|Are island plant communities more invaded than their mainland counterparts?
Montserrat Vila, Joan Pino, Ana Montero & Xavier Font
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 commu- nities. Twenty distinct plant community types were compared between island and mainland coun- terparts.
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 in- vaded communities were ruderal and riparian.
Main conclusion: Our results indicate that para- oceanic island communities such as the Balearic Islands are less invaded than their mainland counterparts. This difference reﬂects a smaller re- gional alien species pool in the Balearic Islands than
Vila, M. (corresponding author: montse.vila@ebd. csic.es) & Montero, A. (firstname.lastname@example.org): 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 Bar- celona, 08193 Bellaterra, Spain.
Font, X. (email@example.com): Plant Biology Department, University of Barcelona, Diagonal 645, 08028 Barcelo- na, Spain.
in the adjacent mainland, probably due to differ- ences in landscape heterogeneity and propagule pressure.
Keywords: alien plants; Balearic Islands; community similarity; Mediterranean communities; para-ocea- nic islands; releve; species richness.
Nomenclature: Bolos & Vigo (1984–2001), Rivas- Martinez et al. (2001).
Islands have been a focus of attention in the bio- logical 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 sus- ceptible to invaders than continents (D’Antonio & Dudley 1994; Jeschke 2008), especially when com- paring their establishment and success of spread. Possibly, invasibility (i.e. intrinsic resistance of com- munities 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 as- sessment independent of climate, human history and insularity status (Chytry et al. 2008a, b). While stu- dies 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 differ- ences 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 composi- tions are very different from their mainland counterparts (Brown & Lomolino 1998).
Mediterranean Basin islands, which are con- sidered 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 re- present 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 Is- lands 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 hy- pothesized that island communities would be more invaded than their mainland counterparts, but pat- terns of invasion would be consistent between islands and mainland communities.
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 con- nected 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, in- cluding Cabrera and Dragonera, among others. Their geographical location is between 11110 and
41190 E and 401050 and 381380 N. The climate is typi-
cally 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 propor- tions 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
01150 E and 31150 E and 401300 N and 421400 N. Due
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 be- tween man and climate in Western Europe and the Mediterranean region. Forest and shrubland dom- inate 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 con- centrates in coastal areas.
A remarkably long tradition in botany has per- mitted the accumulation of a great number of ﬂoristic records in Catalonia and the Balearic Is- lands. This was formerly dispersed in both published work (more than 500 references from journals, books, dissertations and local atlases) and un- published 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 fol- lowing the 10-km UTM grid. These releves have been performed using the sigmatist method (Braun- Blanquet 1964), which assigns to each species an abundance–dominance code associated to a cover percentage range according to the following scale:
15 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 dif- ferent paired community types of Mediterranean distribution were selected (App. 1). Thus, extra- Mediterranean 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 percen- tage 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 commu- nities (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 sur- rogate for sampling period, this lack of interaction indicates that, on average, a particular community type was sampled during the same period in Catalo- nia and in the Balearic Islands. In fact, traditionally, many communities were sampled in parallel by the same school of botanists (Vigo 1998).
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 commu- nity type, the ﬁrst-order Jackknife richness estimator was calculated based on 100 randomiza- tion runs. Mean accumulation curves and Jackknife estimates were computed using EstimateS vs.
8.0 (R.K. Colwell, http://viceroy.eeb.uconn.edu/
Differences in the occurrence of alien species (i.e. percentage of plots with any alien) between is- lands and mainland across communities were tested with a paired t-test. Differences in alien species richness, alien species percentage, alien cover per- centage 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 commu- nities were tested with a Tukey–Kramer test.
Overall, 55 alien species were found in the da- taset (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 spe- cies 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 Ba- learic Islands (Fig. 3).
Of the seven alien species occurring in both the Balearic Islands and Catalonia, there were no sig- niﬁcant differences between mainland and islands in percentage occurrence across plots (paired t-test 5
0.805, P 5 0.452). However, except for Oxalis pes-
caprae and Agave americana, these species were
20 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.
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 be- tween 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 percen- tage of alien species was signiﬁcantly higher in the mainland than in the islands (Table 1). The percen- tage 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 be- tween the other communities (Tukey–Kramer tests not shown). There was a signiﬁcant interaction be- tween 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 dif- ferent 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 (Cata- lonia: 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 relation- ship. In fact, a single alien species could range from o5% to 90% cover.
In total, 1678 native species were found in the da- taset (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 in- teraction 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
Alien occurrence 6.21 ± 2.42 16.23 ± 4.57 2.85, P 5 0.010
Number of aliens 0.04 ± 0.01 0.19 ± 0.02 588.56, Po 0.0001
Percentage of aliens 0.36 ± 0.08 1.38 ± 0.12 655.30, Po 0.0001
not differ, and in two community types it was even lower in Catalonia than in the Balearic Islands,
Percentage of alien cover
0.40 ± 0.11 1.92 ± 0.23 271.57, Po 0.0001
namely ruderal perennial forb communities and cal-
careous pseudo-steppe communities (Fig. 3).
Fig. 3. Number of native species and percentage of alien species per plot (mean ± SE) for different communities in Cata- lonia 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 commu- nities, 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 pseudo- steppe communities, L095 Xeric perennial grasslands, L0123 Calcareous Mediterranean scrub, L0125 Riparian galleries and thickets, L0147 Mediterranean macchia, L0148 Mediterranean sclerophyllous forests.
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 (super- indexs). V 5 vegetative, S 5 sexual reproduction; W 5 wind, Z 5 animal, U 5 unspeciﬁc seed dispersal. See Fig. 3 for community nomenclature.
Agave americana V,S,U
5.13 (L073, L147)
Amaranthus deﬂexus S,U
3.39 (L062, L085)
Chenopodium ambrosioides S,U
6.53 (L062, L063, L065, L073, L085)
Conyza bonariensis S,W
10.26 (L062, L063)
12.53 (L062, L065, L073, L074, L085, L095, L148)
Ipomoea purpurea S,U
1.31 (L062, L063)
Oxalis pes-caprae V,U
58.97 (L021, L063, L074, L095, L125, L147)
0.78 (L063, L073)
Xanthium spinosum S,Z
5.74 (L062, L063, L065)
by a lower occurrence of alien species, lower com- munity 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 nutrient- rich and intermittently wet areas. Invasion in such communities is consistent with the theory of ﬂuctu- ating 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 occu- pancy of alien species is small in both regions (Lambdon et al. 2008). On average, alien species in- vaded 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 com- munity 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 agri- cultural 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 at- tributed to spatial sampling bias. Even if sample size was smaller in the islands than on the mainland, re- sults from the species accumulation curves matched the observed alien species richness (Fig. 2). Further- more, 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 in- vasion 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 Catalo- nia. 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 ad- jacent 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 inva- sion 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 rich- ness 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 an- thropogenic features are surrogates for propagule
pressure (Lockwood et al. 2005; Chytry et al.
These invasion patterns mirror other island studies that have found alien success to be more closely related to environmental variation, differ- ences 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 opportu- nities for species dispersal occur (Sax & Gaines
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CGL-2009-07515 and CGL2006-13421-C04-01, and the
Junta de Andalucıa Excellence Project RNM-4031.
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Received 4 May 2009; Accepted 29 November 2009.
Co-ordinating Editor: Dr. Valerio Pillar.
L009 Phragmition australis
Mainland Islands Mainland Islands Mainland Islands Mainland Islands
Reedbeds 49 10 2.04 0.00 1.97 ± 0.97 0.00 ± 0.00 1986.76 ± 11.25 1983.30 ± 7.07
L015 Isoetion Hydrophytic vegetation
20 20 20.00 0.00 1.06 ± 0.01 0.00 ± 0.00 1990.60 ± 5.53 1990.00 ± 6.81
L026 Crithmo- Limonion
L032 Juncion maritimi
L035 Crucianellion maritimae
L062 Chenopodion muralis
meadows Chamaephytic vegetation of sea cliffs Brackish rushbeds
Fixed dune dwarf scrubs Ruderal annual forb communities
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 1990.02 ± 8.81
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 10.94 ± 2.18 0.00 ± 0.00 1993.88 ± 6.30 1992.71 ± 3.53
65 14 86.15 28.57 57.35 ± 0.00 3.00 ± 0.00 1984.83 ± 17.17 1985.07 ± 8.31
L063 Silybo-Urticion Tall-forb
76 38 26.31 36.84 23.89 ± 3.00 8.89 ± 1.86 1986.12 ± 14.26 1986.74 ± 8.67
L065 Hordeion leporini
L068 Salsolo- Peganion
L073 Bromo- Oryzopsion miliaceae
L074 Echio- Galactition
L089 Helianthemion guttati
Ruderal grasslands Ruderal salty scrub Ruderal
Ruderal perennial forb communities Trampled rud- eral grasslands Siliceous
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 26.89 ± 7.16 0.00 ± 0.00 1988.50 ± 17.65 1988.80 ± 6.05
21 11 9.52 0.00 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 Publication year
L092 Thero- Brachypodion
L095 Brachypodion phoenicoidis
L0123 Rosmarino- Ericion
L0125 Rubo ulmifolii- Nerion oleandri
Calcareous pseudo-steppe communities Xeric perennial grasslands Calcareous Mediterranean scrub
galleries and thickets
Mainland Islands Mainland Islands Mainland Islands Mainland Islands
202 71 0.00 0.00 0.00 ± 0.00 0.00 ± 0.00 1996.42 ± 7.54 1994.48 ± 3.44
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 8.82 ± 2.25 1.00 ± 0.00 1996.17 ± 7.86 1988.80 ± 6.61
121 222 0.83 0.90 1.99 ± 0.99 2.99 ± 0.99 1986.84 ± 16.64 1987.89 ± 8.84
L0148 Quercion ilicis Mediterranean sclerophyllous forest
*i.e. percentage of plots with any alien.
364 25 1.10 0.00 4.99 ± 1.40 0.00 ± 0.00 1991.04 ± 7.65 1991.52 ± 2.02