Ecosystem Change

If there is no struggle, there is no progress.Frederick Douglass

Ecological Succession

Progressive change in species composition, ecosystem function and structure following a disturbance Minor changes in structure and function accumulate

over time Ecological function of ecosystem changes

Initiated by a disturbance

Directional change in structure

Follows a predictable pattern

Successional Classifications (seres) Primary succession: establishment of plant

communities on newly formed habitats lacking plants Lava flows, sand dunes, landslides, etc.

Secondary succession: return to vegetation following a disturbance

There is some blurring of the classifications

Some examples Tornado levels a strip of forest

Trees are all gone, but seed bank is still there.

Secondary succession will follow

Severe fire burns through organic layer of soil and destroys the seed bank and nutrients Primary succession would proceed even

though plants were there before

Disturbances vary along with their impact

Succession as a Deterministic

Process Frederick Clements (1916)

Distinct steps…ends in a climax community

Clementsian Succession

Established species alter the environment Allow new species to move in and establish

Directional change in composition – aka. Community structure – is maintained by the continuous alteration of the environment

Within the community (specified): Succession always follow the same pattern as

it develops to climax community If interrupted, it will follow the path again. This is why considered deterministic

Stability and Climax Community In Clement’s view,

The concept of climax community assumes: The species colonizing and establishing themselves in a

given region can achieve stable equilibrium Stable equilibrium – forces to change system = forces to

keep system the same…no change results Therefore, the climate community = stable equilibrium

Clementsian Example

Hardwood trees, a climax community, has a disturbance in the form of deforestation.

The community will always proceed this way The transition will occur in predictable

manner.

Barring any further disturbances, the hardwood community will be reached and will be stable.

Is deterministic pathway the

only way? Idea of stable community fell out of favor

Individualistic Perspective (Gleason, 1926) The relationship between coexisting species (communities)

as the result of similarities in their requirements and tolerance to the environment.

Partly result of chance Succession is not deterministic

The challenge

Gleason argued that Clements explanation of succession could not explain retrogressive successions Plant community simplifies and loses biomass

over time

Clements vs. Gleason

Clements

Assume long term stability

Deterministic

Interdependence among species

Gleason

Assume environment can deteriorate over time

Random

No relationship between species – together because of similar env. requirements

Stability Refuted Succession viewed as a phenomenon that

rarely attains equilibrium

Equilibrium related to nature of disturbance Disturbance acts at variety of scales Magnitude of disturbance varies Many disturbances remove only part of the

previous plant community

Disturbance Variations in the definition (general

agreement): Any relative discrete event in space and time

that disrupts an ecosystem, community, or population structure and changes resources, substrate, or the physical environment – Pickett and White, 1985 Discrete in time (as opposed to chronic stress or

background environmental variability) Cause a notable change (perturbation) in the

state of the system

Consequences of Disturbance

Total habitat destruction

Creation of new habitat (transformation)

Fragmentation (loss of certain habitat, isolation of habitats)

Increase patch number, isolation, edge

Decrease patch size, connectivity, interior

Alter local climate/microclimate, hydrology, biota diversity, behavior, health, persistence)

The good in disturbances Dependent on temporal scale of analysis

Example: Forest fire

Short term = disturbance Long term = required to maintain seed bank and

regeneration of intermediate tree growth (pines)

• Intermediate disturbance hypothesis• Highest diversity

when disturbances occur at certain time intervals.

Succession Related to Biodiversity

An example Mt. St. Helens eruption – May 1980

Good place to see the different aspects of succession and disturbance at work.

The disturbance

Eruption Survival Survival of organisms was strongly influenced by

characteristics of disturbance processes, local site conditions, and biological factors

Pyroclastic flow and avalanche debris: Almost no organisms survived the blast Those that survived:

Plants with underground buds, burrowing animals, and organisms protected by snow, topography, or other features

Having diverse refuges facilitated survival of some organisms

Life history attributes: Many organisms not present yet

Anadromous fish – still at sea Migratory birds

Larger, local populations suffered higher mortality than smaller species or migratory species

Surviving groups included all of the primary trophic levels – herbivores, predators, scavengers and decomposers

Complex food webs quickly developed in the emerging ecosystems

Surviving species established new interactions and began to process the dead organisms from the pre-eruption system.

Eruption Survival

Timing

Eruption in early morning Allowed nocturnal animals to be protected in

subterranean burrows

Eruption in early spring Snow and ice created refuges and many

plants had not broken out of winter dormancy at higher elevations

Early successional stage of many recently harvested forest sites Profusion of wind dispersed seeds of pioneer

plant species