Page 4 - Russo_Terlizzi_1998
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Giovanni Fulvio Russo & Antonio Terlizzi

                                                                                           of other  sites ;  station
           100%                                                                            points  have  saturations
                                                                                           which  follow  one  anoth-
            80%                         ~  v                       ~                       er  consistent  with  their
                           ~                                                               Marettimo  were  ordered
                                                                                           geographic  al  position:
            60%                                                    l                       S.  Domino  and  Gal-
                                                                                           lipoli,  Lampedusa  and
                                                                                           respectively,  proceeding
            40%                         ~            v                                     from  the  positive  to  the
                                                                                           negative  portion  of the
            20%                                                                            factor,  which  co uld
                                                                                           therefore  be  imerpreted
                                                                                           as  an  East-W est  longiru-
                   S.  Dommo     Gal h poli   Lampedusa    Marettimo      Medas            dinal  gradient.  Along
                                                                                           F2,  apart  from  th e
                  D  air-lift exclusive  D  Common species  D  hand-net exclusive          strong  polarisation of the
                                                                                           samples  collected  by  air
                                                                                           lift  a t  Galli poli,  a  clear
                    Figure 4. Percent of species collected by both che methods and exclusively  by each single method.   separation of the samples
                                                                                           according  to  the  collect-
                                                                                           ing  method was obtained:
        the  samples,  collected  by  both  a1r  lift  and  hand-towed  net.   ali  the samples  by air lift had positive scores,  while all  those by
        Alvania  discors,  Vitreolina  philippi  and  Gibbemla philippii  were   hand-towed net were  in the negative part of the factor.
        present  in  all  air  lift  samples,  while Jujubinus  striatus  and  Bit-
        tiltm retimlatmn were presem in ali hand-towed net samples.  The   DISCUSSION
        bivalve  species,  almost  exclusively  collected  by  air  lift,  were   Remarkable  differences  in  the strucrure of the  mollusc  associa-
        well  represented,  in  quamitative  terms,  in  ali  the  stations  and   tions  living  in  the fìve  seagrass  beds of the Mediterranean  were
        especially in Medas.                                 observed.  This  high structural variability may  be imerpreted as
           Each  prairie was  well  characterised  by the high quamitative   the result of a number of imeracting factors.
        dominance  of particular species: S.  Domino by Rissoella  inflata;   First of all,  differently-strucrured  malacological assemblages
        Gallipoli  by  Bittium  retimlatum,  Bittium  latreillii,  Nassarius   were  obtained  by  the  two sampling  methods,  as  clearly  shown
        incrassatus,  Calliostoma conulum, }ujubinus striat11s and Venericardia   by  the ordination model,  in which along  F2  the samples  by  net
        antiquata;  Lampedusa  by  Rissoa  auriscalpimn,  Tricolia pullm and   have an opposi te polarisation with respect to those by air-lift.
        Aft,ania  lineata;  Marettimo  by  Rissoa  variabilis,  Alvania  discors   The material obtained by  the two sampling tools  was  differ-
        and Vitreolina philippi end Medas  by Bittium retimlatum,  Nassar-  em  in both qualitative and quantitative terms. Despite the fact
        im incrassatm,  Gibbemla philippii and Mmmlm costulat11s.   that  the  hand-net  covers  a  larger  sampling  area  (about  20  m2
                                                             compared to  l  m 2)  and collects many more individuals, the suc-
        Structural analysis                                  tion-sampler  gives  more  complete  information  on  the  overall
        A  fìrst  strucrural  analysis  was  carried  out starting  from  a  82  x   species  composition living  in a particular prairie stand. Samples
        10 matrix (species x (stations x methods), mathematically treat-  collected  by air-lift are  richer  in  species  and  more  variable,  the
        ed  to obtain a CA ordination model.  The  model showed,  along   appararus  being  more  effective at  the  base of the  leaves,  where
        the  first  factor,  a  strong  opposition  between  the  samples  from   the malacological stock is  richer in species and reflects  the vari-
        the  Medas  (both  hand-net  and  air  lift)  and  ali  other  samples   ety  of the  substrate.  Conversely,  samples  collected  by  hand-net
        and, along  the second factor,  the opposition of the samples of S.   are  less  rich  in  species  and  less  variable,  this  tool  being  more
        Domino to ali others (both factors were signifìcam).   effective on the  leaves  which are  inhabited  by a  more homoge-
           A  second  54 x  9 matrix (species  x samples)  was  then  set  up   neous  malacolog ical  stock .  Therefore,  the  suction-sampler
        by erasing  the samples  of the Medas,  in order  to eliminate dis-  would  be  preferable  when  the  qualitative  aspects  are  to  be
        tortion in the first  factor of the  model.  In the new  CA ordina-  emphasized  with respect  to the quantitative ones, as  is  the case
        tion obtained from  this matrix (Fig. 7) the fìrst  two factors were   for  collections for  strucrural analyses  of communities;  the hand-
        significant  an d  showed,  along  F l,  the  sa me  pattern prese m   net  may  be  more  useful  when  the  quamitative  aspects  prevail
        along  F2  in  the  previous  model,  that  is,  the  opposition  of the   over the qualitative ones, as  in the case of population analyses.
        samples  from  S.  Domino  to  ali  others.  In addition, some  other   Secondly,  in  the  large-scale  strucrural  comparison  of the
        imeresting  ordination  patterns  can  be  observed  in  this  latter   associations from  the five  Posidonia  beds,  both sampling  meth-
        model.  Along Fl, the two sample poims (air-lift  and hand-net)   ods  were effective and gave similar  information.  This  is  clearly
        of the same si te have a saturation which is  more similar to that   shown by the AC (Fl /F2) ordination model, where the intra-site

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