Biodiversity Data Journal :
Research Article
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Corresponding author: Gianniantonio Domina (gianniantonio.domina@unipa.it)
Academic editor: Enrico Vito Perrino
Received: 16 Mar 2021 | Accepted: 04 May 2021 | Published: 27 May 2021
© 2021 Giulio Barone, Gianniantonio Domina, Emilio Di Gristina
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:
Barone G, Domina G, Di Gristina E (2021) Comparison of different methods to assess the distribution of alien plants along the road network and use of Google Street View panoramas interpretation in Sicily (Italy) as a case study. Biodiversity Data Journal 9: e66013. https://doi.org/10.3897/BDJ.9.e66013
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The survey by foot in the field is compared to the survey from a car, the photo-interpretation of Google Street View (GSV) panoramas continuously and at intervals of 1.5 km and the photo-interpretation of Google Earth aerial images on a 10 km stretch of road in Sicily. The survey by foot was used as reference for the other methods. The interpretation of continuous GSV panoramas gave similar results as the assessment by car in terms of the number of species identified and their location, but with lower cost. The interpretation online of aerial photos allowed the identification of a limited number of taxa, but gave a good localisation for them. Interpretation of GSV panoramas, each of 1.5 km, allowed the recognition of twice as many taxa as the interpretation of aerial photos and taking half the time, but did not allow a complete localisation.
None of these methods alone seems sufficient to carry out a complete survey. A mixture of different techniques, which may vary according to the available resources and the goal to be achieved, seems to be the best compromise.
To further test the capabilities of the survey using the interpretation of GSV panoramas every 1.5 km along the roads, we proceeded to study the alien plants along 3500 km of the road network on the island of Sicily. This survey identified only 10% of the known species for the region, but allowed us to trace the distribution of invasive species whose distribution is currently poorly recorded.
road ecology, roadside, alien invasive plants, remote sensing
Alien plants are plants that have been transported by humans intentionally or accidentally outside their area of origin (
There are already some examples of monitoring the presence of alien species along road networks (e.g.
Car surveys have the advantage of covering larger areas in shorter periods with fewer resources, when compared to on-foot surveys (
Currently on the internet, there are freely-available sources of images at ground level from which it is possible to obtain information on the plants present. The most famous and complete of all is certainly Google Street View (hereafter GSV). This is a technology featured in Google Maps and Google Earth that provides interactive panoramas, acquired with a complex camera system placed on moving cars, from positions along many streets in the world. Recently, it has been suggested as a cost-effective alternative for habitat assessment and the detection of alien plants along roads (
In this study, we wanted to extensively test the potential of the analysis of GSV images on different habitats, to know whether this method is user-friendly, reliable and cost-effective. This method was compared with the survey on foot, by car and by aerial view. Sicily was a perfect test area because it is well documented from a floristic point of view and able to offer considerable environmental variations within a few tens of kilometres.
Sicily is the largest Mediterranean island located at the centre of the Mediterranean Basin separated from the rest of Italy by a 3 km strait. It has an approximately triangular shape and is surrounded by 14 islands that are inhabited all year round. The coordinates of the extreme points of the main island are respectively: 38 ° 18'5 "N for the northernmost point; 36 ° 38'48" N for the southern extreme; 12 ° 25'28 "E for the westernmost point and 15 ° 39'8" E for the easternmost point. Sicily is a predominantly hilly region (61.4% of the Island), while 24.5% is mountainous and the remaining 14.1% is flat. Its highest mountain is Mount Etna at 3,329 m. Sicilian rivers are all limited in flow rate and length (
The presence of humans on the Island dates back 14,000 years ago. The Island has always been used for agriculture. Urban development and the great expansion of the road system on the Island began around 1960 (
Two analyses were conducted:
A comparison was done on a 10 km stretch of coastal main road, between the reliefs on foot, by car, continuous interpretation of GSV panoramas, interpretation of GSV panoramas, each of 1.5 km and manual interpreting of Google Earth aerial photographs.
The area under investigation includes the roadway and 5 m on each side of the road no. 113 between Buonfornello (
The reliefs on foot and by car were done in September 2020; the photographic shots of the same stretch of road date back to September 2018; the aerial photos available on Google Earth date back to August 2019. The survey by car was carried out before the one on foot so that the operator was not influenced by the results of the more complete survey. The car ran around 35-40 km/h. The photo interpretations of GSV and Google Earth images were undertaken in September 2020, before the surveys in the field. Interpretation of GSV panoramas, each of 1.5 km, was done in seven points. It was decided to consider the survey on foot as a benchmark and to verify the difference in results of the other techniques with respect to this one. The measurements with the other methods were repeated five times and the expected loss analysis was performed [Expected_loss = (x_med-target)^2+sigma^2] to take into account both variability and proximity to the target (precision and accuracy) by combining them together (
A ranking of the methods was determined by calculating a total score for each method. By assigning a relative weight of importance to each method quality feature and giving a relative score to each of them, based on the real outcome of the pilot experiment, the highest total score was found (
A survey on the entire Sicilian road network with GSV interpreting panoramas at 1.5 km from each other on all the highways and on a selection of the main and the local roads was done. The selection of main and local roads was done to cover the whole territory of the region with a homogeneous network (Fig.
Overall, 2,350 panoramas were interpreted in 35 hours, along more than 3,500 km of roads. For each observation, the following data were recorded: coordinates in decimal degrees (WGS84), type of road (Highway, Main Road and Local Road), image capture date and occurrence of alien taxa. The altitude for each point was extracted from a digital terrain model of 2 × 2 m resolution (from http://www.sitr.regione.sicilia.it). Data concerning the taxa identified were added in a second stage: family, life form, origin according to
The taxa identification was based on the skills of the authors, with the help of relevant local literature (
The relief on foot resulted in a list of 34 specific and subspecific taxa. The one by car and by GSV panoramas interpretation along the entire route resulted in the same 21 taxa. The relief interpretation of GSV panoramas, each of 1.5 km, resulted in 10 taxa and the interpretation of Google Earth aerial images resulted in five taxa only (Table
Taxa identified in the 10 km of road. The number of segments of 100 m in which each taxon was recorded on foot and mean of the number of segments by car, with continuous interpretation of GSV panoramas, interpreting a GSV each 1.5 km and with Google earth aerial view is indicated.
Taxon |
Life form |
On foot |
By car |
GSV complete |
GSV each 1.5 km |
Google Earth |
Acacia saligna (Labill.) H.L.Wendl. |
P scap |
8 |
8 |
8 |
1 |
5 |
Agave americana subsp. americana |
P caesp |
1 |
1 |
1 |
||
Agave sisalana Perrine |
P caesp |
2 |
2 |
2 |
1 |
1 |
Ailanthus altissima (Mill.) Swingle |
P scap |
6 |
5 |
5 |
1 |
|
Amaranthus retroflexus L. |
T scap |
1 |
||||
Arundo donax L. |
G rhiz |
80 |
73 |
75 |
3 |
70 |
Asclepias fruticosa L. |
P caesp |
2 |
1 |
2 |
||
Asparagus setaceus (Kunth) Jessop |
G rhiz |
2 |
||||
Bidens pilosa L. |
T scap |
4 |
||||
Boerhavia coccinea Mill. |
T scap |
8 |
6 |
6 |
||
Cascabela thevetia (L.) Lippold |
P scap |
1 |
||||
Cenchrus setaceus (Forssk.) Morrone |
H caesp |
3 |
||||
Erigeron bonariensis L. |
T scap |
7 |
1 |
1 |
1 |
|
Erigeron canadensis L. |
T scap |
4 |
2 |
2 |
||
Ipomoea indica (Burm.) Merr. |
G rhiz |
7 |
7 |
7 |
1 |
|
Lantana camara subsp. aculeata (L.) R.W.Sanders |
P caesp |
2 |
2 |
2 |
||
Leucaena leucocephala subsp. glabrata (Rose) Zárate |
P scap |
2 |
2 |
2 |
||
Medicago sativa L. |
H scap |
1 |
||||
Melia azedarach L. |
P scap |
1 |
1 |
1 |
||
Mirabilis jalapa L. |
G bulb |
1 |
1 |
|||
Opuntia ficus-indica (L.) Mill. |
P succ |
11 |
10 |
11 |
1 |
|
Oxalis pes-caprae L. |
G bulb |
1 |
1 |
|||
Eryobotrya japonica (Thunb.) Lindley |
P scap |
1 |
||||
Ricinus communis L. |
P scap |
53 |
49 |
49 |
2 |
|
Robinia pseudoacacia L. |
P scap |
7 |
7 |
7 |
1 |
2 |
Saccharum biflorum Forssk. |
H caesp |
4 |
4 |
4 |
2 |
|
Setaria adhaerens (Forssk.) Chiov. |
T scap |
1 |
||||
Sorghum halepense (L.) Pers. |
G rhiz |
15 |
12 |
12 |
2 |
|
Symphyotrichum squamatum (Spreng.) G.L.Nesom |
T scap |
10 |
5 |
5 |
||
Tropaeolum majus L. |
T rept |
1 |
||||
Vachellia karroo (Hayne) Banfi & Galasso |
P scap |
3 |
3 |
3 |
||
Vitis × ruggerii Ardenghi, Galasso, Banfi & Lastrucci |
P lian |
1 |
1 |
1 |
||
Washingtonia robusta H.Wendl. |
P scap |
2 |
||||
Xanthium italicum Moretti |
T scap |
5 |
3 |
2 |
||
TOTAL |
258 |
207 |
208 |
14 |
80 |
The results obtained by car and from the complete observation of the entire route with GSV give better results with highly visible species (e.g. trees, shrubs, plants with large flowers etc.) than with inconspicuous species (annuals, hemicriptophytes, bulbs etc.) (Table
Recorded life forms in 10 km using the different methods. The percentages are calculated with respect to the value of the "on foot" survey. Ch: Chamaephyte, G: Geophyte, H: Hemicryptophyte, He: Helophyte, NP: Nanophanerophyte, P: Phanerophyte, T: Therophytes, bienne: biannual, bulb: bulbose, caesp: caespitose, lian: lianoid, rhiz: rhizomatous, scap: scapose, succ: succulent, suffr: suffruticose.
On foot |
By car |
GSV complete |
GSV each 1.5 km |
Google Earth aerial |
||||||
No. |
% |
No. |
% |
No. |
% |
No. |
% |
No. |
% |
|
P scap |
84 |
100.00 |
75 |
89.29 |
75 |
89.29 |
5 |
5.95 |
7 |
8.33 |
P caesp |
7 |
100.00 |
6 |
85.71 |
7 |
100.00 |
1 |
14.29 |
1 |
14.29 |
P lian |
1 |
100.00 |
1 |
100.00 |
1 |
100.00 |
0 |
0 |
0 |
0 |
P succ |
11 |
100.00 |
10 |
90.91 |
11 |
100.00 |
1 |
9.09 |
0 |
0 |
H scap |
1 |
100.00 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
H caesp |
7 |
100.00 |
4 |
57.14 |
4 |
57.14 |
0 |
0 |
2 |
28.57 |
G bulb |
2 |
100.00 |
2 |
100.00 |
0 |
0 |
0 |
0 |
0 |
0 |
G rhiz |
104 |
100.00 |
92 |
88.46 |
94 |
90.38 |
6 |
5.77 |
70 |
67.31 |
T scap |
40 |
100.00 |
17 |
42.50 |
16 |
40.00 |
1 |
2.50 |
0 |
0 |
T rept |
1 |
100.00 |
0 |
0.00 |
0 |
0.00 |
0 |
0.00 |
0 |
0 |
TOTAL |
258 |
100.00 |
207 |
80.23 |
208 |
80.62 |
14 |
5.43 |
80 |
31.01 |
Comparison between the five tested methods. Taxa identified and time spent are measured. The cost is estimated and rounded to the nearest unit.
Parameter |
By foot |
By car |
GSV complete |
GSV each 1.5 km |
Google Earth aerial |
Viewshed |
100% |
90% |
90% |
50% |
90% |
Taxa identified (No.; %) |
34; 100% |
21; 62% |
21; 62% |
10; 29% |
5; 15% |
Expected loss | - | 340.42 | 178.03 | 9268 | 3702.08 |
Mapping of single individuals |
yes |
yes |
yes |
yes |
only for trees and shrubs |
Choice of the season |
yes |
yes |
no |
no |
no |
Weather influence |
yes |
yes |
no |
no |
no |
Possibility of frequent updates |
yes |
yes |
no |
no |
no |
Measurement of plant population extension |
no |
no |
no |
no |
yes |
Safety for the researcher |
medium / low |
medium |
high |
high |
high |
Access to highways |
no |
yes |
yes |
yes |
yes |
Time per 10 km (min) |
160 |
17 |
15 |
5 |
10 |
Cost (€) |
26 |
15 |
3 |
1 |
2 |
Comparison between the five tested methods. Relative scores to each method are based on the real outcome of the pilot experiment.
Parameter |
Relative weight of importance |
By foot |
By car |
GSV complete |
GSV each 1.5 km |
Google Earth aerial |
Viewshed |
0.044 |
1.00 |
0.90 |
0.90 |
0.50 |
0.90 |
Taxa identified |
0.2 |
1.00 |
0.62 |
0.62 |
0.29 |
0.15 |
Expected loss |
0.044 |
1.00 |
0.96 |
0.98 |
0.00 |
0.60 |
Mapping of single individuals |
0.044 |
1.00 |
1.00 |
1.00 |
1.00 |
0.50 |
Choice of the season |
0.044 |
1.00 |
1.00 |
0.00 |
0.00 |
0.00 |
Weather influence |
0.044 |
0.00 |
0.00 |
1.00 |
1.00 |
1.00 |
Possibility of frequent updates |
0.044 |
1.00 |
1.00 |
0.00 |
0.00 |
0.00 |
Measurement of plant population extension |
0.044 |
0.25 |
0,25 |
0.25 |
0.25 |
1.00 |
Safety for the researcher |
0.044 |
0.25 |
0.50 |
1.00 |
1.00 |
1.00 |
Access to highways |
0.044 |
0.00 |
1,00 |
1.00 |
1.00 |
1.00 |
Time per 10 km |
0.2 |
0.00 |
0.89 |
0.91 |
0.97 |
0.94 |
Cost |
0.2 |
0.00 |
0.42 |
0.88 |
0.96 |
0.92 |
Total |
1 |
0.4 |
0.7 |
0.8 |
0.7 |
0.7 |
The panoramic photos, used for this survey on the whole Sicily, date back to 2009-2019. However, 56%, including all highway shots, are not older than 3 years and 81% are not older than 5 years. (Fig.
Biological and chorological spectra of the investigated florula on the Sicilian road network.
Life form |
No. taxa |
% |
Origin |
No. taxa |
% |
|
P scap |
13 |
32.5 |
America |
19 |
47.5 |
|
P caesp |
4 |
10 |
Africa |
5 |
12.5 |
|
P succ |
3 |
7.5 |
Asia |
5 |
12.5 |
|
P lian |
2 |
5 |
Australia |
3 |
7.5 |
|
NP |
3 |
7.5 |
Canary Is. |
2 |
5 |
|
Ch suffr |
2 |
5 |
Europe |
2 |
5 |
|
H caesp |
2 |
5 |
Paleotrop. |
2 |
5 |
|
H bienne |
1 |
2.5 |
Europe. Asia |
1 |
2.5 |
|
G bulb |
2 |
5 |
Madagascar |
1 |
2.5 |
|
G rhiz |
3 |
7.5 |
Total |
40 |
||
He |
1 |
2.5 |
||||
T scap |
4 |
10 |
||||
Total |
40 |
The most commonly recorded species are: Arundo donax L. (396 occurrences), Ailanthus altissima (Mill.) Swingle (171), Acacia saligna (Labill.) H.L.Wendl. (170) and Opuntia ficus-indica (L.) Mill (133) (Table
Taxa identified along the whole Sicilian Road Network using GSV panoramas interpretation, each of 1.5 km.
Taxon |
No. of occurrences |
Arundo donax L. |
396 |
Ailanthus altissima (Mill.) Swingle |
171 |
Acacia saligna (Labill.) H.L.Wendl. |
170 |
Opuntia ficus-indica (L.) Mill. |
133 |
Ricinus communis L. |
68 |
Rhus coriaria L. |
46 |
Cenchrus setaceus (Forssk.) Morrone |
42 |
Robinia pseudoacacia L. |
36 |
Agave americana subsp. americana |
29 |
Oxalis pes-caprae L. |
23 |
Symphyotrichum squamatum (Spreng.) G.L.Nesom |
19 |
Ipomoea indica (Burm.) Merr. |
17 |
Saccharum biflorum Forssk. |
12 |
Phoenix canariensis H.Wildpret |
11 |
Agave sisalana Perrine |
7 |
Erigeron canadensis L. |
7 |
Vachellia karroo (Hayne) Banfi & Galasso |
7 |
Erigeron bonariensis L. |
4 |
Carpobrotus acinaciformis (L.) L.Bolus |
3 |
Cyperus alternifolius subsp. flabelliformis Kük. |
3 |
Lantana camara subsp. aculeata (L.) R.W.Sanders |
3 |
Mirabilis jalapa L. |
3 |
Washingtonia robusta H.Wendl. |
3 |
Cardiospermum grandiflorum Sw. |
2 |
Eucalyptus camaldulensis subsp. camaldulensis |
2 |
Isatis tinctoria subsp. tinctoria |
2 |
Leucaena leucocephala subsp. glabrata (Rose) Zárate |
2 |
Senecio angulatus L.f. |
2 |
Aeonium arboreum (L.) Webb & Berthel. |
1 |
Alnus cordata (Loisel.) Duby |
1 |
Austrocylindropuntia cylindrica (Lam.) Backeb. |
1 |
Canna indica L. |
1 |
Melia azedarach L. |
1 |
Mesembryanthemum cordifolium L.f. |
1 |
Opuntia stricta (Haw.) Haw. |
1 |
Parasenegalia visco (Griseb.) Seigler & Ebinger |
1 |
Parkinsonia aculeata L. |
1 |
Solanum elaeagnifolium Cav. |
1 |
Solanum lanceolatum Cav. |
1 |
Xanthium italicum Moretti |
1 |
Total |
1235 |
The taxa surveyed mostly are naturalised in Sicily (26 taxa), 12 are invasive and only 2 are casual (Table
Invasive, naturalised and casual taxa per type of road, altitudinal range, cartographic unit, protected area, bioclimate and land use. Using GSV, each of 1.5 km, on the whole Sicilian road network.
Type of road |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
Highway |
601 |
393 |
65.4 |
323 |
82.2 |
70 |
17.8 |
0 |
0.0 |
Main |
1352 |
676 |
50.0 |
515 |
76.2 |
159 |
23.5 |
2 |
0.3 |
Local |
397 |
166 |
41.8 |
129 |
77.7 |
37 |
22.3 |
0 |
0.0 |
Total |
2350 |
1235 |
52.6 |
968 |
78.4 |
266 |
21.5 |
2 |
0.2 |
Cartographic unit |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
2.1 Northern coast |
329 |
222 |
67.5 |
181 |
81.5 |
41 |
18.5 |
0 |
0 |
2.2 Eastern coast |
138 |
107 |
77.5 |
78 |
72.9 |
29 |
27.1 |
0 |
0 |
2.3 Southern and Western coast |
144 |
100 |
69.4 |
76 |
76.0 |
24 |
24.0 |
0 |
0 |
3.1 Western Sicily and inland Palermo |
507 |
300 |
59.2 |
256 |
85.3 |
42 |
14.0 |
2 |
0.7 |
3.2 Hilly inland |
507 |
221 |
43.6 |
159 |
71.9 |
62 |
28.1 |
0 |
0 |
4.2 Mts of Palermo |
23 |
8 |
34.8 |
6 |
75.0 |
2 |
25.0 |
0 |
0 |
4.3 Sicani Mts |
43 |
10 |
23.3 |
2 |
20.0 |
8 |
80.0 |
0 |
0 |
4.4 Madonie Mts |
62 |
16 |
25.8 |
13 |
81.3 |
3 |
18.8 |
0 |
0 |
4.5 Erei Mts |
77 |
18 |
23.4 |
15 |
83.3 |
3 |
16.7 |
0 |
0 |
4.6 Nebrodi Mts |
121 |
30 |
24.8 |
21 |
70.0 |
9 |
30.0 |
0 |
0 |
4.7 Peloritani Mts |
29 |
21 |
72.4 |
17 |
81.0 |
4 |
19.0 |
0 |
0 |
4.8 Etna Mt. |
162 |
88 |
54.3 |
76 |
86.4 |
12 |
13.6 |
0 |
0 |
4.9 Iblei and Siracusa Mts |
208 |
94 |
45.2 |
67 |
71.3 |
27 |
28.7 |
0 |
0 |
Altitude m a.s.l. |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
0-300. Plain |
1607 |
1026 |
63.8 |
813 |
79.2 |
211 |
20.6 |
2 |
0.2 |
301-600. Hill |
381 |
141 |
37.0 |
102 |
72.3 |
39 |
27.7 |
0 |
0.0 |
601-2000. Mountain |
362 |
68 |
18.8 |
52 |
76.5 |
16 |
23.5 |
0 |
0.0 |
Biolclimate |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
1. Lower Thermomediterranean |
1069 |
701 |
65.6 |
562 |
80.2 |
137 |
19.5 |
2 |
0.3 |
2. Upper Thermomediterranean |
608 |
374 |
61.5 |
289 |
77.3 |
85 |
22.7 |
0 |
0 |
3. Lower Mesomediterranean |
408 |
119 |
29.2 |
88 |
73.9 |
31 |
26.1 |
0 |
0 |
4. Upper Mesomediterranean |
185 |
35 |
18.9 |
23 |
65.7 |
12 |
34.3 |
0 |
0 |
5. Supramediterranean |
80 |
6 |
7.5 |
5 |
83.3 |
1 |
16.7 |
0 |
0 |
Land use |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
1.1 Urban fabric |
285 |
189 |
66.3 |
147 |
77.8 |
42 |
22.2 |
0 |
0 |
1.2 Industrial- commercial and transport units |
19 |
12 |
63.2 |
9 |
75.0 |
3 |
25.0 |
0 |
0 |
1.3 Mine- dump and construction sites |
4 |
4 |
100.0 |
3 |
75.0 |
1 |
25.0 |
0 |
0 |
1.4 Artificial- non-agricultural vegetated areas |
3 |
0 |
0.0 |
0 |
0 |
0 |
0 |
0 |
0 |
2.1 Arable land |
564 |
213 |
37.8 |
168 |
78.9 |
45 |
21.1 |
0 |
0 |
2.2 Permanent crops |
900 |
553 |
61.4 |
437 |
79.0 |
115 |
20.8 |
1 |
0.2 |
2.3 Pastures |
276 |
166 |
60.1 |
133 |
80.1 |
32 |
19.3 |
1 |
0.6 |
3.1 Forests |
90 |
15 |
16.7 |
7 |
46.7 |
8 |
53.3 |
0 |
0 |
3.2 Scrub or herbaceous vegetation associations |
177 |
74 |
41.8 |
55 |
74.3 |
19 |
25.7 |
0 |
0 |
3.3 Open spaces with little or no vegetation |
31 |
9 |
29.0 |
7 |
77.8 |
2 |
22.2 |
0 |
0 |
5.1 Inland waters |
1 |
0 |
0.0 |
0 |
0 |
0 |
0 |
0 |
0 |
Type of area |
Obs. |
Obs. with aliens |
% |
Invasive |
% |
Naturalised |
% |
Casual |
% |
Protected area |
249 |
101 |
40.6 |
69 |
68.3 |
32 |
31.7 |
0 |
0 |
Non-protected area |
2101 |
1134 |
54.0 |
898 |
79.2 |
234 |
20.6 |
2 |
0.2 |
Four species were accidentally introduced, while 22 were introduced as ornamentals, 10 for forestry or for stabilisation of dunes or road embankments. Only Opuntia ficus-indica is a food plant, although its spread along the road network is, in part, also due to ornamental or fence purposes. Agave sisalana Perrine, Arundo donax, Rhus coriaria L. and Ricinus communis L. are species introduced into Sicily for industrial purposes, but, once their exploitation ended, they remained on the Island and spread.
The large number of taxa recorded reproduces by seeds (24), 12 reproduce both by seeds and vegetatively and four almost exclusively through vegetative reproduction (Suppl. material
Coasts and inland areas with Thermomediterranean climate have the highest percentage of alien taxa, whereas higher mountains with Mesomediterranean or Supramediterranean bioclimates have the lowest percentages of alien taxa (Fig.
The method that gave the best results in terms of number of species is the survey by foot. Taking into account all the other parameters considered, including cost and time, the photo-interpretation of GSV panoramas was found to be preferable for monitoring the occurrence of alien plants along the roads. This method showed some limitations compared to images provided by satellites. The former are updated no more than once per year (
Despite the limitations that emerged, we wanted to make a quick survey of the entire Sicilian road network, using the GSV panoramas interpretation every 1.5 km that gave the same score as the other expeditive methods tested. The study of the entire regional territory, using GSV images spaced each 1.5 km, recorded only 10% of the alien species known for Sicily. This is due to the limited territory overall observed and to the impossibility of identifying a large number of taxa from photos done from a moving car, such as those used for GSV shots. In addition, using GSV panoramas, it is not possible to choose the season in which to carry out the surveys (
For the observed case study, it was recorded that, where the roads cross well-tended agricultural areas, the presence of alien species is very low. The annual tillage of the land tends to limit the presence of perennial alien species, with the sole exclusion of Arundo donax that was favoured in these contexts for the uses made of it which continue to the present day (
The interpretation of GSV photos can provide a minimal fraction of the species present in an area; however, in 35 hours, it was possible to carry out 2350 surveys distributed throughout the Sicilian road network which allowed us to map the distribution of widespread species, such as Acacia saligna, Ailanthus altissima and Opuntia ficus-indica species often neglected in floristic studies and herbaria (
Roadside surveys are a useful tool for compiling and updating alien plants inventories, especially with limited time availability and small inventory budget (
The GSV sampling methodology, applied to the identification of alien plants on roadsides, can definitely act as a starting point for further field investigations and for developing management policies at a local level.
Our sincere thanks to Stefano Barone (University of Palermo) for the discussions and suggestions regarding the statistical analysis.