Lecture 3: Community and Ecosystem Ecology - Dynamics, Dynamics, Succession, and Biodiversity

Introduction to Week 13 Ecology: Communities and Ecosystems

This lecture, the third in the Week 13 series, focuses on how unique species interactions influence dynamics at the community and ecosystem levels.
Learning Objectives: Analyze the influence of species interactions on broader ecological scales and understand the organizational levels within ecology.

Levels of Organization in Ecology

Ecological systems are structured hierarchically. Prior lectures covered populations and species. This lecture elevates the focus to:

Communities: All populations of various species found within a single given location. Discussion at this level is limited to biotic (living) components.
Ecosystems: The highest level of organization, consisting of the community integrated with its physical environment. This includes all abiotic (non-living) features.
Abiotic features include:
Temperature ranges.
Water availability.
Nutrient availability (e.g., soil chemistry, nitrogen levels).
Sunlight and climate conditions.

Community Dynamics and Species Interactions

Communities are composed of populations that are closely tied together through various species interactions.
From Simple Interactions to Complex Webs:

Species interactions are often simplified into "two-player" models, such as predator-prey dynamics (e.g., a fox and a hare) or pollination (e.g., a flower and a pollinator).
However, in a community, these interactions form a complex food web. This web illustrates cascading effects, where an interaction between two species impacts numerous other organisms in the system.
Naming Communities:
Communities are typically named or described based on their dominant plants and animals.
Wetland Community: Characterized by marsh-like reedy grasses, frogs, amphibians, and reptiles.
Forest Community: Defined by the presence of trees.
Desert Community: Defined by cacti and lizards.
Community Boundaries:
There are no clear, discrete boundaries to a community because species ranges do not overlap perfectly.
Realized niches and the principle of competitive exclusion ensure that species are distributed uniquely across space.
Environments often feature "gray areas" where habitats blend (e.g., where a marshy wetland transitions into a forest habitat).

Keystone Species: The Arch of the Community

Definition: A keystone species is one that has a role in its ecosystem that is disproportionately more important than its abundance would suggest.
Architectural Metaphor: The term is derived from the keystone found at the top center of a bridge arch. Just as removing the keystone causes the arch to collapse, removing a keystone species can lead to the collapse of the entire community.
Case Study: Sea Otters and Kelp Forests:

Kelp forests are underwater communities that provide habitat for numerous fish and marine animals.
Kelp is the primary food for sea urchins; sea urchins are the primary food for sea otters.
Consequences of Removal: When sea otters are removed, the urchin population grows unchecked. The overabundant urchins consume the kelp forests, destroying the habitat and causing the ecosystem to collapse for all other associated species.

Ecosystem Functions: Integrating Biotic and Abiotic Factors

An ecosystem is the community plus the environment (Biotic + Abiotic).
Abiotic factors such as climate, light, and soil chemistry define how the system works and influence species distribution.
For example, a plant lives in the African Savannah not only because of its interactions with other organisms but because the sunlight and water levels are suitable for its survival.

Patterns of Organism Distribution and Dispersion

Organisms disperse within their environment in three predictable patterns driven by biotic and abiotic interactions:

Uniform Distribution:
- Individuals are evenly spaced across the environment.
- Driven by resource competition (e.g., trees in a forest needing specific amounts of water and light) and animal territorial behavior.
Random Distribution:
- There is no predictable pattern to where individuals are located.
- This is the rarest distribution pattern in nature.
- Examples include plants with wind-dispersed seeds (e.g., dandelions). It occurs when species interactions (like predator-prey) are weak and resources are abundant.
Clumped Distribution:
- Individuals occur in patches or clusters.
- Examples: Schools of fish, herds of elephants, or clusters of plants.
- Driven by patchy resource availability (e.g., specific nutrient clusters in soil) and social behaviors for mating or protection from predation.

Ecosystem Engineers

Definition: Certain species act as ecosystem engineers by significantly altering the physical (abiotic) environment, which in turn affects the community.
Example: Beavers:

Beavers build dams that change water systems, creating streams, lakes, and wetlands.
This modification alters the abiotic environment by changing water flow and soil (e.g., erosion, mineral/nutrient availability).
These changes support a wider variety of habitats and thus increase overall biodiversity.
Removing ecosystem engineers or their structures (like beaver dams) can have significant cascading effects on the ecosystem.

Disturbance and Ecological Succession

Disturbance: Any event that disrupts a community, removes organisms, or changes resource availability.
Natural Disturbances: Wildfires, heavy storms (tornadoes, hurricanes, windstorms), flooding, and droughts.
Human Disturbances: Open harvesting (deforestation, hunting), pollution, and urbanization.
Succession: The predictable process through which an ecosystem rebuilds itself following a disruption.
Types of Succession:

Primary Succession:
- Occurs after severe disturbances that wipe out everything, leaving only bare rock or soil (e.g., a volcanic eruption).
- This process is slow, often taking hundreds of years.
Secondary Succession:
- Occurs after disturbances that eliminate some species but leave the soil and some underlying community intact (e.g., a fire or windstorm).
- This process is faster, typically taking 5 to 20 years.
  The Successional Progression:

Pioneer Species: The first to return, typically small plants, mosses, lichens, and microbes.
Intermediate Species: Shrubs and small trees.
Climax Community: The final, stable stage of the ecosystem.

The Intermediate Disturbance Hypothesis

This hypothesis suggests that a moderate level of disturbance is ideal for maintaining the highest level of biodiversity.
Reason: Moderate disturbance eliminates some competition, preventing dominant species from taking over and allowing more species to persist.
Levels of Disturbance:

Weak/Mild Disturbance: Not enough to manage competition effectively.
Severe Disturbance: Prevents the system from ever reaching a stable climax community, leading to an unhealthy system.
Management: Natural resource managers use prescribed burns and selective harvesting to mimic intermediate disturbance.

Biodiversity: Levels and Importance

Biodiversity is defined on three levels:

Genetic Diversity: Variety of genes within a species. High genetic diversity prevents the "extinction vortex" and "inbreeding depression."
Species Diversity: The variety of different species within a community.
Ecosystem Diversity: The variety of ecosystem types in a region (e.g., prairies, forests, and wetlands in South Dakota).
Benefits of Biodiversity:

Drives productivity and nutrient cycling (health of the world).
Provides Ecosystem Services: Natural goods and resources with monetary value, including food, medicine, and recreational spaces.

Take-Home Messages

Populations influence species interactions, which shape communities, which then influence the whole ecosystem.
Understanding these levels is vital for managing global challenges:
- Climate change.
- Loss of species through extinction.
- Invasive species.
- Disease management (e.g., COVID-19, a virus that jumped from wildlife to humans).
Healthy, stable ecosystems are essential for providing clean air, water, and resources for human health and survival.