Ch 20: Succession and Stability in Ecological Communities and Ecosystems

Community & Ecosystems

Basic Features of Community Structure

Ecological communities are defined by the abundance and diversity of species, as well as the relationships among organisms at different trophic levels.

• Species abundance: The number of individuals of each species present.

• Species diversity: The variety and relative abundance of different species.

• Trophic relationships: The feeding connections among organisms (e.g., producers, consumers, decomposers).

Important Processes in Ecosystems

• Productivity: The rate at which biomass is produced by autotrophs (primary producers).

• Nutrient cycling: The movement and exchange of organic and inorganic matter back into the production of living matter.

• Succession: The process of change in the species structure of an ecological community over time.

Introduction to Succession

Definition and Types

Succession is the gradual change in plant and animal communities in an area following disturbance. It is a fundamental concept in ecology describing how ecosystems recover and develop over time.

• Primary succession: Occurs on newly exposed geological substrates (e.g., bare rock, sand, volcanic ash) where no soil exists. Pioneer species such as mosses and lichens colonize first.

• Secondary succession: Occurs after a disturbance (e.g., fire) that does not destroy the soil. Pioneer species arise from roots, seeds remaining in the soil, or seeds carried in from surrounding areas.

• Climax community: The late successional community that remains stable until disrupted by disturbance. Examples include temperate forests and grasslands.

Community Changes During Succession

Patterns of Change

Succession leads to predictable changes in community structure, including increases in species diversity and changes in species composition.

• Species diversity: Typically increases rapidly in the early stages of succession, then slows as the community approaches the climax stage.

• Species composition: The presence and abundance of different species change over time. Not all groups increase in density throughout succession.

Examples

• Glacier Bay (Primary Succession): Species richness increased rapidly in the early years and more slowly during later stages. Different plant groups reached maximum diversity at different times.

• Piedmont Plateau (Secondary Succession): Forests cleared and cultivated, then abandoned, showed increases in plant and bird species richness over time.

• Rocky Intertidal Communities: Succession occurred over months, with species richness leveling off quickly.

Summary Table: Succession Timing Across Communities

Community Type	Time to Climax
Intertidal (boulders)	1.5 years
Piedmont plateau	150 years
Glacier Bay	1500 years

Additional info: The timing of change in richness within a community is not necessarily the same for all growth forms.

Ecosystem Changes During Succession

Structural and Functional Changes

Succession is accompanied by predictable changes in ecosystem structure and function.

• Biomass: Total mass of living organisms increases.

• Primary production: Rate of production of new biomass by autotrophs increases.

• Respiration: Total ecosystem respiration increases as biomass accumulates.

• Nutrient retention: Ecosystems become more efficient at retaining nutrients over time.

Example Table: Ecosystem Changes at Glacier Bay

Successional Stage	Soil Depth	Organic Content	Nitrogen
Pioneer	Low	Low	Low
Alder	Medium	Medium	Medium
Spruce	High	High	High

Additional info: During succession, nitrogen, organic matter, and soil depth increase, while phosphorus content, pH, and bulk density decrease.

Recovery of Nutrient Retention

• After disturbance, succession allows for increased plant biomass and improved nutrient retention.

• Export of nutrients from the ecosystem decreases as succession progresses.

Mechanisms of Succession

Three Alternative Models

Succession can be explained by three main models: facilitation, tolerance, and inhibition.

• Facilitation: Early successional species modify the environment, making it more suitable for later species but less suitable for themselves. Example: Cedars make soils more acidic, favoring acid-tolerant species.

• Tolerance: Any species can colonize early; early species do not facilitate later species. Succession proceeds as species tolerant of the conditions establish and persist.

• Inhibition: Early species modify the environment to make it less suitable for both themselves and later species. Example: Allelopathic chemicals produced by oaks and walnuts inhibit growth of other plants.

Mechanisms Table

Model	Effect on Later Species	Example
Facilitation	Environment improved for later species	Cedars acidify soil
Tolerance	No effect; any species can colonize	Mixed colonization
Inhibition	Environment less suitable for all	Allelopathy in oaks

Case Studies

• Mt. St. Helens Eruption (1980): Disturbance set the stage for primary succession; complex blend of facilitation, tolerance, and inhibition observed.

• Glacier Bay (Deglaciation): Inhibition and facilitation affected spruce establishment during major successional stages.

Abiotic Factors Influencing Succession

• Size of disturbed area

• Distance from disturbed area to source population

• Severity of disturbance

Characteristics of Early and Late Successional Species

• Early successional species: Rapid dispersal, tolerance of harsh conditions

• Late successional species: Superior competitors over time

Community and Ecosystem Stability

Definitions and Concepts

• Stability: Absence of change; persistence of a community or ecosystem despite disturbance.

• Resistance: Ability to maintain structure and function in the face of disturbance.

• Resilience: Ability to recover after disturbance.

Understanding Stability

• Stability is poorly understood; factors influencing resistance and resilience are complex.

• No general patterns have been established, but scale matters.

Scale-Dependent Stability

• Stability may appear different depending on the spatial, temporal, or structural scale of measurement.

• Example: Park Grass Experiment (England, 150 years) showed stability at meadow scale, but variation at species level.

Levels of Stability

Scale	Observation
Large (meadow)	Very stable; persisted as meadows
Growth forms	Stable proportions of grasses, legumes, other plants
Individual species	High variation; some increased, some decreased, some unchanged

Complex Interactions

• Stability results from interactions between biotic and abiotic factors.

• Resistance and resilience may differ depending on the type of disturbance.

• Whether natural communities are stable depends on measurement criteria.

Additional info: Sycamore Creek, Arizona, is an example of complex stability dynamics in response to disturbance.