Ecosystem Ecology: Food Webs and Nutrient Cycling

Course Information

  • Course: PCB 3043

  • Term: Spring 2026

  • Week 15 - Tuesday

Announcements and Reminders

  • Seminar Summary: Due Today (Tuesday) Apr 21 at 11:59 PM.

  • Course Evaluation: Please complete! Feedback is very valuable.

  • Final Exam (Midterm #3): Scheduled for Tuesday, April 28, from 7:30 - 9:00 AM in King 1024.   - Covers material from Communities and Ecosystems units (March 26-April 23).   - Study guide will be posted on Canvas by Thursday.   - Bring a pen or pencil!

Last Week's Class Summary: Ecosystem Ecology

  • Topics Covered:   - Ecosystem Production:     - Primary production     - Controls on Net Primary Production (NPP)     - Global patterns of NPP     - Secondary production   - Energy Flow:     - Trophic levels     - Trophic transfer efficiency     - Energy and biomass pyramids     - Bottom-up and top-down controls

Learning Check

  • Figure Quiz: Identifies types of pyramids or webs in ecology. Options included:   - a) Energy pyramid   - b) Biomass pyramid   - c) Energy or biomass pyramid   - d) Food web   - e) NPP web

Trophic Structure in Ecosystems

(A) Terrestrial Ecosystems

  • Trophic Levels:   - Secondary carnivores   - Primary carnivores   - Herbivores   - Primary producers

(B) Aquatic Ecosystems

  • Similar trophic levels mapped out with specific species illustrated:   - Cutthroat trout (Oncorhynchus clarkii)   - Tui chub (Siphateles bicolor)   - Zooplankton   - Phytoplankton

Top-Down vs Bottom-Up Control

Comparison

  • Bottom-Up Control:   - Resource availability (e.g., nutrients) increases NPP.   - Example: Addition of nutrients leads to increased herbivore populations.

  • Top-Down Control (Trophic Cascade):   - Predators influence the structure and abundance of lower trophic levels.   - Example: Adding cutthroat trout affects secondary carnivores and herbivores.

Trophic Cascades – Generalizations

  • Changes in abundance of top specialist predators lead to trophic cascades.

  • Omnivory can buffer these effects.

  • Trophic cascades more typical in simpler ecosystems, especially aquatic ecosystems.

Terrestrial Trophic Cascade Example

  • Location: Costa Rica

  • Experiment Overview:   - Studied impacts of beetles on herbivory rates in Piper plants.   - Treatments: Control, Beetles present, No Beetles   - Conditions: Fertile vs infertile soil, high light vs low light.

  • Results:   - Beetles reduced ant abundance fivefold.   - Herbivory increased threefold, leaf area decreased by half.   - Soil fertility or light had no initial effect.   - Follow-up studies indicated interaction of soil nutrients and light.

Concepts of Ecosystem Processes

  • Energy Flow:   - Ecosystems depend on the flow of energy and cycling of matter among different trophic levels.   - Each level influences the whole system, from primary producers to tertiary consumers.

Complexity in Food Webs

(A) North American Desert Example

  • Illustrated species relationships in a complex food web, emphasizing the interdependence of species.

Key Concepts in Food Web Complexity

  • Omnivory: Organisms that consume multiple trophic levels.

  • Cannibalism: Consumption of individuals of the same species.

  • Ontogenetic Diet Shifts: Changes in diet as organisms grow.

Trophic Subsidies

  • Definition: Cases where food webs receive energetic inputs from external sources that are essential for sustaining higher trophic levels.

  • Examples:   - Allochthonous inputs such as terrestrial detritus into river food webs.   - Energetic trophic subsidies observed in Gulf of California islands.

Nutrient Supply and Cycling

  • Nutrient Definition: Chemical elements required for organism metabolism and growth.

  • Biogeochemistry: Study of transformation of elements based on physical, chemical, and biological factors. Focuses on nutrient supply to primary producers.

Nutrient Requirements by Organism Type

  • Bacteria, Plants (corn), Animals (humans) example demonstrating elemental compositions:   - Oxygen (O), Carbon (C), Nitrogen (N), among others.

Nutrient Availability and Transformations

  • Key Processes:   - Nitrogen Fixation: Conversion of nitrogen gas (N2) into bioavailable forms.     - Requires nitrogenase enzyme, primarily an anaerobic process.   - Mutualistic relationships in plants help in nitrogen fixation during ecological succession.

Decomposition and Nutrient Cycling

  • Detritus: Dead organic materials are crucial for nutrient availability.

  • Decomposition Process: Nutrient release powered by detritivores.

  • Mineralization: Conversion of organic matter to inorganic nutrients, facilitated by decomposers.

Factors Influencing Decomposition Rates

  • Higher temperatures increase decomposition rates while moisture levels have variable effects depending on the context.

  • Optimal conditions maximize nutrient release for plant growth.

Summary of Nutrient Cycling

  • Nutrient Cycling Definition: Movement of nutrients through ecosystems, from uptake by organisms to release through decomposition, followed by uptake again.