Download Complexity and Stability - Powerpoint for Nov. 2.

The Nature of Communities
and Ecosystems
Stability
A stable community or ecosystem is
one that has the ability to replace
itself – exist in place for more than
one generation
Components of Stability
2 major components:
1) resistance - the ability of a community or
ecosystem to avoid disturbance - how most
people think of stability
2) resilience - the speed with which a community
or ecosystem returns to its former state
following a disturbance that has displaced it
from its initial condition
Ecosystems and Stability
Grassland – South Africa
Rainforest – Puerto Rico
Additional Components of Stability
• Local stability describes the tendency of a
community to return to its original state
following a small disturbance
• Global stability describes the tendency of a
community to return to its original state
following a large disturbance
Adaptive Capacity of an Ecosystem
Adaptive Capacity of an Ecosystem
Adaptive Capacity of an Ecosystem
- Chesapeake Bay
Adaptive Capacity in 3D
Current Adaptive Capacity
From Local vs. Global Stability
• dynamically fragile - a community which is
stable only within a narrow range of
environmental conditions
• dynamically robust - a community which is
stable within a wide range of environmental
conditions
Complexity and Stability
Current understanding - no clear relation
between complexity and stability
1) complex, fragile communities of relatively constant
environments (the tropical rainforests) are more susceptible to
outside, unnatural disturbances than are simpler, more robust
communities which experience regular climatic fluctuations
(most temperate communities)
2) In stable environments you would expect to find K selected
species (high competitive ability, high survivorship, low
reproductive output) and such species will resist disturbance
3) In unstable environments you would expect to find r selected
species (low competitive ability, low survivorship, but high
reproductive output) that have little resistance but high
resilience
Why is the World Green?
Boreal Forest Outlined in Green
Spiny Water Flea
Spiny Water Flea Invasion
Spiny Water Flea Current
Distribution
Spiny Water Flea Food Web
Mary
Power
Eel River with steelhead
Eel River without steelhead
Direct and Indirect Effects
• Direct effects - effect of 1 species on another
resulting from physical interaction between the
two – interference competition, inadvertant
interference, mutualisms, parasitism, predatorprey
• Indirect effects - an effect of one species on
another that is not caused by a physical
interaction between the two - these can only
happen when more than two species are
present
Pisaster starfish
Pisaster and Mytilus
californianus
Food web with Pisaster
Yellow – predator; red – filter feeder; blue – grazer; green - algae
Mytilus californianus
Food web without Pisaster
Yellow – predator; red – filter feeder; blue – grazer; green - algae
Strong vs. Weak Interactors
1) Non-interactors - species does not affect population
of those species with which it interacts
2) weak interactors - species only influences those
species with which it interacts directly - effects may
be large
3) strong interactors - species that directly and
indirectly effects other species - these species are
the most important in the community or ecosystem
because a change in their numbers may cause
changes in the entire ecosystem – keystone species
Aerial view
Of 1989
Yellowstone
Fire
Disturbance and Succession
Disturbance
• Disturbance - any agent which causes complete or partial
destruction of the community resulting in the creation of bare
space
• Disturbance agents: both physical and biological processes
may cause disturbances, though we usually focus on physical
processes • Physical - fires, ice storms, floods, drought, high winds,
landslides, large waves
• Biological - severe grazing, predation, disease, things that
inadvertently kill organisms - digging and burrowing
Wind Damage –
July 4, 1999 Derecho
Wildfire – Southern California
October 22, 2007