WK9 - COMMUNITIES & ECOSYSTEMS: Part 1: Food Webs

Communities are studied at a larger scale than individuals or populations.
Food webs represent the trophic structure of a community, focusing on feeding and nutrition (who eats whom).
Species within a community are interconnected through feeding relationships, forming a network known as a food web.
The specific arrangement of these connections can vary across different biological communities over time and in different environments.

A food chain is a linear sequence illustrating the transfer of energy between species.
Typically starts with an autotroph or primary producer (e.g., plants) that obtains energy from the sun.
Examples:
- Tree (autotroph) → Aphid (primary consumer) → Spider → Warbler (bird) → Hawk (predatory bird)
Food webs are sets of interconnected food chains.
A food web contains multiple food chains with species that can occupy multiple trophic levels.

Nodes: Individual species within the food web.
Feeding Links: Connections between species, representing who eats whom.
Trophic Levels: Hierarchical levels based on feeding relationships (primary producers, primary consumers, secondary consumers, etc.).
Predators: Typically at the top of the food web.
Basal Resources: Primary producers at the bottom of the food web.

Food webs can be analyzed by looking at:
- The flow of energy, nutrients, or matter through the community.
- Web structure (linkages, omnivory).
- The impact of different species (identifying keystone species).
Previous lectures introduced the ant fungus microbe food web:
- Plants are harvested by ants (negative interaction for plants).
- Ants cultivate fungus (positive interaction for both).
- Pathogenic fungus attacks cultivated fungus (negative for cultivated fungus, positive for pathogenic fungus).
- Bacterium on ants produces antibiotics to kill pathogenic fungus, acting as a hero in the system.

Chain Lengths: How long interaction chains are in a community.
*Characterizing Major and Minor Pathways:
*Marine food web example illustrates characterizing energy flows as major or minor.
*Blue arrows represent major flows; black arrows represent minor flows. Simplification involves focusing on major energy flow pathways.
Interaction Specialization:
*Interaction frequency and specialization determine energy flow importance.
*Species dependent on single prey are specialized (e.g., baleen whales on krill), while generalist omnivores consume various prey.
Flow of Matter:
*Instead of energy, matter flow can be analyzed using grams per meter squared per year ingested.
*Dominant colors indicate common matter transfer levels.
*Removing weak interactions can simplify food web diagrams, but clarity isn't guaranteed.

Guilds are assemblages of species exploiting a common resource in a similar way.
Examples:
- Sticky-tongued frogs consuming flying insects.
- Web-spinning spiders trapping flying insects.
Simplification by lumping species based on what they eat and how they eat it, not just taxonomy.

Number of Nodes: Number of species in the food web.
Trophic Level: Position of a species in the food web (primary, secondary, etc.).
Interaction Type: Herbivory, predation, parasitism, cannibalism.
Quantitative Measures:
- Ratio of possible to realized interactions.
- Interaction strength.

Connectance: A measure of the density of cross-linkages in a food web.
Linkage Density: A measure of trophic links per species (links/node).
Connectance = (Actual # of feeding links) / (Possible # of feeding links)
Linkage Density = (Number of actual interactions) / (Number of species)
Connectance in food webs do not necessarily increase as you increase the number of species.

Energy Limitation Hypothesis:
- Energy transfer between trophic levels is inefficient (approximately 10% efficiency).
- For $L$ links, available energy is $0.1^{L-1}$ . With five links, available energy at the top level is 0.0001.
Energy and Space Hypothesis:
- More productive habitats should support longer chains.
- Larger habitats support longer chain lengths.

The Productivity Space Hypothesis combines energy and space and suggests
*For large ecosystems:
*More space and energy allow a longer maximum chain length
*For small ecosystems:
*Lack of space eliminates the potential longer chain length which would've been reached with high productivity.

Population fluctuations at lower trophic levels can be magnified up the food chain.
Top predators are exposed to large fluctuations, increasing their risk of extinction and limiting food chain lengths.
Dynamic instability makes it hard for predators because of their too variable and dynamic food source.

Omnivores feed at multiple trophic levels.
Historically, omnivory was thought to be rare due to the dynamic instabilities it could cause.
Later studies show that omnivory is more common than previously thought.
Omnivory is when a species feeds on multiple trophic levels leading to competition within various levels and doesn't necessarily lead to food web instability.

Species with a disproportionately large impact relative to their biomass or abundance.
Distinguished from "dominant" species that have a large impact due to their high biomass.
- Dominant species have high biomass and impact due to their abundance.
- Keystone species have a large impact relative to their biomass or abundance.
Example: Sea stars (Pisaster) in intertidal food webs.

Robert Payne's classic experiment:
- Removed sea stars from intertidal zones and observed significant changes.
- Without sea stars, bivalve and barnacle populations exploded, leading to a collapse in species diversity.
Mechanism is Competition for Space:
- Sea star removal changes food web structure and decreases raw numbers of species.
- Removal leads to predatory escape of bivalves and gooseneck barnacles, resulting in community dominance by bivalves.