Temmerman, S.

Wang, C.

Open Marine Archive

Does alternative stable states theory and catastrophic shift theory 
apply for bare intertidal flats and vegetated marshes? 
Wang Chen and Stijn Temmerman 
Department of Biology, University of Antwerp, Universiteitsplein 1-C, B-2610 Antwerp, Belgium  
E-mail: Chen.Wang@student.ua.ac.be  
The vegetation of intertidal marshes provides important ecosystem services to coastal societies, 
such as the reduction of wave-induced coastal erosion and attenuation of landward flood 
propagation during storm surges. These positive effects of intertidal marsh vegetation may change 
in time, because intertidal areas may shift from bare mudflats to vegetated marshes or vice versa. It 
is therefore important to allow the understanding and prediction of pending shifts between bare 
and vegetated states of intertidal areas. The concept of catastrophic shifts between bare mudflats 
and vegetated marshes has been explored only recently by numerical modeling and up to now only 
few empirical evidence has been presented in the literature. In our study, we tested the hypothesis 
that bare tidal flats and vegetated marshes can be considered as alternative stable landscape states 
with the occurrence of rapid catastrophic shifts between the two stable states. We studied this by 
analyzing a detailed historical record of intertidal elevation surveys and aerial pictures from the 
Westerschelde estuary (SW Netherlands). We examined the bimodal distribution of elevations in the 
intertidal areas. We tested the rapid elevation shifts from bare flats to vegetated marshes. Finally, 
we studied whether we can predict such shifts based on the intertidal elevation. Our results 
indicated that vegetated intertidal marshes lie within a small range of high elevations, while bare 
intertidal flats dominate a different range of lower elevations. The intermediate elevations are less 
frequent. Analysis of the areas that shifted from bare flats to vegetated marshes revealed that the 
shifts occur relatively rapid and that the shifts always seem to have occurred once the peak of the 
elevation distribution had exceeded a threshold value around -0.6m. Study of pioneer patches in 
2004 showed that most of pioneer patches occurred in the elevation range of -0.8 m to -0.4 m, 
which includes the aforementioned threshold elevation where the system shifts from bare to 
vegetated states. Probability maps were constructed as a prediction for the shift from bare to 
vegetated state. The success of prediction is highly different for the onshore and offshore flats. A 
high probability on the onshore flats mostly results in a shift to vegetation by the next time step, 
while not necessarily on the offshore flats. In conclusion, our results indicate empirical evidence for 
the application of alternative stable states theory and catastrophic shift theory in intertidal 
ecosystems. 
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Does alternative stable states theory and catastrophic shift theory apply for bare intertidal flats and vegetated marshes?

D  lt ti  t bl  t t  th  d t t hi  hift th  oes a erna ve s a e s a es eory an ca as rop c s eory
apply for bare intertidal flats and vegetated marshes?
Wang Chen and Stijn Temmerman   
Ecosystem management research group Department of Biology University of Antwerp Belgium   ,   ,   ,  
E-mail: Chen Wang@student ua ac be . . . .
Introduction
Theory: Sudden large shifts may occur in ecosystems between alternative stable states due to positive feedbacks.                
H th i Tid l fl t d h b h lt ti t bl l d t t ith b t hiftypo es s: a  a s an  mars es e ave as a erna ve s a e an scape s a es w  a rup  s s.
Study area: Western scheldt estuary with semi diurnal tidal regime and the local mean tidal range of 4 88m     -           . .
Methods: GIS analysis of historical records of intertidal elevation surveys (DTM), vegetation maps and aerial pictures.               
DTM Vegetation map Five-year mean high water level
(Year)
  
(Year)
     
(m NAP) 
1931 1935 2 36.
1963 1959 2.53
1992 1988 2.70
2004 2004 2 6.7
Bimodal distribution of intertidal elevations Predicting shifts from bare to vegetated based on threshold elevation
Vegetated intertidal marshes lie within a small range of high elevations while bare intertidal flats Pioneer patches are considered to initiate the shift from bare to vegetated state The probability          ,     
d i t b d f l l ti Th i t di t l ti l f t
            .   
f t bli h t f i t ti f th hift l l t d b d th d t fom na e a roa er range o  ower e eva ons. e n erme a e e eva ons are ess requen .  or es a s men  o  p oneer vege a on, proxy o  e s , was ca cu a e  ase  on e a a o  
2004.  
The probability map of getting vegetated was then generated for the four time steps The             .  
d ith th f l f d h b th f ll i ti t Aaccuracy was assesse w  e map o  new y orme  mars es y e o ow ng me s ep.  
high probability on the onshore flats mostly results in a shift to vegetation by the next time step.
100%
) 
(% (a) 1931
50cy
 
0r
ac
Rapid elevation shifts from bare flats to vegetated marshes c
u
0 <0.5 1 2 3A
c
The change from bare to vegetated state was always associated with a much more rapid
A Probability (%)                
increase of the peak of the elevation distribution and when it exceeded a threshold value              .
1931    
100)
 
(%
)
(b) 1963
50cy
 (  
ra
c
0
cu
r
0 <0 5 1 2 3A
cc .A
Probability (%)  
1963    
100)
 
(% (c) 1992
50cy
 (  
ra
c
0
cu
r
0 <0.5 1 2 3A
cc
A
Probability (%)  
1992    
2004   
(%)
Conclusion
Our study provided empirical evidences for the rapid non-linear shifts between low-lying bare high-elevated vegetated marshes, but low occurrence of intermediate elevations.             
flats and higher-elevated vegetated marshes in a macro-tidal intertidal ecosystem
        
2) The elevation shift from bare flats to vegetated marshes is relatively rapid         . 
1) The elevation distribution is bimodal with a high occurrence of low elevated bare flats and
            .   
3) The shift can be predicted as soon as a threshold elevation is exceeded     ,      -                  .
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Does alternative stable states theory and catastrophic shift theory apply for bare intertidal flats and vegetated marshes?

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