Ecological Resilience
An active learning module about
ecosystem change
Example Box-and-Arrow Model
Excessive force:
Drop, throw, smash
the egg
Spin the egg
Egg
Roll the egg
Heat
Hard-Boiled
Egg
Broken Egg
System
• A group of interacting or interdependent
parts
• Maintains its existence and functions as a
whole through the interaction of its parts
Your “seedpod” is a static system.
Ecosystems are dynamic systems,
with interactions that change over time.
Resilience
• The capability or degree to which an
ecosystem can resist perturbation and
remain within the functional boundaries
that characterize it without “flipping” to a
different set of functional boundaries.
• The amount of change a system can
undergo and remain in the same regime,
retaining the same structure, function &
feedbacks.
Resilience and Ecosystems
Resilient systems maintain
both properties and
processes when disturbed.
A resilient “seedpod” will
retain its structure AND
continue to hold the ball.
A resilient forest ecosystem
will retain its plant and
animal species AND
processes of nutrient
cycling and disturbance.
Do these sites have different properties or different processes?
Do these sites have different properties or different processes?
Resilience and Response Diversity
Which ping-pong ball is more vulnerable to disturbance?
Response diversity increases resilience of
ecosystems. Structures made out of only
one material are less resilient than those
made out of multiple materials.
Resilience
• A dynamic property of systems
• Not good or bad—depends on human values
– Dictatorships & eutrophic lakes highly resilient but
may not be desirable from certain human
perspectives
• Ability of a system to recover more important
than the speed of recovery
– Compare popsicle sticks and pipe cleaners
• Opposite of resilience = vulnerability
States
The state of a system is defined by
the values of the state variables
that constitute the system.
“Seedpods” made of the same
materials can be in different
configurations, but the state is
defined by whether the seedpod
does or does not hold the ball.
Ecosystems can be in different
states when they have different
species, or even when they have
the same species but the systems
function differently.
Alternate States in Ecosystems
• Forest:
• Forest, sufficient nutrients to support regeneration
• Cleared, eroded land, insufficient nutrients and soil
• Rangeland:
• Grassland system, nutrients evenly distributed
• Shrubland, nutrient islands and barren interspace
• Lake:
• clear water, abundant fish
• Murky water, fish die
Do these sites have different properties or different processes?
Do they represent alternate
stable states?
Threshold
Point at which a system
crosses into a new set
of stable states.
Levels of disturbance
that your “seedpod” can
withstand before it stops
holding the ball and
enters a new state
where it cannot hold the
ball.
Thresholds in Ecosystems
• Thresholds:
– Lake: amount of P in
sediments
– Australian
Rangeland: amount
of grazing pressure,
rainfall & fire
frequency
– Forest (Easter
Island): amount of
soil N
George et al 1992
Modelling Ecosystem Change: a State and Transition Model
for sagebrush & cheatgrass
Relatively high diversity native
plant community
Native
SBS
Degraded
SBS
T1
“Improved”
SBS
R1
II Weedy SBS
I Natural Sagebrush Steppe
R2
Cheatgrass dominates,
low diversity
T4
T3
T2
Sagebrush eliminated via
aerial spraying. Grasses
dominate, low diversity.
III Chemically Improved SBS
Thresholds, Regimes & States
• Threshold:
– Levels in controlling (slow) variables where feedback
to the rest of the system change
– Point at which a system crosses into a new “regime”
or set of stable states
• Regime:
– A set of states that system can exist in a still behave
in the same way—have the same basic structure and
function.
• State:
– The state of a system is defined by the values of the
state variables that constitute the system.
Thresholds & Alternate States
• Threshold:
– Lake: amount of P in sediments
– Australian Rangeland: amount of grazing pressure,
rainfall & fire frequency
– Forest (Easter Island): amount of soil N
• Alternate States:
– Lake:
• clear water, abundant fish
• Murky water, fish die
– Rangeland:
• Grassland system, nutrients evenly distributed
• Shrubland, nutrient islands and barren interspaces
– Forest:
• Forest, sufficient nutrients to support regeneration
• Cleared, eroded land, insufficient nutrients and soil