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

MaryPower

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, predator-prey

• 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 viewOf 1989Yellowstone 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