures: The number of measurements > 30 °C and the number of measurements > 40 °C were significantly lower in UNDR compared to EDGE and BTW. The number of measurements < 0 °C was also lowest in UNDR, while there were no differences between EDGE and BTW.

Carabid beetles

General results

We captured a total of 51 species and 1,889 individuals in all three solar parks, thereof 32 species (943 individuals) in Sachsenhagen, 31 species (417 individuals) in Bad Nenndorf, and 25 species (529 individuals) in Rehren (Table 5 in the Appendix). Within the different locations of the solar parks, most carabid beetles were found in EDGE with 38 species and 1,125 individuals, followed by BTW with 34 species and 648 individuals; in contrast, we found considerably fewer species and individuals in UNDR (21 species, 116 individuals) (Table 5).

By far the most common species was Poecilus versicolor which made up 52.4% of our total catch. Further frequent species were Harpalus latus (8.5%), Amara lunicollis (5.1%), Carabus nemoralis (3.9%), Clivina fossor (3.7%), and Poecilus cupreus (3.3%). Rank-abundance plots (Fig. 4) indicated a more even distribution of species abundances in UNDR, where Carabus nemoralis was the most abundant species, compared to EDGE and BTW. In the latter, Poecilus versicolor was highly dominant (Fig. 2c, Table 5).

Fig. 2
figure 2

Comparison of a species richness, b total activity density, c activity density of the dominant species Poecilus versicolor, and d activity density of all carabids without P. versicolor between the three solar park locations (n = 12 samples per area). The three locations differed significantly with regard to all variables (***p < 0.001). Shown are boxplots (thick black line = median) and raw data points. EDGE edge area of the solar park, BTW area between the solar panels, UNDR area under the solar panels. For detailed statistics see text and Table 3. (Color figure online)

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Table 3 Comparisons of species richness, activity densities, and CWM of traits (Mean ± SE, n = 12 samples per location) between the three solar park locations
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All species found are classified as non-threatened (Red List category “Least Concern”) in the German Red List (Schmidt et al. 2016). According to the Red List of Lower Saxony (Assmann et al. 2003), Harpalus luteicornis and Ophonus laticollis are classified as Endangered and Microlestes maurus as Vulnerable; Acupalpus exiguus is classified as Near Threatened. Each of these four species was only found in one of the three solar parks and with 1–3 individuals (and within 1–2 traps) each (Table 5).

Carabid diversity, activity densities, and CWM of traits

The species richness (number of species per trap) of carabid beetles significantly differed between the three locations within the solar parks and was about twice as high in EDGE and BTW compared to UNDR (Table 3; Fig. 2a). The total activity density of carabid beetles was also significantly different: In EDGE we observed almost two times more individuals as in BTW and almost 10 times more individuals as in UNDR (Table 3; Fig. 2b). However, this pattern was partly driven by the dominance of Poecilus versicolor in EDGE, where the number of individuals was about three times higher than in BTW and about 120 times higher than in UNDR (Table 3; Fig. 2c). The activity density of carabid beetles without P. versicolor was also significantly different between the three locations, however, without any significant difference between EDGE and BTW (Table 3; Fig. 2d).

The community weighted means (CWM) of humidity and shading were both significantly different between the three locations (Table 3). The value for CWM humidity was significantly lowest in EDGE with no significant differences between BTW and UNDR. In contrast, CWM of shading was not significantly different between EDGE and BTW but remarkably higher in UNDR (Table 3). Although the community weighted mean of body size was slightly higher in UNDR compared to EDGE and BTW, this difference was not significant (Table 3).

Carabid species composition

The constraining variables (location, mean temperature, grass cover, bare ground cover) accounted for 39% of the variance, while 49% remained unexplained. The conditional effect (solar park ID) accounted for 12% of the variance. The first axis explained 35% of the variance explained by the constrained variables and the second axis explained 6%. The species composition of carabid beetles was significantly affected by the location within solar parks (Table 4). Accordingly, EDGE, BTW, and UNDR were characterized by specific assemblages with considerably overlap between EDGE and BTW (Fig. 3a). Additionally, bare ground cover had a significant effect on the species composition and was associated with UNDR (Fig. 3a). In contrast, mean temperature and grass cover had no significant effects on the species composition of carabid beetles (Table 4).

Table 4 Effect of explanatory variables on the species composition of carabid beetles in the surveyed solar parks
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Fig. 3
figure 3

a Species composition of carabid beetles in different solar park locations. Species composition was analyzed using partial distance-based redundancy analysis based on the Morisita-Horn index as a similarity measure after stepwise forward model selection. b Venn diagram indicating the number of exclusive and shared carabid species. EDGE edge area of the solar park, BTW area between the solar panels, UNDR area under the solar panels. For statistics see text and Table 4. (Color figure online)

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Our analysis of shared and exclusively occurring species in the respective locations of the solar parks showed that 13 species (25%) were found in all three solar park locations (Fig. 3b). In addition, BTW and EDGE shared a relatively large number of species (12 species; 24%), which were not present in UNDR. The number of exclusive species was highest in EDGE (11 species; 22%), followed by BTW (7 species; 14%) and UNDR (4 species; 8%). Most of the species that exclusively occurred in one of the three locations had very low activity densities (usually only 1–2 individuals, see Table 5).

Discussion

Our findings confirm the results of previous studies on vegetation structure and temperature (Armstrong et al. 2016; Lambert et al. 2023; Wu et al. 2014), which also found reduced vegetation cover and more balanced temperatures (through buffering of both high and low temperatures) in the areas under solar panels compared to non-covered areas. In addition to these clear differences in biotic and abiotic habitat characteristics, we showed that different locations within solar parks strongly varied in the community structure of carabid beetles.

Most importantly, our results demonstrate that the carabid beetle fauna of areas covered by solar panels is severely impoverished in terms of species richness and activity densities. In addition, the beetle assemblages of UNDR also differed considerable from EDGE and BTW, which in turn were very similar. This clear difference in species composition is mainly caused by the fact that many of the species frequently found in BTW and EDGE were missing in UNDR (e.g. Amara convexior, A. lunicollis), and that the activity densities of the most shared species of the three solar park locations (e.g. Poecilus versicolor, Harpalus latus) were generally lowest there (Table 5). As the areas under the solar modules were already hardly used by these adaptable, widespread, and common species (cf. GAC 2009; Schmidt et al. 2016), the use of UNDR by more demanding xero-thermophilic species is likely to be even more limited or this area may be avoided completely. This could be particularly relevant for less mobile (non-flying) species. If this is confirmed, the issue would be particularly relevant for large solar parks, as they could act as a barrier for certain carabid species and contribute to further landscape fragmentation. However, other carabids such as forest species might tolerate or even prefer the shaded conditions under the panels in comparison to the locations between and surround the panels. Thus, future studies considering other landscapes (e.g. with larger forest areas) are needed to improve our understanding about the relevance of solar parks for biodiversity conservation.

In contrast to the predominantly clear differences from UNDR, EDGE and BTW differed only slightly. Thereby, it is not surprising that EDGE had a somewhat higher total species number (Table 5) and higher activity densities than BTW, as the edge areas of solar parks—as with other habitats (cf. Boetzl et al. 2016, 2024)—are subject to the greatest spillover effects from adjacent habitats such as woody structures and fallows (see Table 1; Fig. 1). However, the higher activity densities in EDGE were exclusively related to Poecilus versicolor, a common and widespread, generalist open land species that mainly colonizes (also intensively used) grassland and other farmland habitats (Fartmann et al. 2018; GAC 2009). The minor differences in habitat characteristics, species richness and activity densities (without P. versicolor), and the large overlap in their assemblages thus show that the narrow gaps (3.5–4 m wide) between the panel rows can, to some extent, be of similar importance as habitat for carabid beetles as the EDGE areas. This also highlights that many species do not only occur at the edge of solar parks, but also penetrate into their central areas. The negative influence of the solar panels within the gaps (BTW) is therefore rather small compared to areas directly covered by the panels (UNDR).

With regard to the carabid beetle trait composition, the higher CWM for humidity in UNDR and BTW indicate that the soil moisture was higher than in EDGE. This is caused by the solar panels (especially in UNDR), which can prevent the soil from drying through shading and slowing down wind speeds (Lambert et al. 2021; Uldrijan et al. 2021). Nevertheless, CWM values of UNDR and BTW (mean 3.2) as well as of EDGE (2.5) refer to meso-hygrophilic species (Irmler and Gürlich 2004), which somewhat relativizes the supposedly large difference between the three locations. In accordance to the supposed shading effect of the solar panels, the CWM for shading was higher in UNDR than in EDGE and BTW. However, the value is still rather low (1.7), which indicates that the beetle communities in UNDR do not consist of particularly shade-preferring species, but (as in BTW and EDGE) predominantly of open land species.

Based on our findings, we would like to emphasize the following: Previous studies on the biodiversity of solar parks have primarily focused on investigating gaps between panel rows and evaluated the biodiversity value of solar parks on the basis of the results from these findings (e.g. Montag et al. 2016; Parker and McQueen 2013; Walston et al. 2024). However, as our results indicate, these assessments lack differentiation, as there are significant differences in species composition, species richness, and abundances between different locations within solar parks, at least for carabid beetles—but probably also for other insect groups such as butterflies, grasshoppers (cf. Herden et al. 2009) or bumblebees (cf. Graham et al. 2021 for pollinators). Therefore, future studies on different species groups should always include all relevant solar park locations and also consider the areas under solar panels, as they account for a major part of solar parks.

In this regard, it should also be noted that in UNDR not only the activity densities of carabid beetles were significantly reduced, but consequently also their biomass. Since carabids serve as food for numerous organisms of higher trophic levels (e.g. for many bird species, especially in the breeding season) (Larochelle 1980; Wilson et al. 1999), this might also have adverse effects on biotic interactions and food webs (see also Barré et al. 2023 on the quality of solar parks as foraging habitat for bats). It is also important to note that the gaps between the panel rows were rather wide (3.5–4 m) in our solar parks. In other solar parks, especially those where panels are aligned in an east–west direction, considerably larger areas are covered with panels. This probably has an even more negative impact on carabids (and certainly also on other species groups) and presumably also on species that feed on them.

Another important finding of our study is, that we almost exclusively found common and widespread, non-threatened generalists in the surveyed solar parks—even in the more species-rich locations EDGE and BTW. The few regionally endangered species only occurred within individual solar parks and with very low activity densities, and therefore certainly did not harbor any larger populations within the facilities. One reason for the lack of endangered species is certainly the immediate surrounding of the solar parks, which, as with many solar parks in Germany (Badelt et al. 2024; Niemann et al. 2017), consisted of industrial estates, traffic infrastructure and intensively used agricultural landscapes. Therefore, immigration of these species from the immediate vicinity of the solar parks was not really to be expected either. However, as the solar parks already existed for more than 10 years at the time of the study, there was a chance for endangered species, e.g. mobile (flight-capable) xero-thermophilic species, to migrate from the wider surrounding, which apparently was also not the case. However, as we did not carry out any surveys outside our parks and therefore do not have any knowledge about the species inventory of their surroundings, it is not possible to say whether the absence of threatened or more demanding species in our solar parks was a result of their habitat characteristics and poor habitat quality (e.g. due to the low structured and grass-dominated vegetation as a result of initial grass seeding and regular mulch mowing) or whether there were simply no source populations in their surroundings. Future studies should therefore also include the surroundings of solar parks and assess whether certain endangered species can benefit from the (micro)habitats provided by solar parks or whether these facilities have a negative impact on them.

Implications for conservation and research demand

Our results show that different locations within solar parks differ considerably with regard to their importance as habitat for carabid beetles. While beetle assemblages under solar panels (UNDR) were severely impoverished, EDGE and BTW differed only slightly and showed significantly higher species numbers and activity densities. Thus, the narrow (3.5–4 m) gaps between the panel rows offered similar habitat conditions and habitat potential for carabids as the edge areas. Therefore, between the panel rows and at the edge of solar parks, larger areas should be kept free from construction with solar panels in order to provide habitats for carabid beetles and other insects. Another option might be to mount the solar panels at higher levels or to use solar trackers in order to reduce the shading impact of the panels by increasing their distance to the ground and/or preventing permanent coverage; however, there is still a need for further research on this issue.

Overall, the investigated solar parks almost exclusively provided habitat for widespread and common non-threatened generalists, while species of conservation concern were extremely rare. We assume that the habitat quality within the solar parks might be limited for more demanding species due to the regular mulch mowing and the grass-dominated, low-structure vegetation. However, EDGE and BTW offer great opportunities for the implementation of measures to increase structural diversity (e.g. by establishing wildflower strips or by using special seed mixtures for solar parks) and thus potentially to improve the habitat potential for carabid beetles and biodiversity as a whole (cf. Meyer et al. 2023; Tölgyesi et al. 2023; Walston et al. 2024).

There is a considerable need for further research on the importance of solar parks as habitat for carabid beetles and factors influencing their habitat function and quality. Thereby, the following aspects should be particularly considered:

  • Succession of the carabid fauna during the ageing of solar parks,

  • Influence of management type (mowing, grazing) and intensity (e.g. intervals and timing of management) on carabid beetles and other species groups,

  • Influence of solar panel characteristics (e.g. height, orientation) and spacing between panel rows on the carabid fauna,

  • Habitat function and quality of solar parks in other seasons (e.g. importance as overwintering habitat), landscapes (e.g. forest-dominated areas) and climatic regions (e.g. in the Mediterranean),

  • Influence of the surrounding area on diversity and species composition of carabid beetles within solar parks

  • Comparison of the habitat quality of solar parks to other land use types, e.g. arable land or grassland, as solar parks are regularly established on these sites,

  • Development and monitoring of measures to improve solar parks for carabid beetles and other species groups and, in particular, for species of conservation concern.

In such studies, all relevant locations of solar parks (UNDR, BTW, EDGE) should always be included in order to obtain meaningful and differentiated results and a comprehensive evaluation of the biodiversity value of solar parks.