Assessment of Ecosystem Sustainability and Management Measures in the Danube Floodplains in Slovakia by the Bioindicative Value of Spiders (Araneae)

Abstract

The aim of this research was to record changes in the population structure of epigeic spider assemblages in the Central European Danube Delta (Slovakia) as a result of habitat management measures and the impact of human intervention. During this research (2020–2023), we assessed the impact of management measures on newly planted forest stands and the effect of grazing in semi-natural conditions, and carried out diversity monitoring in flooded meadows. A total of 6344 individuals belonging to 89 spider species were collected by pitfall traps and identified. Using spatial modelling, we observed the following: (i) there are differences between the structures of managed and unmanaged forest stands (larger number of taxa); (ii) the differences in the number of individuals between study plots and years were statistically significant; (iii) the trend analysis of spider communities showed that study plots that underwent management intervention are expected to see an increase in the number of individuals in the future; and (iv) in the areas that did not undergo management, the number of species was stable. Using spiders as bioindicators could therefore answer the question of whether anthropogenic disturbance disrupts ecological stability. This approach utilizes spiders to assess the sustainability of the landscape.

1. Introduction

The Danube is the second-longest river in Europe. Thousands of square kilometres in Central Europe host the extensive system of floodplain forests of the inland Danube Delta, which is influenced by the specific hydrological regime of the river, which is characterised by flooding. The Danube Delta is one of the most threatened ecosystems in Europe; therefore, for its protection, it is necessary to record the current state of biodiversity and subsequently maintain or ensure its restoration [1]. River regulation, catchment deforestation, and channelisation have also affected the hydrological conditions of the Danube [2,3]. Interventions on river channels and floodplains for agriculture and settlements, urbanisation, hydropower generation, and navigation date back to medieval times, or at least to the early industrialisation period. During the 20th century, human activities and civil engineering operations caused river ecosystem fragmentation and habitat destruction and modified dynamic system properties, such as the flooding regime; longitudinal, lateral, and vertical connectivity; biogeochemical cycles; and biodiversity patterns [3,4]. The araneocoenoses of the Danubian wetlands are markedly affected—more so than other species—by human activities such as simulated floods, the regulation of water regimes, and the restoration of river systems. Both the presence and absence of floods have a significant effect on the structures of epigeic spider communities [5].

Certain epigeic arthropods (e.g., spiders or beetles) are used as environmental bioindicators for monitoring and studying ecological changes and habitat disturbances [6]. Spiders are the most sensitive group of bioindicator animals and have specific habitat requirements. They respond rapidly to changes in conditions by modifying their community structure, abundance, and diversity [5]. These adaptive changes in invertebrates, combined with the impact of flooding, groundwater level fluctuations, and human management of some communities (e.g., molluscs, spiders), confirm the strong influence of these factors on other components of the environment, such as vegetation, soil, and microclimatic conditions in some habitats [7]. Research has shown that the operation of waterworks and power plants on the Danube has a direct and long-term impact on the succession process [8,9]. The impact of moisture on spider communities in Danube floodplain forests has significantly changed the functional composition of these communities and species. The occurrence of most spiders is strictly limited by physical conditions, such as temperature, humidity, wind, and light intensity, and by biological factors, such as vegetation type, food supply, competition, and predation [9].

Environmental factors are important factors affecting spider diversity, especially in open habitats. Matveinen-Huju [10] analysed spider communities from 70 habitat types using a correspondence analysis that correlated with shade and moisture. Beta-diversity was significantly higher in open habitats than in forests. Bonte et al. [11] emphasised the importance of maintaining the full range of moisture conditions in open habitats, because spider communities are limited by changes in vegetation structure (succession), atmospheric and soil moisture, and the occurrence of natural and anthropogenic disturbances. Light intensity and moisture are among the most important abiotic factors characterising spider habitats [12]. Among these factors [13], vegetation has been reported to have a major influence on spider communities. The composition (plant species richness) and structure (height of the plants) of vegetation can reflect the diversity of potential invertebrate prey and habitats for spiders.

The management and protection of these habitats requires an understanding of the relationships between taxa and environmental variables [2]. The most frequent of these are the reconnection of dead arms of the Danube, the planting of native vegetation in devastated areas, the establishment of pastures with native livestock, and the restoration of meadow remnants [6]. Epigeic spiders significantly reflect the impacts of land use intensification. In an integrated annual crop–livestock system, grazing intensification can negatively affect the abundance and richness of terrestrial spiders [14]. Therefore, low stocking densities of cattle in pastoral environments offer more favourable conditions for spiders. At low grazing intensities, there are higher densities of herbaceous plants with greater heights and diversity, resulting in a complex vegetation structure beneficial to spiders [15]. This provides diverse materials and spaces in the litter for web construction, increasing the abundance and diversity of spiders.

Several ecological and environmental levels must be considered to assess spider communities in specific habitats and characterise their status. For this purpose, a range of different materials and a level of knowledge about the ecological requirements of individual species are essential.

This research aimed to analyse the diverse and divergent responses of epigeic spider assemblages in the Danube inundation to the diversity of habitat conditions, as well as to confirm and clarify the relationships between spiders and the environment and human-made changes. The specific tasks of the study were: i. Characterise the spiders of Danubian floodplain forests in Central Europe, and their relationship with environmental management; ii. Evaluate the importance and impact of management on spider coexistence in floodplain areas.

2. Materials and Methods

2.1. Study Sites

The study sites were located in south-western Slovakia, within areas characterised by similar forest or meadow vegetation. These regions (natural landscapes) share uniform climatic and geomorphological conditions and are part of the extensive inundation zones of the Danube River. Research plots were established in 2020, and we continued our research in permanently monitored plots for four years, until 2023. For the present study, we chose three sites without human disturbance (S1 = forest Kraľovská lúka, S2 = meadow Veľký Lél, S3 = pasture Veľký Lél) and three sites subject to human activities to compare impact of management (S4 = meadow Kľúčovec—linking the dead arm of the Danube to restore the flood regime of the meadow, S5 = new forest Veľký Lél—re-ultivation of biotope and completely new planting of floodplain forest, S6 = pasture Bodíky—intensive cattle grazing in the former wet meadow) (Figure 1). The basic characteristics of the studied floodplain habitats, including DMS coordinates, are as follows:

• S1, floodplain forest Salici-Populetum Kraľovská lúka (47°53′36.7″ N 17°30′32.0″ E)
• S2, flooded meadow Veľký Lél (47°44′60.0″ N 17°56′09.9″ E)
• S3, extensive pasture Veľký Lél (47°45′09.3″ N 17°56′58.8″ E)
• S4, torso of non-flooded meadow Kľúčovec (47°47′07.2″ N 17°44′11.4″ E)
• S5, newly planted forest Veľký Lél (47°44′48.3″ N 17°55′20.2″ E)
• S6, intensive pasture Bodíky (47°54′33.5″ N 17°27′52.9″ E)

2.2. Sampling

During the research, spiders were collected from the monitored sites in the Danube River floodplain using pitfall traps. The traps consisted of glass jars (0.7 L capacity, 9 cm diameter) partially filled with an 8% formaldehyde solution used as a preservative. At each study site, traps were arranged in a linear transect, spaced approximately 5–10 metres apart. The obtained samples were sorted under a binocular stereomicroscope according to systematic classification. For long-term preservation, the specimens were transferred into 70% denatured alcohol. We identified the obtained spider specimens to the species level (and the sex of the individuals) following the standard keys [16,17,18].

2.3. Habitat Requirements

For the overall assessment and meta-analysis of the Danube spider fauna, we evaluated a total of 6344 individuals from the six Danubian sites.

Individual spider species were classified according to their habitat preference using relevant literature data [18,19] and our databases. Likewise, we followed standard methods for characterising spiders in terms of their moisture requirements [17].

2.4. Data Analyses

The distribution of epigeic spiders due to biotopes was analysed using principal component analysis (PCA), looking for relationships between species and study plots where management treatments were not conducted (S1–S3) and plots where management interventions were made (S4–S6). We tested the statistical significance of biotopes using the Monte Carlo test (iteration 499) in the Canoco5 programme [20].

We tested the normality of the data distribution using the Shapiro–Wilk W test in Python 3.12 [21]. This test confirmed the violation of the normality of the data distribution (p-value = 0.0001). Consequently, we used the non-parametric Friedman test to evaluate the difference in the number of individuals between the study areas and years. This confirmed a statistically significant difference (p ≤ 0.001). We also used multivariable regression to predict what the population size would be in 2027. We determined the spider’s diversity and balance of the study areas (S1–S6) using the Shannon diversity index (H’) and Margalef’s richness index to measure and compare species richness across different ecosystems, and Pielou index equitability to measure species distribution in assemblages.

3. Results

In total, we recorded 6344 individuals, representing 89 spider species across six study plots. Among the collected specimens, Pardosa agrestis (23.58%) was the most abundant species, followed by Trochosa ruricola (15.02%) and Pardosa lugubris (13.98%) (

Table 1. Abundance of spiders at study sites of the Danube River inundation area.

Taxon/Study Sites	RQ	S1	S2	S3	S4	S5	S6	∑ N	D (%)
Agroeca brunnea (Blackwall, 1833)	s-h	0	0	1	0	0	0	1	0.02
Agroeca cuprea Menge, 1873	dry	0	0	1	0	1	0	2	0.03
Alopecosa pulverulenta (Clerck, 1757)	s-h	0	1	16	60	0	0	77	1.21
Araeoncus humilis (Blackwall, 1841)	dry	1	0	1	6	0	0	8	0.13
Arctosa lutetiana (Simon, 1876)	?	6	0	0	0	6	40	52	0.82
Aulonia albimana (Walckenaer, 1805)	dry	0	1	0	0	8	0	9	0.14
Centromerus sp.	?	0	0	0	0	6	0	6	0.09
Centromerus sylvaticus (Blackwall, 1841)	h	11	0	4	10	8	0	33	0.52
Ceratinella scabrosa (O.P. Cambr., 1871)	h	13	0	0	0	0	0	13	0.20
Civizelotes gracilis (Canestrini, 1868)	dry	0	0	0	2	0	0	2	0.03
Civizelotes pygmaeus Miller, 1943	dry	0	0	0	0	2	0	2	0.03
Clubiona brevipes Blackwall, 1841	dry	0	0	1	0	0	0	1	0.02
Clubiona pallidula (Clerck, 1757)	s-h	0	0	0	0	2	0	2	0.03
Clubiona sp.	?	0	0	5	0	0	0	5	0.08
Diaea dorsata (Fabricius, 1777)	s-h	0	0	0	1	0	0	1	0.02
Diplocephalus picinus (Blackwall, 1841)	s-h	0	1	0	0	0	0	1	0.02
Diplostyla concolor (Wider, 1834)	h	9	0	21	4	0	0	34	0.54
Drassodes pubescens (Thorell, 1856)	dry	0	4	2	0	1	0	7	0.11
Drassyllus praeficus (L. Koch, 1866)	dry	0	0	0	0	12	0	12	0.19
Drassyllus pumilis (C.L. Koch, 1839)	dry	0	0	7	32	0	0	39	0.61
Drassyllus pusillus (C.L. Koch, 1833)	s-h	0	65	0	30	21	31	147	2.32
Drassyllus villicus (Thorell, 1875)	dry	0	0	4	0	12	0	16	0.25
Erigone dentipalpis (Wider, 1834)	s-h	0	5	0	0	0	0	5	0.08
Gnaphosa modestior Kulczyński, 1897	?	0	0	0	0	1	0	1	0.02
Gnathonarium dentatum (Wider, 1834)	h	0	0	0	2	0	0	2	0.03
Gongylidium rufipes (Linnaeus, 1758)	h	3	0	0	0	0	0	3	0.05
Hahnia pusilla C.L. Koch, 1841	s-h	0	0	1	0	0	0	1	0.02
Hahnia sp.	?	0	0	3	0	0	0	3	0.05
Haplodrassus signifer (C.L. Koch, 1839)	dry	0	0	1	10	4	0	15	0.24
Haplodrassus sp.	?	0	0	0	3	14	0	17	0.27
Harpactea lepida (C.L. Koch, 1838)	s-h	0	0	0	0	0	1	1	0.02
Lepthyphantes gen. sp.	?	18	1	0	0	0	3	22	0.35
Liocranoeca striata (Kulczyński, 1882)	h	80	5	7	7	4	0	103	1.62
Mermessus trilobatus (Emerton, 1882)	s-h	0	0	0	0	1	0	1	0.02
Micrommata virescens (Clerck, 1757)	s-h	0	1	0	0	0	0	1	0.02
Microneta viaria (Blackwall, 1841)	dry	2	0	0	0	0	0	2	0.03
Mioxena blanda (Simon, 1884)	dry	0	0	2	0	0	0	2	0.03
Misumena vatia (Clerck, 1757)	s-h	0	0	0	1	0	0	1	0.02
Moebelia penicillata (Westring, 1851)	s-h	1	0	0	0	0	0	1	0.02
Neriene clathrata (Sundevall, 1830)	h	4	2	6	0	0	0	12	0.19
Oedothorax apicatus (Blackwall, 1850)	s-h	0	69	0	0	0	2	71	1.12
Oedothorax retusus (Blackwall, 1850)	s-h	0	3	0	0	0	0	3	0.05
Ozyptila simplex (O.P. Cambridge, 1862)	s-h	0	1	0	2	0	0	3	0.05
Ozyptila praticola (C.L. Koch, 1837)	h	113	4	47	0	6	0	170	2.68
Pachygnatha degeeri Sundevall, 1830	dry	42	268	10	10	0	0	330	5.20
Pachygnatha listeri Sundevall, 1830	h	0	11	0	0	0	0	11	0.17
Palliduphantes insignis (O.P. Cambr., 1913)	dry	0	0	0	0	2	0	2	0.03
Palliduphantes pallidus (O.P. Cambr., 1871)	dry	4	0	0	0	0	0	4	0.06
Pardosa agrestis (Westring, 1861)	dry	0	624	0	84	0	788	1496	23.58
Pardosa amentata (Clerck, 1757)	h	3	2	0	0	0	0	5	0.08
Pardosa lugubris Walckenaer, 1802	e-dry	176	226	83	28	262	112	887	13.98
Pardosa prativaga (L. Koch, 1870)	h	0	67	0	0	0	103	170	2.68
Pardosa sp.	?	22	0	2	3	0	0	27	0.43
Philodromus sp.	?	0	2	0	0	0	0	2	0.03
Phrurolithus festivus (C.L. Koch, 1835)	dry	8	2	0	1	6	0	17	0.27
Piratula hygrophila (Thorell, 1872)	h	424	24	12	0	10	0	470	7.41
Pisaura mirabilis (Clerck, 1757)	dry	9	0	3	1	14	0	27	0.43
Porrhomma oblitum (O. P.-Cambr., 1871)	h	1	0	0	0	0	74	75	1.18
Robertus lividus (Blackwall, 1836)	s-h	0	1	2	7	0	0	10	0.16
Tallusia experta (O.P. Cambridge, 1871)	h	0	0	3	1	0	0	4	0.06
Tapinocyba biscissa (O.P. Cambridge, 1873)	s-h	0	0	4	0	0	0	4	0.06
Tapinocyba pallens (O. Cambridge, 1873)	s-h	0	0	2	0	0	0	2	0.03
Tegenaria campestris (C. L. Koch, 1834)	s-h	1	2	4	2	0	0	9	0.14
Tenuiphantes flavipes (Blackwall, 1854)	s-h	0	0	13	1	0	0	14	0.22
Tenuiphantes tenebricola Kulczyński, 1887	s-h	0	0	6	0	0	0	6	0.09
Thanatus arenarius L. Koch, 1872	dry	0	0	0	6	0	0	6	0.09
Thanatus sp.	?	0	0	3	0	0	0	3	0.05
Thanatus striatus C.L. Koch, 1845	h	0	1	0	0	1	0	2	0.03
Theridion sp.	?	0	4	0	0	0	0	4	0.06
Tibellus oblongus (Walckenaer, 1802)	dry	0	1	0	0	0	0	1	0.02
Titanoeca quadriguttata (Hahn, 1833)	dry	0	1	1	0	0	0	2	0.03
Titanoeca shineri L. Koch, 1872	dry	0	0	0	0	6	0	6	0.09
Tmarus piger (Walckenaer, 1802)	dry	1	0	0	0	0	0	1	0.02
Trachyzelotes pedestris (C. L. Koch, 1837)	dry	92	1	8	3	35	0	139	2.19
Trochosa ruricola (De Geer, 1778)	h	8	437	81	0	203	224	953	15.02
Trochosa terricola Thorell, 1856	s-h	20	22	0	161	91	0	294	4.63
Xerolycosa miniata (C.L. Koch, 1834)	dry	0	0	0	121	9	117	247	3.89
Xysticus acerbus Thorell, 1872	dry	0	0	2	2	0	0	4	0.06
Xysticus audax (Schrank, 1803)	dry	0	1	0	0	0	0	1	0.02
Xysticus sp.	?	0	0	0	0	5	0	5	0.08
Xysticus cristatus (Clerck, 1757)	s-h	0	3	0	6	0	7	16	0.25
Xysticus kochi Thorell, 1872	dry	0	3	0	2	0	0	5	0.08
Zelotes apricorum (L. Koch, 1876)	dry	5	0	14	41	4	0	64	1.01
Zelotes aurantiacus Miller, 1967	dry	0	0	0	0	1	0	1	0.02
Zelotes electus (C.L. Koch, 1839)	?	0	0	0	0	0	2	2	0.03
Zelotes sp.	?	25	15	12	0	13	10	75	1.18
Zelotes subterraneus (C.L. Koch, 1833)	dry	0	14	0	0	7	0	21	0.33
Zora silvestris Kulczyński, 1897	dry	8	1	0	0	0	0	9	0.14
Zora spinimana (Sundevall, 1833)	s-h	0	0	0	1	0	0	1	0.02
∑ N		1110	1896	395	651	778	1514	6344

Explanations: sites without human disturbance—S1 = forest Kraľovská lúka, S2 = flooded meadow Veľký Lél, S3 = extensive pasture Veľký Lél; sites with human management activities—S4 = non-flooded meadow Kľúčovec, S5 = newly planted forest Veľký Lél, S6 = intensive pasture Bodíky; RQ—humidity requirements of spiders (h—humid, s-h—semi-humid, dry—eurytopic, ?—none classified).

). Species composition and abundance varied across the plots, with higher diversity observed in areas without human intervention.

In the typical floodplain forest (S1), we recorded 29 species, with the hygrophilous spider Piratula hygrophila dominating (38.2%), followed by the eurytopic-dry species Pardosa lugubris (16%) and hygrophilous Ozyptila praticola (10.2%). In the newly planted floodplain forest (S5), we identified 33 spider species. Three species dominated, namely the eurytopic-dry Pardosa lugubris (33.7%), hygrophilous Trochosa ruricola (26%), and semi-hygrophilous Trochosa terricola (11.7%)—all large adult spider species.

In the typical wetland-meadow (S2), we recorded 38 species and 1896 individuals, the most of all studied areas. The meadow species Pardosa agrestis dominated (31.4%), followed by the hygrophilous Trochosa ruricola (22%) and Pachygnatha degeeri (13.5%), which prefer dry habitats. We identified 33 species of spiders and higher indices of diversity (

Table 2. Values of Shannon diversity index, species richness according to Margalef and Pielou’s evenness index of spiders in floodplains of the Danube River (at the study sites without human disturbance S1 = forest, S2 = wetland-meadow, S3 = extensive pasture; at study sites with human management activities S4 = non-flooded meadow, S5 = new forest, S6 = intensive pasture).

	S1	S2	S3	S4	S5	S6
No. taxa	29	38	37	33	33	14
No. individuals	1110	1896	395	651	778	1514
Shannon H’	2.165	1.989	2.733	2.443	2.189	1.612
Margalef	3.993	4.902	6.021	4.939	4.807	1.775
Equitability J	0.6429	0.5467	0.7569	0.6987	0.6262	0.6107

) in the remnant of the original meadow (S4), where the dead arm was being restored. Hygrophilous Trochosa ruricola (24.7%) was the most dominant, followed by xerophilous Xerolycosa miniata (18.6%) and the meadow spider Pardosa agrestis (11.7%).

In the semi-natural pasture (site S3), which was grazed by horses, we recorded 395 individuals of 37 spider species. The highest indices of diversity were recorded there. The eurytopic species Pardosa lugubris (21%) dominated, followed by hygrophilous Trochosa ruricola (20.5%) and Ozyptila praticola (11%). In managed pasture S6, where cattle grazed intensively over a substantially small area, we only identified 14 spider species, the fewest of all plots. The lowest indices of diversity were recorded there. The grassland eurytopic species Pardosa agrestis was predominant (52%), but hygrophilous Trochosa ruricola had a lower representation of 14.8%.

We applied multivariate PCA (SD = 2.7 on the first ordination axis) to identify species associations with study plots under different management regimes—undisturbed plots (S1–S3) and managed plots (S4–S6). The values of explained variability for species data were 36.42 on the first ordination axis and 59.91 on the second cumulative ordination axis (Figure 2). Data from Table 2 were used for this analysis.

On the ordination plot, we observed an arrangement of species into four squares. The first quadrant was formed by spiders with an attachment to the non-flooded meadow (S4), specific to this habitat type, such as M. vatia, Z. spinimana, X. miniata or R. lividus. The second quadrant comprised species typically found in natural floodplain forest (S1) and newly planted forest (S5)—C. sylvaticus, T. pedestris, and P. mirabilis. The third square represents extensive grassland (S3), where the hygrophilous species P. hygrophila, O. praticola and P. amentata were abundant, among others. The fourth quadrant shows two study sites—a flooded meadow (S2) and an intensive pasture (S6)—with a high abundance of P. prativaga, T. ruricola and P. agrestis. Management interventions significantly affected spider structure in the non-flooded meadow (S4) and intensively used pasture (S6). On the other hand, the flooded meadow (S2) and extensive pasture (S3), both without management, were among the most well-preserved habitats, and even they were in opposition to the spider coenoses that prefer the drier habitats of the non-flooded meadow and intensive pasture (Figure 2).

The Shapiro–Wilk W test confirmed that the data were not normally distributed (p = 0.0001). We therefore used the non-parametric Friedman test to evaluate differences in the number of individuals between study areas and years, finding a statistically significant difference (p < 0.000001). The results show that in study areas S1, S2, and S3, where no management took place, no differences were observed in the number of individuals between years in the case of S3. However, for the floodplain forest (S1), there was a significant increase in the number of individuals in 2022 (after flooding) and a decrease again the following year. For the meadow (S2), we observed a decrease in the number of individuals in 2021 and then an increase in 2022 and 2023 (Figure 2). In 2021, there were floods (natural and simulated) on the Danube, which had a negative impact on abundance in that year, but in the following years, we observed an increase in both diversity and abundance in both the floodplain forest (S1) and wetland-meadow (S2). The semi-natural extensive pasture (S3) was not affected by higher water levels. There have been changes in study areas S4, S5, and S6, where management adjustments have taken place (Figure 3). Management adjustments have had a positive impact on the torso meadow (S4), where an increase in individuals has been recorded each year. Similarly, at the intensive pasture site (S6), we recorded an increase in the number of individuals in 2022 and 2023, so the management adjustments have had a positive impact on spider abundance. In the planted floodplain forest (S5), we did not confirm a significant impact of human activities, as the number of individuals was similar from year to year.

The proportion of habitat-bound spiders in the Danube inundation is a significant indicator for assessing environmental stability. It reflects the current state of the habitat, as well as changes resulting from both human interventions and natural environmental factors. We compared the similarity of coenoses based on spiders’ habitat moisture requirements using a qualitative–quantitative analysis of a single linkage cluster (in terms of the Bray–Curtis index). The dendrogram displayed two branches at a dissimilarity level of 0.72 (Figure 4). The first branch connected two flooded forests (Natural forest S1 and newly planted forest S5) and a meadow with extensive grazing (S3) based on the high abundance of hygrophilous and eurytopic spiders. The second branch grouped a pasture with intensive cattle grazing (S6) and two meadows—a flooded meadow (S2) and a meadow with new hydrological regulation (S4). This branch shows habitats with a high abundance of both hygrophilous and xerophilous spiders.

We used multiple regressions to examine the annual changes in spider abundance and projected their trends up to 2027. The model explained 80.9% of the variability (R2 = 0.8090), i.e., there was high linkage (high dependence) between the variables under study. Using this model, we predicted a gradual increase in the number of individuals across all monitored sites by 2027 (Figure 5). The results of the spider assemblages trend analysis for study areas S4, S5, and S6, where management interventions have been implemented, predict a future increase in the number of individuals compared to study areas S1, S2, and S3, which did not experience management interventions. In the unmanaged areas, the number of species is stable and not subject to human disturbance.

4. Discussion

The Central European Danube Delta, as a Ramsar site, is still under human pressure. European floodplain forests originally provided high diversity, but their richness has been reduced due to strong human influence [22]. In recent decades, managed activities have been carried out; many river channels have been re-naturalised, and the hydrological regime is under human control. Nowadays, periodic floods are replaced by simulated floods with a completely different character and magnitude [5].

A total of 66,771 invertebrate individuals belonging to 15 arthropod taxa were collected at these study sites between 2020 and 2023 [6]. Spiders, as an important component of the epigeon, constituted more than 12%. Over four years, we compared natural habitats with habitats affected by management measures in the Danube River systems based on research on epigeic spider communities. We studied newly planted forest stands (with a predominance of poplars) and the impact of intensive grazing in these semi-natural conditions, and monitored diversity in original grassland communities, in which the hydrological regime was restored.

The highest taxa diversity (Shannon diversity index (H’)) and evenness were found in the extensive grazing habitat (S3), where the area is unmanaged and without human influence. The second highest diversity (H’) was found in the newly planted forest (S5), where management works were carried out. Conversely, in the classical floodplain forest (S1), diversity was low, but communities were more stable and balanced. At the end of the four years, spider communities in the newly planted forest (S5) appeared to be more stable with higher species co-dominance and a higher proportion of conspecific species, similar to those in the classical floodplain forest (S1) on the Danube [6]. Similar results were also found by [7,23,24], who assumed that the degree of degradation plays a decisive role in the formation of terrestrial communities. In accordance with [24,25], the proportion of forest generalists was high, as they quickly colonised pioneer sites. Forest restoration and management measures led to an increase in diversity. Microhabitats with varying moisture content provided suitable conditions for the penetration and colonisation of newly created habitats, and their proportion in coenoses increased rapidly in newly planted forests, as noted [26]. Selected epigeic arthropods (e.g., Araneae, Coleoptera, and Hymenoptera) serve as suitable bioindicators of the environment and accurately reflect habitat condition [27,28].

Our research confirmed that spiders are dominant and very suitable invertebrates for qualitatively assessing environmental conditions. Consistent with [29,30], we found that an important aspect is that the indirect effects and impacts of human activities cause changes in species diversity. For example, humidity, which is one of the most important factors in riparian habitats, is essential for the existence of wetland species and not only determines the abundance and species composition of spiders in these habitats [31] but also reflects the real status of the study area. Spiders respond rapidly to change and can therefore be used to assess habitat quality [5]. The results showed that changes in their abundance over time and space reflect changes in abiotic environmental factors and human activities, i.e., the environmental status of the habitat.

Effective management measures in flooded areas of the Danube therefore require an integrated approach that takes into account both local habitat characteristics and specific landscape-level factors [6,7,32]. Indeed, repeated monitoring using spiders as bioindicators can answer whether anthropogenic disturbance disrupted ecological stability. This approach would enable the use of spiders to assess landscape sustainability.

5. Conclusions

The impact of management measures on the flooded areas of the Danube inland delta was based on research on spider communities. (i) Newly planted forest stands were characterised as pioneer habitats with initial colonisation by large adult spiders with nocturnal activity [33]. The restoration of floodplain forests with the original vegetation structure should be monitored even 10 years after planting. (ii) Extensive grazing significantly contributed to spider species diversity. However, intensive grazing significantly reduced the diversity of araneocoenoses and corresponds to grassland communities in the stage of habitat degradation. (iii) Management interventions in the meadow (re-naturalisation of the dead arm of the Danube) in the agricultural landscape created a very diverse habitat for many spider species, where the assemblage structure corresponds to a mixture of meadows and agro-coenoses. The naturally flooded meadow habitat appeared to be very stable. After the completion of management measures and wetland restoration, the monitored habitats will be suitable refuges for typical floodplain spiders.