Species Interactions in Communities

Indirect Interactions
Trophic Cascades
Trophic Facilitation
Competitive Networks
Predator Mediated Coexistence
Interaction Strength
Species with Community Level Impacts
  - Foundation Species
  - Ecosystem Engineers
  - Keystone Species

In communities, multiple species interactions generate a multitude of direct and indirect connections.
Direct Interactions:
  - Interaction between two species including trophic and non-trophic interactions.
    - Examples include:
      - Predation
      - Competition
      - Mutualism
      - Parasitism
Indirect Interactions:
- Relationship between two species is mediated by a third (or more) species.

Indirect interactions can have substantial effects on community structure.
Often identified when species are experimentally removed from a system to study the impact of direct interactions.

Effects of species removal:
- Can reveal indirect interactions and community responses.
Trophic Cascades:
  - Rate of consumption or abundance at one trophic level results in changes in species abundance or composition at multiple other trophic levels.
  - Example: In the Anacapa marine reserve, abundant lobsters keep their urchin prey in check, allowing kelp forests to thrive.
  - Data from Behrens & Lafferty (2004) showcased significant effects:
    - Reserve: 26% cover with 2.4/m² of urchins and .012/m² of lobsters.
    - Fished Area: 5% cover with 32.8/m² of urchins and .002/m² of lobsters.

Trophic Cascades in Salt Marsh Ecosystems:
- Reference link: https://www.youtube.com/watch?v=b9xVEeYAs3w

White arrows: Flow of energy
Blue arrows: Direction of trophic control
Plus/Minus signs: Indicate changes in biomass or abundance at each trophic level

Definition: A consumer is indirectly benefited by a positive interaction between its prey and other species.
Example: Commensalism illustrated, where experiments indicated that Juncus enhances the photosynthetic rates of Iva.
- Results: Photosynthetic rates of Iva were lower without Juncus, but removing Iva had no effect on Juncus.

Definition: Competitive interactions among multiple species where every species negatively affects every other species.
- This dynamic might enable competitive coexistence and maintain species richness.
Concept Example:
  - Species A can outcompete Species B.
  - Species B can outcompete Species C.
  - Species C can outcompete Species A, leading to no single species dominating.

Anticipated changes due to climate change include:
  - Increased ocean acidification, negatively impacting calcifying organisms (e.g., corals, mollusks, crustaceans).
  - Higher CO2 concentrations may benefit primary producers.
  - Increased ocean temperatures will affect growth rates and thermal stress levels.

Definition: Interaction strength is the effect of one species on the abundance of another species.
Measured by experimenting with the removal of individuals of one species (interactor species) and measuring the effects on other species (target species).
Metric: Relative interaction intensity (RII) calculated with the formula: $RII = rac{(C - E)}{(C + E)}$
  - Where:
    - C = Number or biomass of target individuals in the presence of the interactor (Control).
    - E = Number or biomass of target individuals in the absence of the interactor (Experimental).
Outcomes:
  - RII will be negative if there is a direct negative interaction (predator or competitor) or an indirect negative interaction.
  - RII will be positive for positive interactions.
  - Strength of interaction determined by the absolute value of RII.

Researchers experimentally removed gulls and collected data for RII calculation:
Using the formula: $RII = rac{(C - E)}{(C + E)}$
  - Example results:
    - Calculation details:
      - RII calculation showed multiple values including:
  - (10-100)/(10+100) = -90/110 = -0.81
  - -2500 / 3500 = -0.71
  - 50 / 150 = 0.33
  - 500 / 5500 = 0.09
  - 400 / 600 = 0.67

Foundation Species:
- Define as species that provide biotic habitat and food for other species.
- Examples include trees, corals, seagrass, oysters.
Ecosystem Engineers:
- Define as species that create, modify, or maintain physical habitats.
- Examples include trees, beavers.
Keystone Species:
  - Define as species with effects disproportionate to their size or abundance, often via indirect effects.
  - Examples include sea otters, sea stars, beavers.
  - The designation of keystone status can depend on the context.

Trees as Foundation and Ecosystem Engineers:
  - Provide habitat through leaves, branches, and trunks.
  - Facilitate nutrient richness in the forest environment, nurturing invertebrates and other biota.
  - Stabilize the forest floor through root systems.
Beavers as Keystone Species:
- Notable for their ability to rapidly change environments, leading to increased wetland areas which benefit multiple other species.

Sea stars, specifically Pisaster ochraceus, exhibit classic keystone species behavior.
Effects of their Removal:
- Consequences include decreased species richness and increased algae cover.
- This highlights their critical predatory role in coastal ecosystems.

Definition: Predator-mediated coexistence indicates that predation reduces dominant competitors, thereby facilitating coexistence among various species.

Core Concepts Covered:
  - Indirect interactions
  - Trophic cascades
  - Trophic facilitation
  - Competitive networks
  - Predator-mediated coexistence
  - Interaction strength
  - Species with community-level impacts including foundation species, ecosystem engineers, and keystone species

Open floor for questions regarding species interactions in communities and their implications.