Spatial ecology
Alternate stable states and
restoration of intertidal systems
Johan van de Koppel
Netherlands Institute of Ecology (NIOO-KNAW)
Centre for Estuarine and Marine Ecology
Spatial ecology
Netherlands Institute of Ecology
Yerseke
Spatial ecology
This talk
• Restoration & alternate stable states
• Shallow lakes, for some background
• Salt marshes as a new case study
• Alternate stable states in non-pond
ecosystems.
Spatial ecology
Restoring ecosystems
Pristine
state
X
Degrade
d
state
• Human interference degrades ecosystem
• Restoration often involves reversal of interference
• Ecosystem may stay in degraded state despite of this
reversal
Spatial ecology
Classical example: lake ecosystems
Spatial ecology
Classical example: lake ecosystems
Spatial ecology
Alternate stable states
Increased phosphorous loading
X
Positive feedback
Spatial ecology
(a simplified view)
Increased
turbidity
+
+
Increased algal
growth
Increased
phosphorous input
Decreased growth
of submergend
vegetation
+
From Scheffer et al,
Nature 2001
Spatial ecology
Catastrophe fold
From Scheffer et al,
Nature 2001
Spatial ecology
Typical features
• Bistability: two states are stable
• Sudden shifts: small perturbation lead to
dramatic changes
• Hysteresis: degradation route differs from
recovery root
• Bimodality: state variables cluster around two
values when measure repeatedly in time or space
• Positive feedback: changes accelerate
Spatial ecology
Salt Marshes
• Salt marshes as a case study
• Decline of marsh area in Westerschelde
• Restoration a big issue in Zeeland
. .
Midden-Subatlanticum
Midden-Subatlanticum
Early Roman
Roman Times
Times
Spatial ecology
50
50 AD
Tijdsinterval
Tijdsinterval
250
250 jaar
jaar
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kaart 11
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... ....
kaart 10
Noordzee, zeegaten
en getijdegeulen
.
Rivieren
.
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Lagunes
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
.
Archeologische
vindplaatsen
.
Midden-Subatlanticum
Midden-Subatlanticum
Late
Late Roman
Roman Times
Times
Spatial ecology
350
350 AD
AD
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..
Tijdsinterval
Tijdsinterval
150
150 jaar
jaar
kaart 13
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kaart 12
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Noordzee, zeegaten
en getijdegeulen
Rivieren
.
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Lagunes
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
.
Archeologische
vindplaatsen
41
Midden-Subatlanticum
Midden-Subatlanticum
Merovingian
Merovingian Time
Time
Spatial ecology
500 AD
kaart 13
Tijdsinterval
Tijdsinterval
150
150 jaar
jaar
.
kaart 14
Noordzee, zeegaten
en getijdegeulen
.
Rivieren
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Lagunes
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
.
Archeologische
vindplaatsen
43
Laat-Subatlanticum
Laat-Subatlanticum
Middle
Middle Ages
Ages
Spatial ecology
1000
1000 AD
AD
.
Tijdsinterval
Tijdsinterval
250
250 jaar
jaar
kaart 15
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kaart 16
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Noordzee, zeegaten
en getijdegeulen
Rivieren
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Inversieruggen in het
schorrenlandschap
Kustveenmoeras
.
Hogere gronden: Pleistoceen zand aan maaiveld
.
Archeologische
vindplaatsen
47
.
.....
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Laat-Subatlanticum
Laat-Subatlanticum
Late Middle
Middle Ages
Ages
Spatial ecology
1250
1250 AD
AD
.. . ..
.... . ... .. kaart 16
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Tijdsinterval
Tijdsinterval
250
250 jaar
jaar
kaart 17
. . ..
.
.. ....... .......
.. .....
. ..
.
..
.
Noordzee, zeegaten
en getijdegeulen
Rivieren
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
Bedijkt gebied
49
Laat-Subatlanticum
Laat-Subatlanticum
Modern
Modern Times
Times
Spatial ecology
1530
1530 AD
AD
kaart 17
Tijdsinterval
Tijdsinterval
280
280 jaar
jaar
kaart 18
Noordzee, zeegaten
en getijdegeulen
Rivieren
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
Bedijkt gebied
51
Laat-Subatlanticum
Laat-Subatlanticum
Modern Times
Times
Spatial ecology
1950
1950 AD
kaart 19
Tijdsinterval
Tijdsinterval
200
200 jaar
jaar
kaart 20
Noordzee, zeegaten
en getijdegeulen
Rivieren, zeearmen,
kanalen
Strandwallen en duinen
Getijdegebied: platen,
slikken en schorren
Kustveenmoeras
Hogere gronden: Pleistoceen zand aan maaiveld
Bedijkt gebied
55
Spatial ecology
Spatial ecology
Problems with the Westerschelde
Spatial ecology
Eroding cliffs in salt marshes
Spatial ecology
Restoring the Westerschelde
The effects of dredging have to be compensated
(EU-directive)
Measures:
•Retreat: “ontpolderen”
•Building dams to reduce wave & flow effects
•Effects are mixed: “pioneer zone” doesn’t reestablish quickly
Spatial ecology
Feedback processes in
salt marshes
Research of
Bregje van Wesenbeeck
Feedback processes
on salt marshes
Spartina
+
Higher elevation
More nutrients
Decreased erosion
Increased sedimentation
+
Spatial ecology
Sediment
Predictions of a
simple feedback model
Spatial ecology
transition zone
Spartina monoculture
bare mudflat
Plant biomass
10
5
0
1
2
Tidal
inundation stress
Environmental
stress
3
How to test for
alternate stable states
• Bimodality
Equilibrium
vs.
Biomass
Biomass
• Threshold effects
Biomass
Spatial ecology
Threshold
time
• Bistability
time
Spatial ecology
Bimodality in salt-marsh vegetation
Biomodality in vegetation biomass
0.12 A: marsh
Hypothesis: plant biomass
follows a biomodal distribution
in the pioneer zone.
0.08
0.04
0
0.2 0
0.2 0.4
0.12 B: pioneer zone
Frequency
Spatial ecology
0.08
Figures:
Biomass (deviation from
average) vs frequency
0.04
0
0.3
0.2 0
0.2 0.4
C: tidal flat
Clear evidence for bimodality!
0.2
0.1
0
125 meter
Biomass estimated
using NDVI
0.2 0
0.2 0.4
NDVI
Spatial ecology
Are there biomass thresholds?
Hypothesis:
The growth and survival of Spartina anglica (pioneer plant)
increases with biomass
3 biomass categories,
reflecting life-history stages:
• Seedlings
• Rhizomes
• Tussocks
Spatial ecology
Tussock development
A: Paulinapolder
B: Krabbekreek
100
rhizomes
seedlings
tussocks
% survival
80
60
40
20
0
May
Sep
Jan
May
Sep
Jan
May
time ( months)
Van Wesenbeeck et al submitted
Sep
Jan
May
Sep
Jan
Spatial ecology
Stability?
Spatial ecology
Studying stability
Hypothesis: Vegetation patches remain stable
once the switched to the high-biomass state.
Testing: overlay analysis of remote sensing
pictures covering an extensive period of salt
marsh development.
Spatial ecology
GIS study of salt-marsh development!
GIS analysis by
Daphne van der Wal,
NIOO - CEME
1982
1998
Spatial ecology
Stability of vegetation patches
A: 1982-1998
B: 1998-2004
Spatial ecology
Lessons to be learned
Salt marshes revealed bimodality and threshold effects,
but no conclusive evidence for alternate stable states!
• Concept of alternate stable states is a simplification, can
only explain ecosystem dynamics in part.
• Local feedbacks vs. large-scale feedbacks
• It’s validity depends on definition of appropriate spatial
and temporal scales.
• Questions on whether alternate stable states exist or not
will never be solved if we don’t address the issue of
spatial scale.
Spatial ecology
Dynamics in space
Interactions between water
& vegetation
Spatial ecology
Intertidal flat
positive
Gully
0
negative
Feedback effect
Tussock
Distance
30
25
Research of
Bregje van Wesenbeeck
A
AB
biomass (grams)
B
20
competition
15
C
no competition
10
5
=> Scale-dependent feedback
0
0
0.5
distance classes
4
Spatial ecology
Salt marsh
Development
Spatial ecology
Modeling spatial feedback
• Explicit hydrodynamics: Delft 3D
• Linked to vegetation model
• Simulation for 30 years
• Includes scale-dependent feedback
positive
Gully
0
negative
Feedback effect
Tussock
Distance
Intertidal flat
Spatial ecology
Modeling spatial
feedback
Modeling by Stijn Temmerman
Spatial ecology
Feedbacks in space
• Spatial feedback is important in salt-marsh development
& restoration
=> Can block development
• Positive feedback does much more that generate
alternate stable states: important factor determining
the spatial structure of a salt marsh
• What about the cliffs?
Spatial ecology
Spatial dynamics on a really
large scale
Spatial feedbacks may lead to complex
ecosystem dynamics.
Salt marsh: making a simple model
Plants
+
Schematic cross-section through a salt marsh
Wave erosion
+
Spatial ecology
Silt/Clay
Dune
or
Dike
Flow erosion high
Assumptions:
• Positive feedback between sediment erosion and
plant growth & erosional losses.
• Erosion of sediment due to (tidal) currents is high
at the seaward edge (left), and low at the dike/dune
edge of the salt marsh.
• Wave erosion is more severe on sloping sediments
Flow erosion low
Spatial ecology
Modelling salt-marsh development
Sedimentstion (I) and erosion
(2 nd
term)
Erosion dependent
on slope
dS
dt
a
dS
(
)
I − emax ⋅
⋅τ ⋅ 1 − α ⋅x ⋅S − dS ⋅ ⋅S
P+ a
dx
dP
P
S
c dS

r ⋅ 1 −  ⋅P ⋅
− m ⋅P − dP ⋅
⋅ ⋅P
c + P dx
 K S + b
dt
Plant growth
Plant mortality
Wave destruction
dependent on slope
Spatial ecology
Simulated salt marsh development
Spatial ecology
Simulated salt marsh development
Disturbance
(e.g., a storm)
Spatial ecology
Model results
Complex dynamics of salt marshes can be explained by a
simple feedback between sedimentation and plant growth.
Effects of positive feedback:
•
Good: Buffering of physical
gradient.
•
Bad: increased vulnerability:
small disturbances lead to runaway collapse.
Van de Koppel, Van der Wal, Bakker and Herman, AmNat 2005
Spatial ecology
Empirical testing
Prediction 1: Salt marshes become increasingly steep at their
seaward edge.
Analysis of marsh cross-sections of a developing marsh
confirms prediction (Jan Bakker, University of Groningen)
Prediction 2: Cliff erosion and regrowth in front of the cliff
can occur simultaneously.
GIS analysis of salt marshes in the Westerschelde confirms
this prediction (Daphne van der Wal)
Van de Koppel, Van der Wal, Bakker and Herman, AmNat 2005
Spatial ecology
Do salt marshes develop as predicted?
Clay thickness was
measured along 33
transect across the
salt marsh.
Data: Group of Jan
Bakker,
RU Groningen
Spatial ecology
Positive feedback and salt marshes
Clay layer thickness
30
Data Harm van Wijnen
& Jan Bakker, RU Groningen
Clay layer thickness (cm)
25
20
40 years
15
30 years
10
20 years
5
10 years
0
0
200
400
600
800
1000
1200
Distance from the alt-marsh edge
1400
1600
1800
Positive feedback in a spatial context!
Salt marsh becomes steeper with age
0.12
Average slope of salt marsh
Spatial ecology
y = 0.002x - 0.0181
2
R = 0.9295
0.10
0.08
0.06
0.04
Data Harm van Wijnen
& Jan Bakker, RU Groningen
0.02
0.00
0
10
20
30
40
Age
50
60
70
Spatial ecology
Positive feedback in a spatial context!
Data from Schiermonnikoog
confirms that the salt marsh edge
becomes steeper with age.
Spatial ecology
GIS study of salt-marsh development!
GIS analysis by
Daphne van der Wal,
NIOO - CEME
Hellegat polder
1982
Spatial ecology
Simultaneous erosion and regrowth!
Spatial ecology
Salt marsh development and
contributing forcing factors
Vegetation change per unit area
HP
TP
PP
ZG
BL
0,6
1982-1998
Are salt-marsh dynamics & structure
only determined by forcing factors?
• Tidal currents
• Proximity of channels
Or is there intrinsic forcing?
=> self-organization?
0,2
Area (ha/ha)
• Wave energy
0,4
0
-0,2
-0,4
Vegetation loss
-0,6
-0,8
Vegetation expansion
HP
BH
ZP
Alternate stable states
& salt marshes
Spatial ecology
• The concept of alternate stable states can only be used as
an approximation of salt-marsh dynamics.
• Positive feedback may generate dynamics that are far
more complicated than that of alternate stable states
models.
• New concepts of the implications of positive feedback are
required, that take into account space (unless you work in
small ponds)