A Comparative Study of Shellfish Assemblage Patterns between a Man-made and a Natural Tidal Flat

Transcription

1 A Comparative Study of Shellfish Assemblage Patterns between a Man-made and a Natural Tidal Flat Naotomo OTA & Takashi KAWAI (Department of Civil Engineering, Anan National College of Technology, Japan)

2 study site habitat endangered tiger beetle Created in Mar Cicindela lewisi Yoshino river

3 objectives Successful settlement of the tiger beetle in the newly- habitat (H. Otsuka, session 9B) terrestrial & high intertidal Information on other shore-associated organisms? Will the man-made habitat acquire fauna? This study aims to clarify: 1.How does the man-made habitat compare with the nearby habitat in terms of shellfish assemblages? 2.What is the time scale of change? mid & low intertidal

4 Physical characteristics of the shore costal vegetation transplanted from the habitat sediment transplanted the topsoil imitation of shore sheltered wave action slope of the shore sediment dredged from near sea bottom

5 Methods (quadrat sampling) Jun Oct (summer mainly, 14 times) 8 quadrats (33*33*15(depth)cm) / site on two tide levels(+50 & +100cm from MLS) 2 and 2 shore sediment sieved by 2mm mesh mollusks and crabs were selected and measured Created shore C2 300m C1 N2 N3 N1(Lost in Nov.2009) Natural shore

6 Assemblage density and number of species number of individuals No. of indi./m : stable : decreasing 0 stay at low level 98% Number of species No. of sp./3.4m > > (max 6 species)

7 Simpson s index of diversity 10 Species diversity with dominant species composition Sep-08 Nov Oct-10 Jun Jun-08 opposite pattern in two shore Sep-08 Nov-09 Oct-10

8 Assemblage similarity after 28 months Oct10 Jul10 35 Oct10 Oct10 Jul10 Nov09 Mar10 Nov09 Mar10 Oct08 Oct08 Feb09 Feb09 Jun09 Jun08 Jun09 Jun08 Nov09 Jun09 Feb09 Oct08 Jun09 Feb09 Oct08 Jun08 Jun08 Oct10 Jul10 Nov09 Mar10 Oct10 Mar10 Jul10 Similarity is decreasing! initial stage Jun D Stress: 0.1 Non-metric Multi-Dimensional Scaling site n1 n2 n3 c1 c2 Similarity distance=similarity Transform: Square root Resemblance: S17 Bray Curtis similarity 0 (low) < similarity < 100 (high)

9 Summary of assemblage characteristics shore shore density stable no. of species higher diversity high to low density decreased & stay at low no. of species lower diversity decreasing in similarity low to high Q. Have we an alternative, similar habitat for shellfish assemblage? No, two habitats are completely different.

10 Ruditapes & Batillaria both shores 2yr. decline in Ruditapes 10 Sep Oct-10 shore 2yr. lacking of Batillaria Sep-2008 Sep-08 Oct-2010 Oct-10

11 Ruditapes/1m 2 6 reference sites Strong 1500 recruitment in Low initial density Decline in R. phlippinarum a synchronized pattern? 2km upward no. of indi./1.7m 2 Shore creation in March study period? Aug Feb Aug Feb Aug Importance of a regional perspective! Both and man-made shores follow the region-wide pattern in Ruditapes populations.

12 Differences in life history of R. phlippinarum (a) (b) 6.4mm 4.2mm Recruitment : two modes (fall & spring) peak in May 15.7mm 11.5mm : only spring peak in June growth >> shell width (mm)

13 Lacking of B. cummingii in the shore no planktonic stage in B. cumingii direct development juvenile crawling out from laid egg B. cumingii longer period may be needed to settle importance of dispersion ability in snail (Armitage & Fong (2004) ) 0 always absent from shore no detectable recruitment after 53 months creation!

14 important ecological aspects for creation regional perspective particularly in recruitment life historical difference recruitment and growth dispersion ability of organism we can fly! no planktonic stage (T_T) planktonic larvae!

15 Discussion Q. How long does it take to become similar with one? 53 months (Mar.2007-Aug.2011) is not enough! 10~20 years are need to approach the state of habitats in salt marshes( Broome & Craft 2009 ). However, epibenthic species tend to be similar, more quickly. continuous sampling is needed!