Lecture 8: Ecosystems, Global Cycles, and Biodiversity

Module 2: Ecosystems, Global Cycles, and Biodiversity Introduction

Welcome to Module 2: This module transitions from the physical world (Module 1) to biological processes and their interaction with the physical environment.
Module Topics:
- Ecosystems (covered today)
- Water (next lecture)
- Carbon (future lecture)
- Nutrients (Nitrogen and Phosphorus)
- Biodiversity
- Biomes Activity Day
- Midterm 1 (covers Modules 1 & 2) is scheduled for October 1. Ensure this is on your schedule.
Study Resources: Study guide questions are available on bCourses (one of the top links) for every lecture.
Lecture Objective: Understand how biological processes interact with the physical world.

Lecture 2 Learning Objectives

Define the biosphere and ecosystem.
Explain the components and structure of ecosystems.
Discuss energy transfer and its efficiency within ecosystems.
Explore patterns of primary productivity.

Slide Organization for Study

Slides are color-coded in the upper corner.
Slides with the same color represent a unit or group of related topics that should be studied together for better organization.

The Biosphere

Definition: The biosphere is a thin layer residing at the Earth's surface, encompassing most of the Earth's land surface and all its oceans.
Extent:
- Ranges up to $20$ kilometers high into the atmosphere.
- Most is associated with the Earth's surface.
- Extends down to about $500$ meters below sea level.
Earth Systems Overlap: The biosphere interacts and overlaps with other key Earth systems:
- Atmosphere (covered in Module 1)
- Hydrosphere (water, to be covered soon)
- Lithosphere (rocks and geological layers)
Interaction: Organisms within the biosphere constantly interact with these other parts of the Earth system.

Organization of the Biosphere: Nested Subunits

The biosphere is organized into nested subunits, starting from the smallest and building up:
- Species: The smallest subunit. Organisms of the same kind that can interact and typically reproduce with one another. (Definition nuances not covered in this class).
- Population: All members of a single species living in a given area. These are interacting members within that area.
- Community: A characteristic assemblage of two or more groups of interacting species. This includes a variety of organisms such as plants, animals, microbes, and fungi.
- Ecosystem: Comprises a community and its physical environment. The physical environment supports the community, and the community, in turn, interacts with ecosystem-level processes (both biotic and abiotic components are key).
- Biosphere: The largest 'house' that contains all these nested subunits, representing the global sum of all ecosystems.

Detailed Ecosystem Definition

Core Idea: An ecosystem is the minimal entity with properties necessary to sustain life.
- Ecosystems can vary greatly in size, from vast coastal or tropical regions to very small scales (examples below).
Components:
- Biotic Components: All living organisms within the area.
- Abiotic Components: The physical, non-living environment (e.g., climate, soil, water).
Scale:
- Spatial: Ecosystems can be defined at any spatial scale. Their boundaries are typically defined by the questions being asked, as well as natural boundaries.
  - Global Scale: Example: "How does carbon loss from plowed soils influence global climate?"
  - Watershed Scale: Example (Strawberry Creek): "How does deforestation influence the water supply in neighboring towns?"
  - Forest Scale: ( $1 ext{ km}$ scale) Example: "How does acid rain influence forest productivity?"
  - Micro Scale (Endolithic): ( $1 ext{ mm}$ scale) Example (organisms living within rock layers): "What are the biological controls over rock weathering?"
- Temporal: Ecosystems are dynamic and change over time; they do not always look the same at a single snapshot.
Processes (Functional Parts):
- Flows of Energy: Transfer of energy between organisms and their environment, primarily through food.
- Cycling of Elements: The movement and recycling of nutrients and other materials (e.g., carbon, nitrogen, phosphorus).

Ecosystems as Open Systems

Conceptual Model: Ecosystems can be represented as a 'box' that is inherently open.
Open System Principle: Energy and materials (matter) can pass into the ecosystem from the surrounding environment and can pass back out of the ecosystem.
Internal Transfers: Within the ecosystem, energy and materials transfer among various members, functional groups, and between biotic and abiotic components.
Linkages: These components are linked by:
- Energy Flows: Primarily driven by food consumption.
- Nutrient Cycling: The continuous movement of essential materials.

Biotic and Abiotic Interactions in Detail

Key Definition Point: Biotic components (living) constantly interact with abiotic components (non-living) within an ecosystem.
Biotic Components: Organisms that are interacting in the ecosystem, varying based on the ecosystem and question's scale.
Abiotic Components: Non-living elements that organisms interact with. There's a coupled relationship where the environment influences organisms, and organisms influence the environment.
Categories of Abiotic Features:
- Environmental Conditions: Long-term patterns characterizing the ecosystem.
  - Climate: Temperature and precipitation patterns.
  - Seasonal Cycles.
  - Water Chemistry (e.g., pH).
  - Atmosphere.
  - Soils.
  - Topography (e.g., mountain side, coastal valley).
- Resources: Abiotic elements that can be consumed or transferred among organisms.
  - Nutrients.
  - Water.
  - Sunlight.
  - Space.
Feedback Example: Soil provides nutrients to plants. Plants grow, adding organic matter, potentially altering soil chemistry (e.g., making soils more acidic if adapted). This creates a feedback loop and adaptation cycles.

Energy and Materials Flow in Ecosystems

Primary Energy Source: The Sun powers most ecosystems.
Functional Groups of Organisms: Organisms are organized into groups based on their function in the system:
- Producers (Autotrophs): Plants, algae. They harvest energy (primarily from the sun) and convert light energy into stored chemical energy within carbon bonds. They are the main conduit for energy entry into the ecosystem.
- Consumers (Heterotrophs): Animals (herbivores eating producers, carnivores eating other animals). They transfer energy by consuming other organisms.
- Decomposers (Heterotrophs): Worms, fungi, bacteria. They break down dead organic material and waste from producers and consumers, harvesting energy from these compartments. Everything eventually ends up with the decomposers.
Energy Flow Diagram:
- Energy from the sun enters via producers.
- Energy flows (black arrows) represent transfers between organisms.
- Some energy is used by organisms to support their bodies.
- Some energy is lost from the ecosystem as heat through metabolic processes (respiration).
Materials (Nutrients) Flow:
- Abiotic resources such as nutrients (gases, water, minerals) are vital for building organic molecules.
- These materials are also brought into the ecosystem primarily by primary producers.
- Materials flow between organisms, similar to energy. Many nutrients are tightly cycled within the ecosystem, with some eventual return and recycling through global cycles (biogeochemical cycles).
- Key takeaway: Both energy and materials enter ecosystems through the producers.

Trophic Interactions and Functional Categories

Trophic: Derived from Greek for 'food' or 'nourishment'. Describes feeding relationships.
Autotrophs ('Self-Feeders'):
- Primary Producers: (e.g., plants, algae) Generate their own nourishment using external energy sources.
- Photoautotrophs: Use light energy (most common in this class).
- Chemoautotrophs: (e.g., deep-sea vent bacteria) Use chemical reactions to produce food (an exception to solar energy reliance).
Heterotrophs ('Other-Feeders'):
- Consumers: (e.g., animals) Obtain nourishment by ingesting other organisms.
- Detrital Feeders & Decomposers: (e.g., fungi, bacteria, worms) Obtain nourishment by breaking down dead organic matter. (Note: Fungi are heterotrophs, even if consumed like vegetables).

Photosynthesis and Respiration: The Core Processes

Photosynthesis (Energy Entry): Performed by primary producers (plants, algae) to bring energy into the ecosystem.
- Chemical Equation: $(6CO2 + 6H2O + ext{light energy} ightarrow C6H{12}O6 + 6O2)$
- Mechanism: Converts light energy into stored chemical energy in high-energy carbon bonds (sugar) and releases oxygen.
- This sugar is how carbon enters the ecosystem.
Respiration (Energy Harvest): The process by which all organisms (autotrophs and heterotrophs) break down organic carbon to harvest energy.
- Chemical Equation: $(C6H{12}O6 + 6O2 ightarrow 6CO2 + 6H2O + ext{energy})$
- Result: Sugar (organic carbon) and oxygen are used to release carbon dioxide, water, and energy.
- The released energy powers metabolic processes, reconfigures proteins, etc.
- Underlies redox chemistry (electron transfers).
Energy Transformation Summary: Sunlight (light energy) is converted to chemical/stored energy, which can then be used by the organism or other members of the ecosystem.

Ecosystem Input and Output Model

Inputs:
- Energy from the Sun ( $ ightarrow$ )
- Carbon Dioxide $(CO_2)$ ( $ ightarrow$ )
- Water $(H_2O)$ ( $ ightarrow$ )
- These enter via autotrophs through photosynthesis, producing sugar and oxygen.
Outputs:
- Energy as Heat ( $ ightarrow$ )
- Carbon Dioxide $(CO_2)$ ( $ ightarrow$ )
- Water $(H_2O)$ ( $ ightarrow$ )
- These leave via all organisms through respiration.

Quick Quiz: Sun-Driven Processes

Question: Which of the following are driven by energy from the sun? (Select all that apply)
- Atmospheric circulation: Yes.
- Energy transferred through ecosystems: Yes.
- The water cycle via evaporation: Yes.
- Surface ocean circulation: Yes.
- Deep ocean circulation: No. Deep ocean circulation is driven by density differences caused by salinity and temperature gradients (thermohaline circulation), specifically brine exclusion (when ice forms, salt is excluded, increasing remaining water's density), which causes the dense water to sink due to gravity. While temperature plays a part in changing water properties, the direct energy input for circulation is not solar.

Trophic Levels and Food Webs

Food Chain: A simplified series of interactions showing how energy (and materials) pass between individuals as 'links in a chain', representing feeding relationships.
Trophic Level: A feeding level within a food chain (derived from 'trophic' meaning nourishment).
- 1st Trophic Level: Primary Producers (e.g., plants, algae). The base of the food chain.
- 2nd Trophic Level: Primary Consumers (Herbivores that eat primary producers).
- 3rd Trophic Level: Secondary Consumers (Carnivores or omnivores that eat primary consumers).
- 4th Trophic Level: Tertiary Consumers (Top predators that eat secondary consumers).
Decomposers: Feed on nonliving organic matter from all trophic levels (e.g., fallen leaves, unassimilated food, animal waste, bones). They are crucial for nutrient recycling.
Food Web: A more complex and realistic model showing all the interconnected feeding relationships within an ecosystem. It illustrates that organisms often eat multiple types of food and can be eaten by multiple predators, leading to intricate interactions. Omnivores can occupy multiple trophic positions.

Energy Transfer Efficiency and Loss

Assimilation: The process of taking in and integrating carbon and nutrients from food into an organism's body.
- Primary Producers: Assimilate solar energy through photosynthesis ( $total ext{ assimilation}$ ).
  - Some is used for respiration (metabolic processes, releasing heat).
  - Some is used for growth and reproduction (integrated into the organism's biomass).
- Consumers (e.g., a snail eating leaves):
  - Not all ingested food is assimilated; some passes through as feces (not used).
  - Assimilated material is used for respiration and growth/reproduction (integrated into its biomass, e.g., for making eggs or increasing body size).
Energy Loss: There is significant energy loss at each step of consumption and transfer within an ecosystem.
- Factors Leading to Loss:
  - Not consumed: Some biomass at a trophic level is never eaten.
  - Undigested: Of what is eaten, some is not digested or assimilated.
  - Respiration (Metabolism): Organisms use assimilated energy for metabolic processes, releasing heat.
- Only a portion of the energy from one trophic level becomes biomass for the next trophic level.

Biomass Pyramid and Trophic Level Transfer Efficiency (TLTE)

Biomass Pyramid (Terrestrial Example): Illustrates the drastic reduction in biomass at successively higher trophic levels.
- 1st Trophic Level (Primary Producers): e.g., $100$ units of biomass.
- 2nd Trophic Level (Primary Consumers): e.g., $10$ units of biomass.
- 3rd Trophic Level (Secondary Consumers): e.g., $1$ unit of biomass.
- 4th Trophic Level (Tertiary Consumers): Very little remaining energy.
Energy Loss Percentage: In the given example, a $90$ \% loss of energy occurs at each transfer, meaning only $10$ \% is retained.
Trophic Level Transfer Efficiency (TLTE): A measure of the efficiency of energy transfer between trophic levels.
- Formula: (TLTE = rac{ ext{Production at current trophic level}}{ ext{Production at previous trophic level}} imes 100 ext{%})
- Example: From 2nd to 3rd trophic level (1/10 imes 100 ext{%} = 10 ext{%} efficiency).
- TLTE can vary between ecosystems.

Real-World Food Chain Implications

Terrestrial Agriculture: Our human food system typically uses about $3$ trophic levels (e.g., primary producers $
ightarrow$ sheep/cows $
ightarrow$ humans).
Aquaculture/Seafood: Food chains can be much longer, implying greater energy loss.
- Example: Phytoplankton $
  ightarrow$ zooplankton/scallops/mussels $
  ightarrow$ herring $
  ightarrow$ salmon/sea bream $
  ightarrow$ tuna.
- Tuna can be at trophic level $5$ , meaning significant energy loss from the initial primary production.
- Analogy: Eating at such high trophic levels on land would be like