landscape-scale patterns of alien plant species on coastal dunes: the case of iceplant in central...

Landscape-scale patterns of alien plant species on coastal dunes: the

case of iceplant in central Italy

M. Laura Carranza, Marta Carboni, Silverio Feola & Alicia T. R. Acosta

AbstractQuestion: We investigated the spatial pattern ofcoastal landscapes invaded by iceplant (Carpobrotusaff. acinaciformis) focusing on two questions: (1)Does the spatial structure of iceplant patches differfrom that of native natural costal dune cover types?;(2) Is the distribution of iceplant patches related toother cover types?Location: Tyrrhenian coast of Central Italy.Method:On the basis of a detailed land-cover map, wecalculated structural metrics for iceplant patches andfor each native coastal dune cover category (meanpatch size and patch shape index) and compared themby means of ANOVA. To assess the spatial associationbetween iceplant patches and the different cover types,we implemented an electivity analysis which analysesthe frequency of common borders.Results: The mapped coastal dune cover types in-cluded beaches, dunes and sand plain variants,related to the typical Tyrrhenian coastal dune vege-tation zonation. Iceplant-dominated vegetationpresented elongated and irregularly shaped patches,which were not significantly different from most ofthe natural cover types suggesting a natural spreadalong the territory analysed. Iceplant patches werepositively associated with beach, mobile and inter-dune cover types indicating that these habitats areexposed to further alien spread. Iceplant patcheswere also positively associated with artificial sur-faces highlighting this cover type as a possiblesource of propagule pressure.Conclusions: The proposed landscape approachcombining patch-based metrics with edge-basedmetrics provided a comprehensive description ofthe invaded coastal landscape. From an applied

research perspective, this landscape approach couldbe useful in identifying the correct managementstrategies for alien-invaded areas.

Keywords: Between patch boundary; Electivity; Ice-plant (Carpobrotus aff. acinaciformis (L.) L. Bolus);Land-cover map; Landscape metrics; Mediterraneancoast; Spatial pattern.

Nomenclature: Conti et al. (2005).

Introduction

Landscape ecology and invasion theory wereconsidered predominantly separate entities until thelast decade. Despite a large number of contributionsfocusing on various aspects of the colonization,spread and control of alien species in numerous ha-bitats (Pysek & Hulme 2005; Richardson & Pysek2008), surprisingly few studies have addressed theseissues through a landscape approach (With 2002;Bruno et al. 2004; J�rgensen & Kollmann 2009).Although there is no doubt that landscape patternscould be a significant driver in invasion processes(Higgins et al. 1999; Hobbs 2001; Zechmeister et al.2007; Essl & Dirnbock 2008), the analysis of thedistribution patterns of alien invasive species usinglandscape metrics has been little explored.

Invasive plants (sensu Pysek et al. 2004) arecharacterized by remarkable dynamics of spreadthat allow them to colonize large areas in regionswhere they were introduced. One of the principles oflandscape ecology is that landscape function isgreatly influenced by the distribution of habitats(Forman 1995). In relation to invasion processes, ithas been demonstrated that habitat connectivity andthe presence of corridors or other man-made com-munication infrastructures offer preferential routesfor the spread of alien species (Christen & Matlack2006; Proche et al. 2007; Richardson & Thuiller2007; Tiebre et al. 2008). For example, the con-struction and improvement of roads promote alieninvasion (Forman & Alexander 1998; Trombulak &Frissell 2000). Moreover, some studies have linkedthe presence of invasive species to human settle-ments (Song et al. 2005) while others have shown

Carranza, M.L. (corresponding author, carranza@

unimol.it) & Feola, S. ([email protected]): En-

vironmetrics Laboratory, Dip. STAT, University of

Molise, Contrada Fonte Lappone, I–86170, Pesche

(IS), Italy.

Carboni, M. ([email protected]) & Acosta,

A.T.R. ([email protected]): Department of Biology,

University of ‘Roma Tre’, Via le Marconi 446, I–00146

Roma (RM), Italy.

Applied Vegetation Science 13: 135–145, 2010DOI: 10.1111/j.1654-109X.2009.01065.x& 2009 International Association for Vegetation Science

mailto:[email protected]









that colonization rates of invasive aliens tend to in-crease near agricultural fields (Elmore et al. 2003).Previous studies also indicate that disturbed andfragmented natural habitats are more susceptible toinvasion (With 2004; Song et al. 2005) and patchesinvaded by exotic species may facilitate alien estab-lishment elsewhere in the landscape (Pauchard &Alaback 2004). In this context, the application oflandscape metrics for describing invaded habitatpatterns can provide a valuable tool for betterunderstanding of the relationship between invasionprocesses and landscape characteristics.

Invasion by the alien iceplant (Carpobrotus spp.,Aizoaceae) is considered one of the most severethreats to numerous terrestrial plant communities incoastal habitats (D’Antonio 1993; Vila et al. 2006).Species of the genusCarpobrotus (i.e.C. acinaciformis,C. edulis, iceplant, mat-forming succulents nativeto South Africa, Wisura & Glen 1993) have been in-troduced in different parts of the world as ornamentalplants and for erosion prevention (Weber 2003).Iceplant species are widely naturalized on coastalhabitats outside their native range (Vila et al. 2006)and establish almost monospecific cover patchesin the invaded habitats, blocking the natural spon-taneous vegetation (Traveset et al. 2008a). Theecological impacts of iceplant include aggressivecompetition with native species (D’Antonio &Mahall1991; Sheppard et al. 2005), the destabilization of na-tive dune communities (Campos et al. 2004; Acosta etal. 2008), the modification of soil pH (Vila et al. 2006)and probably the alteration of ecosystem function interms of successional dynamics. Consequentially, ice-plant is included in the list of the most invasive alienplants in the World (Weber 2003) and among theworst invasive alien species threatening biodiversity inEurope (EEA 2005; Sheppard et al. 2005).

The invasive potential of iceplant has beeninvestigated in different parts of the world, but par-ticularly in Mediterranean ecosystems (D’Antonio1993; Vila et al. 2000, 2006; Lloret et al. 2005;Traveset et al. 2008b) and the importance of map-ping its local distribution for biological controlmeasures has also been recognized (Underwoodet al. 2003). However, valid landscape metrics fordescribing alien distribution within fine-scale coastalecosystems are still lacking. Moreover, a criticalcomponent of effective land management to controlplant invasion in relation to habitat conservation isthe identification and active protection of habitats athigh risk of future invasion (Hobbs & Humphries1995). In the same way, it is also important to iden-tify and minimize land uses that promote alieninvasion (Bradley & Mustard 2006).

Coastal dune systems are natural formationscharacterized by a strong vegetation zonation asso-ciated with the sea–inland environmental gradient(Van der Maarel 2003). Although landscape struc-ture in coastal areas has been poorly investigated,previous results suggest that in most cases, coastalpatches, which are largely determined by environ-mental constraints, are mainly relatively smallbut elongated along the coast (Carboni et al. 2009).In this context, an interesting point to explore iswhether new alien patches invading the coastallandscape assume structural characteristics that dif-fer from native ones, reflecting in this way specificspatial colonization patterns.

Moreover, because of the strong zonation ofcoastal ecosystems, their sustainability is also greatlyinfluenced by the specific contiguity between naturalhabitats (Acosta et al. 2000). Thus, how iceplantpatches are distributed compared with other covertypes at a landscape scale represents a further pointto investigate. If iceplant patches are positively asso-ciated to specific natural habitats, we can pinpointhabitats that are particularly exposed to alienspread. Comparably, in relation to sources ofintroduction, if iceplant patches are positively asso-ciated with specific artificial or agricultural covertypes, we can identify cover types related to iceplantpropagule sources and, in this way, infer those covertypes that are mainly associated with the invasion ofthis alien species.

In consideration of the above, the present worksets out to describe dune landscape composition incoastal ecosystems of Central Italy, where iceplantplants are present, analysing in detail the spatialconfiguration of alien iceplant patches within thiscoastal landscape. In particular, we focus on twoquestions:

(1) Does the spatial structure of iceplant patchesdiffer from that of native natural costal dunecover types?

(2) Is the distribution of iceplant patches related toother cover types?

Materials and Methods

Study area

The study was carried out on the Tyrrheniancoast of Central Italy (Latium region). A recentvascular flora survey (Izzi et al. 2007) revealed thewidespread presence of iceplant along the shores ofthe province of Rome. In this area, as a case study,

136 Carranza, M. L. et al.

we selected two sites with recorded iceplant pre-sence, both located approximately 30 km fromRome (Fig. 1). Owing to the overall similarity ofthe invaded coastal landscape of this area ( see theSupporting information, Fig. S1), we aggregated thetotal 10 km of mapped coast in the two sites to a sin-gle data set for iceplant spatial configuration analysis.In Central Italy, recent dunes (Holocene) mostly oc-cupy a narrow strip along the seashore. In naturalconditions, vegetation zonation follows the sea-to-inland ecological gradient, ranging from annualcommunities on the strandline zone of the beach toMediterraneanmacchia on the inland stabilized dunes(Stanisci et al. 2004). However, agriculture, reforesta-tion, building activities, coast-bound tourism andcoastal erosion have greatly modified the originalcoastal landscape (Ercole et al. 2007).

Coastal landscape map

We produced a 1:5000 scale vector land covermap of the coast of Latium derived from high-resolution panchromatic digital aerial orthophoto-graphs (dated 2002) in a Geographic InformationSystem (GIS) environment (ARCVIEW version 3.1;ESRI 2000, Redlands, CA, US). The mapped areacoincides with the geological classes of Holocenicdunes and beaches (Accordi et al. 1988). Land-covermapping was based on the CORINE land-coverclassification (European Commission 1993) and ex-panded to a fourth level of detail for natural andsemi-natural areas (Acosta et al. 2005). Twenty-fivecover types were identified and mapped (Table 1).The map, verified through field surveys, presents aglobal accuracy of 0.77 (see the Supporting Informa-tion, Table S1). In the study area,Carpobrotus species

tend to form wide monodominant evergreen carpetswhich allow the accurate mapping from aerial photos.Thus, the cover type 3.3.1.6. ‘Vegetation dominatedby iceplant’ was mapped in addition to the other cov-er types already proposed for Mediterranean coastaldunes (Acosta et al. 2005). This category was mappedwith good accuracy, with similar values to the othermapped classes (Table S1).

Fig. 1. Study area. Tinted squares indicate the location ofthe coastal zone analysed.

Table 1. CORINE land-cover legend expanded to afourth level of detail. This table includes only the categor-ies observed in the study area. Mapped cover types are inbold. �Natural coastal cover types. Per cent cover of thedifferent first level CORINE categories are shown be-tween brackets.

Lev. 1 Lev.2 Lev.3 Lev. 41. Artificial surfaces (32.23 %)

1.1. Urban fabric1.1.1. Continuous urban fabric

1.1.2. Discontinuous urban fabric

1.2. Industrial, commercial and transport units1.2.1. Industrial or commercial units

1.2.2. Road and rail networks and associated land

1.3. Mine, dump and construction sites1.3.2 Dump sites

1.4. Artificial non-agricultural vegetated areas1.4.1. Green urban areas

1.4.2. Sport and leisure facilities

2. Agricultural areas (3.78 %)2.1. Arable land

2.1.1. Non-irrigated arable land

2.3. Pastures2.3.1. Pastures

3. Forests and semi-natural areas (60.56 %)3.1. Forests

3.1.1. Broad-leaved forest3.1.1.1. Holm-oak and cork-oak forests

3.1.1.3. Mixed broad-leaved mesophilous and

mesothermophilous forests

3.1.1.6. Igrophilous forests

3.1.1.7. Non-native broad-leaved forests

3.1.2. Coniferous forest3.1.2.1. Mediterranean pine forests

3.2. Shrub and/or herbaceous3.2.1. Natural grassland

3.2.1.1. Continuous grasslands

3.2.3. Sclerophyllous vegetation3.2.3.1. Fixed dune mediterranean macchia�

3.2.4. Transitional woodland shrub3.2.4.1. Transitional shrub vegetation

3.3. Open spaces with little or no vegetation3.3.1. Beaches, dunes, and sand plains

3.3.1.1. Upper beach annual vegetation�

3.3.1.2. Embryo dunes vegetation�

3.3.1.3. Mobile dunes perennial vegetation�

3.3.1.4. Interdune perennial vegetation�

3.3.1.5. Interdune annual grasslands�

3.3.1.6. Vegetation dominated by iceplant�

4. Wetlands (2.83 %)4.1. Inland wetlands

4.1.1. Inland marshes4.1.1.1. Riparian cane formations

5. Water bodies (0.6 %)5.1. Inland waters

5.1.1. Water courses

Landscape-scale patterns of alien plant species on coastal dunes 137

Mapped patches of iceplant were verified in thefield and checked in order to identify the species ofthe genus. Field examination showed that mostpatches belonged to C. acinaciformis (L.) L., andvery few of them appeared to belong to C. edulis (L.)N.E. Br. according to Pignatti’s classification(1982). However, it is important to note that thetaxonomy of this genus is still controversial in theMediterranean area and hybridization between thesetwo species has been verified (Suehs et al. 2004). Forthese reasons, and for the purposes of this work, weconsidered the studied taxon as C. aff. acinacifomis(iceplant throughout the manuscript).

To provide a brief description of the invadedcoastal dune landscape, we calculated the extensionof all mapped categories on the basis of the landcover map, focusing in particular on first levelCORINE categories.

Patch structure analysis

To analyse the spatial pattern of the invadedcoastal dune landscape, we calculated a set of struc-tural metrics focusing only on natural coastal dunecover categories (with asterisks in Table 1). Weconsidered the individual polygon as a patch and thegroup of polygons of the same cover type as a cate-gory. After recording the percentage cover of eachcategory compared with the total natural coastaldune cover types and the total number of patches ofeach category, we focused on two patch metrics thathave been previously reported as ecologically mean-ingful (Riitters et al. 1995; Schindler et al. 2008) andwhich have been proven to be useful in describingpatch spatial structure in a coastal landscape con-text (Carboni et al. 2009): mean patch size (MPS)and mean shape index (MSI).

The MPS is the average size in m2 of all patcheswithin each cover category, and corresponds to theratio between the total extension covered by eachcategory and the number of patches in that cate-gory. This metric was selected because it is related tohabitat fragmentation or to local environmentalheterogeneity (Forman 1995).

TheMSI is themean of the shape index (SI) valuesfor all the patches belonging to each cover category,

SI ¼ p

2ffiffiffi

pp ffiffiffi

ap

where p and a are, respectively, the perimeter (m) andarea (m2) of the patch (McGarigal &Marks 1995). TheSI attains its minimum value (SI51) for circles (themost compact shapes in vector data) and increases(with no theoretical upper limit) for more convoluted

or elongated shapes. For coastal environments, wehypothesize that natural formations assume stretched-out forms parallel to the coastline, and therefore havehigh MSI values. If alien patches were strongly struc-tured by human activity, we may expect them topresent simpler shapes than natural ones (Forman1995), while if they naturally spread in the coastallandscape, we would expect to observe similar shapesto natural formations.

After normalizing both patch-based metrics(MPS and MSI) distributions using the Box-Coxtransformation, the significance of differences amongcoastal dune-cover categories was assessed by one-way ANOVA, followed by post-hoc pairwise compar-isons (Tukey unequal n HSD).

Patch distribution analysis

The distribution of boundaries in the landscapecan bemore informative than themere presence of themapped cover categories because these describe theposition of patch types in relation to other patch types(Rescia et al. 1994; He et al. 2000). Furthermore, ithas been demonstrated that boundary analysis is veryeffective for assessing conservation status in coastaldunemosaics (Acosta et al. 2000; Carboni et al. 2009).In this work, iceplant boundaries are investigatedthrough an adjacency analysis (Turner 1990). We cal-culated the shared perimeter in metres betweeniceplant patches and other land-cover categories. Inorder to assess any preference of spatial associationbetween iceplant patches and the different categoriesmapped, we calculated the electivity index of Jacobs(1974) as used by Pastor & Broschart (1990).

Eij lnðrijÞð1� pijÞðpijÞð1� rijÞ

where Eij5 the electivity index for shared perimeterprobabilities calculated for patch types i and j, withrij5 the proportion of shared perimeter of type iaround type j, and pij5 the proportion of shared peri-meter of type i around all other patch types except type j.

Eij is then the odds-ratio of finding category iadjacent to category j divided by the odds-ratio offinding category i in contact with all the mappedcategories in the general area, and log-transformedto obtain symmetry around 0 (Jacobs 1974). Nega-tive values of Eij thus indicate negative associationand positive values indicate positive association.The electivity indices were then tested against the w2

distribution to identify significant associations (seePastor & Broschart 1990 for details).


Results

Landscape composition and patch structure analysis

Landscape composition in the study area is char-acterized by the dominance of natural categories(forested and semi-natural cover types according tothe CORINE land cover classification), followed byurban and artificial ones (Table 1; see Table 2 for thecorrespondence of natural cover categories with ma-jor coastal dune plant communities and EuropeanDirective Habitat types-EC 43/92 – European Com-mission DG Environment 2003). The naturalcategories, covering 61% of the area, include coastaldune natural types (24.5% of the area) and othernatural fixed dune and inland types mainly re-presented by broad-leaved forests, non-native pineforests and grasslands (Table 1).

As is to be expected considering the fine-scalemapping of sandy coastal landscapes, six of the four-teen natural cover categories at the fourth level ofdetail, express beaches, dunes and sand-plain variants(3.3.1.) (Table 1). The native cover types, althoughfragmented, can easily be related to the typical Tyr-rhenian coastal dune vegetation zonation (Stanisci etal. 2004) even though in some tracts the coastal woodyvegetation (mainly Mediterranean macchia) is absent.It should be noted that all these coastal native covertypes correspond to EU habitats of interest (Eur-opean CommissionDGEnvironment 2003) (Table 2).

Regarding patch size, iceplant-dominated vege-tation is not significantly different from the native

coastal dune categories. Most native coastal habitatsform a quite even mosaic, where no category presentsa significant difference in MPS or clearly dominates(Fig. 2) with two exceptions: the upper beach annualvegetation (3.3.1.1.) distributed in larger patches andthe interdune perennial vegetation (3.3.1.4.) in smallerones (ANOVA F5 15.940, with Po0.001, df betweengroups5 6, within groups5 606).

The SI indicates that iceplant presents elongatedand irregularly shaped patches and is not significantlydifferent from most of the native cover types (Fig. 3).The exceptions are: those cover types related to theexternal margin of the coastal zonation facing the sea,which include the previously mentioned upper beachannual vegetation and the embryo dune vegetation(3.3.1.2.), both with very long and irregularly shapedpatches, and the perennial interdune vegetation(3.3.1.4.) that covers a limited area distributed on onlyseven small and relatively round patches (Figs 2 and3) (ANOVA F5 16.889, with Po0.001, df betweengroups5 6, within groups5 606).

Patch distribution analysis

Ten cover types share boundaries with iceplantpatches (Fig. 4). In particular, most of the iceplantpatches are in contact with two native cover types: theupper beach annual vegetation (3.3.1.1.) and the mo-bile dune vegetation (3.3.1.3.). The electivity analysisshows that the association between iceplant patchesand the different beach, mobile and transition dunecover types is significantly positive (Fig. 4). Although

Table 2. Correspondence between the cover types identified, major coastal dune plant communities and European DirectiveHabitat types (European Commission DG Environment 2003). �A priority natural habitat of European interest (EC 43/92).

Cover type Community type Description Habitat type

3.3.1.1. Upper beachannual vegetation

Ephemeral beachcommunity with Cakilemaritima

An annual, nitrophilous and ephemeral community,exposed to wind action and occasionally to breaking waves

1210 Annual vegetationon drift lines (Cakileteamaritimae)

3.3.1.2. Embryo dunesvegetation

Perennial herbaceouscommunity withElymus farctus

A pioneer, perennial and halophilous community,dominated by dune-forming plants with low vegetationcover and poor sandy substrate

2110 Embryonic shiftingdunes

3.3.1.3 Mobile dunesperennialvegetation

Perennial herbaceouscommunity withAmmophila arenaria

A perennial herb community growing on mobile dunes,dominated by the rhizomatous tussock, Ammophila arenaria

2120 Shifting dunes alongthe shoreline with Ammophilaarenaria

3.3.1.4. Interduneperennialvegetation

Perennial interdunecommunity withCrucianella maritima

A perennial herb community, partially sheltered from windsand dominated by chamephytic species

2210 Crucianellion maritimaefixed beach dunes

3.3.1.5. Interdune annualgrasslands

Annual interdunecommunity withVulpiafasciculata

An annual grass community, generally widespread whereperennial communities have disappeared due to disturbance

2230 Malcomietalia dunegrasslands

3.3.1.6 Vegetationdominated byiceplant

Perennial patches dominated by the mat-forming succulentCarpobrotus aff. acinaciformis with few other species, mainlygeophytes and hemicriptophytes.

3.2.3.1 Fixed duneMediterraneanmacchia

Mediterraneanmacchia with Phyllireaangustifolia

An evergreen macchia, sheltered from winds and dominatedby shrub species with high cover values and less exposed tothe harsh coastal conditions

2250 Coastal dune withJuniperus sp. (J. macrocarpaand J. turbinata)�


the shared perimeter of iceplant with interduneperennial vegetation (3.3.1.4.) is relatively limited,we observed a significantly positive association withiceplant patches, which is particularly noteworthyconsidering the low cover values of this EU habitat ofinterest. Conversely, iceplant patches tend to haveshort shared perimeters with fixed-dune cover types.Moreover, iceplant patches are never in contact withagricultural areas or with other internal fixed-dunecover types such as broad-leaved forests, native pineforests and grasslands even if together they represent

almost 40% of the mapped area. However, althoughthere are no contacts with agricultural areas, there is asignificant positive association between iceplant pat-ches and some artificial surfaces, mainly discontinuousurban fabrics (1.1.2.) and continuous urban settle-ments (1.1.1.) (Fig. 4).

Discussion and Conclusions

The proposed landscape approach combiningpatch-based metrics with edge-based metrics pro-vided a comprehensive description of the invadedcoastal landscape. We were able to identify naturalcoastal cover types that reflect the dune vegetationzonation of Tyrrhenian sandy coasts which is re-lated to the strong beach–inland environmentalgradient (Stanisci et al. 2004). According to thisgradient, we observed that coastal native patches aretypically long and narrow, and parallel to thecoastline. This trend is particularly evident in thosecover types that are closer to the sea, whereenvironmental conditions are particularly harsh. Atthis spatial resolution, we find relatively small aver-age dimensions for native patches and only theupper beach is characterized by large, long patches.However, the upper-beach cover type includes alarge proportion of bare sand as it was not possibleto map strand annual plants separately even thoughwe used high-resolution orthophotos.

Fig. 2. Percentage of cover (% cover), number of patches (NUMP), and mean patch size (MPS)1SE of the different coastaldune land cover types. Letters represent homogeneous subsets for normalizedMPS according to Tukey’s test at a5 0.05 (ANOVA

F5 15.94, Po0.001, df between groups5 6, within groups5606). A dark-tinted bar is used to underline invaded class cover.

Fig. 3. Mean shape index (MSI) � SE of the differentcoastal dune cover types. Letters represent homogeneoussubsets for normalized MSI according to Tukey’s testat a5 0.05 (ANOVA F5 16.889, Po0.001, df betweengroups5 6, within groups5 606). A dark-tinted rectangleis used to indicate mean values of invaded patches.


Iceplant-dominated patches do not assume aspecific spatial pattern and are not significantlydifferent from most of the native coastal dunecategories, neither from the point of view of the di-mension of patches nor of their shape. Iceplant doesnot present regular man-made boundaries typical ofplanted or strongly humanly-shaped patches (For-man 1995; Saura & Carballal 2004). Conversely, theoccurrence of monodominant patches with a similarspatial pattern to those of native cover types sug-gests that iceplant is particularly well-suited for thecolonization of the coastal environment and is agood competitor, invading the coastal landscape bysubstituting the native vegetation rather than occu-pying empty niches (Moragues & Traveset 2005;Vila et al. 2006; Traveset et al. 2008a, b).

We found a significant positive spatial associa-tion of iceplant patches with artificial surfaces,underlining the strong relationship between ananthropogenic landscape and the presence and dis-tribution of iceplant. By studying C. edulis inCalifornian coasts, Underwood et al. (2003) foundthat landscapes shaped by human activity enhanceextensive iceplant invasions into remaining habitats.In addition, in Italy, as in many Mediterraneansandy coastal areas, suburban houses are often builtwith gardens facing the sea and in direct contactwith foredunes and interdunes. Iceplant is com-monly planted in residential gardens and probablyfunctions as a man-made corridor that supports thecontinuous introduction of iceplant onto the neigh-bouring landscape, as described by Sobrino et al.(2002) for Spanish coasts. It thus represents animportant source of propagule pressure sensuWilliamson & Fitter (1996). By contrast, the invasion

of iceplant does not appear to be directly related toagricultural practices as has been mentioned for otherinvasive aliens (Elmore et al. 2003). In this context it iscrucial to consider how current and future land-usealternatives might affect the dispersal and coloniza-tion of iceplant in coastal areas.

The absence of contacts of iceplant with well-represented fixed dune cover types, such as broad-leaved woods, pine woods and grasslands, and itsconsistent adjacency with beach, mobile and inter-dune cover types, underlines a preferential estab-lishment on these open sectors of the analysed area.The ability of iceplant to invade open coastal areasmay suggest that the species can tolerate a widerange of environmental conditions and expandaccordingly. Furthermore, the electivity analysis, byhighlighting iceplant’s preferential association,allowed us to identify the upper beach, the mobiledunes and the interdune perennial vegetation as thecover types that are most exposed to the risk of fur-ther iceplant invasion. Iceplant’s striking capacityfor local vegetative spread in the Mediterranean-in-vaded range, which certainly contributes to theinvasion success of the genus, suggests that there isthe threat of iceplant-patch expansion to contiguousnatural habitats. New shoots (rhizomes) of this alienare known to ‘explore’ the environment (Travesetet al. 2008b), extending the outline of the plant intonew contiguous patches. Therefore, the current pre-sence of iceplant can be used to identify invasionfoci, or starting points for further spread, to beconsidered a high priority for management (Moody&Mack 1988; Hobbs & Humphries 1995). The needfor a clear and targeted iceplant invasion preventionpolicy becomes apparent when we consider that two

Fig. 4. Relative importance of the contacts between iceplant patches and the other cover types expressed in percentages ofCarpobrotus aff. acinaciformis total edge. Signs on the right, show the results of iceplant class electivity analysis. Symbols are1 for positive association, – for negative association and 0 for independence. Association shown for w245.99, 1 df, Po0.01.


out of the three cover types with which iceplantforms strong positive associations are habitats ofEuropean interest according to the Directive 43/92(European Commission DG Environment 2003):shifting dunes along the shoreline with Ammophilaarenaria (EC code 2120) and the Crucianellion mar-itimae fixed-beach dunes (EC code 2210). The latter,the only native land cover type with very low covervalues and high fragmentation (relatively roundpatches), presents a critical situation not only in thestudy area but it is also an endangered habitatthroughout Mediterranean coasts (European Com-mission 2008).

In conclusion, patch structure indices, normallyused to asses fragmentation of natural habitats, werevery useful to compare iceplant monodominant pat-ches with coastal dune native patches. However,patch boundary metrics, used extensively to relate theeffects of the spatial distribution of different habitatson landscape processes (Ricotta et al. 2003), wereuseful for describing spatial distribution of iceplantpatches and, in particular, to make inferences aboutthe probable alien propagule sources and to highlightnatural cover types more prone to alien invasion.Moreover, since many single alien species have beensuccessfully detected and mapped, both through sa-tellite and aerial imagery (Everitt et al. 1995;Anderson et al. 1996; Bradley & Mustard 2006), ourapproach could also prove useful for analysing pat-terns of other invasive plants in other contexts andregions. However, the utility of this approach dependson the alien species of interest and the habitat typeswithin which the alien is present. In this study, wesuccessfully mapped and measured a relatively lowand creeping, but distinctly shaped and coloured alienwithin a coastal landscape. For less distinct, sparselydistributed alien species, positive identification mightnot be practicable and a realistic distribution map ofinvaded patches would be very difficult.

Our work illustrates that the integration ofremote sensing for land cover identification andpatch-based metrics in conjunction with edge-basedmetrics can enhance the understanding of invasionpatterns and potentially improve conservation ef-forts. From an applied research perspective, thislandscape approach could also be useful to identifythe correct strategies to manage invaded areasand to prevent further invasions. Analysing the dis-tribution patterns of alien invasive species at alandscape scale is a key issue in establishing man-agement priorities for invasion control and habitatprotection (Hobbs & Humphries 1995; Collinghamet al. 2000; Marvier et al. 2004). A landscape analy-sis can assist in identifying targets, invasion fronts

and particularly endangered habitats, which are allcrucial steps in control. On these bases, it is possibleto identify the most effective iceplant managementstrategies helping to contain the high economicimpacts of this alien and the financial costs asso-ciated with both manual and mechanical controls(D’Antonio & Meyerson 2002; Sheppard et al.2005).

Acknowledgements. We are grateful to the Editor and

three anonymous reviewers who helped to improve an

early version of the manuscript. We also acknowledge the

financial support of Italian Ministry of Education

(MIUR), PRIN-COFIN 2005-2007.

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Supporting Information

Additional supporting information may be found inthe online version of this article:

Figure S1. Land-cover maps of the coastal tractsanalysed showing the first level of CORINE legendalong with their relative histograms expressed in percent.Table S1. Error matrix and accuracy analysis for thenatural coastal cover types. 3.3.1.1.5Upper beach an-nual vegetation; 3.3.1.2.5Embryo dunes vegetation;3.3.1.3.5Mobile dunes perennial vegetation; 3.3.1.4.5 Interdune perennial vegetation; 3.3.1.5.5 Interduneannual grasslands; 3.3.1.6.5Vegetation dominated byiceplant; 3.2.3.1.5Fixed dune Mediterranean macchia


(for the accuracy analysis see Acosta et al. 2005 for adetailed explanation of methods).

Please note: Wiley-Blackwell is not responsiblefor the content or functionality of any supportingmaterials supplied by the authors. Any queries

(other than missing material) should be directed tothe corresponding author for the article.

Received 16 October 2009;

Accepted 19 October 2009.

Co-ordinating Editor: Dr Ralf Ohlemuller


landscape-scale patterns of alien plant species on coastal dunes: the case of iceplant in central...

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