BIO330 Chapter 4: Ecosystems Ecology - Detailed Notes
Ecosystems Ecology
Lesson Outcomes
Explain the components of trophic structure.
Explain the concepts and mechanisms of energy flows through an ecosystem.
Describe biogeochemical cycles within and between ecosystems.
Introduction to Ecosystems
Ecosystem Definition: An ecosystem includes all the organisms (multiple communities) in a given area as well as the abiotic factors with which they interact.
Difference from a Community: A community consists only of the living organisms in a specific area, excluding abiotic factors. An ecosystem includes both.
Difference from a Biome: A biome spans a large geographical area and is defined by characteristic environmental conditions determining which species can survive within it.
Trophic Structure
Definition: Trophic structure refers to the different feeding relationships in an ecosystem that determine the route of energy flow and the pattern of chemical cycling.
Represented by a food chain or food web, showing the flow of material and energy among different kinds of organisms.
Trophic Levels within a Food Chain
Trophic Level 1: Producers
Plants (producers) obtain their energy directly from the sun.
Trophic Level 2: Primary Consumers
Animals that eat plants (herbivores).
Trophic Level 3: Secondary Consumers
Animals that eat plant-eating animals (carnivores).
Trophic Level 4: Tertiary Consumers
Animals that eat carnivorous animals (top carnivores).
Decomposers and Detritivores: Break down the organic matter accumulated in the bodies of producers and other consumers.
Trophic Structure within a Food Web
In reality, organisms typically feed on multiple types of organisms and are eaten by several others.
These relationships form a series of branching lines called a food web.
Decomposers and detritivores are typically not shown in food web diagrams.
Trophic Level 1: Producers
Primary Producers: Autotrophs or photosynthetic organisms that use light energy to synthesize sugars and other organic compounds.
Examples:
Plants (terrestrial ecosystems)
Phytoplankton (limnetic zone of lakes and in the open sea)
Algae and aquatic plants (shallow freshwater and marine ecosystems)
Energy enters an ecosystem through photosynthesis; producers in terrestrial ecosystems capture about 1% of the solar energy that falls on their leaves and convert it to chemical energy.
Trophic Levels 2 and Above: Consumers
Consumers: Heterotrophs that obtain organic food molecules by eating other organisms or their by-products.
Primary Consumers: Feed directly on green plants (terrestrial) or phytoplankton (aquatic).
Examples: Insects, snails, grazing mammals, fruit-eating birds, and zooplankton.
Secondary Consumers (Carnivores): Feed on primary consumers (herbivores).
Tertiary Consumers (Carnivores): Feed on secondary consumers.
The consumer at the end of the food chain/web is known as the top carnivore.
Trophic Level: Decomposers
Decomposers chemically break down the organic matter accumulated in the remains of other organisms and feed on it.
Examples: Bacteria and fungi, which secrete enzymes that digest organic material (external digestion) and then absorb the breakdown products.
Saprotroph: A decomposer that feeds on decaying organic matter and obtains nutrients by extracellular digestion of dead organic matter.
Trophic Level: Detritivores
Detritivore is a decomposer that feeds on detritus (dead organic matter; decomposing plants or animals) and obtains nutrients by internal digestion.
Examples: Snails, millipedes, crabs, earthworms, dung flies, maggots.
Energy Flows Through Ecosystems
All organisms require energy for growth, maintenance, reproduction, locomotion, etc.
Primary producers use light energy to synthesize energy-rich organic molecules, which can subsequently be broken down to make ATP.
Consumers acquire their energy-rich organic molecules through the food chain/web.
Energy decreases as it moves up trophic levels; energy is lost as metabolic heat when organisms from one trophic level are consumed by organisms from the next level.
At each level up the food chain, only 10% of the energy is passed on to the next level, while approximately 90% of the energy is lost as heat.
Energy Flows Through Ecosystems: Global Energy Budget
Earth is bombarded by 10^{22} joules/day of solar radiation (1 joule = 0.239 calories).
Most solar radiation is absorbed, scattered, or reflected by the atmosphere.
Much of the solar radiation that reaches the biosphere lands on bare ground and bodies of water.
Only a small fraction strikes algae and plant leaves, and only 1-2% of the visible light that does reach algae and leaves is converted to chemical energy via photosynthesis.
Energy Flows Through Ecosystems: Primary Productivity
Primary Productivity: The amount of light energy converted to chemical energy (organic matter) by the autotrophs of an ecosystem during a given time period.
Gross Primary Productivity (GPP): The total organic matter produced, including that used by the autotrophs (photosynthetic organisms) for respiration.
Net Primary Productivity (NPP): A measure of the amount of organic matter produced in a community in a given time that is available for heterotrophs.
NPP = GPP - R where R is the energy used by the autotrophs for respiration.
Productivity (NPP) of Different Biological Ecosystems
High NPP Ecosystems:
Swamps & marshes and tropical rainforests produce between 8000-9000 kcal of organic matter per square meter per year.
Low NPP Ecosystems:
Deserts produce less than 800 kcal/m²/year.
Open oceans produce less than 1600 kcal/m²/year.
Energy Flows Through Ecosystems: Secondary Productivity
Secondary Productivity: The rate at which an ecosystem’s consumers convert the chemical energy of the food they eat into their own new biomass.
Gross Secondary Productivity (GSP): The total energy or biomass assimilated by consumers (food eaten minus fecal loss).
Net Secondary Productivity (NSP): The biomass or energy gain by consumers after respiratory loss (R).
Energy Flows Through Ecosystems: Secondary Productivity Details
Herbivores (primary consumers) manage to eat only a small fraction of the plant materials produced, and they are unable to digest all the organic compounds that they ingest.
Approximately only 17% of the energy consumed by a herbivore is used for growth (new biomass). The rest is passed as feces (50%, that will be consumed by decomposers) and used for cellular respiration (33%, that will be lost from the ecosystem).
Carnivores (secondary consumers) are slightly more efficient at converting food into biomass because meat is more easily digested than vegetation.
However, secondary consumers use more energy they assimilate for cellular respiration, which dramatically decreases the amount of chemical energy available to the next trophic level.
Heterotroph Energy Utilization
A heterotroph assimilates only a fraction of the energy it consumes.
Example: If a caterpillar eats 200 J of plant material (1 J = 0.239 calories):
About 50% (100 J) is lost in feces.
About 33% (67 J) is used to fuel cellular respiration.
About 17% (33 J) is converted into new biomass.
Energy Flows Through Ecosystems: Ecological Efficiency
Ecological Efficiency: The percentage of energy transferred from one trophic level to the next level, or the ratio of net productivity at one trophic level to net productivity at the level below.
Ecological efficiency varies among organisms, ranging from 5% to 20% (average 10%).
Approximately 80-95% of the energy available at one trophic level never transfers to the next level.
This multiplicative loss of energy from a food chain can be represented by a pyramid of energy (also known as pyramid of productivity).
Pyramid of Energy: Trophic levels are stacked in blocks, with primary producers forming the foundation of the pyramid. The size of each block is proportional to the energy or productivity of each trophic level.
Energy Flows Through Ecosystems: Ecological Pyramids
Ecological Pyramids: Graphical representation showing the feeding relationship and distribution of organisms, energy, or biomass within an ecosystem.
Ecological pyramids measure different characteristics of each trophic level:
Pyramid of energy
Pyramid of numbers
Pyramid of biomass (normal or upright, and inverted)
Energy Flows Through Ecosystems: Pyramid of Energy
Pyramid of energy compares the amount of energy flow at each trophic level over a period of time.
Only 1/1000 of the chemical energy fixed by primary producers via photosynthesis can flow all the way to tertiary consumers.
This partly explains why food chains/webs usually include only 3-5 trophic levels; energy from the top trophic level (top carnivores) is insufficient to support another trophic level.
Energy Flows Through Ecosystems: Pyramid of Numbers
Pyramid of numbers compares the number of organisms at each trophic level.
The size of each block is proportional to the number of individual organisms present in each trophic level.
The number of organisms is largest at the bottom and narrows towards the apex.
The number of producers are very large but relatively small in size, while the number of top predators are very small but tend to be fairly large in size.
Energy Flows Through Ecosystems: Pyramid of Biomass
Pyramid of biomass compares the mass of biological materials at each trophic level.
Most pyramids of biomass show a sharp decrease in biomass at successively higher trophic levels (normal pyramid of biomass, also known as upright pyramid of biomass).
However, some aquatic ecosystems demonstrate inverted pyramids of biomass.
Energy Flows Through Ecosystems: Inverted Pyramid of Biomass
In some aquatic ecosystems, a small population (low biomass; 4 g/m²/year) of phytoplankton supports a large population of zooplankton (high biomass; 21 g/m²/year), thus demonstrating an inverted pyramid of biomass.
Inverted pyramids of biomass occur because the zooplankton (consumers) consume the phytoplankton (producers) very quickly, causing the phytoplankton to never develop a large population size or biomass.
However, phytoplankton have a short turnover time with high productivity (grow and reproduce rapidly).
Biogeochemical Cycles: Introduction
The movement of energy and matter through ecosystems is related because both occur by the transfer of substances through photosynthesis, feeding relationships, and decomposition.
Unlike energy, matter can be recycled within and between ecosystems.
Examples: Carbon and nitrogen are cycled (used and reused) between biotic and abiotic components of the ecosystems.
Physical processes such as evaporation and precipitation move matters through the abiotic component of the ecosystems >>> organisms capture these matters through photosynthesis and transform them into organic compounds, and passed through biotic components via food chain/web >>> eventually these matters return to the abiotic component via respiration, decomposition, etc.
Biogeochemical Cycles: General Model of Nutrient Cycling
Most nutrients cycle within the biosphere among four major compartments or reservoirs.
The biological and geological processes that move nutrients from one compartment to another.
Biogeochemical Cycles: The Water Cycle
Evaporation exceeds precipitation over the oceans.
The result is a net movement of water vapor (carried by wind) from the ocean to the land.
The excess of precipitation over evaporation on land results in the formation of surface and groundwater systems that flow back to the sea.
Over the sea, evaporation forms most water vapor. However, on land, 90% or more of the vaporization is due to plant transpiration.
Biogeochemical Cycles: The Carbon Cycle
The reciprocal processes of photosynthesis and cellular respiration are responsible for the major transformations and movements of carbon.
The return of CO_2 to the atmosphere by respiration closely balances its removal by photosynthesis.
However, the burning of wood and fossil fuels adds more CO_2 to the atmosphere.
Atmospheric CO_2 also moves into or out of aquatic ecosystem, where it is involved in a dynamic equilibrium with other inorganic forms, including bicarbonates.
Biogeochemical Cycles: The Nitrogen Cycle
Most of the nitrogen cycling through food webs is taken up by plants in the form of nitrate.
Most of the nitrate comes from the nitrification of the ammonium that results from the decomposition of organic materials.
The addition of nitrogen from the atmosphere involves relatively small amounts compared to the recycling that occurs in the soil or water.
The amount of nitrogen returned to the atmosphere by de-nitrification is also relatively small compared to that recycled in soil or water.
Biogeochemical Cycles: The Phosphorus Cycle
Phosphorus does not have an atmospheric component and tends to cycle within the soil and water.
Phosphorus losses from terrestrial ecosystems caused by leaching are balanced by gains from the weathering of rocks.
Phosphorus is also lost from the ecosystem because of chemical precipitation and gradually accumulates in sediments.
This phosphorus may become available to ecosystems again through geological uplifting.
Ecosystems Ecology
Lesson Outcomes
Explain trophic structure components.
Explain energy flow mechanisms in ecosystems.
Describe biogeochemical cycles within ecosystems.
Introduction to Ecosystems
Ecosystem: Organisms (multiple communities) and abiotic factors in an area.
Community vs. Ecosystem: Community = living organisms only; Ecosystem = living + abiotic factors.
Biome vs. Ecosystem: Biome spans large areas with specific environmental conditions.
Trophic Structure
Definition: Feeding relationships determining energy flow and chemical cycling, shown via food chains/webs.
Trophic Levels
Level 1 (Producers): Plants use sun energy.
Level 2 (Primary Consumers): Herbivores.
Level 3 (Secondary Consumers): Carnivores that eat herbivores.
Level 4 (Tertiary Consumers): Top carnivores.
Decomposers/Detritivores: Break down organic matter.
Food Web Trophic Structure
Organisms feed on multiple types, forming a food web.
Decomposers/detritivores not typically shown.
Trophic Level 1: Producers
Primary Producers: Autotrophs using light energy.
Examples: Plants, phytoplankton, algae.
Producers capture ~1% of solar energy.
Trophic Levels 2+
Consumers: Heterotrophs eating other organisms.
Primary Consumers: Eat plants/phytoplankton.
Examples: Insects, snails, zooplankton.
Secondary Consumers: Eat primary consumers.
Tertiary Consumers: Eat secondary consumers.
Top carnivore at the end of the chain.
Trophic Level: Decomposers
Decomposers break down organic matter via external digestion, e.g., bacteria, fungi.
Saprotroph: Decomposer on dead matter.
Trophic Level: Detritivores
Detritivores feed on detritus via internal digestion, e.g., snails, earthworms.
Energy Flows
Energy needed for life processes.
Producers make ATP; consumers eat organisms.
Energy decreases up levels; 10% passed on, 90% lost as heat.
Global Energy Budget
Earth bombarded by 10^{22} J/day solar radiation.
Only 1-2% converted to chemical energy by producers.
Primary Productivity
Primary Productivity: Light energy converted to chemical energy by autotrophs.
GPP: Total organic matter produced.
NPP: Organic matter available for heterotrophs; NPP = GPP - R
Productivity of Ecosystems
High NPP: Swamps, rainforests (8000-9000 kcal/m²/year).
Low NPP: Deserts, open oceans (< 1600 kcal/m²/year).
Secondary Productivity
Secondary Productivity: Consumer conversion of food to biomass.
GSP: Total energy assimilated.
NSP: Energy gain after respiration.
Secondary Productivity Details
Herbivores use ~17% for growth, rest lost as feces/respiration.
Carnivores more efficient but use more energy for respiration.
Heterotroph Energy Utilization
Heterotrophs assimilate a fraction of energy.
Example: Caterpillar eats 200 J; 100 J lost, 67 J respiration, 33 J biomass.
Ecological Efficiency
Ecological Efficiency: Energy transfer % between levels (5-20%).
Pyramid of energy represents energy loss.
Ecological Pyramids
Show feeding relationships/distribution.
Pyramid of energy.
Pyramid of numbers.
Pyramid of biomass.
Pyramid of Energy
Compares energy flow at each level.
Only 1/1000 energy reaches tertiary consumers.
Chains limited to 3-5 levels.
Pyramid of Numbers
Compares number of organisms.
Number largest at the bottom, narrows towards the apex.
Pyramid of Biomass
Compares mass at each level.
Normal pyramid decreases at higher levels; inverted in some aquatic ecosystems.
Inverted Pyramid of Biomass
Small phytoplankton supports large zooplankton due to rapid consumption and turnover.
Biogeochemical Cycles: Introduction
Energy and matter move via photosynthesis, feeding, decomposition.
Matter recycled between ecosystems (e.g., carbon, nitrogen).
General Model of Nutrient Cycling
Nutrients cycle among four compartments via biological/geological processes.
Water Cycle
Evaporation > precipitation over oceans.
Water vapor moves to land, forming surface/groundwater.
90% land vaporization from plant transpiration.
Carbon Cycle
Photosynthesis and respiration drive carbon transformations.
Burning fossil fuels adds CO_2 to the atmosphere.
CO_2 in equilibrium with aquatic forms.
Nitrogen Cycle
Plants uptake nitrate from nitrification of ammonium.
Small nitrogen addition from atmosphere; denitrification returns small amount.
Phosphorus Cycle
No atmospheric component; cycles in soil/water.
Losses from leaching balanced by rock weathering.
Accumulates in sediments, available via geological uplifting.