Community Ecology

Ecological Community

  • Definition: A group of species that coexist and interact within a defined area.

  • Characteristics:

    • Each species has its unique interactions with other species.

    • Communities can be studied based on energy flow and biomass.

    • Vary in size and scope, can be defined precisely or arbitrarily.

    • The number of species is determined by the available energy.

Primary Producers

  • Definition: Organisms that convert sunlight (or some inorganic molecules) into chemical energy, which is then used to form energy-rich organic molecules.

  • Also known as: Autotrophs.

  • Role: Primary producers are consumed by heterotrophs.

Gross Primary Production (GPP)

  • Definition: The amount of stored chemical energy produced from photosynthesis.

  • Key points:

    • Not all of GPP is available to consumers.

    • Some GPP is used by producers for maintenance and reproduction.

    • Net Primary Production (NPP): The portion of GPP that is available to consumers after producer expenditures.

Measuring GPP

  • Global GPP can be quantified through indirect means such as satellite technology that detects solar-induced chlorophyll fluorescence.

  • GPP levels can vary significantly from low to high (kgC/m²/year).

Trophic Levels and Food Chains/Webs

  • Trophic Levels: The different nutritional levels in a food chain or web.

    • Examples of trophic levels:

    • 1st Level: Primary producers (e.g., live and dead maple leaves)

    • 2nd Level: Primary consumers

    • 3rd Level: Secondary consumers

    • 4th Level: Tertiary consumers

    • 5th Level: Quaternary consumers

  • Interaction: Arrows in diagrams indicate the direction of energy flow from the consumed organism to the consumer.

Energy Transfer in Trophic Levels

  • 2nd Law of Thermodynamics: Energy transfer is never 100% efficient.

  • Reasons for energy loss at each trophic level:

    • Heat loss during energy transformation.

    • The digestibility of biomass: not all parts can be ingested or utilized.

    • Availability of food sources.

  • Ecological Efficiency: About 10% of energy at one trophic level is transferred to the next.

Pyramid Diagrams

  • Most communities support 3 to 5 trophic levels due to energy loss.

  • Higher trophic levels typically have fewer individuals with lower reproduction rates, smaller population sizes, and larger body sizes.

  • Much primary producer biomass in forests is not digestible, leading to fewer consumers supported.

  • High division rates in marine algae can support exceptionally high consumer populations.

Productivity and Species Richness

  • The amount of energy (sunlight) available limits overall productivity in communities.

  • Increasing water availability typically enhances productivity.

  • Paradoxically, extremely high productivity levels can lead to a decline in species diversity due to competitive exclusion.

Species Interactions and Community Structure

  • Trophic Cascades: The interactions of one consumer can lead to indirect effects on other species within the community.

  • Example: In Yellowstone National Park, overgrazing by elk depleted streambank trees, leading to erosion.

  • The reintroduction of wolves stabilized the stream banks by culling the elk population.

Keystone Species

  • Definition: Species that have a disproportionately large impact on their communities.

  • Functions:

    • Enhance species richness by increasing resources (food, space) that are limited.

  • Example: Sea otters control sea urchin populations, preventing overgrazing of giant kelp, which is vital for many aquatic species.

Diversity Index

  • Measurement: Species diversity can be quantified using the Shannon Index (H).

  • Interpretation: A high H value indicates high diversity based on the predictability of sampling a species.

  • Calculation: Sum of the proportion of total individuals that belong to each species.

Patterns of Diversity

  • Diversity can vary within a single community or between multiple communities.

  • Regions can exhibit different levels of diversity, categorized as alpha (within a community), beta (between communities), and gamma (regional).

Case Study: Freshwater Diversity in England

  • Comparison of rivers, ponds, and ditches.

    • Rivers: High species richness but low beta diversity (similar species).

    • Ponds: Variable species richness with high beta diversity (less overlap).

    • Ditches: Lowest alpha diversity but contribute to gamma diversity.

Determinants of Species Richness

  • Key Factors Influencing Diversity:

    • Geographic Location: Distance from the equator affects energy and water availability.

    • Size of Land Mass: Continents allow for easier migration, while islands restrict movement and, thus, diversity.

Island Biogeography

  • Population dynamics depend on:

    • Birth and death rates, immigration, and emigration.

    • Isolated populations, like those on islands, often have low immigration rates, resulting in limited diversity.

  • Larger islands tend to support more species due to the species-area relationship (more habitat available).

  • Equilibrium: Occurs when the number of species arriving (immigration) matches the number going extinct.

E.O. Wilson's Research on Island Species

  • Methodology:

    • Conducted surveys on small islands: counted all arthropods and followed recolonization post-fumigation.

  • Findings: Each island was able to regain original species richness after recolonization.

Disturbances in Communities

  • Definition: Events that can dramatically change community composition.

  • Examples of Disturbances:

    • Tree falls, floods, hurricanes, climate change, volcanic eruptions, fires.

    • Disturbances create new opportunities for species colonization.

Cyclical Disturbances

  • Certain species depend on disturbances for establishment.

    • Example: Weeds require light for germination.

    • Some seeds germinate only after fire events (e.g., lodgepole pines).

    • Karrikin is a compound found in smoke that enhances germination in various plants.

Succession

  • Definition: The process of community reassembly following disturbances.

  • Stages in Succession:

    • Starts with annual plants that quickly reproduce and contribute biomass.

    • Food webs become increasingly complex until reaching a stable community (climax community).

  • Types of Succession:

    • Primary Succession: Follows significant disturbances like glacier recession.

    • Secondary Succession: Occurs in less severe events like agricultural land abandonment.

Specific Succession Studies

  • Succession after Glaciation (Example from Alaska):

    • Sediment exposure timeline and colonizing plant species at different intervals (20, 100, 150-200 years).

Climax Forests

  • Eastern Deciduous Forests of the US: Characterized by species such as beech and sugar maple, adapted to grow in shade.

Climax Forests in Iowa

  • Dominated by white oak, bur oak, and shagbark hickory.

  • True climax forests are complicated by seedling shade intolerance, requiring disturbances to regenerate oaks.

Species Richness and Community Stability

  • Research Focus: Relationship between species richness, functional diversity, and ecosystem functions like net primary productivity (NPP).

  • Hypothesis: NPP increases with higher species richness and functional diversity.

  • Experimental Setup:

    • 289 experimental plots with various species and functional plant groups.

    • Measurement of aboveground biomass produced in each plot.

    • Functional groups studied include cool-season grasses, warm-season grasses, legumes, woody plants, and forbs.

  • Predictions:

    • NPP will increase with species richness and functional diversity.

  • Results & Conclusions:

    • NPP was observed to increase with both richness and diversity up to a certain point.

Low Diversity and Fragility of Communities

  • Case Study: The Midwest agriculture region, particularly concerning corn and soybean diversity.

  • Challenges: Low diversity with only two species makes communities fragile against pests.

  • Mitigation Efforts: Planting strips of prairie plants in agricultural fields to increase biodiversity and reduce nitrogen runoff.

  • Historical Example: Southern Corn Blight in the late 1960s led to significant loss of the corn crop due to susceptibility in widely planted varieties, resulting in a billion-dollar loss.