Chap17

CHAPTER 17: SPECIES INTERACTIONS AND COMMUNITY STRUCTURE

Definitions and Key Concepts

  • Community: A group of interacting species that occur together, inhabiting a common area.

    • Interactions: Predation, competition, mutualism, and other forms that influence species coexistence and community dynamics.

  • Community Structure: Refers to the various attributes describing a community, including:

    • Number of species

    • Relative abundance of species.

    • This structure is shaped by interactions among species within the community.

Food Webs

  • Food Web Definition: A summary of the feeding interactions within a community, providing an overview of energy and nutrient fluxes among biotic components of an ecosystem.

  • Community structure is influenced not just by competitive interactions but collectively by all species interactions within the community.

Complexity of Food Webs

  • Food webs can exhibit high complexity:

    • Example: A food web with only 10 fish species.

    • Removal of weak feeding relationships can simplify the community structure but still leaves it complex.

    • Food webs can vary over time and in different spatial contexts.

Food Chains vs. Food Webs

  • Food Chain: A sequence that outlines energy and nutrient flow in one specific pathway through a food web.

  • Energy transfer between trophic levels is generally low, about 10% efficiency, leading to pyramidal structures (both in energy and numbers).

Strong and Weak Interactions

  • Strong Interactions (Strong Links): Trophic connections that significantly influence community structure.

    • They affect abundance and diversity and may not always represent the greatest flow of energy.

  • Weak Interactions: Less impactful connections on community structure.

  • Foundation Species: High biomass species that substantially influence community dynamics.

Food Web Characteristics

  1. Food Chain Length:

    • Defined as the number of trophic links in the longest food chain.

    • Typical lengths are short (3-4 links).

    • Influenced by:

      • Primary Productivity: Limits energy available at the base trophic level.

      • Trophic Efficiencies: Governed by thermodynamic principles, restricting energy transfer.

      • Optimal Foraging: Evolutionary pressure encouraging feeding on lower trophic levels for better energy availability.

  2. Connectance (C):

    • Formula: C = \frac{n}{S(S-1)}, where:

      • n = number of links,

      • S = number of species.

    • Represents the percentage of existing links in a food web compared to possible links.

  3. Compartmentalization:

    • Larger food webs are often divided into compartments with strong interactions within but weak interactions between compartments.

  4. Stability:

    • Refers to the ability of a community to persist over time, influenced by:

      • Food chain length,

      • Connectance,

      • Compartmentalization.

Types of Food Webs

  • Connectedness Web: Shows the feeding relationships among organisms.

  • Energy Flow Web: Quantifies and shows connections based on energy flux.

  • Functional Web: Highlights the influence of populations on the growth rates of other populations.

Predator-Prey Dynamics

  • Fluctuating Environment: Consumers may rely on brief, intense feeding bouts, leading to ephemeral connections that may be weaker.

Direct and Indirect Interactions in Food Webs

  • Direct Interactions: Include predation, herbivory, and competition within a single trophic level.

  • Indirect Interactions: Occur when one species impacts another through a third species.

    • Examples include indirect commensalism, effects across trophic levels.

Apparent Competition

  • A phenomenon where:

    • Negative effects arise when two species share a predator or herbivore.

    • One species increase the population of a predator affecting another.

Keystone Species

  • Definition: A species that significantly controls community structure despite potentially low abundance.

    • The keystone concept, studied by Robert Paine in the 1960s, emphasizes that predators can enhance biodiversity.

    • Example: The predatory sea star increases overall diversity by controlling mussel populations.

Community Structure and Consumer Activities

  • The feeding activities of influential consumers control community structure but require an understanding of food web structure.

  • Example: The impact of grazing snails (Littorina littorea) on algal diversity varies with their density.

Top-Down vs. Bottom-Up Effects

  • Top-Down Control:

    • Influenced by consumers at higher trophic levels (e.g., keystone species).

    • Trophic cascades illustrate how changes in predator biomass affect lower trophic levels.

  • **Bottom-Up Control:

    • Driven by nutrient availability and primary production effects on community dynamics.

The Debate on Trophic Cascades

  • Trophic Cascade Model: Suggests that manipulating the biomass of top predators leads to cascading effects down to phytoplankton.

    • Example: Reduced piscivore (predatory fish) biomass leads to increased herbivore biomass and decreases in phytoplankton biomass.

Fishing Impact on Marine Food Webs

  • Research by Daniel Pauly highlights the decline of mean trophic levels in fisheries from 1950 to 1994, which indicates unsustainable fishing patterns, particularly evident in the Northern Hemisphere.

Summary of Bottom-Up Control

  • Bottom-up processes underpin the existence of consumer dynamics; however, both top-down and bottom-up perspectives should be integrated for a comprehensive understanding of ecosystem dynamics.

Productivity Factors in Ecosystems

  • Soil Moisture & Nitrogen: Control forest productivity affecting invertebrate and songbird populations through bottom-up effects.

  • Forest landscape topography significantly influences resource distribution and, subsequently, ecosystem interactions.