Research Article |
Corresponding author: Alice Misuri ( alice.misuri@unifi.it ) Academic editor: Gianluigi Bacchetta
© 2024 Alice Misuri, Michele Mugnai, Michele Giunti, Lorella Dell’Olmo, Lorenzo Lazzaro.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Misuri A, Mugnai M, Giunti M, Dell’Olmo L, Lazzaro L (2024) Invasive alien plants at Capraia Island (Italy): distribution and threats to Natura 2000 habitats. Plant Sociology 61(1): 31-44. https://doi.org/10.3897/pls2024611/02
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Island ecosystems face significant threats from biological invasions due to their unique biomes. In over a quarter of these ecosystems, the number of alien plants surpasses the total count of native ones, leading to notable impacts. This concern is particularly pronounced in the Mediterranean Basin, a globally important hotspot for plant diversity. In line with this, our study aimed to investigate the current distribution of six main alien plant species (Chasmanthe floribunda, Nicotiana glauca, Opuntia ficus-indica, Opuntia stricta, Senecio angulatus, and Zantedeschia aethiopica) on Capraia Island in the Tuscan Archipelago, Italy. We identified invaded vegetation types and N2000 habitats (sensu 92/43/EEC “Habitats” Directive), created a detailed map of the current distribution of these six target species, and analyzed the relationship between their distribution and environmental factors. The six target species were found to be more widespread in areas of the island with higher levels of anthropogenic disturbance, with only O. stricta also spreading into natural habitats. Overall, the invasion covers an area of 71 hectares (3.6% of the island’s surface), of which over 70 hectares are invaded by O. stricta. The habitats most impacted by alien plant invasion include 5330 “Thermo-Mediterranean and pre-desert scrub”, 5320 “Low formations of Euphorbia close to cliffs”, 1240 “Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.”, and 6220* “Pseudo-steppe with grasses and annuals of the Thero-Brachypodietea”, all primarily invaded by O. stricta. Environmental factors most correlated with alien plant distribution include the distance from infrastructure, altitude, and slope aspect.
Biological invasions, distribution map, management, Mediterranean ecosystems, Mediterranean island, Opuntia stricta
Biological invasions represent one of the main causes of biodiversity loss at the global level, following habitat fragmentation and degradation (
About 6% of all alien plants are considered to be invasive (IAPs), as they do represent a real threat to biodiversity, human health and economy, exerting numerous deleterious impacts at different levels (
For this reason, the Convention on Biological Diversity CBD (http://www.biodiv.org) has repeatedly emphasized the primary importance of protecting higher-risk areas, placing special emphasis on biodiversity conservation and sustainable development on islands (
Focusing on the Tuscan Archipelago, Capraia is the third largest island, but the second as far as invasion rate is considered, after Elba, the largest one (
Capraia, like the other islands of the Tuscan Archipelago, is included in the N2000 network, whose main objectives are the protection of habitats and species considered vulnerable at the European level and their conservation in a favorable status (sensu 92/43/EEC “Habitats” Directive). However, this does not mean a total ban on human activities within these areas, but rather their integration. So, the creation of the N2000 network does not completely prevent access to these areas where, actually, various human activities can be carried out. Indeed, anthropic activities result to be the main driver of biological invasions (
Accordingly, within this article we aimed to map the distribution of these six priority IAPs and identify which vegetation types and N2000 habitats (sensu 92/43/EEC “Habitats” Directive) are most invaded. For each species, through a series of field surveys, we monitored and updated previous data with the aim of producing an updated distribution map. Then we assessed the extent of the invaded areas and evaluated whether these included N2000 habitats. Additionally, we analyzed environmental data, such as slope, slope aspect, distance from human infrastructure and altitude, to investigate if there was a relationship between these factors and alien species distribution.
The information collected by means of this research is pivotal to define an ecological framework for future management actions to be carried out on Capraia Island.
Capraia Island (WGS84: 43°2'58.29"N, 9°50'6.38"E) is the third largest island (19.2 km2 of surface) of the Tuscan Archipelago (Fig.
The vegetation is dominated by Mediterranean shrubland, while forest is almost absent (
Capraia Island is almost totally included in the Special Areas of Conservation (SAC)/Special Protection Areas (SPA) (IT5160006), which extends for 19 km2, except for the port area and the inhabited center. Furthermore, 77% of the island territory belongs to the Tuscan Archipelago National Park. The island hosts 14 terrestrial habitats of conservation interest (sensu 92/43/EEC “Habitats” Directive) of which two are priority, i.e. 6220* “Pseudo-steppe with grasses and annuals of the Thero-Brachypodietea” and 3170* “Mediterranean temporary ponds” (see Table
Vegetation types and N2000 habitat (sensu 92/43/EEC “Habitats” Directive) present in Capraia Island. The first column represents the macro-categories of vegetation used for our analysis while the second column represents the N2000 habitats corresponding to or included within the macro-vegetation categories. All these data come from the cartographic analysis carried out for the SAC Management Plan (
Macro-Vegetation types | N2000 Habitats as described in |
NOTE |
---|---|---|
Tall scrub with Erica sp. and Arbutus unedo | - | - |
Orchards and tree crops | - | - |
Riparian Nerium oleander galleries | 92D0 | Fully corresponding to the habitat |
Low scrubs with Cistus sp. and/or Erica sp. and their mosaics | 3120; 3170*; 6220*; 8220; 8230 | Habitats with partial cover in the mosaic of vegetation |
Mosaics of Mediterranean xeric grasslands and Mediterranean temporary ponds | 6220*; 8220; 8230 | Habitats with partial cover in the mosaic of vegetation |
Mosaics of siliceous cliffs vegetation and Mediterranean xeric grasslands | 6220; 3120; 3170* | Habitats with partial cover in the mosaic of vegetation |
Euphorbia dendroides scrub | 5330 | Fully corresponding to the habitat |
Scrub mosaics with Euphorbia dendroides, Erica sp. and Cistus sp. | 5330 | Habitat with partial cover in the mosaic of vegetation |
Vegetation of abandoned areas | 6220* | Habitat with partial cover in the mosaic of vegetation |
Artificial pine forests | - | - |
Coastal vegetation | 1240; 1430; 3120; 3170*; 5330; 5320; 8220 | Habitats with partial cover in the mosaic of vegetation |
Quercus ilex forests and thickets | 9340 | Fully corresponding to the habitat |
Acquatic vegetation of the Stagnone pond | 3120; 3140; 3150; 6420 | Fully corresponding to a mosaic of this habitats |
The entire island was investigated with extensive field explorations focused on areas where the presence of six plant alien species (Chasmanthe floribunda, Nicotiana glauca, Opuntia ficus-indica, Opuntia stricta, Senecio angulatus, and Zantedeschia aethiopica; see Fig.
Due to the remarkable spread of both species of Opuntia, O. ficus-indica and O. stricta, we classified the areas invaded by them into density classes according to a logarithmic scale with 5 intervals: class 1 = 1–10 individuals per hectare, class 2 = 10–100 individuals per hectare (50 average individuals estimated), class 3 = 100–1000 individuals per hectare (550 average individuals estimated), class 4 = 1000–4000 individuals per hectare (2500 average individuals estimated), and class 5 >4000 individuals per hectare (10000 average individuals estimated). These estimates of individuals do not correspond to an actual mathematical average, but rather to empirical estimates performed approximately through visual observation carried out in the field, aided by binoculars for the inaccessible areas of the island. As for the other study species, we considered square meters of invaded area, since these are species that create monospecific mats.
Summary table showing the different biological and ecological characteristics of the six target invasive species on Capraia Island (
Species | Families | Area of origin | Life-form | Dispersal means | Reproduction system | Introduction pathway |
---|---|---|---|---|---|---|
Opuntia stricta (Haw.) Haw. | Cactaceae | Southern USA, Caribbean | Succulent perennial | Animals | Vegetative and sexual | Fruit-producing |
Opuntia ficus-indica (L.) Mill. | Cactaceae | Central America, Mexico | Succulent perennial | Animals | Vegetative and sexual | Fruit-producing |
Senecio angulatus L. f. | Asteraceae | South Africa | Perennial herb | Wind | Vegetative and sexual | Ornamental purpose |
Nicotiana glauca Graham | Solanaceae | Southern America | Evergreen shrub, tree | Animals | Vegetative and sexual | Ornamental purpose |
Chasmanthe floribunda (Salisb.) N.E.Br | Iridaceae | South Africa | Perennial herb | Animals | Vegetative and sexual | Ornamental purpose |
Zantedeschia aethiopica (L.) Spreng | Araceae | South Africa | Prennial herb | Water, animals | Vegetative and sexual | Ornamental purpose |
After collecting the data in the field, the IAPs distributional data were reported into geographic information systems (GIS) environment using ArcGIS Software vers. 10.8.1 (https://desktop.arcgis.com). The digitization was aided with the use of the land aerial photos of the Tuscany Region, OFC 2019 20 cm - 32 bit color – RGB and OFC 2016 20 cm - 32 bit color – RGB (available at https://www502.regione.toscana.it/geoscopio/ortofoto.html) and other satellite orthophoto sources, Google Satellite and Bing Virtual Earth. Distribution data were initially digitized using two vectorial topologies, points and polygons, the first for punctual data and the other for polygonal data. The point data were used in the case of small areas or few individuals that could not be reasonably outlined on the field maps during the surveys due to scale constraints, noting the areas extension and position. To allow further analyses of the spatial extent of the plant distribution, punctual data were then transformed into polygonal data and then merged with them, to obtain a single polygonal geographic feature of the distribution of IAPs. From this geographical layer we extrapolated the current extent of the surface invaded by each species.
To assess which vegetation types and N2000 habitats were mostly invaded, we retrieved the latest updated cartographic data concerning the vegetation and N2000 habitats on the island, issuing from the cartographic analysis included in the SAC Management Plan (
Ultimately, we explored the connection between and the IAPs distribution and environmental factors. We studied the species presence/absence in a 10 × 10 m cell grid in function of the following environmental factors: slope, slope aspect, distance from human infrastructure and altitude, using a Generalized Linear Model (GLM) with binomial distribution of the errors. Slope aspect was expressed as northerness following the formula: northerness = cosine [(aspect in degrees * π)/180)]. The analyses were conducted in R environment (R version 4.2.3) and the graphs were produced using the ggplot2 R package version 3.4.2 (
The total invaded area amounts to about 71 ha (3.6% of the island’s area). The most widespread species resulted to be Opuntia stricta with over 70 ha, while Nicotiana glauca is the least common one with 51 m2 in the island (Table
As far as N2000 habitats are concerned, Chasmanthe floribunda, Nicotiana glauca, Opuntia stricta, Senecio angulatus and Zantedeschia aethiopica show a different distribution pattern. The most invaded habitat is 5330 “Thermo-Mediterranean and pre-desert scrub”, with 10 ha massively occupied by Opuntia stricta. The other invaded habitats are: 5320 “Low formations of Euphorbia close to cliffs”, 1240 “Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.” and 6220* “Pseudo-steppe with grasses and annuals of the Thero-Brachypodietea”.
Concerning the other species, Senecio angulatus is mostly found in 1240 habitat, and it occurs forming very small patches in 3120 habitat “Oligotrophic waters containing very few minerals generally on sandy soils of the West Mediterranean, with Isoëtes spp.” and in the 3170* priority habitat “Mediterranean temporary ponds”. Instead, Zantedeschia aethiopica is present within the 92D0 habitat “Southern riparian galleries and thickets (Nerio-Tamaricetea and Securinegion tinctoriae)”. Finally, Chasmanthe floribunda and Nicotiana glauca invade a truly irrelevant portion of habitat 6220*, 3170* and 3120 respectively (Table
For all the alien species we found a significant and negative correlation among the probability of presence and the distance from human infrastructure, with a higher presence closer to human settlements. Altitude has a significant effect on the alien species Opuntia stricta, Chasmathe floribunda, O. ficus-indica, and Senecio angulatus showing a higher probability of occurrence at lower altitudes, near the sea. Only for Opuntia stricta, O. ficus-indica and Senecio angulatus we found a significant effect of slope aspect, with O. stricta growing preferably in south-facing areas, while O. ficus indica and Senecio angulatus in north-facing areas. Finally, Nicotiana glauca, Opuntia ficus-indica, and Senecio angualtus seem to prefer areas further from steep cliffs, showing a significant and negative correlation with slope (Fig.
Area invaded at Capraia Island by the six alien species examined. The whole invaded area amounts to about 71 ha (3.6% of the island’s area). The most widely distributed species is Opuntia stricta with over 70 ha, while Nicotiana glauca is the least widespread with 51 m2 in the island.
Species | Invaded area (m2) |
---|---|
Chasmanthe floribunda | 445,41 |
Nicotiana glauca | 50,56 |
Opuntia ficus-indica | 971,05 |
Opuntia stricta | 708875,10 |
O. stricta density class 1 | 114107,63 |
O. stricta density class 2 | 294136,26 |
O. stricta density class 3 | 145594,30 |
O. stricta density class 4 | 147219,49 |
O. stricta density class 5 | 7817,42 |
Senecio angulatus | 1291,04 |
Zantedeschia aethiopica | 234,08 |
Tot | 711867,24 |
Areas invaded by Opuntia stricta, Senecio angulatus, Chasmanthe floribunda, Nicotiana glauca and Zantedeschia aethiopica at Capraia Island divided into N2000 habitats.
Habitat | Chasmanthe floribunda | Nicotiana glauca | Opuntia stricta | Senecio angulatus | Zantedeschia aethiopica | Total m2 |
---|---|---|---|---|---|---|
1240 | 17839.77 | 18.12 | 17857.90 | |||
3170* | 0.01 | 1207.80 | 1.60 | 1209.41 | ||
5320 | 23798.82 | 23798.82 | ||||
5330 | 100449.38 | 100449.38 | ||||
6220* | 0.51 | 12782.86 | 12783.37 | |||
8220 | 1349.52 | 1349.52 | ||||
92D0 | 271.39 | 48.86 | 320.25 | |||
3120 | 0.01 | 875.45 | 1.60 | 877.06 | ||
8230 | 0.03 | 0.03 | ||||
Total m2 | 0.51 | 0.01 | 158575.03 | 21.32 | 48.86 | 158645.73 |
Area invaded by Opuntia stricta and O. ficus-indica divided into density classes by vegetation type. Density classes according to a logarithmic scale with 5 intervals: class 1 with 1–10 individuals per hectare, class 2 with 10–100 individuals per hectare (50 average individuals estimated), class 3 with 100–1000 individuals per hectare (550 average individuals estimated), class 4 with 1000–4000 individuals per hectare (2500 average individuals estimated), and class 5 with >4000 individuals per hectare (10000 average individuals estimated).
Areas invaded by Opuntia stricta and O. ficus-indica divided into N2000 habitats (sensu Habitats Directive, 92/43/EEC) and density classes. Density classes according to a logarithmic scale with 5 intervals: class 1 with 1–10 individuals per hectare, class 2 with 10–100 individuals per hectare (50 average individuals estimated), class 3 with 100–1000 individuals per hectare (550 average individuals estimated), class 4 with 1000–4000 individuals per hectare (2500 average individuals estimated), and class 5 with >4000 individuals per hectare (10000 average individuals estimated).
Repeated Measurement ANOVA table to analyse the relationship between environmental factors (distance from infrastructure, northerness, altitude and slope) and the six invasive species. Significance codes: p value < 0.001 ‘***’; p value < 0.01 ‘**’; p value < 0.05 ‘*’, p value < 0.10 ‘˙’.
Species | Enviromental factors | Df | Deviance Resid. | Df Resid. | Dev | Pr(>Chi) | |
---|---|---|---|---|---|---|---|
Opuntia stricta | Distance from infrastructure | 1 | 25092.40 | 193714 | 45537.00 | <0.001 | *** |
Northerness | 1 | 3547.80 | 193713 | 41989.00 | <0.001 | *** | |
Altitude | 1 | 1841.00 | 193712 | 40148.00 | 0.014 | * | |
Slope | 1 | 1125.80 | 193711 | 39022.00 | 0.054 | ||
Chasmanthe floribunda | Distance from infrastructure | 1 | 213.40 | 193714 | 545.38 | <0.001 | *** |
Altitude | 1 | 24.80 | 193713 | 520.54 | <0.001 | *** | |
Nicotiana glauca | Distance from infrastructure | 1 | 161.99 | 193714 | 263.45 | <0.001 | *** |
Slope | 1 | 4.10 | 193713 | 259.40 | <0.001 | *** | |
Senecio angulatus | Distance from infrastructure | 1 | 330.15 | 193714 | 645.44 | <0.001 | *** |
Northerness | 1 | 70.82 | 193713 | 574.62 | <0.001 | *** | |
Altitude | 1 | 40.15 | 193712 | 534.47 | <0.001 | *** | |
Slope | 1 | 25.90 | 193711 | 508.57 | <0.001 | *** | |
Zantedeschia aethiopica | Distance from infrastructure | 1 | 100.65 | 193714 | 288.07 | <0.001 | *** |
Variation of the alien species density and the environmental factors (distance from infrastructure, northerness, slope and altitude): Opuntia stricta (A, B, C, D), Chasmanthe floribunda (E, F, G, H), Nicotiana glauca (I, J, K, L), Opuntia ficus-indica (M, N, O, P), Senecio angulatus (Q, R, S, T), Zantedeschia aethipica (U, V, W, X).
The results of our study provide a detailed picture of the current status of plant invasions at Capraia Island through an accurate field check on the distribution of the main IAPs. Almost 4% of the island’s surface is invaded by the six alien species analyzed, of which the most widespread is Opuntia stricta with over 70 ha, while the most threatened habitats are 5330, 5320, 1240 and 6220*. Notably, the habitat 5330, characterized by the dominance of Euphorbia dendroides, is a relatively rare Mediterranean habitat in Italy, reaching its highest latitudinal extent along the northern border of the peninsula, particularly in Liguria and Tuscany. The four aforementioned habitats are often in close spatial contact with each other and are all typical of Mediterranean areas with hot and dry summers. This likely contributes to their suitability for the establishment and spread of Opuntia stricta, a species well-adapted to these environmental conditions as it is thermophilic, xerophilic and heliophilous with CAM metabolism and specialized organs for water reserve (
Although there are numerous specific case studies investigating the distribution of some IAPs (e.g.
As to coastal and island ecosystems, while several articles evaluated the impacts of IAPs on coastal sand dune environments (e. g.
Out of the six IAPs examined, five (Nicotiana glauca, Opuntia ficus-indica, O. stricta, Senecio angulatus and Zantedeschia aethiopica) emerged as invasive species posing significant threats, as indicated by two risk assessments conducted on alien plant species within the Tuscan Archipelago (
All six target species show an ecological preference for low-altitude areas close to the coastline and areas characterized by high levels of human disturbance. The significant influence of human activity, expressed both as tourist pressure and the expansion of settlements, on the richness and composition of alien flora in small Mediterranean islands is clearly emphasized in literature (
In conclusion, this study contributes to a better knowledge of the N2000 habitats invaded by the main six alien species in Capraia Island. We reported the most threatened habitats and gave a complete picture of the current invasion process in Capraia, which shows no signs of slowing down. Precisely for this reason, although O. stricta is currently the only species on which there is no doubt about the urgency of timely management intervention, it is also important to consider that currently its impact mechanisms and outcomes on native plant communities are not well known.
Because of the importance of a more in-depth study to apply effective management strategies, we are conducting new research to better understand the impact mechanisms exerted on some N2000 habitats by Opuntia stricta, the most widespread species on Capraia Island. Finally, it would be important to monitor the invasive capacity of the other species and investigate the possible effects of their invasion, aware that the longer the time interval from the moment of introduction, the more difficult and costly possible future interventions are.
The authors acknowledge the support of NBFC to the University of Florence, funded by the Italian Ministry of University and Research, PNRR, Missione 4 Componente 2, “Dalla ricerca all’impresa”, Investimento 1.4, Project CN00000033.
Vegetation types and N2000 habitat (sensu Habitats Directive, 92/43/EEC) present in Capraia Island
Data type: docx
Explanation note: table S1. Vegetation types and N2000 habitat (sensu Habitats Directive, 92/43/EEC) present in Capraia Island. The first column represents the macro-categories of vegetation used for our analysis which include both the most specific vegetation types in the second column and N2000 habitats in the third column, both coming from the cartographic analysis carried out for the SAC Management Plan (