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Article

Solanum elaeagnifolium (Solanaceae) Invading One in Five Natura 2000 Protected Areas of Greece and One in Four Habitat Types: What Is Next?

by
Nikos Krigas
1,*,
Nefta-Eleftheria Votsi
2,
Ioulietta Samartza
1,
Georgios Katsoulis
3 and
Maria A. Tsiafouli
3,*
1
Institute of Plant Breeding and Genetic Resources, Hellenic Agricultural Organization Demeter, P.O. Box 60458, Thermi, 57001 Thessaloniki, Greece
2
Institute for Environmental Research & Sustainable Development, National Observatory of Athens, I. Metaxa & Vas. Pavlou, P. Penteli (Lofos Koufou), 15236 Athens, Greece
3
Department of Ecology, School of Biology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
*
Authors to whom correspondence should be addressed.
Diversity 2023, 15(2), 143; https://doi.org/10.3390/d15020143
Submission received: 29 December 2022 / Revised: 16 January 2023 / Accepted: 18 January 2023 / Published: 20 January 2023
(This article belongs to the Special Issue Recent Studies of Invasion Ecology in the Mediterranean Basin)
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Abstract

:
Invasive alien plants have severe impacts on biodiversity and ecosystem services worldwide. To assess the invasion of Solanum elaeagnifolium Cav. (a major alien invasive plant; Solanaceae) in Greek protected areas (PAs), we conducted an extensive drive-by survey across the country crossing half of the Greek Natura 2000 sites. The occurrence data were then linked in GIS with (i) the boundaries of the Natura 2000 sites, (ii) the mapped habitat types in the invaded sites, and (iii) the Corine land cover, from which we calculated the ecosystem services. The results showed that approximately 24% of the investigated PAs were invaded by S. elaeagnifolium to varying degrees of extension and density of the populations. A variety of 29 different habitat types of Annex I of the Directive 92/43/EEC (including a priority habitat and seven habitat types of national importance) were found to be invaded (one in four habitat types present in Greece). In the invaded Natura 2000 sites, we recorded human activities/threats that were mostly related to agricultural activities of high intensity, while the potential in ecosystem service provision was relatively low for the category of regulating services in the invaded sites. Our study provides the first nationwide report of the distribution of S. elaeagnifolium in Natura 2000 sites of Greece, providing baseline maps and information for future monitoring. Our results highlight the need of an effective management strategy across the Natura 2000 network to effectively protect the local biodiversity, which should include management measures for invasive species, mitigating habitat fragmentation, and soil degradation, as well as the adaptation of eco-enhancing management strategies for the provision of multiple ecosystem services in agriculture.

1. Introduction

In light of the ongoing environmental crisis that is threatening global biodiversity, biological invasions have come to play a vital role in the equation. In fact, according to the United Nations Intergovernmental Platform for Biodiversity and Ecosystem Services (IPBES), biological invaders threaten about 20% of the Earth’s surface, including important biodiversity hotspots [1,2,3]. Approximately 58% of biological extinctions are related to invasive species [4,5]. The situation is troublesome, especially in Europe, where, during the last century, the number of invasive species has increased four-fold [5,6]. Therefore, alien invasive species are considered a major threat to biodiversity, posing a global threat to ecosystem functioning and services [7,8], human health, and the economy [9,10].
Protected areas (PAs) function as protection zones for habitats and species, safeguarding them from adverse effects of human activities, and they play a fundamental role in biodiversity conservation, ecosystem functioning, and ecosystem service provision [11,12]. PAs being high in biodiversity act as natural barriers to invasions [5,13,14,15]. Europe has developed the most extended system of PAs worldwide [5]. The Natura 2000 network is the largest coordinated network of PAs in the world, spreading across 27 EU countries [16] and comprising over 18% of terrestrial areas and around 8% of marine habitats [17]. However, apart from being shelters of native species [5], PAs are also prone to biological invasions depending on several factors, such as human activities, elevation, slope, distance from the hydrological network, and human population density [18]. Even though PAs are usually identified as “refugia” against plant invasions [5], several studies have attempted to depict the situations of invasive species in such sites, e.g., [3,19]. Invasive alien species pose direct impacts on PAs through competition and habitat alteration, often leading to the reduction of population abundance or modification of population traits of native species. On the ecosystem level, invasive plants are considered major sources of shifts in fire regimes, driving the transformation of entire ecosystems, while plant invasions can also affect the biogeochemical cycles in ecosystems [20]. Such impacts represent significant threats to ecosystems, let alone PAs; thus, monitoring and management of these problematic species is considered a priority [19,20].
Solanum elaeagnifolium Cav. (Solanaceae), commonly called Silverleaf nightshade, is a perennial invasive plant in many countries worldwide [21,22]. Its native range is the Southern USA and Northern Mexico [21] and probably Argentina [23], whereas the species is considered a noxious weed worldwide, including the Mediterranean Basin, the Mediterranean-type climate zones of Central and South America, and subtropical and South Africa, as well as South Asia and the Oceania [24]. The first introduction of the species in the Mediterranean Basin dates back to over 90 years ago, probably accidentally, in Thessaloniki, Greece [22,25,26]. The most recent introduction of S. elaeagnifolium in a European country was recorded in 2021 in Portugal [27].
As S. elaeagnifolium is of global concern, to date, several studies have investigated the ecology of this species, including its invasiveness status and impacts on both the ecosystem and economy. Therefore, the invasion of S. elaeagnifolium to an area may have negative effects on crop production, livestock, human life, the biodiversity, and the environment [25]. To date, there are only a few systematic species—specific studies in Europe regarding the presence and distribution of invasive plant species in PAs, including Natura 2000 sites. In a recent study, the distribution pattern of many invasive plant species has been investigated in Natura 2000 sites in Greece, leading to an estimate of 73 naturalized plant taxa recorded in 159 studied sites [3]. In the case of S. elaeagnifolium, Krigas et al. [22] recorded its presence across the Greek territory and found links between the presence of the species and higher maximum temperatures and precipitation in the summer and low precipitation in the winter and soil disturbance [20]. Additionally, Formozis et al. [28], found that the invasive species is present “outside and along forest edges” while investigating the presence of S. elaeagnifolium in Mediterranean forest ecosystems, but it is not found in a typical forest environment. They also revealed that the leaf extracts of specific tree species act as inhibitors to the seed or root fragments reproduction of S. elaeagnifolium [28].
The mapping of invasive species distribution is one of the fundamental methods for the development of a long-term management plan to protect biodiversity [29]. In this regard, the core idea of the current study was to identify the distribution of S. elaeagnifolium across the Greek Natura 2000 sites and to investigate the relationship between the species’ presence, habitat types, human activities, and ecosystem services. It was attempted to obtain a better understanding of the species’ current success informing future invasion potential, with the aim to facilitate the development of monitoring and management plans for the prevention of further spread into other PAs of Greece. Therefore, the current study represents a critical baseline for future monitoring and conservation efforts facing the threat of its invasion.

2. Materials and Methods

2.1. Study Area and Field Survey

The methodology followed in this survey is described in detail by Krigas et al. [22]. In brief, to record the presence of S. elaeagnifolium populations in PAs, we performed a so-called drive-by survey passing repeatedly during 2000–2020 by almost half of the Natura 2000 sites in Greece [30]. The Greek Natura 2000 network currently includes 446 sites in total, namely 239 Sites of Community Interests (SCIs), 181 Special Protection Areas (SPAs), and 26 SPAs/SCIs.
The sampling plan covered the basic road network connecting all major Greek cities (capitals of local prefectures) with smaller satellite cities and other settlements (in total, 150 routes connecting 54 Greek cities); see also [22]. This drive-by survey transected with or crossed about half of the sites of the Greek Natura 2000 network, namely 120 out of 265 SCIs or SCIs/SPAs and another 17 out of 30 SPAs that do not spatially overlap with SCIs (there are 181 SPAs in Greece, but 151 out of them partially overlap with SCIs). Sites with largely marine coverage (>95%) were not included in the sites to be investigated. Figure 1 shows the distribution of the investigated Greek Natura 2000 sites (see Natura 2000 site codes in Supplementary Materials Table S1).
From the center of every city, we recorded per kilometer [22] the distribution of S. elaeagnifolium populations along the basic city exit roads leading to different destinations, including Natura 2000 sites (Figure 2). The examined road network included multilane highways; old interstate highway systems; and regional, local, and rural roads (covering a total length of 19,993.8 km, including all major national highways and 12–25% of the remaining road network). Each of the selected routes was surveyed 3–5 times during 2000–2020 (sampling effort of ca. 100,000 km), and the respective presence/absence records were aggregated.
The observations of the naturalized wild-growing populations of S. elaeagnifolium were performed along road verges connecting different destinations (e.g., from a Greek city to a specific Natura 2000 site). All distribution records were made by two scientists while driving a car at a low speed (40–70 km/h), and car stops were made whenever a population was encountered to record the geographical coordinates with portable GIS devices and to observe with binoculars its presence in the surrounding area (radius of ca. 1 km). To avoid possible confusion with other cooccurring plant species, all surveys were performed in the summer and autumn, during which S. elaeagnifolium populations dominate the landscape and are quite evident from a distance [22].

2.2. Data Set Compilation

All distribution records of S. elaeagnifolium taken in the wild were later inserted into the GIS environment (version 10.1, ArcGIS® software by ESRI), and a polygonal grid cell of 1 km in size was adjusted on them digitally. This layer (hereafter distribution cells of S. elaeagnifolium) comprised 15,723 km2 in total (11.92% of the Greek territory) and was the main layer on which other data layers were linked accordingly (cell-by-cell) for data mining from the following geodatabases [31,32]:
(i)
The boundaries of the Greek Natura 2000 sites (Natura v30.shp, Hellenic Ministry for the Environment),
(ii)
The habitat types in the Greek Natura 2000 sites (Identification and description of habitat types in areas of interest for the conservation of nature during 1999–2001, Hellenic Ministry for the Environment), as well as the “conservation status” and the “representativity” (Natura 2000 database and Standard Data Forms, 2017).
(iii)
The Corine Land Cover, 2012 (Copernicus Land Monitoring Services, Εuropean Εnvironment Agency). We preferred this older version of the CLC, as it is better linked to the years the survey took place (starting from 2000).
We then linked the area covered by the distribution cells of S. elaeagnifolium with thematic information accordingly (for the methodology, see [31,32]). In this way, we were able to identify the invaded sites out of the investigated Natura 2000 sites (Figure 1, Supplementary Materials Table S1).

2.3. Data Analysis

We calculated the degree of invasion based on the percentage area of each site invaded (calculated from the number of invaded cells to the number of cells covering the site area) and the S. elaeagnifolium population density in the invaded areas (calculated from the number of its populations per cell).
The invaded Natura 2000 sites were characterized as follows:
-
Sites with no invasion (NO: 0% of area invaded);
-
Sites of low invasion where the invaded area corresponded to 0.1–5% of the total site area (LOW: 0.1–5% of area invaded);
-
Sites of moderate invasion where the invaded area corresponded to 5.1–10% of the total site area (MODERATE: 5.1–10% of area invaded);
-
Sites of high invasion where the invaded area corresponded to 10.1–20% of the total site area (HIGH: 10.1–20% of area invaded);
-
Sites of very high invasion where the invaded area corresponded to >20% of the total site area (VERY HIGH: >20% of area invaded).
We investigated links between the invasion status of the Natura 2000 sites (i.e., invaded or non-invaded) and the intensity and the type of human threats/activities within the sites. Data on human threats/activities that may have an influence on the biodiversity of Natura 2000 sites have been collected by experts under the responsibility of each EU member state, according to Standard Data Forms based on the Commission Decision concerning the site information format (European Commission, Document C, 2011, 4892) and are included in the Natura 2000 database. These data are publicly available for all Natura 2000 sites in Europe. To retrieve data on threats/activities for our study, we used the European Natura 2000 database, version of December 2017 (NATURA_ENG_Dec2017.mdb). Specifically, for the Greek Natura 2000 sites that were investigated in this study, we used 1918 records of 144 threats/activities that were included in the above-mentioned database. We calculated the percentage contribution for each of the different threats/activities to the total of records for the sites that were found not invaded, as well as for those that were found invaded. In graphs, we kept those threats/activities that covered, on average (i.e., for both categories of sites), at least 3% of the total records. For the “conservation status” and the “representativity” of habitat types present in the Natura 2000 sites invaded by S. elaeagnifolium, we used data records of the European Natura 2000 database, version December 2017 (NATURA_ENG_Dec2017.mdb). For all the habitat types, we calculated the percentage contribution (%) of each of the categories of “conservation” and “representativity” of the total. It should be noted that status “A” for the criterion “conservation status” stands for “excellent conservation” of the structure and functions, and “B” stands for “good conservation” of the structure and functions, while “C” stands for “average or reduced conservation”. The degree of “Representativity” provides a measure of how typical a habitat type is based on the definition of habitat types in Annex I of the Directive 9243/EEC, namely “A”, “B”, and “C” standing for “excellent”, “good”, and “significant”, respectively. For a typical non-significant presence, the characterization “D” is used. In this case, the conservation status was not evaluated.
We attempted to investigate relations among the profiles of the invaded Greek Natura 2000 sites regarding their potential in delivering different types of ecosystem services. For the invaded Natura 2000 sites, we identified Corine land cover types, and we calculated the area of each one. We further calculated the capacity of each land cover type to deliver ecosystem services (supporting, provisioning, regulating, and cultural services) based on an assessment matrix of the broadly used methodology [33,34]. More specifically, each land use type corresponds to a certain score ranging from zero (0), when there is no relevant ecosystem service capacity, to five (5), when there is a high relevant capacity. Scores of ecosystem services belonging to the same category (supporting, provisioning, regulating, or cultural) were then summed together. We finally related and graphically presented the amount of area of the Natura 2000 sites potentially providing ecosystem services to the total score of each of the four designated ecosystem service categories [33,34].

3. Results

3.1. Greek Natura 2000 Sites Invaded and Degree of Invasion

Figure 3 shows the investigated Natura 2000 sites according to the degree of invasion (no invasion or low, moderate, high, and very high invasion). From the 137 investigated sites (Table 1, Supplementary Materials Table S2), a number of 104 sites (83 SCI, 8 SPA/SCI, and 13 SPA) were not found to be invaded by S. elaeagnifolium, while the remaining 33 (25 SCI, 4 SPA/SCI, and 4 SPA) were invaded (Table 1, Figure 3). Out of them, seven were found highly invaded (S. elaeagnifolium has invaded from 21% up to 91.7% of the area of the sites GR1220001 (wider area of Lagkada and Volvi Lakes; LIMNES VOLVI KAI LAGKADA—EVRYTERI PERIOCHI), GR1260002 (Strymonas River mouth; EKVOLES POTAMOU STRYMONA), GR1260003 (Ai Giannis and Eptamyloi areas; AI GIANNIS—EPTAMYLOI), GR1270004 (Agios Mamas wetlands; LIMNOTHALASSA AGIOU MAMA), GR1270013 (Nea Fokaia wetlands; YGROTOPOI NEAS FOKAIAS), GR2310007 (Lake Amvrakia; LIMNI AMVRAKIA), and GR2510003 (Aronafplia-Palamidi area; AKRONAFPLIA KAI PALAMIDI). In another three sites, namely GR1220002 (wider area of Axios-Loudias-Aliakmonas Delta-Axioupoli; DELTA AXIOU—LOUDIA—ALIAKMONA—EVRYTERI PERIOCHI—AXIOUPOLI), GR1420005 (Tempi Valley aesthetic forest; AISTHITIKO DASOS KOILADAS TEMPON), and GR1420009 (Kalamaki Canyon and Zarkos Mountains; STENA KALAMAKIOU KAI ORI ZARKOU), the degree of invasion was found to be high, namely from 10–20%. In the further three sites, the degree of invasion was moderate (5.1–10% of area invaded), while in the last 20 invaded sites, the degree of invasion was low (0.1–5% of the area invaded).
Regarding the S. elaeagnifolium invasion in terms of population density, Table 1 shows the number of populations recorded in the invaded cells. The site GR2440002, although with a moderate degree of invasion, presented a high population density of S. elaeagnifolium, and 12 cells were found invaded with more than two populations per cell, while site GR1220002 was found invaded in six cells. The sites GR1150010 and GR1420009 were found invaded in three cells, each with more than two populations.

3.2. Habitat Types Prone to Invasion

The results of our investigation linking vegetation characteristics (habitat types) to the sites found invaded by S. elaeagnifolium populations are shown in Table 2. We observed a variety of invaded habitat types, ranging from wetland to forest habitat types. Out of the habitat types invaded, the habitat type 6110* (Rupicolous calcareous or basophilic grasslands of the Alysso-Sedion albi) is listed in ANNEX I of the 92/43/EU Directive, and it is a habitat type of priority. The habitat types 5420 (Sarcopoterium spinosum phrygana), 5350 (Pseudomaquis), 92A0 (Salix alba and Populus alba galleries), 92D0 (Southern riparian galleries and thickets Nerio-Tamaricetea and Securinegion tinctoriae), and 3280 (Annual communities at alluvial banks of Euro-Siberian rivers) were found most commonly invaded (i.e., were found in most sites).
As regards the conservation status of these invaded habitats (Table 3), we found that the conservation status was mostly assessed of “good conservation B” (48.39%) or “average or reduced conservation C” (30.65%), while their “Representativity” was, in most cases, “good—B” (51.61%).

3.3. Human Threats/Activities and Degree of Invasion

In the investigated Greek Natura 2000 sites, we found 144 different human activities/threats in total (1918 individual activity counts). Figure 4 shows the percentage of activities of low, moderate, and high intensity in the investigated Greek Natura 2000 sites with no invasion (No I; 0%), and these are contrasted to those with invasion by S. elaeagnifolium (sites with low, moderate, high, and very high invasion pooled together). The invaded sites presented a relatively higher percentage contribution of the activities/threats of high intensity, while the non-invaded sites presented a relatively higher percentage contribution of activities/threats of low intensity. As for activities/threats of moderate intensity, both categories of sites presented equal percentages.
Figure 5 shows the percentage occurrence of the most important activities/threats found in the sites invaded by S. elaeagnifolium (sites with low, medium, high, or very high invasion pooled together) and in the non-invaded ones. In the invaded sites, the dominant human activities/threats found were “fertilization” and “irrigation”, followed by “disposal of household waste” and “leisure and tourism” (Figure 5). In the non-invaded sites, the dominant human activities/threats found were “hunting”, “cultivation”, and “grazing”.

3.4. Ecosystem Services

The potential scores of the four designated ecosystem service (ES) categories in relation to the area potentially providing those ES in the Greek Natura 2000 sites invaded by S. elaeagnifolium are presented in Figure 6. As regards the ES category of ecological integrity, most of the area in the invaded Natura 2000 sites presented a relatively high potential score (i.e., 19–21) of ecosystem integrity. As regards the category of regulating services, most of the area presented a medium to low potential in providing regulating services (score 4–7). On the other hand, most of the area showed a relatively high potential in the provisioning services category (i.e., score > 18), while the potential in providing cultural services was, for most of the area, low (score < 3) (Figure 6).

4. Discussion

4.1. Solanum elaeagnifolium as Successful Invader

Solanum elaeagnifolium is generally considered a notorious alien invader in Greece. The current study has come to add more knowledge about the invasion potential of this species, further supporting and supplementing our previous study [22]. Our results present, among other things, the level at which S. elaeagnifolium has invaded Greek Natura 2000 sites, outlining an alarming situation. This first-time documentation can serve as the baseline for the necessary future monitoring and management actions.
The invasion success of silverleaf nightshade is impressive, surpassing all invasion barriers [35,36]. Several studies indicate that the success of the species establishing in new habitats is closely related to its reproductive mechanisms [21,22,25]. S. elaeagnifolium develops a deep root system and forms rhizomes and root fragments that can spread in diverse ways to new areas, thus creating with ease new plantlets asexually, especially when disturbed by human-induced disturbance and activities [22]. S. elaeagnifolium plants also produce a high amount of seeds, ensuring sexual reproduction [21,22,25], thus supporting the “Propagule Pressure” hypothesis [35,36,37,38]. The species is also adapted to a wide range of habitats and soil textures, and it can tolerate drought stress and saline conditions [21,29,39,40]. Additionally, it has been reported that S. elaeagnifolium can posit allelopathic effects by producing exudates that affect neighbor plants’ growth [21,39], thus supporting the “Novel Weapon” hypothesis [37,40,41].

4.2. Habitat Types of Annex I of the Directive 92/43/EEC

Greece is a biodiversity hotspot [42,43], and the Natura 2000 sites have been designed to embrace and safeguard this diversity, hence covering almost 18% of the country’s territory [17]. As regards the habitat types, out of the 264 included in Annex I of the Habitats Directive 92/43/EEC, 89 are found in Greece [44], corresponding to a percentage of 34% of the total. Among them, 15 are of conservation priority. Out of the 82 terrestrial habitat types listed in ANNEX I of the Directive 92/43/EEC of Greece [44], in our study, 22 have been found to be invaded at the investigated Natura 2000 sites. This relatively high percentage (i.e., 27%) indicates that it is not ensured that pristine habitat types (and PAs including them) can act as natural barriers to invasions, as stated in many studies (e.g., [ 5,13,14,15]).
In addition, it seems that there is no barrier to the ecosystem category that can be potentially invaded. Specifically, for the category non-marine coastal and halophytic habitat types and coastal and inland dunes of Greece, 5 habitat types were found invaded (i.e., 1210, 1240, 1310, 1410, and 2110) out of 17 in total present in Greece, thus representing a percentage of 29%. For the category of freshwater habitats, this holds true for two habitat types (i.e., 3150 and 3280) out of ten extant in Greece (i.e., 20%). For the categories sclerophyllous vegetation (matorral) and grasslands, this applies for 4 habitat types (i.e., 5420, 6110*, 6210, and 6420) out of the 17 extant in Greece (i.e., 24%). For the category of rocky habitats and caves, this holds true for two habitat types (i.e., 8210 and 8220) out of six terrestrial ones present in Greece (i.e., 33%). Finally, 9 habitat types (i.e., 9150, 9260, 9320, 9340, 9350, 9540, 92A0, 92C0, and 92D0) out of 27 different forest types also present in Greece (i.e., 33%) were found invaded in our study. The seven habitat types that are important for Greece belong to a variety of categories, including communities at alluvial banks (i.e., 32B0), reed beds (i.e., 72A0), East Mediterranean gariques (i.e., 5340), pseudomaquis (i.e., 5350), thermophilous oak forests (i.e., 924A), and Greek holm oaks (i.e., 934A).
It has to be noted that, among the invaded habitat types, the habitat type 6110* (Rupicolous calcareous or basophilic grasslands of the Alysso-Sedion albi) is a habitat of conservation priority. This habitat type is species-rich, including many endemic species [44], thus especially important for local biodiversity. In addition, the responsibility of preserving this habitat at the European level is high for Greece, both because of its infrequent presence and because of the multiple threats it is subjected to [44]. Although not priority habitats at the European level, the habitat types building galleries near waterbodies such as the Salix alba and Populus alba galleries, Platanus orientalis and Liquidambar orientalis woods (Platanion orientalis), and Southern riparian galleries and thickets (Nerio-Tamaricetea and Securinegion tinctoriae) are of high value for biodiversity conservation, especially concerning bird populations and mammals [44]. These habitat types (easily accessible most of the times) are popular for recreation activities, exposing them to disturbances leading to soil degradation and acting in favor of S. elaeagnifolium [22].
In almost 50% of the cases, the invaded habitat types were of Conservation status and Representativity “B”, indicating some degree of disturbance and degradation. Only 13% and 23% of the cases had Conservation status and Representativity of “A” (i.e., good). Our findings highlight that the conservation status (or “health”) of habitat types might be more important than the category or the high diversity of the habitat type per se. As our study only recorded S. elaeagnifolium close (up to 1 km) to roads, all these parts of habitats affected might be considered as edge habitats due to road proximity. The negative effect of edges on biodiversity and environmental characteristics has been demonstrated in many studies [28,45]. Hence, our study demonstrates that roads paving the way for a S. elaeagnifolium invasion across the landscape [22] acts in the same fashion for PAs. Our results highlight the significance of roadless areas [46] for the protection of local biodiversity against invasive species. It is very important to take into consideration that future planned constructions of windmill parks in PAs of Greece [46] will probably include the expansion of edge habitats due to the construction of service roads providing favorable conditions for the further invasion of S. elaeagnifolium [22].

4.3. Invasion of Solanum elaeagnifolium and Ecosystem Services

For our study, we used the straightforward and easy-to-implement approach [33] to evaluate the potential ecosystem service provision in the studied areas. We found that most of the area in the invaded Natura 2000 sites presented a relatively high potential in the category of ecosystem integrity, a medium to low potential in the category of regulating services, a relatively high potential in the provisioning services category, and a low potential in cultural services. Those characteristics are in line with our findings regarding human activities/threats. Specifically, we found that, in the invaded sites, the dominant human activities/threats were “fertilization” and “irrigation”, followed by “disposal of household waste” and “leisure and tourism”.
The research to understand the relation between regulating ES and invasive species is still ongoing; there is evidence that invasive species can affect ES, by altering ecosystem processes. Our findings support the idea that sites with low regulative services are susceptible to invasion [47]. In the case of areas invaded by S. elaeagnifolium, this is probably related to activities degrading soil health, thus providing favourable conditions to invasion [22]. S. elaeagnifolium has been typically found to be present in agricultural and ruderal habitats as well as in pastures, on beaches, cities, urbanized areas, and roadsides [21,22,25], i.e., areas with high human influence in general. However, this study highlights how PAs are also suffering from invasion by this notorious species.
The PAs in Europe have been developed after a long-term history of interaction between natural processes and human activity, especially in countries with ancient history such as Greece. In the recent era of the Anthropocene, of biodiversity loss and climate crisis, it is of particular importance to understand complicated interactions of the socioeconomic, cultural, and policy parameters on natural processes to adapt efficient management strategies [48]. The profile of ES potential in the invaded areas in relation to the most common activities/threats, i.e., activities related to agriculture, shows the significant multiple impacts of the latter on biodiversity conservation. It has to be noted that, in Greece, up to 80% of PAs include agriculture-related activities [49], while similar trends occur for most of the Natura 2000 sites at the pan-European level [50]. Thus, the adaptation of innovative, eco-enhancing management strategies, which will lead to the provision of multiple ecosystem services [51] in agricultural systems in PAs and the near surroundings is essential.

5. Conclusions

Considering that many habitat types and endemic species in Greece are under threat, the mapping and monitoring of invasive species is of vital importance in attempts to safeguard biodiversity on a national scale. Our study documented that S. elaeagnifolium has invaded almost one in five Natura 2000 sites in Greece and one in four habitat types. The presence of S. elaeagnifolium jeopardizes the role of the Natura 2000 sites, offering in situ protection for habitat types and important species, along with the ES that these sites of high biodiversity can actually provide. To prevent worsening of this situation, targeted strategy measures and integrated management actions should be implemented promptly [52], especially considering that there is a significant number of other invasive species about to or already invading Greek protected areas apart from S. elaeagnifolium.
S. elaeagnifolium is a noxious and long-established alien invasive weed; thus, its management is challenging, expensive, and unsustainable [53,54]. The current study contributes towards a reliable and effective policy framework of biodiversity conservation, suggesting integrating invasive species mapping and related ES assessment for Natura 2000 sites at the national level. Protected areas, as biodiversity hotspots, constitute genetic resources of huge importance with various and valuable ecosystem services; thus, the need to protect these sites from alien species invasion is manifold and urgent.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d15020143/s1: Table S1: Natura 2000 sites of Greece investigated in this study. Table S2: Site codes and names of the Greek Natura 2000 sites invaded by Solanum elaeagnifolium.

Author Contributions

Conceptualization, N.K., N.-E.V., and M.A.T.; methodology, N.K., N.-E.V., G.K., I.S., and M.A.T.; software, N.-E.V. and G.K.; validation, N.K. and M.A.T.; formal analysis, I.S., N.-E.V., G.K., and M.A.T.; investigation, N.K., N.-E.V., I.S., G.K., and M.A.T.; resources, N.K. and M.A.T.; data curation, N.K., N.-E.V., G.K., I.S., and M.A.T.; writing—original draft preparation, N.K., N.-E.V., I.S., and M.A.T.; writing—review and editing, N.K., N.-E.V., I.S., G.K., and M.A.T.; visualization, N.K., N.-E.V., and M.A.T.; supervision, N.K.; and project administration, N.K. and M.A.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data supporting the reported results can be provided upon request.

Acknowledgments

We would like to thank Efthimia Antonopoulou and Evangelia Bagkou for assistance with the field work. We would also like to thank the anonymous reviewers for their constructive comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Distribution map of the Natura 2000 sites (SCI, SCI/SPA, and SPA) in Greece. The green-colored areas indicate the sites investigated in this study.
Figure 1. Distribution map of the Natura 2000 sites (SCI, SCI/SPA, and SPA) in Greece. The green-colored areas indicate the sites investigated in this study.
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Figure 2. Examples of different routes (represented by squares of 1 km2) followed to record the presence/absence data for Solanum elaeagnifolium, starting from the city center of Thessaloniki to destinations including the Natura 2000 sites (in green).
Figure 2. Examples of different routes (represented by squares of 1 km2) followed to record the presence/absence data for Solanum elaeagnifolium, starting from the city center of Thessaloniki to destinations including the Natura 2000 sites (in green).
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Figure 3. Map of the investigated Greek Natura 2000 sites according to the degree of invasion by Solanum elaeagnifolium.
Figure 3. Map of the investigated Greek Natura 2000 sites according to the degree of invasion by Solanum elaeagnifolium.
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Figure 4. Percentage of activities of low, moderate (medium), and high intensity at the investigated Greek Natura 2000 sites with no invasion (No I; 0%) contrasted to those with invasion by Solanum elaeagnifolium (sites with low, moderate, high, and very high invasion pooled together).
Figure 4. Percentage of activities of low, moderate (medium), and high intensity at the investigated Greek Natura 2000 sites with no invasion (No I; 0%) contrasted to those with invasion by Solanum elaeagnifolium (sites with low, moderate, high, and very high invasion pooled together).
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Figure 5. Percentage occurrence of different activities/threats at the investigated Natura 2000 sites of Greece with no invasion (blue) and invasion (red) by Solanum elaeagnifolium (sites with low, moderate, high, and very high invasion are pooled together). Only activities with an occurrence >3% in either invaded or non-invaded sites are shown in the graph.
Figure 5. Percentage occurrence of different activities/threats at the investigated Natura 2000 sites of Greece with no invasion (blue) and invasion (red) by Solanum elaeagnifolium (sites with low, moderate, high, and very high invasion are pooled together). Only activities with an occurrence >3% in either invaded or non-invaded sites are shown in the graph.
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Figure 6. Potential score of the four designated ecosystem services (ES) categories (ecological integrity, regulating, provisioning, and cultural services) and respective area of the Greek Natura 2000 sites invaded by Solanum elaeagnifolium.
Figure 6. Potential score of the four designated ecosystem services (ES) categories (ecological integrity, regulating, provisioning, and cultural services) and respective area of the Greek Natura 2000 sites invaded by Solanum elaeagnifolium.
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Table
Table 1. Greek Natura 2000 sites invaded by the alien invasive species Solanum elaeagnifolium outlining the percentage of the site area invaded, the total number of cells invaded, and the number of cells invaded by one or more populations.
Table 1. Greek Natura 2000 sites invaded by the alien invasive species Solanum elaeagnifolium outlining the percentage of the site area invaded, the total number of cells invaded, and the number of cells invaded by one or more populations.
Natura 2000 Site CodeSite Area (km2)Percentage (%) of Site Area InvadedNumber of Invaded Cells (Cell Area = 1 km2) Number of Cells with One PopulationNumber of Cells with ≥2 Populations
GR1110004164.380.61110
GR1130009294.562.38770
GR1150010224.852.22523
GR1220001269.4820.4155532
GR1220002336.7611.8840346
GR1250001191.401.57330
GR1260001783.040.64550
GR126000212.9738.55550
GR12600033.2791.66330
GR1270002180.323.88770
GR1270003334.261.55550
GR12700046.3378.97532
GR12700134.4045.50220
GR1420001124.381.61220
GR1420004434.361.61770
GR142000513.3614.97220
GR1420008245.720.81220
GR142000941.6916.79743
GR1430001311.120.64211
GR1440003606.252.8017170
GR2110001287.883.13990
GR2310001355.100.56220
GR231000631.233.20110
GR231000722.0554.4312120
GR2410001116.079.4811101
GR2440002475.477.57362512
GR244000666.855.98440
GR25100033.6654.62220
GR252000569.851.43110
GR2550006533.670.19110
GR300000688.192.27220
GR300001353.921.85110
GR422001171.551.40110
Table
Table 2. Habitat types invaded by Solanum elaeagnifolium in Greek Natura 2000 sites, and number of sites where these invaded habitats were found. Asterisk (*) in habitat code denotes priority habitat according to the EU Directive 92/43/EEC. Habitat types important for Greece and not indexed in Annex I of the EU Directive 92/43/EEC are underlined.
Table 2. Habitat types invaded by Solanum elaeagnifolium in Greek Natura 2000 sites, and number of sites where these invaded habitats were found. Asterisk (*) in habitat code denotes priority habitat according to the EU Directive 92/43/EEC. Habitat types important for Greece and not indexed in Annex I of the EU Directive 92/43/EEC are underlined.
CodeHabitat NameNumber of Sites with Habitats Invaded
1210Annual vegetation of drift lines2
1240Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.1
1310Salicornia and other annuals colonizing mud and sand3
1410Mediterranean salt meadows (Juncetalia maritimi)4
2110Embryonic shifting dunes3
3150Natural eutrophic lakes with Magnopotamion or Hydrocharition—type vegetation5
3280Constantly flowing Mediterranean rivers with Paspalo-Agrostidion species and hanging curtains of Salix and Populus alba7
5420Sarcopoterium spinosum phrygana8
6110*Rupicolous calcareous or basophilic grasslands of the Alysso-Sedion albi1
6210*Semi-natural dry grasslands and scrubland facies on calcareous substrates (Festuco-Brometalia) 1
6420Mediterranean tall humid grasslands of the Molinio-Holoschoenion2
8210Calcareous rocky slopes with chasmophytic vegetation3
8220Siliceous rocky slopes with chasmophytic vegetation1
9150Medio-European limestone beech forests of the Cephalanthero-Fagion1
9260Castanea sativa woods1
9320Olea and Ceratonia forests3
9340Quercus ilex and Quercus rotundifolia forests5
9350Quercus macrolepis forests1
9540Mediterranean pine forests with endemic Mesogean pines4
92A0Salix alba and Populus alba galleries9
92C0Platanus orientalis and Liquidambar orientalis woods (Platanion orientalis)5
92D0Southern riparian galleries and thickets (Nerio-Tamaricetea and Securinegion tinctoriae)6
32B0Annual communities at alluvial banks of Euro-Siberian rivers2
1420Mediterranean and thermo-Atlantic halophilous scrubs (Sarcocornetea fruticosi)6
5340East Mediterranean gariques2
5350Pseudomaquis10
72A0Reedbeds8
924AThermophilic oak forests of the East Mediterranean and Balkans2
934AGreek holm oaks2
* Important orchid sites
Table
Table 3. Conservation status and representativity of the habitat types of Greek Natura 2000 sites invaded by Solanum elaeagnifolium.
Table 3. Conservation status and representativity of the habitat types of Greek Natura 2000 sites invaded by Solanum elaeagnifolium.
A (Excellent)B (Good)C (Average or Reduced)Not Evaluated
Conservation status12.90%48.39%30.65%8.06%
A (Excellent)B (Good)C (Significant)D (Not evaluated)
Representativity22.58%51.61%17.74%8.06%
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Krigas, N.; Votsi, N.-E.; Samartza, I.; Katsoulis, G.; Tsiafouli, M.A. Solanum elaeagnifolium (Solanaceae) Invading One in Five Natura 2000 Protected Areas of Greece and One in Four Habitat Types: What Is Next? Diversity 2023, 15, 143. https://doi.org/10.3390/d15020143

AMA Style

Krigas N, Votsi N-E, Samartza I, Katsoulis G, Tsiafouli MA. Solanum elaeagnifolium (Solanaceae) Invading One in Five Natura 2000 Protected Areas of Greece and One in Four Habitat Types: What Is Next? Diversity. 2023; 15(2):143. https://doi.org/10.3390/d15020143

Chicago/Turabian Style

Krigas, Nikos, Nefta-Eleftheria Votsi, Ioulietta Samartza, Georgios Katsoulis, and Maria A. Tsiafouli. 2023. "Solanum elaeagnifolium (Solanaceae) Invading One in Five Natura 2000 Protected Areas of Greece and One in Four Habitat Types: What Is Next?" Diversity 15, no. 2: 143. https://doi.org/10.3390/d15020143

APA Style

Krigas, N., Votsi, N. -E., Samartza, I., Katsoulis, G., & Tsiafouli, M. A. (2023). Solanum elaeagnifolium (Solanaceae) Invading One in Five Natura 2000 Protected Areas of Greece and One in Four Habitat Types: What Is Next? Diversity, 15(2), 143. https://doi.org/10.3390/d15020143

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