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Original article

Nestedness in centipede (Chilopoda) assemblages on continental islands(Aegean, Greece)

Stylianos Michail Simaiakis a,*, Miguel Angel Martínez-Morales b,1

aNatural History Museum of Crete, University of Crete, Knossos Av., PoBox 2208, GR-71409 Heraklion, Crete, GreecebCentre for Biological Research, Autonomous University of the State of Hidalgo, A.P. 69, Plaza Juárez, Pachuca 42001, Hidalgo, Mexico

a r t i c l e i n f o

Article history:Received 12 June 2009Accepted 29 January 2010Published online 23 February 2010

Keywords:CentipedesDodekanisaGeophilomorphaKykladesLithobiomorphaNODFNTC

a b s t r a c t

In natural ecosystems, species assemblages among isolated ecological communities such as continentalislands often show a nested pattern in which biotas of sites with low species richness are non-randomsubsets of biotas of richer sites. The distribution of centipede (Chilopoda) species in the central and southAegean archipelago was tested for nestedness. To achieve this aim we used distribution data for 53species collected on 24 continental Aegean islands (Kyklades and Dodekanisa). Based on the first-orderjackknife estimator, most of islands were comprehensively surveyed. In order to quantify nestedness, weused the nestedness temperature calculator (NTC) as well as the nestedness metric based on overlap anddecreasing Fill (NODF). NTC indicated that data exhibited a high degree of nestedness in the central andsouth Aegean island complexes. As far as the Kyklades and Dodekanisa are concerned, NTC showed lessnested centipede structures than the 24 islands. Likewise, NODF revealed a significant degree of nest-edness in central and south Aegean islands. It also showed that biotas matrices without singletons weremore nested than the complete ones (Aegean, Kyklades and Dodekanisa). The two commonest centipedetaxa (lithobiomorphs and geophilomorphs) contributed differently to centipede assemblages. In theKyklades and Dodekanisa, geophilomorphs did not show a reliable nested arrangement unlike lith-obiomorphs. In relation to the entire data set, nestedness was positively associated with the degree ofisolation. In the Kyklades altitudinal range best explained nestedness patterns, while in Dodekanisahabitat heterogeneity proved to be more important for the centipede communities. Island area does notseem to be a significant explanatory variable. Some of our results from the Kyklades were criticallycompared with those for terrestrial isopod and land snail nested assemblages from the samegeographical area. The complex geological and palaeogeographical history of the Aegean archipelagopartly accounted for the pattern of centipede assemblages.

� 2010 Published by Elsevier Masson SAS.

1. Introduction

It is well known that the distribution of organisms in nature isnot random. Species on islands within an archipelago or in isolatedhabitats have often been found to exhibit characteristic distribu-tional patterns. According to Patterson and Atmar (2000)geographic or ecological islands often support “nested subsets” ofspecies, where the species comprising smaller local assemblagesconstitute a subset of the species in richer ones. Species presencesin a strongly nested set of communities where sites are arranged inorder of decreasing species richness and species are ranked in orderof decreasing incidence will fill the upper left of the matrix in

a triangular shape (Patterson and Atmar, 1986; Wright et al., 1998)with no or a minimum of unexpected absences or presences(Fischer and Lindenmayer, 2002). In nature, as far as large speciesassemblages are concerned, nestedness is rarely perfect (Wrightet al., 1998). It is also reasonable to suppose that in highly nestedsystems narrowly distributed species occur only on islands withrich faunas while the fauna of depauperate islands consists ofabundant, widespread species (Cutler, 1991).

Over the last twenty years many scientists have tried tomeasureand interpret nestedness patterns e.g., Patterson and Atmar (1986),Atmar and Patterson (1993), Worthen et al. (1996), Wright et al.(1998), Brualdi and Sanderson (1999), Patterson and Atmar(2000), Fischer and Lindenmayer (2002) and Almeida-Neto et al.(2008). The majority of authors have tried to identify the compo-nents of variation in nested distributions and have tested fordifferences in nestedness patterns mostly between vertebrates(Cutler,1991; Simberloff andMartin,1991; Lomolino,1996; Bird and

* Corresponding author. Tel.: þ30 6976614269.E-mail address: simaiakis@biology.uoc.gr (S.M. Simaiakis).

1 Present address: Hidalgo Section of the Society for the Study and Conservationof Birds in Mexico, Fco. I, Madero 58, Omitlán 43560, Hidalgo, Mexico.

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Boecklen, 1998; Conroy et al., 1999; Fernández-Juricic, 2002;Davidar et al., 2002; Fleishman et al., 2002; Hecnar et al., 2002;Mac Nally et al., 2002; Cook et al., 2004; Martínez-Morales, 2005)and plants (Kadmon, 1995; Berglund and Jonsson, 2003; Bruun andMoen, 2003). However, there are recent works concerned withinvertebrates such as orthoptera (Schouten et al., 2007), butterflies(Kerr et al., 2000; Davidar et al., 2002; Fleishman et al., 2002;Summerville et al., 2002), hymenopterans (Kerr et al., 2000),terrestrial isopods (Sfenthourakis et al., 1999, 2004), land snails(Sfenthourakis et al., 1999; Hausdorf and Hennig, 2003) and benthicbiotas (Loo et al., 2002).

The most widely cited method to assess nestedness is theNestedness Temperature Calculator (NTC) algorithm developed byAtmar and Patterson (1993, 1995). It estimates the highest possiblenestedness in a given matrix by reorganizing its topology in a waythat minimizes the unexpectedness of species occurrences asexpressed by their topological deviations from the perfectly nestedpattern. There is also a new consistent metric for nestedness anal-ysis, the Nestedness metric based on Overlap and Decreasing Fill(NODF), an improved method that is supposed to avoid someproblems faced by NTC such as dependence on matrix size andshape (Almeida-Neto et al., 2008). Apart from the NTC and NODF,several other quantitative measurements of nestedness have beenproposed (Cutler, 1991, 1994; Simberloff and Martin, 1991; Wrightand Reeves, 1992; Lomolino, 1996; Wright et al., 1998; Rodríguez-Gironés and Santamaría, 2006; Ulrich, 2006). The rationale forchoosing NTC and NODF metrics over others is based on thepotential of the calculator (NTC) for identifying idiosyncratic speciesand sites (Patterson and Atmar, 2000), as well as the possibility ofthe NODF metric to calculate nestedness independently amongrows and among columns (Almeida-Neto et al., 2008). Also theformer metric is used to allow comparisons with previous worksconcerning the Aegean islands (see Sfenthourakis et al., 1999).

In the Aegean archipelago the complex palaeogeographic historyand the continuous human activities for at least 4000 years havemodified the natural landscape in multiple ways resulting inincreased fragmentation of habitats. Due to the high percentages ofendemism, and the fact thatfloral and faunal elementsmay originatefrom Europe, Asia and Africa, the Aegean appears to be an excellentgeographical region for studying distributional patterns andfaunistic relationships (Foufopoulos and Ives, 1999; Sfenthourakiset al., 1999; Dennis et al., 2000; Chatzimanolis et al., 2003;Chatzaki et al., 2005; Parmakelis et al., 2006; Triantis et al., 2008).

Chilopoda (centipedes) are common and relatively familiar soilanimals, mainly carnivorous, which are found in litter or understones or bark (Lewis, 1981). They rank among the most importantsoil arthropods in sites such as woodland leaf litter, decaying wood,hedgerows, compost heaps (Wallwork, 1976) and coastal habitatswhere the substrate is either shingle beaches ormuddy cliffs (Kettleand Arthur, 2000). Geophilomorpha and Lithobiomorpha are thetwomost diverse orders with approximately 1100 and 1500 speciesrespectively (Andersson et al., 2005). The former comprises blind,elongated, worm-like, soil-dwelling forms (Lewis, 1981), with 27 to191 pairs of legs (Minelli et al., 2000) while the latter includesshort-bodied animals with 15 pairs of legs (Lewis, 1981). Geo-philomorphs prefer moist environments and penetrate soil andhumus (Lewis, 1981), while lithobiomorphs are more agile but, dueto their compact form, tend to be more surface living (Eason, 1964).Furthermore, most lithobiomorphs are widely distributed withoutrestricted environmental demands (generalists), unlike most geo-philomorph species that be able to easily penetrate into the soil andhave more narrow habitat preferences (Eason, 1964).

Our aim was to investigate and explain nestedness patterns ofcentipedes in the central and south Aegean archipelago using boththe NTC and the NODF metrics. We looked for patterns of species

nestedness between the two pivotal insular complexes of thecentral and south Aegean area (Kyklades to the west and Dodeka-nisa to the east) based on the complex geography and history of theregion. The analysis was also applied to the richer centipede orders(geophilomorphs and lithobiomorphs) in order to explore whetherthere was any community structure in these particular sub-groupsof the data. We examined the relationship of island features andspecies life-history traits with possible nestedness patterns. Ourresults were compared with those for other invertebrate groupssuch as land snails and terrestrial isopods of the Kyklades islands.

2. Materials and methods

2.1. Study area

The Aegean area consists of more than 7500 islands, mostlycontinental in their origin. The central and south Aegean archi-pelago comprises three main geographical compartments Crete,Kyklades and Dodekanisa. The distinct geological history of theKyklades and Dodekanisa justifies our approach which was toanalyze the nestedness pattern of these two insular complexes. TheKyklades Islands are found in the central Aegean Sea, betweencontinental Greece and Asia Minor, consisting of 21 large islandswith surface larger than 30 km2 and hundreds of smaller islets. Inorder to compare our results with the study on land snails andterrestrial isopods from the Kyklades (Sfenthourakis et al., 1999),the same 14 islands were included. As far as Dodekanisa is con-cerned (southeastern Aegean archipelago), the centipede faunafrom 10 large islands was analyzed (Fig. 1). According to our map(see Fig. 1) Astypalaia has been included in Dodekanisa froma faunistic point of view (see discussion).

2.2. Field sampling design

Field sampling was designed according to all different type ofecosystems hosted in the Aegean islands and all habitat typesexploited by centipedes. To achieve this goal sampling strategyscheduled after the study of available geographical, geological andvegetational maps of islands, and by in situ investigation of the fieldon each island. We qualitatively collected centipedes for 53 days on24 islands during the wet periods (SeptembereDecember andMarcheMay) of the year 2002e2004 (Table 1). The number of

Fig. 1. Kyklades (14 islands with bold numbers). 1: Serifos, 2: Amorgos, 3: Naxos, 4:Sifnos, 5: Milos, 6: Syros, 7: Paros, 8: Thira, 9: Andros, 10: Kea, 11: Tinos, 12: Mykonos,13: Anafi, 14: Kythnos. Dodekanisa (10 islands with normal numbers). 1: Kos, 2:Kalymnos, 3: Nisyros, 4: Symi, 5: Tilos, 6: Leros, 7: Chalki, 8: Patmos, 9: Astypalaia,10: Leipsoi. Numbers indicate islands in ascending order of species richness accordingto NTC.

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sampling stations per island is related to the area and the habitatheterogeneity of the islands (Table 1). In particular, not only maquisand phrygana that are extensively present on most Aegean islands,but also coniferous forests, deciduous forests, sand dunes,hygrophilous vegetation, coastal habitats, cultivated lands, urbanand suburban environment often occur in the study area. In total,197 sampling stations were accomplished, covering all habitattypes in the study area (e.g. maquis, phrygana) and all geographicsubdivisions of each island (eastewest, northesouth and low-highaltitude direction). Sampling stations were tracks of more or lessequal length distributed over the whole island surface and habitatsexploited by centipedes. Each track had a length of approximately100 meters, a width of approximately 2e3 m, covering an area ofabout 200e300 m2. All samples were collected by hand and weremainly found under stones, leaf litter, fibrous strands of Posidoniaoceanica (washed up to nearby shorelines), soil surface and fallentrunks. In each sampling station, collection of samples wasconsidered complete when no new species were found fora reasonable time period. On certain islands (Syros, Naxos, Paros,Amorgos) repeated samplings were fulfilled to check effectivenessof sampling design in different periods. The spatial distribution ofeach species in the study area is given in Appendix A. Samplingeffort (number of collecting days, number of sampling stations perisland and total number of samples per island) on each island waspresented in Table 1.

2.3. Data set

The centipede fauna of the Kyklades consists of 36 speciesbelonging to 17 genera (see also Simaiakis et al., 2005). Thecomplete set of islands studied hosts all the centipede species of theKyklades, but 17 of the species occur on only one island (singletons,

excluded in some analyses). It is noteworthy that there are nospecies endemic to the central Aegean Islands (Kyklades). As far asDodekanisa is concerned, 45 species were identified belonging to19 genera (see also Simaiakis et al., 2005). Eighteen of these speciesoccur on only one island, whilst there are no locally endemic. Thespecies per island matrix is given in Appendix A.

2.4. Analytical methods

2.4.1. Survey completenessCook (1995) suggested that any measure of nestedness must

depend on a complete species list for each site. True species rich-ness for each island was estimated using the first-order jackknife,one common nonparametric richness estimator that uses species-by-sample data (see Table 1). The EstimateS software package(Colwell, 1997) was used to generate the estimator for true speciesrichness before any nestedness analysis was attempted.

2.4.2. Measure of nestednessThe NTC was used to estimate the degree of nestedness of the

maximally packed matrices, whereas NODF was used to measuretotal matrix nestedness as well as nestedness among islands (rows)and species (columns).

NTC (freely available on: http://www.aics-research.com/nestedness/tempcalc.html) calculates the matrix “temperature”(Tm) of the observed data. Tm provides a standardized measure ofmatrix disorder, ranging from 0 for a perfectly nested matrix to 100for one that is completely disordered (Wright et al., 1998). Thesoftware also calculates the statistical significance of the estimatedTm, whilst it presents the idiosyncratic temperatures of eachspecies and island. The statistical significance of Tm is testedagainst the estimated temperature of null matrices (Ta) with thenumber of cells occupied in the random matrices equal to that ofthe matrix of observed data. In order to compare values of nest-edness among isopods, snails and centipedes from the Kyklades, westandardized the absolute values (Tm) by using the Z-score (Ulrichet al. 2009).

NODF improves the nested-temperature method of Atmar andPatterson (1993), evaluating nestedness among sites only oramong species only (Guimaraes and Guimaraes, 2006). It discrim-inates a nested structure in which fill is minimum, from others inwhich fill is maximum and it is sufficiently insensitive to matrixshape and matrix size but positively correlated with matrix fill(Almeida-Neto et al., 2008). We used ANINHADO to compute thedegree of nestedness. ANINHADO is a Cþþ program (Platform DOSunder Windows) for researchers already familiarized with thenestedness concept and null model analysis developed for fast andautonomous calculation of two distinct measures of nestedness(Guimaraes and Guimaraes, 2006): a) NODF (Almeida-Neto et al.,2008) and b) matrix's Temperature (Atmar and Patterson, 1993).

2.4.3. Assessment of nestedness causalityTo explore the mechanisms that may account for the presence of

nestedness in the communities studied, the order in which islandswere sorted by the NTC algorithmwere correlatedwith island orderbased on ecological features. Pearson productemoment correlationcoefficient was used for these tests in Statistica 6 (StatSoft, Inc.,2001). In detail, from the presenceeabsence matrix packed tomaximal nestedness byNTC,we took the resulting site order (or else“current island position”) and successively checked for its correla-tion with the site order based on area, altitudinal range, distancefrom mainland Greece, Turkey and Crete, degree of isolation (theclosest distance to any mainland), habitat heterogeneity (describedby Simaiakis, 2005 for centipedes), survey completeness (estimatedusing the first-order jackknife nonparametric richness estimator)

Table 1Proportion of survey completeness of the centipede community on the Aegeanislands (D: Dodekanisa, K: Kyklades) based on the first-order jackknife estimator(Colwell, 2006). Sobs: observed number of species; Sexp: expected mean number ofspecies with standard deviation (SD); Scom: survey completeness; Ds: days ofsampling on each island. Sampling effort is shown in terms of number of samplingstations and number of samples collected on each island. EstimateS: Statisticalestimation of species richness and shared species from samples. Version 8. Availablefrom: http://purl.oclc.org/estimates (accessed January 2009).

Islanda Sobs Sexp (mean� SD) Scom Ds Number ofstations/samples

D-Astypalaia 14 17.97� 1.96 0.78 2 11/113D-Chalki 15 21.91� 2.50 0.68 2 8/74D-Kalymnos 22 26.57� 2.32 0.83 3 7/138D-Kos 22 34.92� 3.45 0.63 2 8/163D-Leipsoi 12 17.84� 2.21 0.67 1 4/37D-Leros 16 21.93� 2.36 0.73 2 7/92D-Nisyros 21 26.98� 2.21 0.81 3 13/284D-Patmos 14 21.93� 2.71 0.64 2 7/113D-Symi 18 23.07� 2.20 0.78 2 8/99D-Tilos 16 20.97� 2.20 0.76 2 10/199K-Amorgos 13 14.99� 1.40 0.87 2 12/139K-Anafi 7 7.97� 0.97 0.88 1 4/35K-Andros 11 15.95� 2.17 0.69 4 12/109K-Kea 9 11.96� 1.69 0.75 2 6/78K-Kythnos 7 9.96� 1.69 0.70 2 6/80K-Milos 10 10.99� 0.99 0.91 3 8/175K-Mykonos 8 8.00� 0.30 1.00 1 3/65K-Naxos 12 15.95� 1.94 0.75 3 12/80K-Paros 10 10.99� 0.99 0.91 3 9/114K-Serifos 13 14.94� 2.15 0.87 2 7/76K-Sifnos 12 16.92� 2.12 0.71 2 7/60K-Syros 10 12.97� 1.70 0.77 3 11/113K-Thira 10 12.95� 1.68 0.77 2 8/61K-Tinos 9 11.97� 1.70 0.75 2 9/94

a Sampling method: All samples were collected by hand.

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and species richness (Appendix B). We used the same procedure toestimate whether centipede species (“current species position”)were sorted by their dispersal ability as described in previousworks(Eason, 1964; Lewis, 1981), degree of habitat specificity (defined bySimaiakis et al., 2005 for the centipede fauna of the Aegean archi-pelago) and species incidences (see Appendix C).

2.4.4. Collinearity effectTo check for collinearity among independent variables, we

performed a multiple regression analysis (dependent variable:species richness) and we used a collinearity diagnostic as producedand described by SPSS v. 15 (SPSS 15.0 for Windows, Inc.,1989e2006). VIF (variation inflation factor), indicates whethera predictor variable has a strong linear relationship with the otherpredictor variable (Field, 2005). Although, there are no strict rulesabout what values of the VIF should be cause for concern, Myers(1990) suggests that a value of 10 is a good value at which toworry. Hence, all independent variables with VIF lower than 10were included in the discussion of the causes of nestedness,whereas all variables with VIF higher than 10 were excluded (e.g.,Kyklades: distance frommainland Greece and distance from Turkeywere excluded from the model because VIF values were signifi-cantly higher than 10). However, in some cases the exclusion ofsome variables may be problematic. To ensure that we would notthrow away any informative variable we also used partial correla-tions among the independent and the dependent variables asproduced by SPSS v. 15 (SPSS 15.0 for Windows, Inc., 1989e2006).Regardless of applying partial correlation, values among variableswere only slightly different from those obtained from the Pearsoncorrelation procedure supporting the exclusion of several variablesfrom the discussion of causality of nestedness.

3. Results

Fifty-three centipede species were collected from the 24 centraland south Aegean islands. About one third of the species (18 of 53)was present on only one island (singletons) (Appendix A). Noendemic species have been reported from the area suggesting itsstrong faunistic relationships with the adjacent continents (main-land Greece e Balkan peninsula and, Turkey e Anatolian penin-sula). Singletons were 47% of species in the Kyklades (17 of 36) and40% in Dodekanisa (18 of 45) (Appendix A).

The number of species collected on each island varied between 7(Kythnos, Kyklades) and 22 (Kos, Dodekanisa). Survey complete-ness for the 24 islands ranges between 63 and 100% (mean� SD:0.78� 0.09). According to the expected species richness, on 19 ofthe 24 islands (79%) more than 80% of the species were collected,whereas for the remaining five islands, the mean value of theestimates was higher than 63% (Table 1). In the Kyklades, on 13 ofthe 14 islands (93%) more than 80% of the expected species wererecorded (mean survey completeness� SD: 0.81�0.09). Andros isthe only island less comprehensively surveyed (69% of expectedspecies richness, see Table 1). In Dodekanisa (mean survey com-pleteness� SD: 0.73� 0.07) on 6 of the 10 islands (60%), more than78% of the species were collected, whereas for the rest of the islands(40%) the mean value of survey completeness was lower than 70%(Table 1).

NTC showed that the centipede assemblage of the south Aegeanarchipelago was significantly nested (Table 2). When the Kykladesand Dodekanisa were analyzed separately, the two matrices wereless ordered than the complete one, but still significantly nested(see Table 2). Moreover, the species assemblages of the Aegeanislands without singletons were less nested using NTC (Table 2)whereas in the Kyklades and Dodekanisa matrices without single-tons were more nested (Table 2).

NODF similarly revealed that centipede assemblages weresignificantly nested (Table 3). It indicated that the degree ofnestedness among islands (Aegean, Kyklades and Dodekanisa) washigher than the degree of nestedness in species occupancy(Table 3). Contrary to NTC, NODF showed that matrices withoutsingletons were more nested than the complete ones (Aegean,Kyklades and Dodekanisa) (Table 3). While it was quite complicatedto estimate how nested one ormore columns (species) is in relationto other ones, we accomplished to evaluate nestedness among rows(islands). According to ANINHADO output files (Guimaraes andGuimaraes, 2006), islands located at the lower part of the packedmatrix (Aegean islands) had proper subsets of centipede speciespresent in Kos (NODF¼ 73), Symi (NODF¼ 70) and Kalymnos(NODF¼ 68) (Dodekanisa islands) as well as Serifos (NODF¼ 68)and Amorgos (NODF¼ 67) (Kyklades islands). The Aegean data setwithout singletons was highly nested with Sifnos (NODF¼ 72) andNaxos (NODF¼ 71) instead of Serifos and Amorgos. Astypalaia hadhigher values of nestedness in relation to the Kyklades (NODF¼ 67)than Dodekanisa (NODF¼ 61). When we measured NODF exclu-sively for the Kyklades and Dodekanisa, we evaluated that Serifos(NODF¼ 68), Amorgos (NODF¼ 67), Naxos (NODF¼ 67) and Sifnos(NODF¼ 67) as well as Symi (NODF¼ 67), Kalymnos (NODF¼ 66),Kos (NODF¼ 65) and Nisyros (NODF¼ 61) significantly contributedto thematrix nestedness respectively. Regarding thematrices of theKyklades and Dodekanisa without singletons, apart from the islandof Symi the rest of the islands showed high values of nestedness inrelation to other islands.

The two most diverse centipede taxa along the Aegean islands,namely geophilomorphs and lithobiomorphs, were strongly nestedaccording to NTC (Table 4). Unlike geophilomorphs, the samepattern emerged when we separately examined the lithobiomorphspecies assemblages from the Kyklades and Dodekanisa (Table 4).

Table 2Temperature (Tm, �C) (based on the Nestedness Temperature Calculator) of thecentipede species matrices. (Ta �C): the mean temperature of 1000 permutatedmatrices, (SD): the standard deviation of the permutated matrices, (F%): thepercentage of matrix fill, (S): the number of species in each matrix, (wos): withoutsingletons. p(Ta< Tm): the possibility of randomly finding a cooler matrix, *p< 0.05, ** p< 0.001.

Island groups Tm Ta SD F % S

Aegean 24.84** 56.79 3.76 22.8 53Aegean (wos) 31.95** 63.11 4.71 32.7 35Kyklades 37.67* 52.27 5.76 25.7 36Kyklades (wos) 33.28** 54.68 6.03 43.2 19Dodekanisa 43.93* 54.51 5.75 33 45Dodekanisa (wos) 37.14** 54.60 6.11 50.4 27

Table 3Measure of nestedness for the south Aegean archipelago. Aegean (24 islands),Kyklades (14 islands) and Dodekanisa (10 islands). Nrow: nestedness among all rows(islands), Ncol: nestedness among all columns (species), NODF: total matrix nest-edness, NODF(Er): nestedness of null model where presences are randomly assignedto any cell within the matrix, P(Er): significance of NODF based on this null model,NODF(Ce): nestedness of null model where the probability of a cell aij showa presence is (Pi/Cþ Pj/R)/2, in which Pi is the number of presences in the row i, Pj isthe number of presences in the column j, C is the number of columns and R is thenumber of rows, P(Ce): significance of NODF based on this null model, (wos):without singletons.

Matrix Nrow Ncol NODF NODF(Er) P(Er) NODF(Ce) P(Ce)

Aegean 63.41 39.53 43.52 26.1 0.00 32.8 0.00Aegean (wos) 67.45 51.23 56.17 35.39 0.00 42.76 0.00Kyklades 62.13 39.79 42.61 30.02 0.00 36.54 0.04Kyklades (wos) 73.26 60.88 64.89 45.58 0.00 53.42 0.02Dodekanisa 62.42 50.18 50.72 39.64 0.00 45.73 0.05Dodekanisa (wos) 71.62 67.47 67.94 54.96 0.00 61.14 0.03

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However, except for the Kyklades data set, lithobiomorph matriceswithout singletonswere less nested than the complete one (Table 4).

Pearson productemoment coefficient indicated that the islandorder, as determined by NTC metric for all the 24 Aegean islands,was strongly correlated with species richness (r¼�0.95) anddegree of isolation (closest distance to any mainland, r¼ 0.45)(Table 5). It is notable that area, distance from Crete, number ofbiotopes, altitudinal range and estimated survey completenesswere not correlated with island rank. On the other hand, distancefrom mainland Greece and Turkey were excluded from the modelbecause of strong collinearity with all the other predictor variables.

Regarding the Kyklades islands, due to high collinearity (vari-ables with VIF values higher than 10) the majority of independentvariables were excluded (area, distance from mainland Greece,Turkey and Crete, degree of isolation and habitat heterogeneity).Pearson productemoment coefficient showed that current islandposition, as determined by NTC was negatively correlated withspecies richness (r¼�0.93) and altitudinal range (r¼�0.51)(Table 5). In Dodekanisa, however, current island position wasnegatively correlated with species richness (r¼�0.95) and habitatheterogeneity (r¼�0.66) (see Table 5). Distance variables, areaand altitudinal range were discarded from the statistical analysisbecause of strong multicollinearity. Distance frommainland Greeceand survey completeness were negatively associated, howeverwithout any prominent contribution to nestedness pattern. It is also

relevant that distance fromTurkeywas coidentifiedwith the degreeof isolation (closest distance from any mainland) due to thegeographical position of Dodekanisa, therefore the latter variablewas removed from the analysis.

Table 6 shows how species rank, produced by NTC, was corre-lated with dispersal capacity, habitat specificity and speciesoccurrence on each island. For all 53 species occurring on the southAegean islands, current species position is highly and negativelycorrelated with habitat specificity (r¼�0.77) and species occur-rence (r¼�0.80) and less but still significantly correlated withdispersal capacity (r¼�0.36) (Table 6).

In the Kyklades, species current position derived from NTC washighly and negatively correlated with habitat specificity (r¼�0.52)and species occurrence (r¼�0.79), but not with dispersal capacity(Table 6). Regarding the centipede assemblages of Dodekanisa,species rank was significantly negatively correlated with allanalyzed variables. It was strongly correlated with habitat speci-ficity (r¼�0.76) and species occurrence (r¼�0.92), but lesscorrelated with dispersal capacity (r¼�0.39) (Table 6).

4. Discussion

The occurrence and the quantification of nestedness in ecolog-ical systems provide evidence about processes that affect speciescomposition and intraspecific interactions in a region (Almeida-Neto et al., 2008). Apart from selective extinction and coloniza-tion, habitat fragmentation, niche structure and passive samplingare considered to cause nested patterns (Simberloff and Martin,1991; Kodric-Brown and Brown, 1993; Lomolino, 1996; Wrightet al., 1998; Fischer and Lindenmayer, 2002; Ulrich et al., 2009).According to Almeida-Neto et al. (2008), environmental studies, asdescribed in previous works (Atmar and Patterson, 1993; Cutler,1991, 1994), often search for quantification of nestedness. Duringrecent years many metrics and computational programs havebecome available (Cutler, 1991, 1994; Atmar and Patterson, 1995;Lomolino, 1996; Wright et al., 1998; Brualdi and Sanderson, 1999;Rodríguez-Gironés and Santamaría, 2006; Ulrich, 2006). However,ecologists are always facing the dilemma of choosing the appro-priate method for their particular analysis (Fischer andLindenmayer, 2002). In addition, sampling effort affects measureof nestedness (Cook, 1995).

Using NTC and NODF metrics, we found a high degree of nest-edness for the centipede fauna of all the 24 Aegean islands.Although the degree of nestedness was also statistically significantfor each island complex (Kyklades and Dodekanisa), the matriceswere not so highly nested. One possible explanation for thisdiscrepancy could be the low proportion of survey completeness insome islands. Species survey completeness was quite high for themajority of the islands (more than 80%), but there were some caseswhere we found survey values to be lower on average values oncertain island (Andros, Kos, Leipsoi, Chalki and Patmos). Low surveycompleteness is mainly explained by rare species because their

Table 4Temperature (Tm �C) (based on the Nestedness Temperature Calculator) of thegeophilomorph (geo) and lithobiomorph (lith) centipedematrices. (Ta �C): themeantemperature of 1000 permutated matrices, (StD): the standard deviation of thepermutated matrices, (F%): the percentage of matrix fill, (S): the number of speciesin each matrix, (wos): without singletons. p(Ta< Tm): the possibility of randomlyfinding a cooler matrix, *p< 0.05, **p< 0.001, n.s.: non significant.

Island groups Tm Ta StD F % S

Aegean (geo) 20.03** 41.86 5.28 18.2 26Aegean (geo) (wos) 28.58** 52.86 6.47 30.3 14Aegean (lith) 15.83** 51.28 6.13 24.1 20Aegean (lith) (wos) 18.66** 54.05 6.24 30.8 15

Kyklades (geo) 30.28 n.s 39.96 7.73 21.8 17Kyklades (geo) (wos) 33.49 n.s 44.38 9.24 34.9 9Kyklades (lith) 19.82** 43.95 9.31 27.9 12Kyklades (lith) (wos) 13.87* 36.89 10.16 57.1 5

Dodekanisa (geo) 33.94 n.s 41.83 8.32 25.2 21Dodekanisa (geo) (wos) 25.8 n.s 44.38 9.45 45.5 9Dodekanisa (lith) 31.01* 49.38 7.73 40.5 17Dodekanisa (lith) (wos) 34.08* 48.93 7.51 53.3 12

Table 5Correlation (Pearson productemoment correlation coefficient e r) between currentisland position (cip), after packing to maximal nestedness using NTC, and islandorder based on particular features of all 24 islands as well as of 14 Kyklades islandsand 10 Dodekanisa islands. Island features include area, distance from mainlandGreece (dist Gr), distance from Turkey (dist Tu), distance from Crete (dist Cr), islandisolation (iso), number of biotopes in each island (bio), altitudinal range (alt), surveycompleteness (srv) and species richness (sp). *p< 0.05, **p< 0.001, n.s: non signif-icant. Excl: excluded correlation values due to significant collinearity of independentvariables.

r NTC (24) r NTC (14) r NTC (10)

cip & area 0.14 n.s �0.10 n.s �0.49 (excl.)cip & dist Gr �0.74** (excl.) �0.01 (excl.) �0.47 n.scip & dist Tu 0.68** (excl.) �0.32 (excl.) 0.68* (excl.)cip & dist Cr 0.22 n.s 0.20 (excl.) 0.37 (excl.)cip & iso 0.45* �0.19 (excl.) 0.68* (excl.)cip & bio 0.08 n.s �0.16 (excl.) �0.66*cip & alt �0.17 n.s �0.51* �0.85** (excl.)cip & srv 0.20 n.s 0.02 n.s �0.29 n.scip & sp �0.95** �0.93** �0.95**

Table 6Correlation (Pearson productemoment correlation coefficient e r) between currentspecies position (csp), after packing to maximal nestedness using NTC, and speciesorder based on biological and ecological features of all 55 species as well as of the 38species of Kyklades and the 46 species of Dodekanisa. Species features includedispersal capacity (dc), habitat specificity (hs) and species occurrence (oc). *p< 0.05,**p< 0.001, n.s: non significant.

r NTC (53) r NTC (36) r NTC (45)

csp & dc �0.36* �0.24 n.s �0.39*csp & hs �0.77** �0.52** �0.76**csp & oc �0.80** �0.79** �0.92**

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chance of being sampled in the field is lower that the chance ofan abundant species. This could also interpret the high numberof singletons (mainly rare species) both in the Kyklades andDodekanisa. Thus, it is reasonable to suggest that for smallermatrices, such as the Kyklades and Dodekanisa, possible unex-pected absences significantly affect nestedness measurements (seeTable 2), contrary to largermatrices (24 Aegean islands) that tend tobe more nested (Wright et al., 1998). Moreover, in the absence ofendemics, the large number of singletons might also affect theresults of nestedness analyses. Nestedness values could be inducedto a significant degree by such matrix configurations. However, it isremarkable to note that when we excluded singletons theremaining matrices (Aegean, Kyklades, Dodekanisa) proved to bemore nested (NODF metric). The identity of singletons and anypossible way they affect measure of nestedness will be discussedlater in this work.

The difference in nestedness between the entire matrices andthe matrices without singletons indicate that the latter do notdecrease the degree of nestedness within the system. We observedthat most of the species with a single presence tend to occur onhigher ranked islands (Appendix C). However, this is not the casefor the Kyklades and Dodekanisa (smaller matrix size) where mostof the singletons (unexpected presences or outliers) tend to occuron median and lower ranked islands such as Thira, Kea, Andros,Anafi (Kyklades) as well as Chalki and Leros (Dodekanisa) (Table 2).It could be assumed that this is only an artifact of the method used,but on the other hand, it may reflect the idiosyncratic character ofcertain species that occur on lower parts of the maximally packedmatrices in the Kyklades (i.e. Henia devia, Himantarium gabrielis,Nannophilus ariadnae, Stigmatogaster gracilis, Lithobius catascaphius,L. microps, L. micropodus and Cryptops anomalans) and Dodekanisa(i.e. Dignathodon pachypus, Hydroschendyla submarina, Nannophilusariadnae, Pleurogeophilus mediterraneus, Schendyla nemorensis,Lithobius catascaphius, Lithobius intermissus, Lithobius tidissimus). Itis also important to note that the majority of the above-mentionedspecies are geophilomorphs. Even though notes on spatial andtemporal distribution of these species are scanty, we assume thatsome of these may exhibit a relictual distribution from a previouslywider geographical range established during the intense geologicalchanges of the Aegean archipelago. Geophilomorph habits, such asnarrow, cryptozoic habitat preferences on the one hand, and loss ofhabitats or fragmented habitats due to continuous human activitiesin the Aegean landscape on the other, often make their findingdifficult. This issue and possible sampling artefacts that couldintroduce undesirable bias (Cam et al., 2000) need to be controlledin the future.

Of all the islands, the biogeographical position of Astypalaia inthe Aegean archipelago has been most discussed in the past(Triantis et al., 2008) with evidence supporting affinities withDodekanisa (Sfenthourakis, 1996; Trichas et al., 2008). Astypalaia isone of the most isolated areas of the Aegean archipelago(Dermitzakis and Papanikolaou, 1981). In our case, the finding ofLithobius pusillus (a lithobiomorph species with south Europeanaffinities) only in Dodekanisa, with Astypalaia being the westernlimit of its distribution in insular Greece, strongly supports theDodekanisian character of Astypalaia (see also Simaiakis, 2005). Onthe contrary, by using NODF, it was very interesting to note that theKyklades islands retained more nested centipede species subsets toAstypalaia compared to the subsets of Astypalaia with Dodekanisa(see Appendix A). It would be possible to interpret this patternbased on certain common species absences (“holes”) from theKyklades and Astypalaia (i.e. Lithobius aeruginosus, Lithobiuscrassipes, Lithobius pamukkalensis, Lithobius peloponnesiacus,Pleurolithobius patriarchalis, S. nemorensis). It would also be ratio-nale to expect proper subsets of centipede species occurring on

smaller islands to be present on larger islands, such as Naxos(Kyklades) and Kos (Dodekanisa). Nevertheless, this pattern wasnot clearly observed for the Kyklades and Dodekanisa (completematrices). Only when singletons were excluded from the data setdid Naxos and Kos show highly nested values with the majority ofthe smallest islands. Furthermore, Nisyros being one of the smallestof the 24 Aegean islands had significant nestedness values withother islands. This is more notable if we consider that Nisyros is oneof the newest volcanoes in the Aegean volcanic arc, not older than200,000 years (Vougiokalakis, 1993).

Nestedness within the centipede assemblages was expressedmost strongly when only the species belonging to the orderGeophilomorpha and Lithobiomorpha were analyzed. AlthoughWright et al. (1998) did not find reliable evidence for nestednessamong taxa with different vagility capacities and the fact thathigher dispersal ability is not always related with a greater degreeof nestedness (Kadmon, 1995), in our study the more vagilelithobiomorphs exhibited a greater degree of nestedness thangeophilomorphs in all the 24 Aegean islands, the Kyklades andDodekanisa (Table 4). Moreover, supposing that in nature somedegree of nestedness appears to be the rule (Wright et al., 1998),in the case of the Kyklades and Dodekanisa geophilomorphs werefound to have non-nested communities. Non-nested assemblagesmight result from the large number of geophilomorph singletonsand the highly fragmentary matrices. Even when geophilomorphsingletons were excluded, matrices remained disordered withinsignificant nested patterns (Kyklades and Dodekanisa). On theother hand, while it could be argued that frequent colonizationtends to weaken the effect of extinction in producing a nestedstructure (Wright et al., 1998) and that more vagile taxa wouldshow a lower degree of nestedness, in our study this is notobserved.

To further investigate the patterns of nestedness in the centralAegean archipelago, we tested and compared the performance ofNTC among centipedes, terrestrial isopods and land snails in thoseKyklades islands, for which we have an adequate knowledge(Mylonas, 1982; Sfenthourakis et al., 1999). Apart from the differ-ences concerning (i) dispersal ability and ecological requirementsamong these three animal groups, (ii) the period of data collection,and (iii) the sampling method, when we compared absolute valuesof nestedness among these studies we noticed considerablesimilarities in nestedness pattern within the Kyklades islands(Tm isopods¼ 35.03 �C, p< 0.001, 57 species; Tm snails¼ 35.19 �C,p< 0.001, 87 species; Tm centipedes¼ 36.79 �C, p< 0.001, 36species; see also Sfenthourakis et al., 1999). Whenwe standardizedthe estimated values of nestedness (by applying Z-score; for furtherinformation see Ulrich et al. 2009) to compare complete matrices,significant differences were revealed (Zisopods¼�6.02, p< 0.001;Zsnails¼�8.10, p< 0.001; Zcentipedes¼�2.53, p< 0.005). We noticeda similar pattern when we compare the matrices without single-tons among the three taxonomic groups. However, negativeZ-scores showed that lower T-values were found than expected bythe simulated matrices. More specifically, Z-scores lower than �2.0indicated that more than 95% of the expected nested values werebelow the observed values (Ulrich et al. 2009). Unlike centipedes,singletons in land snails and terrestrial isopods in the Kykladesoccurring on higher sorted islands did not decrease nestedness.Nevertheless, it is remarkable to note that there are no endemiccentipede species along the central and south Aegean archipelago.On the other hand, the occurrence of endemics in the Kyklades (16%in isopods and 20% in land snails) reduces the degree of nestednessdue to independent evolutionary processes contrary to the hierar-chical pattern of nestedness (Sfenthourakis et al., 1999). Conse-quently, based on the above-mentioned differences (vagility,ecological habits, occurrence of endemics, evolutionary processes)

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among the three terrestrial taxa (centipedes, isopods and snails)the apparent convergence of nestedness values seemsmeaningless.As far as current data is concerned, we could assume that this isonly a matter of species structure and/or matrix fill.

The Aegean archipelago has many of islands of different sizeswith awide variation in isolation from twomain sources (mainlandGreece and Turkey). Distance to the closest continent (degree ofisolation) was highly informative for Aegean islands for whichcentipede data was available. The greater the distance to anymainland, the more depauperate is the island fauna (Table 5). Itseems that not only continental Greece (European e Balkanelements), but also Turkey (Asian elements) contributed to thenested centipede fauna. It is well known that the isolation of thesoutheastern islands (Dodekanisa) from the Asia Minor coast is noolder than 20,000 years (Van Andel and Shackleton, 1982;Perissoratis and Conispoliatis, 2003). As previously pointed out(Fattorini et al., 1999; Trichas et al., 2008), a significant number ofDodekanisa islands “behave” as parts of a continuous landmasswith higher numbers of centipede species compared to theKyklades. This probably clarifies the more important role of theAnatolia peninsula in the Aegean centipede community composi-tion taking into account also the lower mean distance of all 24islands from Turkey (102 km) than from Greece (157 km).Regarding the causes of nestedness within the Kyklades, only alti-tudinal range could explain the order in which islands were sortedby NTC. The broader the altitudinal range, the richer the islandfauna (Table 5). On the other hand, along the southeastern islandsof Dodekanisa, habitat heterogeneity seemed to play a significantrole in the occurrence of nestedness. The higher the habitatheterogeneity, the higher species richness. Despite the differentvariables that could be involved in the occurrence of nestedness inthe Kyklades and Dodekanisa, we assume that altitudinal rangecould be a proxy of habitat heterogeneity. However, the exact roleplayed by altitudinal range and the relation with habitats in theformation of nested patterns needs to be investigated in the future.It is also interesting to note that island area in all cases (Aegean,Kyklades, Dodekanisa) and habitat heterogeneity in two cases(Aegean, Kyklades) could not explain a nested pattern in thecentipede community (Table 5). This may imply that islands aremore likely to be occupied by species with high dispersal abilitiesand wide habitat preferences.

In the Kyklades island complex, even though centipedes,isopods and snails show considerable similarities in nestednesspatterns (see Sfenthourakis et al., 1999), there are critical differ-ences concerning the causality of nestedness. Isopod and snailnestedness are best explained by distance from continental Greeceand by absolute shorter distance of mainland, respectively(Sfenthourakis et al., 1999). Hausdorf and Hennig (2005) testedthe hypothesis that the fauna composition of recent taxa on theAegean archipelago reflects palaeogeographical patterns. Whilethey supported the idea that the distribution patterns of landsnails have been influenced by recent and Pliocene distances,Sfenthourakis et al. (1999) suggested that the separation from AsiaMinor (upper Miocene) has probably affected the snail fauna(Fig. 2). On the other hand, the composition of the terrestrialisopod fauna seems to have a distinct history influenced by recentand Pliocene distances (Hausdorf and Hennig, 2005), nestednessbeing explained by the distance from mainland Greece(Sfenthourakis et al., 1999). Additionally, there are studies dealingwith butterflies (Dennis et al., 2000) and tenebrionid beetlefaunas (Fattorini, 2002) suggesting that recent and Pleistocenedistances had a crucial influence on the composition of species inboth cases. In our analysis of the Kyklades islands, as mostcentipedes are quite vagile and the islands do not host anyendemic centipedes, all these considerations of palaeogeography

spanning several million years seems to partially contribute to thenested pattern.

Although data on life-history characteristics of centipedes isscanty, we have described two traits, namely habitat specificityand dispersal ability, based on older and more recent studies(Eason, 1964; Lewis, 1981; Zapparoli, 2002; Andersson et al., 2005;Simaiakis et al., 2005). Apart from the relationship betweenspecies occurrence and species rank produced by the NTC analysis,habitat specificity was strongly associated with species order in allthe island subsets (24 Aegean islands, 14 islands of the Kykladesand 10 islands of Dodekanisa). Our results showed that dispersalcapacity was less correlated but also meaningful for the speciescategorization in all the 24 Aegean islands and Dodekanisa (seeTable 6). Geophilomorph species are more diverse in the Aegeanarchipelago (49%) than lithobiomorphs (38%), scolopendromorphs(11%) and scutigeromorphs (2%), having more restricted habitatpreferences. We suggest that this fact probably explains whyhabitat specificity promotes nestedness within the centipedes.Widespread species or species with broader habitat preferences(i.e. Bothriogaster signata, Cryptops trisulcatus, Eupolybothruslitoralis, Lithobius carinatus, Lithobius nigripalpis, Pachymeriumferrugineum, Scolopendra cingulata and Scutigera coleoptrata) werefound on more islands. This is in accordance with the view thatspecies occupy particular habitats and that the habitats them-selves have a nested arrangement (Simberloff and Martin, 1991;Wright et al., 1998). Highly idiosyncratic temperatures producedby NTC were indicative of both geographically widespread andrestricted species. This tendency partially agrees with terrestrialisopod and land snail nested patterns from the same geographicalregion (Sfenthourakis et al., 1999). While most centipede speciesin the Aegean archipelago and the Kyklades having highly idio-syncratic temperatureswere geophilomorphs (11 of 19 species and7 of 11 species, respectively), in the Dodekanisa lithobiomorphsalso exhibited the same feature (8 of 16 species).

It seems that terrestrial arthropod groups are useful foranswering particular community structure questions. As far as thecentral and south Aegean centipede fauna is concerned a morethorough sampling effort in few islands would likely providea more reliable nested pattern.

Fig. 2. The palaeogeography of Greece in the Tortonian (8 Mya) (Creutzburg 1963;Dermitzakis and Papanikolaou, 1981), recently published by Parmakelis et al. (2006).The map is drawn based on present geography. 1: Kyklades, 2: mainland Greece,3: Dodekanisa, 4: Turkey, 5: Crete, 6: Aegean trench. Gray: land, white: sea, black: lake.

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5. Conclusion

The Aegean archipelago, with its clearly continental formationof the central and south islands, is an excellent area for studyingparticular biogeographical and ecological issues such as theexploration of nested communities. In this study we founda significant degree of nestedness in all island groups. AlthoughNTC and NODF revealed similarities in their degree of nestednessfor identical matrices (Aegean islands), the role of singletons wasmore questionable. While it was difficult to explore the role ofsingle presence species, it was also interesting to show that inclu-sion of singletons lessened nestedness for the Kyklades andDodekanisa when NTC applied, whereas it increased nestednesswhen the NODF metric used. The independent study of geo-philomorphs and lithobiomorphs revealed that nestedness was notonly a general pattern for centipedes, but was also derived fromsome particular centipede taxa. While nestedness was a commonpattern in lithobiomorph assemblages (Kyklades and Dodekanisa),matrices of geophilomorph species (complete or without single-tons) showed non-nested species assemblages. Our attempt torelate certain island features and centipede species life-historytraits to possible assemblages showed that there is not a uniquevariable to explain nestedness in all island groups. In the 24 Aegeanislands the degree of isolation was found to explain nestedness. Inthe Kyklades it was the altitudinal range and in Dodekanisa thehabitat heterogeneity that contributed to nestedness. Regardingcentipede species characteristics, habitat specificity is stronglyassociated with species order in all the island subsets. Usinga previous study on the Kyklades islands (see Sfenthourakis et al.,1999), we have had the opportunity to examine nestednesspatterns comparing different organisms with a variety of dispersalcapabilities and ecological requirements. It was very intriguing tonote the similarity in the quantitative measure of their nestedness,but we assume that this might be a matter of matrix fill. Moreover,it was also surprising to see that nestedness patterns of centipedeand land snails were better explained by shorter distance andabsolute shorter distance respectively, whilst that of isopods wasexplained by distance from continental Greece. We would suggestthat, despite the vast differences between snails and centipedes,the two animal groups may have similar histories in central Aegeanislands, but still more evidence is necessary to compare.

Acknowledgments

We are grateful to Mário Almeida-Neto, Sinos Giokas and twoanonymous referees for the meticulous reading and commenting ofthe manuscript. We thank Panagiota Miltiadous for her kindsupport in the field. We also thank Antony Barber and Helen Readfor correcting and editing the linguistic part of this manuscript.

Appendix. Supplementary data

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.actao.2010.01.007.

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