Urban–Peri-Urban Gradient Effects on Beetle Assemblages: Ecological Contrasts and Climatic Responses in Sibiu, Romania

Abstract

Urban and peri-urban ecosystems play a growing role in biodiversity conservation, yet multi-annual comparative studies from Central-Eastern Europe remain scarce. This study presents the first three-year (2021–2023) dataset comparing ground beetle assemblages between the Dumbrava Forest (peri-urban protected oak forest) and the Sub Arini Park (semi-anthropic urban park) in Sibiu, Romania. Using standardized pitfall trapping (41 traps, 2360.9 m² monitored area), a total of 5008 individuals, belonging to 46 species and 12 families, were recorded. Species richness was slightly higher in Sub Arini (26 species) than in Dumbrava (22 species), forest-associated species (e.g., Pterostichus niger) and generalists (P. melanarius) dominated in the Dumbrava Forest, while P. oblongopunctatus was more strongly associated with forest habitats. Diversity indices showed moderate similarity between communities (Bray–Curtis = 0.46; Jaccard = 0.62). Shannon diversity reached H′ = 2.41 in Sub Arini and H′ = 2.03 in Dumbrava, reflecting higher evenness in the urban park. Predators comprised 65–70% of all beetles, underlining their regulatory function in soil ecosystem balance. Climatic variability—milder winters and warmer summers—favored population fluctuations of forest species and the dominance of eurytopic taxa in the park. These findings demonstrate that peri-urban forests act as climatic refugia for specialists, while urban parks function as dynamic hotspots for generalist diversity. The study provides baseline data for integrating insect monitoring into regional biodiversity management and climate adaptation strategies across Central-Eastern Europe.

1. Introduction

Forests are among the most complex and valuable terrestrial ecosystems, representing major biodiversity reservoirs and providing essential ecological services for human well-being [1]. In Romania, forest ecosystems extend over approximately 0.2–0.3 million hectares and host more than half of the known biocenosis, playing a central role in maintaining ecological stability [2,3,4]. Consequently, forest biodiversity conservation has become a strategic priority, with numerous studies addressing the diversity, distribution, and ecological roles of beetles and other invertebrate groups across different forest types [3,5,6,7,8,9,10,11,12,13,14,15,16,17]. Romania harbors more than 6800 of the 8000 beetle species known in Europe, distributed across 122 families and 15 superfamilies, a richness that places the country among Europe’s main biodiversity hotspots [18,19,20,21,22,23].

Within this national context, Dumbrava Forest, located near Sibiu, has long attracted scientific attention for its exceptional entomofauna, as documented in studies coordinated by the Natural History Museum of Sibiu [24,25,26,27,28,29]. These works emphasized both the faunistic richness of the area and the ecological importance of invertebrates as bioindicators of habitat quality, due to their sensitivity to microclimatic fluctuations, vegetation structure, and anthropogenic pressures [30]. Dead wood, a fundamental structural component of forest ecosystems, has been repeatedly highlighted as a key factor supporting biodiversity, providing microhabitats for insects, fungi, birds, and mammals, and contributing to nutrient cycling and soil formation [3,8,31,32,33,34]. However, increasing human disturbance requires the adoption of sustainable forest management practices to preserve ecological integrity and functional diversity [10,15,35,36,37,38].

Among invertebrates, ground beetles (Coleoptera: Carabidae) stand out due to their taxonomic diversity, ecological functions, and responsiveness to environmental change. They are widely recognized as reliable bioindicators of habitat quality and land-use intensity [39,40,41,42], and have been extensively studied regarding their taxonomy, distribution, and ecological roles [43,44,45,46,47,48,49,50,51,52]. Earlier faunistic surveys in Dumbrava [24,25,26,27,28,29] reported several rare and protected taxa, while more recent investigations highlighted new threats, notably the spread of invasive species such as the oak lace bug Corythuca arcuata (Say, 1832), which affects canopy dynamics and alters microclimatic stability in temperate oak forests [53].

Despite the substantial body of literature on Romanian forest entomofauna, comparative analyses between semi-anthropic urban parks and peri-urban protected forests remain scarce. Such studies are essential for understanding how biodiversity patterns respond to increasing urbanization and climate variability in Central and Eastern Europe. The present study addresses this gap by comparing two representative green infrastructures of Sibiu: Dumbrava Forest (a peri-urban protected oak–hornbeam forest, IUCN category IV) and Sub Arini Park (a semi-anthropic urban park with mixed native and exotic vegetation).

The main objectives of this study are as follows: (1) perform a quantitative and qualitative assessment of beetle populations in both ecosystems, emphasizing rare and protected species; (2) evaluate the ecological roles of beetles in ecosystem functioning and their response to anthropogenic pressures such as grazing, tourism, and logging [54,55,56,57]; (3) analyze community structure using classical ecological indices (abundance, dominance, constancy, and Dzuba index) [58,59]; and (4) investigate vegetation–beetle interactions to identify species with high bioindicator value [60,61,62,63,64,65,66,67].

By integrating multi-annual monitoring with standardized ecological indices, this study provides the first comparative multi-year analysis of beetle diversity in urban and peri-urban ecosystems of Sibiu. The findings fill a significant knowledge gap for Central-Eastern Europe, offering baseline data to support biodiversity monitoring, conservation strategies, and the integration of insect-based indicators into urban planning and climate adaptation policies.

2. Materials and Methods

2.1. Study Area

This study was conducted in two contrasting green infrastructures within Sibiu County, central Romania—the Dumbrava Forest, a semi-natural peri-urban woodland, and Sub Arini Park, an intensively managed urban park. The selection of these ecosystems aimed to capture ecological variability along an urban–peri-urban gradient, representative of biodiversity responses to anthropogenic pressure.

The terms used in this paper are defined as follows: Urban—areas located within the built environment, dominated by artificial surfaces and maintained vegetation (e.g., Sub Arini Park). Peri-urban—transitional landscapes between urban cores and rural matrices, with semi-natural habitats partially affected by anthropogenic use (e.g., Dumbrava Forest). Semi-anthropic—areas where human influence modifies natural vegetation but ecological functionality is still partially retained (Figure 1).

Figure 1. Map of Romania showing Sibiu County and the two study sites (Dumbrava Forest and Sub Arini Park). Insets include an orientation compass, legend, and scale bar for improved readability.

2.1.1. Dumbrava Forest

Located approximately 4 km southwest of Sibiu (45°52′ N, 24°14′ E), Dumbrava Forest covers 993 ha at 510–606 m a.s.l. and represents a remnant of the historical Transylvanian oak–hornbeam forests. It is classified as a protected area of national interest (IUCN Category IV). The dominant forest association is Querco robori–Carpinetum dacicum, with Quercus robur, Q. petraea, and Carpinus betulus as primary canopy species. Subdominant trees include Tilia cordata, Fraxinus excelsior, and Acer campestre, while the understory features Crataegus monogyna, Cornus sanguinea, and Prunus spinosa. The herbaceous layer is diverse, including Galeobdolon luteum and Hedera helix.

2.1.2. Sub Arini Park

Established in 1857 and covering 21.65 ha at 420–440 m a.s.l., Sub Arini Park (45°87′ N, 24°23′ E) is among Romania’s oldest and largest urban green spaces. Its dendrofloral composition includes both native and ornamental species such as Quercus robur, Tilia cordata, Platanus × acerifolia, Ginkgo biloba, and Acer platanoides. The park is subject to continuous management, frequent mowing, and visitor pressure, producing fragmented habitats and microclimatic heterogeneity.

Both sites are characterized by a temperate-continental climate, with a mean annual temperature of 9.3 °C and mean annual precipitation of 675 mm (data: Sibiu Meteorological Station, 2021–2023).

A comparative description of both ecosystems, including research periods, monitoring areas, and floristic composition, is presented in Table 1.

Table 1. Comparative characteristics of the two study ecosystems in Sibiu County, Romania.

Variable/Parameter	Dumbrava Forest (Peri-Urban)	Sub Arini Park (Urban)
Research Location	Situated 4 km southwest of Sibiu, at elevations between 510 and 606 m a.s.l. (maximum altitude at Obreja Peak). Total area: 993 ha. Mean stand age ≈170 years. Declared a protected area of national interest (IUCN Category IV) in 2000 [67].	Located in the central part of Sibiu City. Total area: 21.65 ha. Average stand age ≈150 years. Established in 1857, recognized as the largest and oldest urban park in Sibiu County.
Habitat Type	Mixed deciduous forest (Querco robori–Carpinetum dacicum association) dominated by Quercus robur, Q. petraea, and Carpinus betulus, with Tilia cordata, Fraxinus excelsior, and Acer campestre as secondary species.	Artificially structured urban green space with mixed native and ornamental trees (Quercus robur, Tilia cordata, Platanus × acerifolia, Ginkgo biloba, Acer platanoides), arranged along pedestrian alleys and open lawns.
Monitoring Area	Sampling conducted near county road DJ106A (Sibiu–Rășinari), at ≈510 m elevation. Two circular sampling plots (radius 12 m) encompassing 452.39 m2 each were established.	Sampling conducted along six linear transects (T1–T17), each 250–700 m long, placed between terrain dikes. The monitored area covered the entire 21.65 ha of the park.
Sampling Period	2021: 16 field expeditions over 156 days (March–August). 2022: 19 expeditions over 178 days (April–September).	2023: 25 field expeditions over 184 days (April–November), covering all four seasons.
Sampling Method	Ground beetles and other edaphic taxa were captured using pitfall traps, targeting large, mobile, and predatory beetles. Traps were handmade from two nested PET containers (2 L + 1.5 L) with a PVC funnel. Total: 24 traps, arranged in two circular transects (C1–C12 each), spaced 50 m apart. Each trap had an effective opening surface of 226.08 cm2 (see Figure 2 and Figure 3).	The park was divided into six habitat zones, within which 17 traps were randomly distributed (T1–T17). Traps followed the same PET–PVC model as in Dumbrava Forest and were checked bi-weekly.
Vegetation Surveys	Two phytosociological inventories conducted in 2021–2022 using the Braun–Blanquet method, confirming the dominance of Querco robori–Carpinetum dacicum var. Asperula odorata.	Vegetation surveys relied on previous inventories by Dollu (1965–1966) and Drăgulescu (1972), which identified 80 tree and shrub species. The current survey confirmed persistence of diverse ornamental and native taxa.
Climatic Context (2021–2023)	Temperate-continental climate with mean annual temperature 9.3 °C and mean annual precipitation 675 mm. Data from Sibiu Meteorological Station were used to contextualize interannual variation in beetle activity.	Same regional climate regime: microclimatic variation expected due to heat-island effect, increased radiation exposure, and soil moisture heterogeneity.
Analytical Methods	Ecological indices calculated: Constancy (C), Dominance (D), Ecological Significance (W), Shannon–Wiener (H′), Simpson (D), and Pielou Evenness (J′). Data normalized per trap-day.	Same analytical framework applied for comparability; community similarity measured using Bray–Curtis and Jaccard indices.
Sampling Effort (overview)	Two years (2021–2022) of sampling, 24 traps, total monitored surface ≈904.78 m2.	One year (2023) of sampling, 17 traps, total monitored surface ≈3843.36 m2; data standardized for cross-site comparison.

Note: All quantitative data were standardized per trap-day. Abbreviations: T—temperature; precip.—precipitation; H′—Shannon–Wiener index; J′—Pielou evenness.

2.2. Sampling Design and Field Procedures

To evaluate beetle assemblages across habitats, pitfall traps were used following European biodiversity monitoring protocols [39,68,69,70,71,72,73]. Each trap consisted of two nested polyethylene containers (2 L and 1.5 L) equipped with a PVC funnel and partially filled (⅓) with a water–detergent solution (neutral, non-attractant). Traps were buried flush with the soil surface, stabilized with small stones, and covered with natural litter to minimize evaporation and visual disturbance (Figure 2).

Figure 2. Pitfall trap design and field placement in the study area. (a) Hand-made pitfall trap used to sample ground-dwelling beetles (2 L protective container + 1.5 L collector, PVC funnel, water–detergent preservative). (b) Example of trap location and marking in the field (stake and coded label). Photos: C. Stancă-Moise. Handmade traps (photo Cristina Moise).

A total of 41 pitfall traps were installed: Dumbrava Forest—24 traps, active from March to September (2021–2022), arranged in two circular transects (C1–C12 per circle; 12 m radius; 50 m between circles). Sub Arini Park—17 traps, active from April to November (2023), distributed along six linear transects (T1–T17) spanning 250–700 m.

Each trap represented an independent sampling unit, with a capture area of 226.08 cm², resulting in a total monitored surface of 2360.9 m². The unequal sampling duration between Dumbrava (two years) and Sub Arini (one year) was accounted for statistically by normalizing data per sampling effort (individuals/trap/day) prior to analysis (Figure 3).

Figure 3. (a) Spatial layout of pitfall traps (C1–C19) in Dumbrava Forest; each point represents trap position. Red symbols = affected trees, green = healthy trees. (T1–T3) General view of sampling traps in Sub Arini Park marked with red circles.

2.3. Specimen Processing and Taxonomic Identification

Traps were emptied bi-weekly, and specimens were transferred to labeled containers with preservative solution (70% ethanol). In the laboratory, beetles were sorted and identified under a Nikon SMZ-745 stereomicroscope (made by Nikon Corporation, Tokyo, Japan) using standard morphological keys for Central and Eastern Europe [68,74,75,76,77,78,79,80,81]. Nomenclature followed the Catalogue of Palaearctic Coleoptera [80,81], with synonymy verified through BioLib [74].

All Coleoptera families captured were included in the dataset, not only Carabidae, to ensure a comprehensive understanding of beetle assemblages as recommended by reviewers. Conservation status and distribution were checked using the European Red List of Beetles [82] and the Romanian Red Book of Invertebrates [83]. Voucher specimens were deposited in the Entomological Collection, Faculty of Agricultural Sciences, Food Industry and Environmental Protection, Lucian Blaga University of Sibiu.

2.4. Climate Data and Statistical Analysis

Daily temperature, precipitation, and relative humidity data for the 2021–2023 sampling period were obtained from the Sibiu Meteorological Station (National Meteorological Administration, Bucharest, Romania). For each month, mean minimum and maximum temperature, total precipitation, and average humidity were calculated to contextualize beetle activity and abundance patterns. Vegetation cover and density were estimated visually within a 1 m radius around each trap following Braun–Blanquet classes (1–5), averaged to produce a vegetation density index used in NMDS ordination.

Beetle abundance and species richness were standardized to the sampling effort (individuals/trap/day) to ensure comparability between sites and years. Community-level analyses were performed using classical ecological indices widely applied in biodiversity monitoring [68,69,70,84,85,86,87,88,89]. Species-level abundance data were aggregated to genus and functional guilds (predators, decomposers, herbivores) to produce the summarized results displayed in Figure 4:

Figure 4. Distribution and comparative abundance of beetle families collected in Dumbrava Forest (peri-urban) and Sub Arini Park (urban). Values represent relative percentages of total individuals. Error bars indicate standard deviation between years. Note: Data represent mean values at the species level aggregated by guild; abundance expressed as individuals/trap/day (mean ± SD).

Abundance (A), expressed as the percentage of individuals belonging to a species within the total community:

$$\mathrm{A}={\displaystyle \frac{\mathrm{n}}{\mathrm{N}}}\times 100$$

where n = number of individuals of a given species, and N = total individuals collected.

Dominance (D), classifying species according to their relative abundance (D1–D6), from sporadic to eudominant categories [71,72,73].

Constancy (C), calculated as the proportion of samples in which a species occurred, expressed as a percentage and assigned to classes C1–C5 (from very rare to constant).

Ecological significance (W), or the Dzuba index, which integrates dominance and constancy to estimate the structural importance of each species in the community:

$$\mathrm{W}={\displaystyle \frac{\mathrm{D}\times \mathrm{C}}{100}}$$

Diversity was further evaluated through the Shannon–Wiener index (H′), Simpson index (D), and Pielou evenness (J′), providing complementary perspectives on community structure:

$${\mathrm{H}}^{\prime }=-\sum \left({\mathrm{p}}_{\mathrm{i}}\ln {\mathrm{p}}_{\mathrm{i}}\right)\mathrm{D}=1-\sum {\mathrm{p}}_{\mathrm{i}}^2 {\mathrm{J}}^{\prime }={\displaystyle \frac{{\mathrm{H}}^{\prime }}{\ln \mathrm{S}}}$$

where p_i is the relative abundance of species i and S is total species number.

To assess inter-site community similarity, both Bray–Curtis (BC) and Jaccard (J) indices were computed:

$$\mathrm{B}\mathrm{C}=1-{\displaystyle \frac{2{\mathrm{C}}_{\mathrm{i}\mathrm{j}}}{{\mathrm{S}}_{\mathrm{i}}+{\mathrm{S}}_{\mathrm{j}}}}$$

where C_ij is the sum of the lesser abundances of species common to both sites, and S_i, S_j are the total abundances for each site.

$$\mathrm{J}={\displaystyle \frac{\mathrm{a}}{\mathrm{a}+\mathrm{b}+\mathrm{c}}}$$

where a = number of shared species, b and c = species unique to each site.

Statistical analyses were carried out using R v4.3.2 (packages vegan and stats) and PAST v4.14 for multivariate ordination. Differences in diversity and abundance between Dumbrava Forest and Sub Arini Park were tested using one-way ANOVA (p < 0.05).

To examine environmental effects, Generalized Linear Models (GLMs) with Poisson distribution were applied, using beetle abundance as the dependent variable and climatic predictors (mean temperature, precipitation, humidity) as independent variables. Model selection followed the Akaike Information Criterion (AIC), and validation included residual analysis and dispersion checks to ensure model robustness.

All indices and analyses were selected based on their broad applicability in urban ecology and to guarantee reproducibility and comparability with similar European datasets. Raw occurrence data are available upon reasonable request from the corresponding author.

3. Results

3.1. Overview of Sampling and Taxonomic Composition

Across both ecosystems and three years of monitoring (2021–2023), a total of 5164 individuals representing 46 species and 12 beetle families (Coleoptera) were recorded. The total number of specimens collected was slightly higher in Dumbrava Forest (2890 individuals, 22 species, 10 families) than in Sub Arini Park (2118 individuals, 26 species, 9 families).

Among the recorded families, Carabidae was the most abundant and diverse, comprising 23 species and 61.7% of the total individuals captured, followed by Staphylinidae (11.4%), Silphidae (8.2%), and Tenebrionidae (6.9%). The remaining eight families accounted collectively for less than 12% of the total catch (Figure 4). In both sites, carabids dominated the pitfall trap samples, but the relative contribution of other families (notably Staphylinidae and Scarabaeidae) increased significantly in the urban habitat (p < 0.05; ANOVA).

3.2. Species Richness and Diversity Indices

Species richness was higher in Sub Arini Park (26 species) compared to Dumbrava Forest (22 species), reflecting the structural heterogeneity and higher microhabitat variability of the urban environment. However, overall beetle abundance was greater in the forest, consistent with its more stable microclimatic and trophic conditions.

Diversity indices (Table 2) confirmed these trends: Shannon–Wiener index (H′) averaged 2.31 ± 0.08 in Sub Arini and 2.12 ± 0.06 in Dumbrava; Simpson index (D) values were 0.84 ± 0.02 and 0.81 ± 0.03, respectively; Pielou evenness (J′) indicated more uniform species distribution in Sub Arini Park (0.81 vs. 0.75).

Table 2. Comparative values of ecological indices (mean ± SD) for beetle communities in Dumbrava Forest and Sub Arini Park.

Index	Dumbrava Forest	Sub Arini Park	Significance (ANOVA, p)
Shannon–Wiener (H′)	2.12 ± 0.06	2.31 ± 0.08	0.041 *
Simpson (D)	0.81 ± 0.03	0.84 ± 0.02	0.049 *
Evenness (J′)	0.75 ± 0.04	0.81 ± 0.03	0.038 *
Ecological Significance (W)	5.4 ± 1.1	4.9 ± 1.3	n.s.

Note: Mean ± standard deviation (SD); * p < 0.05 indicates significant differences between sites. n.s.—not significant.

Dominance and constancy analyses revealed that the carabids Pterostichus melanarius Illiger, 1798, Carabus coriaceus Linnaeus, 1758, and Abax parallelepipedus Piller & Mitterpacher, 1783 were consistently present and dominant in Dumbrava, whereas Amara familiaris Duftschmid, 1812 and Harpalus affinis Schrank, 1781 showed increased frequency in the urban site (Figure 5).

Figure 5. Comparative species diversity (H′) and dominance (D) indices for Dumbrava Forest and Sub Arini Park, showing greater evenness and taxonomic richness in the urban park. Note: Error bars represent standard deviation (SD).

3.3. Community Structure and Functional Composition

Beetle communities differed markedly in trophic structure between the two ecosystems.

In Dumbrava Forest, predatory taxa (mainly Carabidae and Staphylinidae) comprised 72% of individuals, while saprophagous (e.g., Silphidae, Tenebrionidae) and phytophagous species were underrepresented (18% and 10%, respectively). Conversely, Sub Arini Park exhibited a more balanced trophic composition, with predators (54%), saprophages (29%), and phytophages (17%), indicating greater habitat mosaic and food-source diversity (Figure 6).

Figure 6. Comparative trophic composition of beetle communities in Dumbrava Forest and Sub Arini Park, expressed as relative abundance of feeding guilds. Note: Data represent mean values at the species level aggregated by guild; abundance expressed as individuals/trap/day (mean ± SD).

Guild-level diversity was significantly higher in the urban site (R² = 0.62, p < 0.05), while total abundance correlated positively with vegetation cover and negatively with disturbance intensity (R² = 0.57).

3.4. Temporal Dynamics and Seasonal Activity

Temporal variation in beetle activity density (Figure 7) revealed two main peaks: (1) a spring peak (April–May) dominated by Carabus coriaceus and Abax parallelepipedus, and (2) an autumn resurgence (September–October) particularly in Sub Arini, linked to Harpalus spp. and Amara spp.

Figure 7. Seasonal activity patterns of beetle assemblages in both ecosystems (mean individuals/trap/day). Lines show mean monthly activity; shaded bands indicate standard error. Note: The model was significant at p < 0.05, but effect magnitude was moderate due to low mean capture rates (0.25–2 ind/trap/day).

The 2021–2022 forest data showed stable seasonal profiles, while the 2023 urban dataset exhibited higher variability and later activity peaks due to warmer microclimatic conditions.

3.5. Inter-Site Community Similarity

Bray–Curtis similarity (BC) and Jaccard index (J) analyses revealed moderate overlap between communities: BC = 0.46, J = 0.39, indicating distinct assemblages shaped by contrasting management regimes. Non-metric multidimensional scaling (NMDS) ordination (stress = 0.09) separated samples from the two ecosystems along the first axis, corresponding to vegetation cover and soil disturbance gradient (Figure 8).

Figure 8. NMDS ordination of beetle assemblages across sampling sites (stress = 0.09). Circles represent Dumbrava traps; triangles denote Sub Arini traps. Color gradient reflects vegetation cover density. Note: Labels were repositioned to avoid overlap.

3.6. Climatic Correlations and Model Outputs

Generalized Linear Models (GLMs) showed that beetle abundance was significantly influenced by mean monthly temperature (β = 0.43, p = 0.017) and precipitation (β = −0.32, p = 0.042), explaining R² = 0.64 of total variance. Model residuals met homoscedasticity assumptions, confirming robustness.

Principal Component Analysis (PCA; Figure 9) grouped species according to their ecological affinities: Axis 1 (47% variance) reflected microclimatic and soil humidity gradients, Axis 2 (29%) captured anthropogenic disturbance and canopy openness. Predatory species (Carabus coriaceus, Pterostichus melanarius) were associated with shaded forest sectors, while generalists (Harpalus affinis, Amara familiaris) clustered within open, disturbed urban plots.

Figure 9. Principal Component Analysis (PCA) of beetle species in relation to environmental variables. Vectors represent significant climatic predictors (temperature, humidity, disturbance index). Note: Data represent mean values at the species level aggregated by guild; abundance expressed as individuals/trap/day (mean ± SD). Labels were repositioned to avoid overlap.

3.7. Species of Conservation and Ecological Importance

Several forest-associated species (Carabus coriaceus, Abax parallelepipedus, Pterostichus niger) were restricted to Dumbrava Forest, suggesting high conservation value of this semi-natural woodland.

Conversely, Harpalus affinis, Amara aenea, and Ontholestes murinus were more frequent in Sub Arini Park, indicating tolerance to anthropogenic stress (Table 3).

Table 3. Species of conservation relevance recorded during the study, with dominance category (D), constancy class (C), and ecological significance (W).

Species	Family	Site of Occurrence	D	C	W	Conservation Status
Carabus coriaceus (Linnaeus, 1758)	Carabidae	Dumbrava	D6	C6	0.92	LC
Abax parallelepipedus (Piller & Mitterpacher, 1783)	Carabidae	Dumbrava	D5	C5	0.77	LC
Harpalus affinis (Schrank, 1781)	Carabidae	Sub Arini	D4	C4	0.69	LC
Silpha carinata (Linnaeus, 1758)	Silphidae	Sub Arini	D3	C5	0.58	LC
Ontholestes murinus (Linnaeus, 1758)	Staphylinidae	Sub Arini	D2	C3	0.41	LC

Abbreviations: LC—Least Concern (IUCN Red List category).

4. Discussion

The comparative analysis of beetle assemblages from Dumbrava Forest and Sub Arini Park provides valuable insights into the ecological structure, diversity, and functional roles of coleopteran communities in two contrasting urban ecosystems of Sibiu. By integrating multi-annual monitoring (2021–2023) with ecological indices, the study reveals both similarities and marked differences shaped by habitat type, vegetation heterogeneity, and anthropogenic pressures.

4.1. General Patterns and Community Structure

The comparative assessment of beetle assemblages between Dumbrava Forest and Sub Arini Park highlights distinct ecological trajectories resulting from divergent management regimes, habitat complexity, and microclimatic conditions. Both ecosystems sustained diverse communities dominated by Carabidae, consistent with previous studies identifying this family as a reliable bioindicator of forest integrity and disturbance gradients [39,40,41,42,60].

In Dumbrava Forest, the prevalence of Pterostichus niger, P. melanarius, and Abax parallelepipedus reflects a community structured around generalist frequently occurring in forest habitats, typically associated with mature oak–hornbeam stands and shaded litter layers [3,43,52]. Conversely, Sub Arini Park, characterized by anthropogenic heterogeneity and mixed exotic vegetation, was dominated by Carabus violaceus, Amara familiaris, and Harpalus affinis, taxa known for their high ecological plasticity and tolerance to heat-island effects [7,11,47].

These findings align with urban–rural gradients documented elsewhere in Central Europe, where peri-urban forests retain relict faunal assemblages while urban parks favor adaptable generalists [8,9,53].

4.2. Species Richness, Diversity, and Evenness

Despite the smaller area, Sub Arini Park exhibited slightly higher species richness and Shannon diversity (H′ = 2.31) compared with Dumbrava Forest (H′ = 2.12), confirming that structural mosaicism and vegetation heterogeneity enhance local species diversity [68,71,90,91,92,93,94]. The higher evenness (J′ = 0.81) observed in the park suggests that no single taxon monopolized resources, while the more pronounced dominance pattern in Dumbrava reflects habitat stability but reduced niche diversity.

These results corroborate the intermediate disturbance hypothesis, which predicts maximal diversity at moderate disturbance levels due to increased niche heterogeneity [3,10,15,35,36,37,38]. However, excessive anthropogenic influence may destabilize communities by favoring opportunistic taxa, as observed in other European cities [7,8,9].

4.3. Functional Guild Composition

Functional guild analysis revealed that predators comprised over two-thirds of the total catch, emphasizing their key role in soil trophic regulation and decomposition processes. Dumbrava’s predator dominance (≈72%) supports the hypothesis that closed-canopy forests favor stable carabid populations through abundant detrital prey [31,32,33,34,39]. In contrast, the higher proportion of saprophagous and phytophagous species in Sub Arini (≈46%) reflects the park’s richer detrital input, ornamental flora, and microclimatic variability.

Such guild restructuring mirrors patterns reported in Mediterranean and Central-European urban greenspaces, where omnivores and decomposers expand under mild microclimates and increased organic matter [7,47,60]. The balance among guilds indicates that both sites maintain functional redundancy—an essential element of ecosystem resilience under climate stress [53,54,55,56,57].

4.4. Temporal Dynamics and Climatic Drivers

Seasonal activity patterns exhibited clear bimodal peaks—spring and autumn—with site-specific timing influenced by temperature and humidity. The first peak (April–May) was dominated by Carabus coriaceus and Abax parallelepipedus, while the second (September–October) was more pronounced in Sub Arini Park, largely driven by Harpalus and Amara species. Although the numerical differences appear small, they are ecologically relevant given the low overall beetle activity densities recorded across sites.

Data from 2021 to 2022 in Dumbrava Forest showed relatively stable seasonal profiles, whereas the 2023 urban dataset displayed greater temporal variability and later activity peaks, reflecting the warmer microclimatic conditions typical of urban environments.

Generalized Linear Models (GLMs) indicate that temperature has a significant positive effect, while precipitation showed a negative influence on beetle abundance, confirming the sensitivity of epigeic beetles to microclimatic fluctuations. These relationships illustrate how thermal and moisture dynamics shape community composition and seasonal activity intensity.

The extended activity period in Sub Arini Park during warmer months exemplifies the urban heat-island effect, a phenomenon increasingly documented across European cities [31,32,33,34,46]. In contrast, Dumbrava Forest, with its more buffered microclimate and dense canopy, maintained stable phenological patterns and preserved assemblages of forest-associated species.

Such temporal differentiation emphasizes the importance of integrating phenological monitoring into long-term biodiversity assessments, as urban warming can alter reproductive cycles, trophic interactions, and the ecological balance between generalist and specialist taxa [39,40,41,42,60].

4.5. Spatial Ordination and Environmental Correlates

Multivariate analyses (NMDS and PCA) revealed clear segregation between forest and park assemblages along vegetation cover and disturbance gradients [83,95,96]. In NMDS space (stress = 0.09), Dumbrava traps clustered in high-shade, high-humidity quadrants, while Sub Arini plots spread across open, disturbed areas. PCA axes associated temperature and disturbance with generalist taxa (e.g., Harpalus affinis), and humidity with forest specialists (Carabus coriaceus, Pterostichus melanarius). These patterns indicate that vegetation structure and soil microclimate are the strongest environmental predictors of community composition, confirming similar findings in temperate Europe [46,97,98,99,100]. They also highlight the importance of maintaining canopy heterogeneity and soil moisture balance as biodiversity safeguards under climate variability.

4.6. Conservation Implications

From a conservation perspective, both habitats demonstrate substantial ecological value, although through different functional pathways. Dumbrava Forest acts as a refugium for forest-associated beetles such as Carabus coriaceus and Abax parallelepipedus, which are widespread but valuable indicators of mature, structurally complex oak–hornbeam stands. The preservation of these habitats should prioritize the maintenance of old-growth trees, coarse woody debris, and stable microclimatic conditions, all of which sustain specialist predators and decomposers characteristic of temperate forests [24,25,26,27,28,29,43,44,45,46,47,48,49,50,51,52,101,102,103,104,105,106,107].

In contrast, Sub Arini Park, despite its anthropogenic management and high visitor pressure, supports a functionally diverse assemblage dominated by generalists and decomposer species. This highlights its role as an urban biodiversity reservoir, contributing to ecological connectivity, pollination services, and public awareness of conservation issues.

Integrating beetle monitoring into local management aligns with European biodiversity and climate adaptation objectives—particularly the EU Biodiversity Strategy for 2030 and the Nature-Based Solutions framework. Adaptive management should therefore focus on selective mowing, retention of deadwood and leaf litter, structural diversification of vegetation, and the creation of shaded–open habitat mosaics. Such measures would enhance ecological resilience, sustain soil processes, and improve the functional complementarity between peri-urban forests and urban green spaces [53,54,55,56,57,108,109,110,111,112,113].

4.7. Broader Context and Future Directions

This study provides one of the first multi-annual datasets from Central-Eastern Europe comparing peri-urban and urban coleopteran assemblages under current climatic conditions. The results reinforce that landscape heterogeneity and sustainable management mediate biodiversity resilience. Future work should expand sampling to other Transylvanian green infrastructures, employ molecular tools (DNA metabarcoding) for finer taxonomic resolution, and integrate remote sensing to track vegetation–fauna interactions. Long-term datasets such as this are essential to detect early ecological signals of urbanization and climate change, bridging local observations with continental-scale conservation monitoring.

5. Conclusions

This study provides a multi-annual comparative assessment of beetle assemblages along an urban–peri-urban gradient in Sibiu, Romania, under present climatic variability. The results reveal the distinct yet complementary ecological roles of the two investigated habitats.

Dumbrava Forest, a semi-natural peri-urban woodland, supports abundant populations of forest-associated carabids such as Pterostichus melanarius, Abax parallelepipedus, and Carabus coriaceus. Their dominance reflects stable microclimatic conditions, structural continuity, and limited disturbance—attributes essential for maintaining long-term biodiversity in temperate oak–hornbeam ecosystems. Conversely, Sub Arini Park, situated within the urban core, hosts a more functionally diverse beetle community dominated by thermophilous and generalist taxa (Harpalus affinis, Amara familiaris), indicating high ecological plasticity and adaptation to anthropogenic stress and habitat fragmentation.

Predatory guilds remained dominant in both systems, confirming the persistence of essential regulatory functions within urban and peri-urban ecological networks. Climatic variables—particularly temperature and precipitation—proved to be key drivers of beetle abundance and seasonal dynamics, emphasizing the sensitivity of epigeic fauna to microclimatic shifts.

From a conservation and management perspective, this case study highlights the ecological complementarity of Dumbrava Forest and Sub Arini Park and provides practical recommendations for biodiversity-friendly management within the Sibiu region. Maintaining old-growth stands, coarse woody debris, and soil microhabitats in Dumbrava Forest, together with selective mowing, native tree planting, and microhabitat diversification in Sub Arini Park, can enhance the ecological resilience and connectivity of both habitats.

Future perspectives should include the establishment of long-term insect monitoring programs and the integration of advanced analytical methods (e.g., trait-based and molecular approaches) to refine species identification and evaluate biodiversity responses to ongoing climate and land-use changes.