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- Of all the sun's energy that reaches the Earth's surface, only 0.1 % is used by living things.
- Yet this amount is responsible for the production of several thousand grams of organic matter per square meter of forest per year.
- It becomes incorporated in net primary production (NPP) – the total energy accumulated per unit time by plants during photosynthesis.

The Fate of Organic Matter in the Community
- Main element of all living matter (95%) à C
- C involved in the accumulation & storage of energy
- C enters into the trophic structure of a community when CO2 is fixed through photosynthesis*
- High-energy molecules of C stored in plants - animals that eat the plants obtain this energy à use much of this energy to carry on their life activities à store an even smaller amount.
- Respiration - C is oxidized through metabolism & released into the atmosphere as CO2


Primary Producers
- Plants = autotrophs in terrestrial ecosystems
- Algae + aquatic plants = autotrophs in near-shores waters of both freshwater & marine ecosystems
- Phytoplankton = autotrophs in open ocean
- Chemosynthetic bacteria = an exception is communities of organisms living around hot water, deep sea vents where producers oxidise H2S (driven by geothermal energy).

Consumers
- Animals à consumers and get their energy either directly or indirectly from producers.
- Consumers that feed directly on producers are called primary consumers (also called herbivores).
- Secondary Consumers - Consumers that feed on primary consumers are called secondary consumers.
- Secondary & higher level consumers are usually carnivores (flesh-eating animals).

Detritivores
- Decomposers - consumers that derive energy from organic wastes & dead organisms.
- Important components of the recycling process à forms a major link between primary producers & higher-level consumers.

Trophic Structure
- Each ecosystem has a trophic structure (hierarchy of levels) that represents the different feeding relationships that determine the route of energy flow and the pattern of chemical cycling.

Trophic levels
- Each step in this series of organisms eating other organisms is called atrophic, or feeding level.
- Trophic levels represent a feeding step in the transfer of energy & organic matter in an ecosystem.
- There is no limit to the number of trophic levels in a particular ecosystem.
- As you move up a higher trophic level, both available energy & biomass decrease.
- Energy is transferred upwards but is diminished with each transfer à being used to maintain the metabolism of the organism & to power its daily activities.
- A small amount of the energy taken in by herbivores (primary consumers) is changed into new animal biomass.
- Biomass is the total mass of all the organisms in atrophic level.

ENERGY FLOW
• Energy cannot be used again - energy in an ecosystem is referred to as a flow rather than a cycle.
• Energy flows to higher trophic levels through food webs.
• Energy needed for growth, maintenance and reproduction.
• Light energy is used by 1o producers to synthesis organic molecules (photosynthesis) & broken down to produce ATP (cellular respiration).
• An ecosystem's entire energy budget is determined by the photosynthetic activity of the system


Ecological Pyramid
• Used to represent the energy relationships among trophic levels.
• There are three types of ecological pyramids:
(a) Pyramid of energy - shows the total amount of incoming energy at each successive level.
(b) Pyramid of biomass
(c) Pyramid of numbers
One of the most important processes in any ecosystem is the flow of energy through the ecosystem.

The Laws of Thermodynamics
1. First law of thermodynamics states that energy is neither created nor destroyed.

Implications:
We should be able to describe the amounts of energy in each trophic level & follow energy as it flows through successive trophic levels.

2. Second law of thermodynamics states that when energy is converted from one form to another some energy escapes as heat.

Implications:
When energy passes from one trophic level to the next, there will be a reduction in the amount of energy in living things & an increase in the amount of heat in their surroundings .

Pyramid of Energy
In an ecosystem, the total energy can be measured in several ways:
1. the number of calories of heat energy produced by burning is equivalent to the energy content of the organic material of the plants.
2. To measure the rate of photosynthesis & respiration - calculate the amount of energy being trapped in the living material of the plants.
3. In general, there is about a 90% loss if energy as we proceed from one trophic level to the next higher level.
4. This loss of energy at the second and subsequent trophic levels is primarily due to the second law of thermodynamics.
- Trophic levels stacked in blocks proportional in size to the energy acquired from the level below. Food chains are usually bottom heavy since only 10% of energy is transferred.


Pyramid Numbers
- Another method to quantify energy in trophic levels because of difficulties in measuring the amount of energy in each level
- Simply count the total number of individual in each trophic level but may not be reliable due to several factors, e.g. size of the organism at different trophic levels that caused an inverted pyramid.
- Usually the number of organisms decreases at each successive level.
- Exceptional case: In a temperate deciduous forest, one tree (producer) can support a large number of insects (primary consumers).
- Like pyramids of biomass, pyramids of numbers show only the amount of organic material present at one time.
- They do not give the total amount of material produced or the rate at which it is produced, as do pyramids of energy.

Pyramid of Biomass
- The trophic levels of an ecosystem can also be represented by a pyramid of biomass, which has each tier symbolising the total dry weight (biomass) of all organisms in an ecosystem's levels at any given time - eliminate the problem of different sizes
- Biomass represents chemical energy stored in the organic matter of a trophic level.
- Most narrow sharply from producers at the base to top-level carnivores at the top.
- Shortcoming: the accumulation of biomass vary with the different length of life span of organisms
e.g. some aquatic systems are inverted since producers can have high turnover rates. They grow rapidly but are consumed rapidly, leaving little standing biomass.
- Biomass of top-level carnivores is usually small compared to the total biomass of producers & lower-level consumers.


Primary Productivity
- Primary productivity is the rate at which light energy is converted to chemical energy by autotrophs of an ecosystem.
- The total is known as gross primary productivity (GPP) which may be determined by measuring the total oxygen produced by photosynthesis.

Net primary productivity
(NPP = GPP - Rs)
Rs = energy used by producers for respiration
NPP = organic mass of plants (growth) & represents storage of chemical energy available to consumers
NPP:GPP ratio - generally smaller for larger producers with elaborate nonphotosynthetic structures (such as trees) which support large metabolically active stem & root systems
• Net Primary Productivity can be expressed as:
(a) biomass (expressed as dry weight since water contains no unable energy) added to an ecosystem per unit area per unit time (g/m2/yr)
or (b) energy per unit time (J/m2/yr).

Primary Production
- Global terrestrial net primary productivity=120 x 1019 tons dry weight per year
- In the sea = 50-60 x 109 tons per year
- Unevenly distributed across the earth
- Most productive systems – swamp & marshland, estuaries, reefs & cultivated land
- Productivity decreases moving away from equator – temperature & radiation are important.

Factors important in limiting productivity depend on the type of ecosystem & temporal changes such as seasons.
- Tropical rainforests are very productive & contribute a large proportion to the planet's overall productivity since they cover a large portion of the Earth's surface.
- Estuaries & coral reefs are also very productive but make only a small contribution to planetary productivity since they do not cover an extensive area.
- The open ocean has a relatively low productivity but makes the largest contribution to overall productivity of any ecosystem due to its very large size.

(A) Freshwater Ecosystem Productivity
1. Varies from the surface to the depths in relation to light intensity;
2. Water temperature is important & seasonal fluctuations in productivity occur in temperate zones;
3. Availability of inorganic nutrients is sometimes limiting, but biannual turnovers bring nutrients to the surface waters.

(B) Marine Ecosystem Productivity
Usually determined by :
- Light intensity & temperature
(a) affect primary productivity of phytoplankton in the open oceans
(b) productivity is highest near the surface and decreases with depth.
- Inorganic nutrients
(a) limiting at the surface of open ocean waters with N & P in especially short supply - a primary reason for the relatively low productivity of open oceans.
e.g. Marine phytoplankton is most productive where upwellings bring nutrient rich waters to the surface; these areas (usually in polar seas) are more productive than tropical seas.

(C) Terrestrial Ecosystem Productivity..
1. Precipitation, temperature & light intensity are factors limiting productivity
2. Productivity increases as latitudes approach the equator because availability of water, heat & light increases in the tropics.
3. Productivity in terrestrial ecosystems may also be limited by availability of inorganic nutrients, e.g. plants require a variety of nutrients, some in large quantities & some in small quantities.
4. Primary production sometimes removes nutrients from the system faster than they can be replenished àlimiting nutrient.
5. N & P are usually limiting nutrients since they are needed in large quantities but are often present in small or moderate amounts in natural environments.
6. Carbon dioxide availability sometimes limits productivity.

Secondary Production
- Definition = the rate of production of new biomass by heterotrophic organisms
- Heterotrophs = organisms with a high requirement for energy-rich organic molecules
e.g. animals, fungi, bacteria
- Not all energy stored in biomass can be converted to productivity at the next trophic level due to loss of energy dissipated as heat.
- Herbivores only consume a small fraction of available plant material.
- 2/3 organic material absorbed - for cellular respiration that degrades compounds into organic waste products & heat
- 1/3 adds biomass to the trophic level.
- Carnivores are more efficient at converting food into biomass but more is used for cellular respiration, further decreasing energy available to the next trophic level.

Ecological Efficiency
- Ecological efficiency is the ratio of net productivity at one trophic level compared to net productivity at the level below.
- It can vary greatly depending on the organisms involved, but is roughly 10%. This means that 90% of the energy available at one trophic level never transfers to the next.

Transfer Efficiency
• To predict the pattern of energy flow, three categories of transfer efficiency is required:
(a) Consumption efficiency (CE)
(b) Assimilation efficiency (AE)
(c) Production efficiency (PE)

CE in Herbivores
• Generally low – may be due to difficulty of plant material utilisation or low herbivore density
• Forest – 5%
• Grassland – 25%
• Phytoplankton dominated community – 50%

CE in Carnivores
• Much less is known
• Vertebrate predators – 50-100% from vertebrate preys, only 5% from invertebrate preys.
• Invertebrate predators – 25% from invertebrate preys.

• Herbivores, detritivores and microbivores – 20-50% (low)
• Carnivores – 80%
• Generally, animals are not well equipped to deal with living and dead plant materials.
• Polymeric composition (lignin, cellulose) of plants
• AE for seeds – 70%
• AE for leaves – 50%
• Organic matters originates from animals – better digestion and assimilation

• Varies according to taxonomic class of organisms concerned
• Invertebrates – high 30-40%, losing little energy as respiratory heat
• Endotherm vertebrates – intermediate ~10%,
• PE increases in size in endotherms, and decrease with size in ecotherms.

Feeding Relationships
- Animals and plants in the biosphere are tied together in complicated networks of feeding relationships.
- The simplest feeding relationship is a food chain.

• Pathways of nutrient flow:
Autotrophic organisms assimilate inorganic resources (N, P, C) into packages of organic molecules (protein, carbohydrates,etc)
Resources for heterotrophs
Food chain

At each link of food chain, we can recognize 3 pathways to the next trophic level:
- Decomposition – organisms die or waste/secretory product à food source for decomposers (bacteria, fungi, detritivores)
- Parasitism – living organism as resource as it is still alive
- Predation – food organism is eaten & killed
- A transfer of food from trophic level to trophic level.
- Food chains - usually branched
- several different primary consumers may feed on the same plant species
- a primary consumer may eat several species of plants.
- The feeding relationships are usually woven into a network of interconnected food chains within an ecosystem -> food web.



Poisons in Food Chains
- A variety of toxic chemicals, including unnatural synthetics, have been & are dumped into ecosystems à many cannot be degraded by microbes & persist for years or decades
- Organisms acquire toxic substances along with nutrients or water, some of which accumulate in their tissues.

Biological Magnification
- This is the process by which toxins become more concentrated with each link in a food chain.
- Results from biomass at each trophic level being produced from a much larger biomass ingested from the level below.

The top-level carnivores are usually most severely affected by toxic compounds released into the environment.
• The pesticide DDT is a well known example of biological magnification à used to control mosquitoes & agricultural pests.
• DDT persists in the environment & is transported by water to areas away from the point of application because it is soluble in lipids and collects in fatty tissues of animals.
• The concentration is magnified at each trophic level & reached such high concentrations (10 X 106 ) in top-level carnivorous birds à calcium deposition in eggshells was disrupted.