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Landscape Ecology ISSN 0921-2973Volume 29Number 9 Landscape Ecol (2014) 29:1541-1550DOI 10.1007/s10980-014-0074-3

Landscape fragmentation, land-use legacyand propagule pressure promote plantinvasion on coastal dunes: a patch-basedapproach

Marco Malavasi, Marta Carboni,Maurizio Cutini, Maria L. Carranza &Alicia T. R. Acosta

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RESEARCH ARTICLE

Landscape fragmentation, land-use legacy and propagulepressure promote plant invasion on coastal dunes:a patch-based approach

Marco Malavasi • Marta Carboni •

Maurizio Cutini • Maria L. Carranza •

Alicia T. R. Acosta

Received: 3 February 2014 / Accepted: 19 July 2014 / Published online: 6 August 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Coastal dunes and sand areas are reported to

be among the habitats most invaded by alien species in

Europe. Landscape pattern could be a significant driver in

invasion processes in parallel with land-use legacy.

Fragmentation of natural habitats combined with the

availability of propagules from the surrounding matrix

may enhance the invisibility of ecological communities.

Based on multitemporal land cover maps (1954–2008)

and a floristic database, we analyzed how habitat

fragmentation, propagule pressure and land-use legacy

have affected alien plants’ presence and richness on

natural dune patches along the Lazio Coast (Central

Italy). Floristic data were derived from an existing geo-

database of random vegetation plots (64 m2). A set of

landscape patch-based metrics, considered to be adequate

proxies of the main processes affecting alien invasion and

richness, was calculated. First, we fit a generalized linear

model (GLM) with binomial errors to assess which

landscape metrics are influencing patch invasion. Sec-

ond, we extracted invaded patches and, with GLMs, we

investigated how landscape metrics affect average alien

species richness. Alien invasion and alien richness seem

to be affected by different processes: although alien

invasion of each patch is strongly associated with its land-

use legacy, the richness of aliens is more affected by

landscape fragmentation and by the propagule pressure to

which patch is exposed. By integrating spatial and

temporal landscape metrics with floristic data, we were

able to disentangle the relations of landscape fragmen-

tation, propagule pressure and land-use legacy with the

presence and richness of alien plants. The methodolog-

ical approach here adopted could be easily extended to

other alien species and ecosystems, offering scientifically

sound support to prevent the high economic costs derived

from both the control and the eradication of aliens.

Keywords Aliens � Landscape pattern � Land use

change � Mediterranean coastal dunes � Patch-based

metrics � Alien invasion � Alien richness

Introduction

Theoretical studies suggest that the spread of alien

species is closely connected to factors operating at the

Electronic supplementary material The online version ofthis article (doi:10.1007/s10980-014-0074-3) contains supple-mentary material, which is available to authorized users.

M. Malavasi � M. Carboni � M. Cutini � A. T. R. Acosta

Dipartimento di Scienze, Universita degli Studi di Roma

Tre, V.le Marconi 446, 00146 Rome, Italy

Present Address:

M. Carboni

Laboratoire d’Ecologie Alpine, Universite Joseph Fourier/

CNRS Grenoble, BP 53, 2233 Rue de la Piscine,

38041 Grenoble Cedex 9, France

M. L. Carranza (&)

EnviX-Lab. Dipartimento di Bioscienze e Territorio,

Universita degli Studi del Molise, C.da Fonte Lappone,

86090 Pesche, IS, Italy

e-mail: carranza@unimol.it

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DOI 10.1007/s10980-014-0074-3

Author's personal copy

landscape level (With 2002, 2004). There is no doubt

that landscape patterns could be significant drivers of

invasion processes (Higgins et al. 1999; Hobbs 2000;

Zechmeister et al. 2003; Essl and Dirnbock 2008;

Gonzalez-Moreno et al. 2013) in parallel with histor-

ical factors such as land use and the time of

introduction (Pysek and Hulme 2005). Surprisingly,

despite a large number of contributions focusing on

various aspects of the colonization, spread and control

of alien species in numerous habitats (Pysek and

Hulme 2005; Richardson and Pysek 2008), few studies

have addressed these issues through a landscape

approach (With 2002; Bruno et al. 2004; Vila and

Ibanez 2011; Thomas and Moloney 2013) and even

fewer have taken into account a historical component

or land-use legacy (Domenech et al. 2005; Kuhman

et al. 2010; Mattingly and Orrock 2013).

Invasive plants have the ability to thrive and spread

aggressively outside their native range. In particular,

the landscape fragmentation of natural habitats may

directly enhance the invasibility of ecological com-

munities (Hobbs 2000). Landscape ‘‘fragmentation’’

(Forman and Godron 1986) entails the reduction of the

extent of natural habitats and the isolation and decline

of the ecological quality of the remaining fragments.

The resulting fragmented pattern has the potential to

boost habitat colonization by alien species from the

surrounding matrix (Carranza et al. 2010; Vila and

Ibanez 2011). Among the agents of fragmentation,

urban sprawl may have consistent effects on alien

invasion by increasing the availability of alien prop-

agules, ready to move from artificial areas to adjacent

natural communities (e.g., O’Shea and Kirkpatrick

2000; McKinney 2006; Thuiller et al. 2006). Last but

not least, the vulnerability of a landscape to invasion

depends not only on its current spatial pattern but also

on the local land-use history and legacy (Vila et al.

2003; Vila and Ibanez 2011). In fact, it is well known

that land-use changes can alter ecosystems, providing

opportunities for exotic species to invade due to the

increased movement of seeds (Hobbs and Huenneke

1992; Trombulak and Frissell 2000). Historical pro-

cesses and unique events profoundly influence local

community structure (Ricklefs 1987), and the time and

circumstances of the introduction of alien species may

play a particularly central role in invasion success

(Lonsdale 1999).

Coastal dune ecosystems are increasingly threa-

tened by direct and indirect human pressures

(Schlacher et al. 2008) and have been reported to

harbor many alien species (Bruno et al. 2004; Dark

2004; Chytry et al. 2008). In Europe, coastal dune and

sand areas (EUNIS B1) (European Environment

Agency 2008) are among the most invaded habitats

by alien species (Chytry et al. 2008). In particular,

during the last decades, beach fruition by tourists, the

expansion of urban areas and the spread of agriculture

and afforestation activities have strongly shaped

Mediterranean coastal landscapes (Alados et al.

2004; Hesp and Martinez 2007; Malavasi et al.

2013), consistently modifying land use and their

spatial patterns (Carboni et al. 2009; Drius et al.

2013). Furthermore, humans have been responsible

not only for landscape transformation but also for

transporting species beyond their native ranges. The

arrival of an exotic species to a landscape generally

results from the intentional or accidental introduction

by humans, which involves overcoming geographical

barriers (With 2004; Carboni et al. 2010b). Although

different coastal dune habitats are known to undergo

different levels of invasion (Carboni et al. 2010a;

Carranza et al. 2011; Chytry et al. 2008), to our

knowledge no studies have considered the role of both

the spatial and the temporal pattern of the landscape in

influencing alien invasions.

In consideration of all the above, the aim of this

work is to investigate whether and how spatial

landscape characteristics and landscape legacy may

affect fine-scale alien plant invasion on natural

Mediterranean coastal dune habitats. In particular,

based on multitemporal land cover maps and on a

floristic database, we propose to investigate the effects

of habitat fragmentation, propagule pressure and land-

use legacy on alien plant invasion and alien richness in

natural dune patches.

Methods

Study area

The study was carried out on the Tyrrhenian coast of

Central Italy and focused on recent coastal dunes

(Holocene). The area includes seven study sites along

the coast of the Lazio region (Fig. 1), with recent

dunes harboring remnant natural vegetation patches

along the seashore. Along the sea-inland ecotone,

abiotic conditions vary greatly, shaping habitat

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zonation along the dune profile in a typical sea-inland

gradient. Under natural conditions, the vegetation

zonation follows this ecological gradient, ranging

from annual pioneer communities on the beach to

Mediterranean macchia on the landward fixed dunes

(Acosta et al. 2000; Carboni et al. 2011). Several

potential hazards threaten these ecosystems, including

intense and rapid urbanization processes. For these

reasons, the natural vegetation has been severely

damaged and reduced along the entire Lazio coast.

Nonetheless, the coastal dunes of Central Italy still

host many EU Directive 92/43 Habitat types (Carranza

et al. 2008). The climate of the area is typically

Mediterranean, with high summer temperatures, dry

summers and most of its annual rainfall distributed in

autumn and winter (Carranza et al. 2008).

Land cover maps

We produced two fine-scale (1:5,000) land-cover

maps of the Lazio coast for the years 1954 and 2008.

Given that the dune ridges of Central Italy are

generally very narrow (Carranza et al. 2008), we

mapped a 500 m wide strip starting from the coastline

toward inland areas. Land cover was visually inter-

preted on screen in a GIS environment (ArcGis 9.2).

The legend conforms to the European standard

CORINE land cover (CLC) expanded to a more

accurate fourth level of detail for natural and semi-

natural areas (see Acosta et al. 2005 for details).

Eleven land cover types were identified and mapped

(see Table 3 in Appendix—Supplementary Material).

The land cover map was verified through field surveys

during 2012. Natural dune plant communities, accord-

ing to Acosta et al. (2005), were mapped in three

different cover types, which enclose six EU habitat

types sensu EU Directive 92/43 (see Table 3 in

Appendix—Supplementary Material). The three nat-

ural dune cover types are: open sand (3.3.1.1.),

corresponding to the beaches with annual pioneer

vegetation growing close to the drift line; partially

vegetated dunes and densely vegetated dunes

Fig. 1 Study area. The investigated sites along the coast are highlighted in black. UTM WGS 84 projected coordinates (Fuse 33 N)

Landscape Ecol (2014) 29:1541–1550 1543

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(3.3.1.2.), corresponding to the herbaceous dune

vegetation growing on fore dunes; and Mediterranean

macchia (3.2.3.1.), corresponding to the woody dune

vegetation growing on back dunes (Malavasi et al.

2013). To make the maps comparable, we interpreted

the year 1954 using the backward editing technique.

On the basis of the most recent map (2008), the other

time level was processed in reverse order by editing,

with information being adapted to the older time level

(Walz 2008).

Floristic data

Floristic data of the natural dune patches were derived

from an existing geo-database of random vegetation

plots performed on the study area during the spring in

the years 2005–2010 (Santoro et al. 2012). Plots were

geographically positioned with 3 m of accuracy using

a GPS. For each 8 m by 8 m (64 m2) plot, the list of all

vascular plant species (including aliens) was com-

piled. A total of 570 plots were available with 158

species recorded. Among these, eight are alien species,

identified according to Celesti-Grapow et al. (2009).

In this work, alien species are defined as plant taxa in a

given area whose presence is due to intentional or

unintentional human involvement (Celesti-Grapow

et al. 2009).

Landscape data

We overlaid the georeferenced plots and the landscape

maps, selecting only those coastal dune patches that

overlapped with at least two floristic plots. In this way,

we obtained a total of 186 patches containing random

plots used for further analyses. For each coastal dune

patch, we calculated a set of landscape metrics as

proxies for three main processes affecting alien

invasion and richness at the landscape level: fragmen-

tation, propagule pressure and land-use change.

Although, in some cases, the same patterns may result

from multiple processes, here we assigned each metric

only to the principal process to which it has been linked

in previous studies and ecological theory (Forman

1995; Carboni et al. 2009; Carranza et al. 2010).

Landscape fragmentation

For each natural dune patch, we calculated two

parameters that have been proven to be useful in

describing the spatial pattern of coastal landscapes

(Carboni et al. 2009; Carranza et al. 2010): patch size

(PS) and shape index (SI) (Table 1). PS is a good

parameter for describing the effects of human distur-

bance on coastal dune habitats: the smaller the size, the

higher the ongoing fragmentation process (Carboni

et al. 2009). We chose SI because of its power to

discriminate between long irregularly shaped patches,

characteristic of well-preserved coastal dune habitats,

and regular round patches, which often dominate in

disturbed and fragmented coastal dunes (Forman

1995; Carranza et al. 2010). Moreover, we calculated

the Euclidean Nearest Neighbor (ENN) based on the

shortest edge-to-edge distance. This parameter is

commonly used to infer the degree of patch isolation

and the level of fragmentation in many different

landscape contexts (Forman 1995).

Table 1 Landscape parameters calculated for each patch and

used as independent variables

Metric Description Process

PS (ha) Size of the focal patch Landscape

fragmentationSI

(adim.)

Shape Index of the focal patch

ENN

(m)

Euclidean Nearest Neighbor to

the nearest neighboring patch

of the same type based on

shortest edge-to-edge distance

SB_Art

(%)

Shared border with artificial

cover types (ART)

Propagule

pressure

SB_Sn

(%)

Shared border with semi-natural

cover types (WET, SHV,

SWV, REF, BFM)

SB_Du

(%)

Shared border with the other

dune cover types (BPV, HDV,

WDV)

DC_Art

(m)

Centroid-to-centroid distance

from the nearest artificial

polygon (ART)

DC_Sn

(m)

Centroid-to-centroid distance

from the nearest semi-natural

polygon (WET, SHV, SWV,

REF, BFM)

DC_Du

(m)

Centroid-to-centroid distance

from the nearest natural dune

polygon (BPV, HDV, WDV)

Durb (m) Difference between 1954 and

2006 distance to the artificial

nearest neighboring patch

based on shortest edge-to-edge

distance

Land use

legacy

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Propagule pressure

As proxies of propagule pressure, we use the spatial

relationships of each patch with adjacent categories

applying boundary-based metrics (Rescia et al. 1994)

and the distance to other polygons (Table 1).

The analysis of shared borders, which allows us to

describe the contiguity among patches (SB_Art:

shared borders with artificial surfaces, SB_Sn: shared

borders with semi-natural cover types, SB_Du: shared

borders with other natural dune habitats), has been

previously used for assessing the conservation status

of coastal dune landscapes (Acosta et al. 2000;

Carboni et al. 2009; Drius et al. 2013). A positive

spatial association of dune cover types with artificial

surfaces is expected for anthropogenic landscapes

(Carranza et al. 2010), leading to higher levels of

propagule pressure in the adjacent natural communi-

ties (e.g., O’Shea and Kirkpatrick 2000; McKinney

2006; Thuiller et al. 2006). On the other hand,

although highly connected natural dune cover types

could guarantee dune integrity in landscapes with low

human pressure (Drius et al. 2013), in highly human

transformed sites, the series of adjacent natural

patches could facilitate the movement of aliens across

the landscape (Alofs and Fowler 2010).

The spatial position of natural dune cover types was

assessed by computing the centroid-to-centroid min-

imum Euclidean distance (DC—in meters) between

each natural dune patch and the patches of other cover

types (Table 1). This metric (DC_Art: distance from

artificial areas; DC_Sn: distance from semi-natural

cover types; DC_Du: distance from other natural dune

habitats) gives information about the proximity of

each patch to urban nuclei and/or to other natural dune

areas and is used here for the first time on coastal dune

mosaics. A shorter distance to urban patches could

enhance alien invasion by increasing the probability of

success of alien propagules, which move from artifi-

cial areas to surrounding natural communities (Thu-

iller et al. 2006). In addition, the proximity of invaded

semi-natural patches may facilitate alien establish-

ment elsewhere in the landscape (Pauchard and

Alaback 2004).

Land use legacy

Finally, we took the local history of each patch into

account by quantifying temporal urbanization trends

in the surrounding areas. Coastal areas have been

mostly shaped by urban expansion in most of the

Mediterranean countries (Hesp and Martinez 2007;

Malavasi et al. 2013). Thus, we selected the urbani-

zation process as a proxy of the overall historical land-

use legacy. First, we measured the shortest edge-to-

edge distance (in meters) between each focal natural

dune patch and artificial patches for the years 1954 and

2006. Then, for each patch, we computed the differ-

ence between these edge-to-edge distances (1954 -

2006) and obtained a Durb value (Table 1). Durb, which

expresses the urbanization trends in the surrounding

areas of each natural patch, is equal to 0 when the

distance between natural dunes and urban patches

across the last 60 years has remained stable, and it

increases as artificial areas expanded closer to the

natural patches (i.e., when natural and artificial areas

became closer).

Statistical analyses

For each natural dune patch, we calculated two

response variables: the presence of at least one

invaded plot within the patch (binary variable: invaded

or not) and the average number of aliens recorded in

the floristic plots within it (quantitative variable:

average alien species richness). In this way, alien

species richness is here considered at the plot level

(mean alien richness of all plots recorded in that

patch). Then, linear models were fit for each response

variable (alien invasion and alien richness) using

different landscape metrics of fragmentation (patch

size, shape index, and Euclidean Nearest Neighbor),

propagule pressure (shared borders and minimum

Euclidean distance to other cover types) and land-use

legacy (temporal variation of the distance to artificial

areas) as independent variables (Table 1). In addition,

by including the geographical coordinates of each

patch (x and y of the centroid) in the model, we also

accounted for spatial autocorrelation among patches.

In addition, the land cover type of each patch was

included in the models to accommodate the fact that

different coastal dune habitats are known to have

different levels of invasibility (Carboni et al. 2010a;

Chytry et al. 2008; Carranza et al. 2011).

First, we fit a generalized linear model (GLM) with

binomial errors to assess which landscape metric

influences patch invasion by alien plants. Second, we

extracted invaded patches and investigated how

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landscape metrics affect the average alien species

richness by using GLMs.

All landscape metrics were log transformed

(excluding the shared borders values), and variance

inflation factor (VIF) analysis was used to reduce

collinearity among all the explanatory variables (R

package ‘‘car’’). Model simplification was performed

by implementing stepwise model selection (forward

and backward) based on the Akaike Information

Criterion (AIC). Finally, correlograms were used to

evaluate any remaining spatial dependencies among

the residuals from the various models (R package

‘‘spdep’’).

All modeling analyses were conducted in the

R-statistical environment (version 2.13.2; R Founda-

tion for Statistical Computing, Vienna, Austria. http://

www.R-project.org.).

Results

Alien invasion

The final model explains approximately 30 % of the

total variance (R2 = 0.28) (Table 2), suggesting that

the selected landscape characteristics can indeed

explain a large amount of variability in patch invasion

patterns. We found a significant relationship between

patch alien invasion and historic factors, expressed by

land-use legacy (Durb) and by the geographic position

of each patch (centr_x) (Table 2). The higher the

recent development of artificial areas in the surround-

ing areas of the patch (Durb; p \ 0.001), the higher the

probability of invasion (Fig. 2). The geographical

coordinates were also retained in the model

(p \ 0.001), suggesting that the geographical position

of the patches in the study area also play an important

role (Fig. 2). Moreover, the minimum adequate model

based on AIC retains shared borders with other dune

cover types (SB_Du) (Table 2). However, the F-test in

the ANOVA table (Table 2) highlights that this last

term was not significant and should be omitted from a

more conservative model (Zuur et al. 2007).

The correlogram showed that no spatial depen-

dency was left among the residuals of the model (see

Fig. 4 in Appendix—Supplementary Material). Given

that geographical coordinates were included in the

model, the absence of spatial dependency in the

residuals was expected.

Alien richness

The minimal adequate model for alien plant richness

within the invaded patches explains almost 40 % of

the total variance (R2 = 0.37) (Table 2). The model

suggests that patch extension (PS; p \ 0.001) and the

length of the shared borders with artificial cover types

(SB_Art; p \ 0.001) are negatively and positively

correlated with alien richness, respectively. The

smaller the patch size, the higher the average number

of alien species in the plots (Fig. 3); the higher the

amount of patch border shared with artificial areas, the

higher the average number of alien species in the plots

within the patch (Fig. 3). The minimal model for alien

richness based on AIC also included the centroid-to-

centroid distance from the nearest natural dune

polygon (DC_Du; p = 0.0173), the Shape Index (SI;

p = 0.766) and the land cover category (Land_Cover;

p = 0.0527) to which the patch belongs (Table 2).

However, F-tests showed that DC_Du, SI and Land_-

Cover were not significant terms and therefore should

be omitted from the final model (Zuur et al. 2007).

Correlograms highlighted limited spatial depen-

dency in both the original response variable (Alien

richness) and the residuals of the model but only when

considering first lag neighbors (see Fig. 5 in Appen-

dix—Supplementary Material). Indeed, the geograph-

ical coordinates were excluded from the minimal

adequate model.

Discussion

Based on our results, we can affirm that the three

analyzed processes affect alien plant invasion within

Mediterranean dune patches at different stages.

Although the presence of alien plants is strongly

related to land-use legacy, alien richness mainly

develops from fragmentation and propagule pressure

forces.

Alien invasion

We found that alien invasion is associated with the

land-use legacy of each patch and with its geograph-

ical position along the coast. The Italian coast has

experienced contrasting histories of human develop-

ment and landscape transformation over the last

60 years (Malavasi et al. 2013), which have most

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likely conditioned the time of arrival and settlement of

alien plants. Within the boundaries of the Lazio

region, some areas endured intensive urbanization

processes but others remained better conserved (Mal-

avasi et al. 2014). Indeed, the high significance of

geographical position for the invasion of dune patches

gives clear evidence that invaded patches are not

randomly distributed across the landscape and along

the regional coast (Pino et al. 2005; Thomas and

Moloney, 2013). The increase in patch invasion is

observed in correspondence with the recent expansion

of artificial areas closer to natural habitats (Durb),

pinpointing the existence of consistent links between

human-driven landscape changes and the presence and

distribution of alien species on coastal dunes. As on

other human-modified landscapes, on coastal dunes,

the recent construction and general improvement of

roads (Forman and Alexander 1998; Trombulak and

Frissell 2000) and urban sprawl (Pauchard et al. 2006)

seem to have promoted alien invasion.

Alien richness

Regarding alien richness, within the invaded patches

our results highlight a crucial role of both landscape

fragmentation, represented by patch extension (PS),

and propagule pressure, expressed by shared borders

with artificial areas (SB_Art) and by the centroid-to-

centroid distance from the nearest natural dune

polygon (DC_Du).

Fig. 2 Logit plot

regression: partial residuals

of alien invasion against

significant landscape

parameters (centr x:

longitude coordinates of the

patch and Durb: difference

between 1954 and 2006

distance to the artificial

nearest neighboring patch

based on the shortest edge-

to-edge distance)

Table 2 Analysis of

Variance (ANOVA) tables

for the Minimal Adequate

Model based on AIC for

plant invasion and richness

For each spatial parameter,

the F-value and significance

code (0 ‘***’, 0.001 ‘**’,

0.01 ‘*’, 0.05 ‘.’, 0.1 ‘’, 1)

are listed. At the end of the

table, R-square values are

displayed for each model

Process Alien invasion Alien richness

F-value p F-value p

Minimal Adequate Model (MAM)

Landscape fragmentation PS 17.575 7.08e-05***

SI 0.089 0.766

ENN

Propagule pressure SB_Art 17.495 7.33e-05***

SB_Sn

SB_Du 2.799 0.096015.

DC_Art

DC_Sn

DC_Du 5.911 0.0173*

Land use legacy Durb 12.856 0.000432***

Geographic centr x 52.856 1.03e-11***

Habitat invasibility Land_Cover 3.055 0.0527.

R-square 0.284 0.371

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We found evidence that on coastal dunes, the

fragmentation of natural habitats enhances the number

of aliens present in each patch. Specifically, we

observed an increase in the per-plot number of alien

plant species in small patches. This increase agrees

with previous results, which indicate that fragmented

natural habitats are more susceptible to invasion

(Harrison et al. 2001; With 2004; Song et al. 2005).

On coastal dunes, the presence of very small patches is

reported as a good indicator of fragmentation (Mal-

avasi et al. 2013), which is often related to a general

decline in habitat conservation status (Carboni et al.

2009).

In accordance with previous works conducted at

different scales, higher alien richness appears to be

strongly related to adjacency with human structures,

which increases propagule pressure. We found that

per-plot alien richness significantly increases with an

increase in shared borders with artificial areas, which

mainly include bathing establishments and summer-

houses with gardens facing the beach. Such structures

are optimal sources of propagules (O’Shea and

Kirkpatrick 2000) as demonstrated by the presence

of many aliens in the natural dune plant communities

that are frequently cultivated on artificial areas for

ornamental purposes. Furthermore, the construction

and improvement of roads aimed to increase touristic

facilities could offer preferential routes for the spread

of alien species (Proches et al. 2005; Richardson and

Thuiller 2007).

Finally, our results pinpoint that adjacency to other

natural dune patches may also facilitate the spread of

alien species. Patches closer to other natural patches

tend to host a higher average number of aliens, most

likely because other invaded natural patches may

facilitate alien establishment elsewhere in the land-

scape (Pauchard and Alaback 2004). For instance,

some aliens widely present on Mediterranean coasts,

such as iceplant (Carpobrotus sp.) and agave (Agave

sp.), with their extensive vegetative growth, lead to the

formation of extensive mats that may facilitate the

further expansion of the species in other neighboring

natural dune patches (Weber 2003; Badano and

Pugnaire 2004). Natural corridors have been claimed

as venues for the spread of alien animal species (Koch

et al. 2006; Urban et al. 2008), but evidence of the role

of ecosystem connectivity on plant invasions is scarce

and controversial.

Conclusion

Integrating spatial and temporal landscape metrics

with floristic data, we were able to investigate the

relationships between landscape fragmentation, prop-

agule pressure and land-use legacy with the presence

and richness of alien plants.

Here, we present evidence that alien invasion and

alien richness are affected by different processes:

although alien invasion is strongly associated with the

history of each patch (here expressed by urbanization

trends in the past 50 years), the richness of aliens is

more affected by the fragmentation of invaded patches

and the propagule pressure to which they are exposed.

Indeed, alien plant richness increases with the inten-

sity of human-driven landscape transformations that

fragment natural habitats and with the presence of

natural and artificial corridors that foster the arrival of

Fig. 3 Regression plot: partial residuals of average alien richness against significant landscape parameters (PS: Patch Size; SB_Art:

shared border with artificial cover types and DC_Du: centroid-to-centroid distance from the nearest natural dune polygon)

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new propagules. Thus, spatial and temporal landscape

patterns and the processes that drive alien invasions

seem to be inherently linked, and our understanding of

the spread and richness of alien species will be

substantially improved by considering them

simultaneously.

Finally, from a practical point of view, our results

offer a scientifically sound basis for orienting the

management and risk analysis of biological invasions.

They can help identify the most effective strategies for

sustainable management, prevent the high economic

costs derived from both the control and eradication of

aliens and, ultimately, prevent the loss of natural

mosaics and associated ecological services.

Acknowledgements This work was partially funded by

ENVEUROPE project (LIFE08 ENV/IT/000339).

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