4/24: Notes on Ecosystem Science and Energy Flow (REC)(DOC)

• Ecosystem Level in Biological Hierarchy

  • Distinction between Community and Ecosystem:

  • Community: Interactions among different species in an area.

  • Ecosystem: Includes the community and all abiotic factors (e.g. sunlight, temperature, water, soil pH).

• Two Main Concepts in Ecosystem Science:

  • Energy Flow

  • Matter Cycling

  • Ecosystem scientists also consider community dynamics, population interactions, and evolutionary aspects.

• Energy Flow:

  • Discussed in terms of trophic structure (feeding structure).

  • Primary Producers:

  • Mainly plants (autotrophs) create their own food via photosynthesis.

  • Some organisms (e.g. certain bacteria) are chemoautotrophs and can synthesize inorganic compounds in harsh conditions; however, most producers are traditional plants.

• Consumers (Heterotrophs):

  • Organisms that do not create their own food; they consume other life forms for energy.

  • Decomposers:

  • While they are also consumers and heterotrophs, their main role is consuming dead organic material, in contrast to other consumers who often eat living organisms.

• Trophic Levels and Food Chains:

  • Defined by organisms that get energy from the same source.

  • Food Chains illustrate the flow of energy (who eats whom). Examples include:

  • Decomposer Food Chain:

    • Starts with dead leaf (former primary producer) → bacteria (primary consumers) → earthworm (secondary consumer) → robin (tertiary consumer) → Cooper's hawk (quaternary consumer).

  • Grazing Food Chain:

    • Starts with green maple leaf (primary producer) → cricket (primary consumer) → robin (secondary consumer/carnivore) → Cooper's hawk (tertiary consumer/carnivore).

• Food Webs:

  • More complex than food chains; represent interconnections in an ecosystem.

  • Example: African Savanna Food Web

  • Producers include acacia trees and grasses.

  • Herbivores include giraffes, impalas, and zebras.

  • Carnivores include lions, cheetahs, and hyenas.

  • Shows how different species are interrelated and identifies potential keystone species, which have a significant effect on community structure despite low biomass (e.g. lions).

• Ecological Efficiency:

  • Generally, energy transfer between trophic levels is inefficient, typically around 10%.

  • Most energy is lost due to various factors (e.g. cellular respiration, unconsumed portions).

• Gross Primary Production (GPP):

  • Total energy produced via photosynthesis by plants.

• Net Primary Production (NPP):

  • GPP minus energy used for cellular respiration; the energy available for herbivores.

  • Typically, NPP is about half of GPP.

• Impacts of Energy Loss:

  • As energy moves up trophic levels, less is available (e.g. 10% GPP is available to herbivores, 10% of their energy to carnivores).

  • An energy pyramid illustrates this loss; fewer trophic levels can support fewer organisms.

• Understanding Ecosystem Health:

  • A food web helps assess the health of ecosystems and the importance of species.

  • Disturbances (e.g. loss of producers) can significantly impact trophic interactions and biodiversity.

• Flow vs. Cycling of Energy:

  • Energy is not recycled; it flows through the ecosystem and eventually dissipates into the environment as heat.

• Lessons Learned:

  • Energy is limited in ecosystems.

  • High ecological inefficiency limits biomass at higher trophic levels.

  • Ecosystem integrity relies on maintaining diverse populations across trophic levels.

Lecture week 14 (4/24)

Energy Flow and Trophic Structure

I. Introduction

A. Previous lectures have dealt with individuals, populations, species, and communities.

Finally, we are at a point where we can talk about Ecosystems. Ecosystems are

communities plus the abiotic conditions that surround them.

B. Ecosystem science is largely concerned with 2 things: energy flow and cycling of

matter. We will cover matter cycling in the next lecture.

C. Energy flow starts with solar radiation striking a part of Earth. Some of it is reflected

back into the atmosphere and space. Some is absorbed by organisms, soil, water, etc.,

increasing their kinetic energy and raising their temperature. Temperature affects the

rate of biochemical reactions within an ecosystem. Some of the incoming sunlight is

absorbed by chlorophyll in plants.

D. Plants use sunlight energy to convert carbon dioxide and water into carbohydrate,

which provides herbivores with a usable form of energy. Herbivores take in these

molecules and transform them. The new molecules can then be used by carnivores.

Additionally, all plants lose leaves and stems, animals defecate, shed skin, hair and/or

feathers, and all eventually die. The molecules of these non-living organic bits

provide energy for use by decomposers, who break down the molecules and return

nutrients to the soil and carbon to the atmosphere. These nutrients can then be taken

up by plants so the process can start over again. But where does the energy go?

E. We can view a natural environment (e.g., forest, coral reef, desert, etc.) as a system

that absorbs, transforms, and stores energy. In this view, physical, chemical, and

biological structures and processes are inseparable. When we view an environment

this way we view it as an ecosystem.

F. Careful: Unlike nutrients and water, energy does not cycle through an ecosystem. In

other words, energy flows through an ecosystem and exits as it is lost through the

process of cellular function. Energy does not get cycled into the soil and then pulled

back into plants.

II. Trophic Structure

A. The Players:

1. Abiotic Environment includes sunlight, soil, climate, temperature, local

weather conditions, water, and nutrients. Keep in mind, nutrients refers to

elements and molecules that an organism takes out of the environment to use.

For plants, this refers to elements and small molecules in the soil and air (and

sometimes directly from other organisms) (e.g., potassium, zinc, ammonium,

etc.). Remember, plants do not get carbon out of the soil. They get it from

the air in the form of CO2. Being consumers, we often confuse nutrients with

food. But plants create their own food. Consumers get their nutrients from

plants and other organisms.

2. Primary Producers Most are plants, bacteria, and algae (Autotrophs) that

produce their own food through photosynthesis. A few are bacteria that

convert inorganic compounds to food in extreme environments.

a. Created chemical energy is used in 2 ways:

i. Maintenance and respiratory costs

ii. Growth and reproduction

3. Consumers are Heterotrophs - organisms that feed on other organisms to

obtain energy for maintenance, growth, and reproduction. Consumers

include:

a. Herbivores - Primary consumers

b. Carnivores - Secondary consumers, or tertiary or quaternary and/or

Primary carnivores or secondary carnivores, etc.

c. Decomposers (Detritivores) – get energy from consuming dead

organisms or their wastes.

B. Trophic level – Organisms that obtain their energy from the same type of source are

said to be in the same trophic level (eg primary consumer vs secondary consumer).

C. Food chain

1. Connects the trophic levels in an ecosystem and describes how energy flows

between trophic levels. Essentially a food chain is a list of who eats whom in

an ecosystem.

2. At higher levels, many food chains merge. Also, all food chains end up going

into the decomposer food chain eventually.

3. Most animals eat more than one type of food, and many are represented at

different trophic levels in different food chains. So the idea of a food chain is

a little over-simplifying.

D. Food web

1. Compact way of summarizing energy flow and documenting the complex

trophic interactions that occur in communities

2. Food webs are highly interwoven. There are typically several links through

each level, and maybe several links through each individual.

3. Food webs cannot be understood solely on the basis of their direct

interactions. That is to say, a food web is not simply a collection of food

chains.

a. Example: A predator may reduce competition between prey species by

keeping numbers low (as in Keystone species).

b. Prey, by their presence or absence, may impact predator/predator

competition.

III. Energy Flow

A. Draw it out – Make a drawing of the transfer of energy from sun to plant to rabbit to

fox. Make it as detailed as possible, accounting for all energy transfer.

B. Does your diagram include...

C. Specifics

1. On average, < 1% of sunlight striking Earth is captured by plants.

2. Plants take in CO2 from the atmosphere and convert it (and water) to sugar

by photosynthesis.

3. The total amount of carbohydrates created by plants is referred to as Gross

Primary Production (GPP)

4. The process of photosynthesis is

12 H2O + 6CO2 + solar energy C6 H12 O6 + 6O2 + 6H2O

Notice the uptake of carbon in the form of CO2. This is the only way in

which plants get carbon. They do not take up carbon from the soil.

5. Energy is stored in the chemical bonds of carbohydrate. What happens to

it?

a. It may be stored as starch.

b. It may be used for growth of plant material.

c. It may be used for reproduction.

d. It may be used for plant maintenance.

6. All of these options require the functioning of cells. And the cells use

carbohydrate to function. This is cellular respiration and is the opposite

of photosynthesis.

C6 H12 O6 + 6O2 + 6H2O 6CO2 + 12 H2O + Energy

Notice that CO2 is given off during cellular respiration.

7. Every time a cell respires, energy is released from the bonds of the

carbohydrates and is used to drive cellular function. Because this process

is not 100% efficient, some of the energy is lost from the system and is not

used by the organism or by the others in the community. This is true for

all cells.

D. Ecological Efficiency

Given that we know a certain amount of energy is lost with cellular function, we can

then describe how efficiently energy moves from one trophic level to the next. We

refer to this as Ecological efficiency. Ecological efficiency is the percent energy

transferred between trophic levels.

Studies indicate the typical ecological efficiency is somewhere between 10-20%. The

majority of research reports that most values are closer to 10% than 20%.

1. Approximately 50% of a plant’s energy usage is in the form of cellular

respiration only (the remainder goes to storage and building bulk plant

matter). That means that energy that is not used for respiration becomes

available energy for herbivores. (Keep in mind that not all plant matter is

available to herbivores due to various defense mechanisms and

availability. See previous notes on interspecies exploitation.) Net

Primary Production (NPP) is GPP minus that energy used for

respiration. In other words, NPP is the amount of matter in plants that is

plant material and that may be available to herbivores.

2. How much of the NPP is harvested by animals? Depending on the

ecosystem, somewhere between 10 and 70%.

3. Pyramid of Productivity shows that productivity (total net biomass

production) is highest at the bottom, or first, trophic level, and decreases

dramatically as you move up through the higher trophic levels.

a. What conclusions can we draw from this?

E. What is your moral of the story?