Are island plant communities more invaded than their mainland
counterparts?
Montserrat Vila, Joan Pino, Ana Montero & Xavier Font
Abstract
Questions: Are island vegetation communities more
invaded than their mainland counterparts? Is this
pattern consistent among community types?
Location: The coastal provinces of Catalonia and
the para-oceanic Balearic Islands, both in NE Spain.
These islands were connected to the continent more
than 5.35 million years ago and are now located
o200 km from the coast.
Methods: We compiled a database of almost 3000
phytosociological releves from the Balearic Islands
and Catalonia and compared the level of invasion
by alien plants in island versus mainland communities. Twenty distinct plant community types
were compared between island and mainland counterparts.
Results: The percentage of plots with alien species,
number, percentage and cover percentage of alien
species per plot was greater in Catalonia than in the
Balearic Islands in most communities. Overall,
across communities, more alien species were found
in the mainland (53) compared to the islands (only
nine). Despite these differences, patterns of the level
of invasion in communities were highly consistent
between the islands and mainland. The most invaded communities were ruderal and riparian.
Main conclusion: Our results indicate that paraoceanic island communities such as the Balearic
Islands are less invaded than their mainland
counterparts. This difference reflects a smaller regional alien species pool in the Balearic Islands than
Vila, M. (corresponding author: montse.vila@ebd.
csic.es) & Montero, A. (anamontero@ebd.csic.es):
Estacion Biologica de Donana (EBD-CSIC), Avda.
Americo Vespucio s/n, Isla de la Cartuja, 41092 Sevilla,
Spain.
Pino, J. (Joan.Pino@uab.es): CREAF (Center for
Ecological Research and Forestry Applications) and
Unit of Ecology, Department of Animal and Plant
Biology and Ecology, Autonomous University of Barcelona, 08193 Bellaterra, Spain.
Font, X. (xfont@ub.edu): Plant Biology Department,
University of Barcelona, Diagonal 645, 08028 Barcelona, Spain.
in the adjacent mainland, probably due to differences in landscape heterogeneity and propagule
pressure.
Keywords: alien plants; Balearic Islands; community
similarity; Mediterranean communities; para-oceanic islands; releve; species richness.
Nomenclature: Bolos & Vigo (1984–2001), RivasMartinez et al. (2001).
Introduction
Islands have been a focus of attention in the biological invasion literature due to their high regional
pool of alien species and the impact that invaders
pose to native species and ecosystem processes (Elton
1958; Lonsdale 1999). However, several studies have
challenged the hypotheses that islands are more susceptible to invaders than continents (D’Antonio &
Dudley 1994; Jeschke 2008), especially when comparing their establishment and success of spread.
Possibly, invasibility (i.e. intrinsic resistance of communities to invasion) does not differ between island
and mainland communities but differences are due to
differences in the history of invasion (Sol 2000).
Recently a comparison of the level of habitat
invasion across regions of Europe, including Great
Britain, found a high inter-regional consistency in
habitat invasion patterns. These results suggest that
habitats are good predictors for invasion risk assessment independent of climate, human history and
insularity status (Chytry et al. 2008a, b). While studies have shown higher regional proportions of alien
species pools (Lonsdale 1999) or higher occurrences
of certain species (Gimeno et al. 2006) in islands
compared to their mainland counterparts, there has
been no rigorous analysis quantifying the differences in alien species richness in such plant
communities to date. Such a comparison should be
conducted in paired homologous
communities.
Communities that are homologous to the mainland
are often difficult to find in remote oceanic islands
whose evolutionary histories and species compositions are very different from their mainland
counterparts (Brown & Lomolino 1998).
Mediterranean Basin islands, which are considered to be hotspots for biodiversity conservation
(Cowling et al. 1996; Medail & Quezel 1999), are at
significant risk from invasive species, which are
threatening endemic species, especially in coastal
areas (Di Castri et al. 1990; Hulme 2004). Most
Mediterranean islands are para-oceanic (i.e. they
have been attached to the continent in the past) and
share many floristic components with the adjacent
mainland (Greuter 1995). They thus do not represent a disproportionately low native species
richness considering area–species relationships
(Medail 2008). Therefore, these islands offer the
opportunity to test whether island communities are
more invaded than their mainland counterparts. We
took advantage of the large datasets available on
vegetation plots and compared the level of invasion
in different community types from the Balearic Islands and the adjacent mainland in Catalonia (both
in NE Spain). We aimed to answer the following
questions: Are island vegetation communities more
invaded than their mainland counterparts? Is this
pattern consistent among community types? We hypothesized that island communities would be more
invaded than their mainland counterparts, but patterns of invasion would be consistent between
islands and mainland communities.
Methods
Study area
The Balearic archipelago (5014 km2) is located
about 200 km east of Spain (Fig. 1). These islands
are considered to be para-oceanic as they were connected to the continent during the Messinian period
(between 5.70 and 5.35 million years ago). The
Balearic archipelago consists of four main islands
(Mallorca, Menorca, Eivissa and Formentera)
and a set of uninhabited small islands and inlets, including Cabrera and Dragonera, among others.
Their geographical location is between 1111 0 and
4119 0 E and 40105 0 and 38138 0 N. The climate is typically Mediterranean, with warm summers, mild
winters and low precipitation. Climate variability
is highly affected by island size and elevation
range, which is highest in Mallorca and lowest in
Formentera and Cabrera. Although the Balearic
Islands encompass similar land uses and have
predominantly calcareous soils, the relative proportions of these differ, as does the topography (Rita &
Payeras 2006).
Catalonia is located in the northeast of Spain,
bounded in the north by the Pyrenees and on the
east by the Mediterranean Sea. It is situated between
0115 0 E and 3115 0 E and 40130 0 N and 42140 0 N. Due
Catalonia
Balearic Islands
25
0
125 km
Fig. 1. Study area in the Balearic Islands (883 plots) and in Catalonia (2053 plots). The shaded area in Catalonia (the inland
province of Lleida) was not included for analysis.
to its contrasting topography, geology and climate,
the vegetation is very heterogeneous (Bolos &
Vigo 1984–2001). For this analysis, only provinces
with typical Mediterranean maritime climate and
located at o120 km from the coast were considered.
The Catalonia study area covered 19 946 km2
(Fig. 1).
The landscape structure of Catalonia and the
Balearic islands reflects the typical interaction between man and climate in Western Europe and the
Mediterranean region. Forest and shrubland dominate the hilly areas, as a result of agricultural land
abandonment in the mid-20th century. In contrast,
lowlands are intensively cultivated or urbanised.
Most of the population and summer tourism concentrates in coastal areas.
Database
A remarkably long tradition in botany has permitted the accumulation of a great number of
floristic records in Catalonia and the Balearic Islands. This was formerly dispersed in both published
work (more than 500 references from journals,
books, dissertations and local atlases) and unpublished information (mainly PhD and Master’s
theses). The Catalonian Database of Biodiversity
(Banc de Dades de Biodiversitat de Catalunya,
BDBC; Font & Ninot 1995) has been devoted to the
collection, organisation and online exploitation of
all these floristic data (see http://biodiver.bio.ub.es/
biocat and http://www.sivim.info). Currently, the
BDBC accounts for about 1 690 000 floristic records
and 20 906 phytosociological releves organised following the 10-km UTM grid. These releves have
been performed using the sigmatist method (BraunBlanquet 1964), which assigns to each species an
abundance–dominance code associated to a cover
percentage range according to the following scale:
1 5 o5%; 1 5 5–10%; 2 5 10–25%; 3 5 25–50%;
4 5 50–75%; 5 5 475%.
From the total BDBC releves, we selected those
alliances having phytosociological classification and
that occurred both in Catalonia (2053 releves) and
in the Balearic Islands (883 releves). In total, 20 different paired community types of Mediterranean
distribution were selected (App. 1). Thus, extraMediterranean communities (i.e. Boreal, Atlantic,
Continental) occurring in Catalonia but lacking in
the Balearic Islands were discarded.
From each releve we calculated the number of
alien and native species (i.e. species richness) and the
percentage of alien species. We also calculated the
percentage cover of alien species, by assigning to
each species the central value of the cover percentage range corresponding to its abundance–
dominance code. A species was considered an alien
if it originated in another region outside mainland
Spain or the Balearic Islands and it was introduced
accidentally or deliberately by humans after 1500
AD. Therefore, only neophytes were considered and
archeophytes (i.e. alien species introduced before the
15th century) were excluded from analysis.
The publication date of releves (plots, hereafter)
considered in this study ranged from 1960 to 2003
(App. 1). It was not significantly different between
islands and the mainland (F1, 2888 5 2.28, P 5 0.13)
but it was significantly different between communities (F19, 2888 5 11.31, Po0.0001). The interaction
between island and mainland status and community
was not significant (F1, 2888 5 1.14, P 5 0.30).
Therefore, assuming the publication year as a surrogate for sampling period, this lack of interaction
indicates that, on average, a particular community
type was sampled during the same period in Catalonia and in the Balearic Islands. In fact, traditionally,
many communities were sampled in parallel by the
same school of botanists (Vigo 1998).
Data analysis
To overcome differences in sample size (i.e.
number of plots), mean species accumulation curves
were computed for alien species in the islands and on
the mainland (Gotelli & Colwell 2001). For all
communities pooled together and for each community type, the first-order Jackknife richness
estimator was calculated based on 100 randomization runs. Mean accumulation curves and Jackknife
estimates were computed using EstimateS vs.
8.0 (R.K. Colwell, http://viceroy.eeb.uconn.edu/
estimates).
Differences in the occurrence of alien species
(i.e. percentage of plots with any alien) between islands and mainland across communities were tested
with a paired t-test. Differences in alien species
richness, alien species percentage, alien cover percentage and native species richness between islands
and the mainland, and among communities were
tested by a general linear approach with Type 3
sums of squares performed with STATISTICA
(version 6, StatSoft Inc., Tulsa, OK, USA) after log
transformation of native species richness data and
square-root (0.51x) transformation for the other
variables. Pair-wise differences between communities were tested with a Tukey–Kramer test.
Results
Number of alien species
Overall, 55 alien species were found in the dataset (53 in Catalonia and only nine in the Balearic
Islands). Two alien species in the Balearic Islands
(Cynara cardunculus and Hedysarum coronarium)
were not found in Catalonian plots. Total alien species richness was lower in the islands than on the
mainland (Jackknife estimates in islands 5 10 ± 1
SD; on mainland 5 42 ± 2 SD; Fig. 2). Alien species
richness per plot was also lower in the islands than
on the mainland (Table 1).
The overall occurrence of alien species was low.
It was significantly higher in Catalonia than in the
Balearic Islands (Table 1). Ruderal salty scrub and
calcareous pseudo-steppe communities were never
invaded. Besides these two communities, 10 of the
other communities were never invaded in the Balearic Islands (Fig. 3).
Of the seven alien species occurring in both the
Balearic Islands and Catalonia, there were no significant differences between mainland and islands in
percentage occurrence across plots (paired t-test 5
0.805, P 5 0.452). However, except for Oxalis pescaprae and Agave americana, these species were
Catalonia
50
45
40
35
30
25
20
15
10
5
0
Balearic Islands
1
3
5
7
9
11
13
15
17
19
21
Number of plots (x 102)
Fig. 2. Species accumulation curves in the Balearic Islands
and in Catalonia for Jackknife estimates of alien species
richness across plots. Error bars are ± 1 SD.
Table 1. Level of invasion (mean ± SE) in the Balearic
Islands and in Catalonia. Differences were tested with a
GLM test (X2 value), except for alien occurrence (i.e.
percentage of plots with any alien), which was tested with
a paired t-test.
Species
Balearic
Islands
Catalonia
Statistic
Alien occurrence
Number of aliens
Percentage of aliens
Percentage of
alien cover
6.21 ± 2.42 16.23 ± 4.57 2.85, P 5 0.010
0.04 ± 0.01 0.19 ± 0.02 588.56, Po 0.0001
0.36 ± 0.08 1.38 ± 0.12 655.30, Po 0.0001
0.40 ± 0.11 1.92 ± 0.23 271.57, Po 0.0001
found in more community types in Catalonia than in
the Balearic Islands (Table 2).
Number of sampling plots was very unbalanced.
However, there was no significant relationship between the number of plots per community type and
the mean percentage of alien species (Catalonia:
Spearman rank correlation, r 5 — 0.75, P 5 0.45;
Balearic Islands: r 5 — 1.28, P 5 0.20). The percentage of alien species was significantly higher in the
mainland than in the islands (Table 1). The percentage of alien species was also significantly different
between communities (X2 5 57.064, Po0.0001, Fig.
3). Ruderal annual forb communities were the most
invaded, followed by tall-forb humid communities,
ruderal tall grasslands and riparian galleries and
thickets. No significant differences were found between the other communities (Tukey–Kramer tests
not shown). There was a significant interaction between community type and mainland/island status
(X2 5 184.224, Po0.0001), indicating that although
most communities were more invaded in Catalonia
than in the Balearic Islands, a few communities
could be as invaded in the islands as in the mainland
(e.g. tall-forb humid communities, ruderal perennial
forb communities and riparian communities; Fig. 3).
Alien percentage cover was also significantly different between islands and the mainland (Table 1),
and differences among communities followed the
same patterns as for percentage of alien species.
There was a positive correlation between the number
of alien species per plot and alien plant cover (Catalonia: Spearman
rank
correlation, r 5 0.36,
Po0.0001; Balearic Islands: r 5 0.38, P 5 0.03).
However, there was a large variation in this relationship. In fact, a single alien species could range from
o5% to 90% cover.
In total, 1678 native species were found in the dataset (1442 in Catalonia and 827 in the Balearic
Islands). The number of native species per plot ranged
from one to 80 species, and on average was larger in
Catalonia (18.09 ± 0.20, mean ± SE) than in the
Balearic
Islands
(15.23 ± 0.32;
X2 5 907.161,
Po0.0001). The number of native species per plot was
also significantly different between communities
(X2 5 15.567, Po0.0001). There was a significant interaction between community type and mainland/
island status (X2 5 242.483, Po0.0001), indicating
that although, in general, native species richness was
larger in Catalonia than in the Balearic Islands, in
some communities the number of native species may
not differ, and in two community types it was even
lower in Catalonia than in the Balearic Islands,
namely ruderal perennial forb communities and calcareous pseudo-steppe communities (Fig. 3).
40
Balearic Islands
Catalonia
35
Number of native species
30
25
20
15
10
5
L147
L148
L148
L125
L147
L123
L095
L092
L089
L085
L074
L073
L068
L065
L063
L062
L035
L032
L026
L021
L015
L009
0
25
Balearic Islands
Catalonia
Percentage of alien species
20
15
10
5
L125
L123
L095
L092
L089
L085
L074
L073
L068
L065
L063
L062
L035
L032
L026
L021
L015
L009
0
Community
Fig. 3. Number of native species and percentage of alien species per plot (mean ± SE) for different communities in Catalonia and the Balearic Islands. Communities follow the BDBC nomenclature (http://biodiver.bio.ub.es/biocat): L009
Reedbeds, L015 Hydrophytic vegetation, L021 Perennial humid meadows, L026 Chamaephytic vegetation of sea cliffs, L032
Brackish rushbeds, L035 Fixed dune dwarf scrub, L062 Ruderal annual forb communities, L063 Tall-forb humid communities, L065 Ruderal grasslands, L068 Ruderal salty scrub, L073 Ruderal tall grasslands, L074 Ruderal perennial forb
communities, L085 Trampled ruderal grasslands, L089 Siliceous pseudo-steppe communities, L092 Calcareous pseudosteppe communities, L095 Xeric perennial grasslands, L0123 Calcareous Mediterranean scrub, L0125 Riparian galleries and
thickets, L0147 Mediterranean macchia, L0148 Mediterranean sclerophyllous forests.
Discussion
Contrary to the general wisdom that islands are
more invaded than the mainland, we found that
most plant communities in the Balearic Islands were
less invaded than their mainland counterparts in
Catalonia. This lower level of invasion is manifested
Table 2. Percentage occurrence across invaded plots of the seven alien species present in both the Balearic Islands and
Catalonia, with indication of the invaded communities (in parenthesis), and reproduction and dispersal modes (superindexs). V 5 vegetative, S 5 sexual reproduction; W 5 wind, Z 5 animal, U 5 unspecific seed dispersal. See Fig. 3 for
community nomenclature.
Species
Balearic Islands
Catalonia
Agave americana V,S,U
Amaranthus deflexus S,U
Chenopodium ambrosioides S,U
Conyza bonariensis S,W
Ipomoea purpurea S,U
Oxalis pes-caprae V,U
Xanthium spinosum S,Z
5.13 (L073, L147)
5.13 (L062)
5.13 (L062)
10.26 (L062, L063)
2.56 (L063)
58.97 (L021, L063, L074, L095, L125, L147)
2.56 (L063)
0.26 (L063)
3.39 (L062, L085)
6.53 (L062, L063, L065, L073, L085)
12.53 (L062, L065, L073, L074, L085, L095, L148)
1.31 (L062, L063)
0.78 (L063, L073)
5.74 (L062, L063, L065)
by a lower occurrence of alien species, lower community alien species richness and lower alien plant
cover. However, there was a high consistency in the
identity of the most invaded communities, which in
general had intermediate levels of native species
richness. These communities are located in nutrientrich and intermittently wet areas. Invasion in such
communities is consistent with the theory of fluctuating resource availability (Davis et al. 2000), which
proposes that pulses in resource inputs or in their use
by organisms (e.g. because of episodic perturbations)
trigger the establishment and spread of invaders.
The frequency of individual alien species across
plots was low and did not differ between the islands
and the mainland, indicating that the niche occupancy of alien species is small in both regions
(Lambdon et al. 2008). On average, alien species invaded more community types in the mainland than
in the islands, except for A. americana and especially
O. pes-caprae. Both species have the advantage that
they reproduce vegetatively. Extensive field surveys
have also found O. pes-caprae invades more community types in the islands than in the mainland,
possibly because of higher dispersal opportunities
in the islands than on the mainland due to soil
transport and machinery sharing between agricultural fields (Gimeno et al. 2006). O. pes-caprae
have a fast growth rate and very effective annual
vegetative reproduction through
underground
bulbs, with a high sprout and establishment rate in a
broad variety of environmental conditions (Vila et
al. 2006, 2008).
The low level of invasion in island communities
compared to mainland communities cannot be attributed to spatial sampling bias. Even if sample size
was smaller in the islands than on the mainland, results from the species accumulation curves matched
the observed alien species richness (Fig. 2). Furthermore, communities with many replicates did not
host higher levels of invasion. Mediterranean scler-
ophylous forest and calcareous scrub in Catalonia
best exemplify this lack of association, since they
were the most represented in the database, with
more than 300 plots each; however their level of invasion ranked very low.
The lower level of invasion in the islands and
also lower native species richness compared to the
mainland can be related to a smaller regional pool of
species in the Balearic Islands compared to Catalonia. In the Balearic Islands, the total number of alien
species has recently been estimated to be 304 species
(Moragues & Rita 2005), while in Catalonia it is
around 450 species or even more, considering that
the most up-to-date regional survey is 16 years older
than in the islands (Casasayas 1989). The regional
pool of native species in mainland East Spain adjacent to Balearic Islands (3134, Bolos & Vigo 1984–
2001) is also two-times larger than in these islands
(1569, Rita & Payeras 2006).
The primary causes of the lower level of invasion in the Balearic Islands than in Catalonia can be
due to a less heterogeneous landscape and lower
propagule pressure. Catalonia is a larger and more
heterogeneous region than the Balearic Islands, and
it therefore provides a greater diversity of habitats.
Both total and alien species richness in Catalonia are
related to habitat diversity at the regional scale
(Pausas et al. 2003; Pino et al. 2005, respectively).
On the other hand, differences in alien species richness at the local scale are suggestive of differences in
propagule pressure (Vila & Pujadas 2001). In fact,
according to the National Statistics Institute (http://
www.ine.es), population density and the density of
road networks in 2003 (the latest database sampling
year) were larger in Catalonia (277.99 hab km — 2
and 0.60 km — 1) than in the Balearic Islands
(181.54 hab km — 2 and 0.43 km — 1). These two anthropogenic features are surrogates for propagule
pressure (Lockwood et al. 2005; Chytry et al.
2008b).
These invasion patterns mirror other island
studies that have found alien success to be more
closely related to environmental variation, differences in disturbance regimes and invasion history
than to community invasibility (Sol 2000; Gabriel
et al. 2001; Teo et al. 2003; Li et al. 2006). However,
the Balearic Islands, although less invaded than
mainland, might not be less vulnerable to invasion,
provided that land-use changes and more opportunities for species dispersal occur (Sax & Gaines
2003).
Acknowledgements. We thank V. Pillar and two anonymous referees for their helpful comments. This study was
partially financed by the Ministerio de Ciencia e Innovacion projects CGL2007-61165/BOS, CSD2008-00040
CGL-2009-07515 and CGL2006-13421-C04-01, and the
Junta de Andalucıa Excellence Project RNM-4031.
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App. 1. Main descriptors (mean ± SD) of plots across community types in mainland Catalonia and the Balearic Islands.
The syntaxonomical nomenclature follows Bolos & Vigo (1984–2001) and Rivas-Martınez et al. (2001).
Code
Alliance
Community
Number of plots
Occurrence*
Total alien species
Mainland Islands Mainland Islands Mainland
L009
L015
L021
L026
L032
L035
L062
L063
L065
L068
L073
L074
L085
L089
Phragmition
australis
Isoetion
Reedbeds
Hydrophytic
vegetation
MolinioPerennial
Holoschoenion humid
vulgaris
meadows
CrithmoChamaephytic
Limonion
vegetation of
sea cliffs
Juncion
Brackish
maritimi
rushbeds
Fixed dune
Crucianellion
maritimae
dwarf scrubs
Chenopodion
Ruderal
muralis
annual forb
communities
Silybo-Urticion Tall-forb
humid
communities
Ruderal
Hordeion
leporini
grasslands
SalsoloRuderal
Peganion
salty scrub
BromoRuderal
Oryzopsion
tall grasslands
miliaceae
EchioRuderal
Galactition
perennial forb
communities
TrifolioTrampled rudCynodontion
eral grasslands
Helianthemion Siliceous
guttati
pseudo-steppe
communities
Islands
Publication year
Mainland
Islands
49
10
2.04
0.00
1.97 ± 0.97 0.00 ± 0.00 1986.76 ± 11.25 1983.30 ± 7.07
20
20
20.00
0.00
1.06 ± 0.01 0.00 ± 0.00 1990.60 ± 5.53
96
35
1.04
8.57
1.98 ± 0.98 1.00 ± 0.00 1990.57 ± 11.75 1989.69 ± 6.23
145
93
6.21
0.00
2.99 ± 0.99 0.00 ± 0.00 1990.37 ± 9.90
47
21
8.51
0.00
2.00 ± 0.00 0.00 ± 0.00 1987.64 ± 10.15 1987.48 ± 7.37
58
17
20.69
0.00
65
14
86.15
28.57
57.35 ± 0.00 3.00 ± 0.00 1984.83 ± 17.17 1985.07 ± 8.31
76
38
26.31
36.84
23.89 ± 3.00 8.89 ± 1.86 1986.12 ± 14.26 1986.74 ± 8.67
95
28
18.95
0.00
16.94 ± 2.58 0.00 ± 0.00 1988.64 ± 15.11 1988.93 ± 5.48
12
38
0.00
0.00
0.00 ± 0.00 0.00 ± 0.00 1987.33 ± 9.51
1978.68 ± 12.40
54
13
40.74
15.38
16.87 ± 3.71 3.84 ± 1.24 1977.37 ± 9.53
1982.77 ± 3.70
74
20
21.62
10.00
14.90 ± 3.48 2.95 ± 0.95 1997.04 ± 8.61
1990.90 ± 6.14
102
10
32.35
0.00
21
11
9.52
0.00
10.94 ± 2.18 0.00 ± 0.00 1993.88 ± 6.30
1990.00 ± 6.81
1990.02 ± 8.81
1992.71 ± 3.53
26.89 ± 7.16 0.00 ± 0.00 1988.50 ± 17.65 1988.80 ± 6.05
3.90 ± 1.90 0.00 ± 0.00 1995.76 ± 1.30
1996.00 ± 0.00
App. 1. (Continued).
Code
Alliance
Community
Number of plots
Occurrence*
Total alien species
Mainland Islands Mainland Islands Mainland
L092
TheroBrachypodion
L095
Brachypodion
phoenicoidis
L0123 RosmarinoEricion
L0125 Rubo
ulmifoliiNerion
oleandri
L0147 OleoCeratonion
L0148 Quercion ilicis
Islands
Publication year
Mainland
Islands
Calcareous
pseudo-steppe
communities
Xeric perennial
grasslands
Calcareous
Mediterranean
scrub
Riparian
galleries and
thickets
202
71
0.00
0.00
0.00 ± 0.00 0.00 ± 0.00 1996.42 ± 7.54
66
52
6.06
3.85
4.96 ± 1.38 1.00 ± 0.00 1985.99 ± 13.50 1987.56 ± 6.64
363
135
0.83
0.00
1.00 ± 0.00 0.00 ± 0.00 1985.34 ± 14.60 1986.50 ± 8.14
23
10
21.74
20.00
Mediterranean
macchia
Mediterranean
sclerophyllous
forest
121
222
0.83
0.90
1.99 ± 0.99 2.99 ± 0.99 1986.84 ± 16.64 1987.89 ± 8.84
364
25
1.10
0.00
4.99 ± 1.40 0.00 ± 0.00 1991.04 ± 7.65
*i.e. percentage of plots with any alien.
8.82 ± 2.25 1.00 ± 0.00 1996.17 ± 7.86
1994.48 ± 3.44
1988.80 ± 6.61
1991.52 ± 2.02