trophic levels

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CHAPTER 10: ECOSYSTEMS (NOTES)
(1) WHAT IS AN ECOSYSTEM?
Definition: a community of plant and animal species living in a specific environment. They
interact and are affected by their immediate environment. E.g. an island, a swamp, a forest, a
desert.
1.1 INTERACTIONS WITHIN AN ECOSYSTEM: TROPHIC RELATIONSHIP
-Trophic relationship = a feeding relationship or a feeding connection found in food chains.
-Food chain:
-
includes species which are linked by their feeding habits. It gives the links
between species being eaten and those who eat them. It also includes the
following layers or trophic levels:
Producers
Consumers
Decomposers
Producers (first trophic level)
-They are able to transform inorganic matter (water, mineral salts) into organic matter (proteins,
lipids, carbohydrates which are all used in the composition of living beings) with or without solar
energy. E.g. plants, algae, bacteria
-They are called autotrophs (self-feeders) since they make their own food without having to
consume another being. Autotrophs are placed at the bottom of the food chain since they are
its foundation.
- Autotrophs provide ecosystems energy throughtheir use of solar energy, CO2, soil nutrients,
water to produce organic matter, therefore also called producers.
- Photosynthesis is the main process used to transform inorganic matter into organic matter
Consumers
-Also called heterotrophs (other-feeders). They obtain energy by feeding on other organisms
-There are four levels of consumers:
(1) Primary (first-order) consumers:
they eat plant material. They are also called: herbivores;
seed-eaters: granivorous; fruit-eaters: frugivorous. E.g.
rabbit, vole, grasshopper, koala, hamster…They are the
second trophic level.
(2) Secondary (second order) consumers: carnivore. They eat primary consumers. E.g. wolf, milk
snake, lion… They are third trophic level.
(3) Tertiary (third order) consumers: carnivore: They eat the secondary consumers. They are
the fourth trophic level
(4) Quaternary (fourth order) consumers: carnivore: They feed on tertiary consumers. They are
the fifth trophic level.
- The higher up on the food chain an organism is the more precarious is its position. An organism
at a higher trophic level will be directly or indirectly dependent on all organisms below it. Why?
- The trophic level of a species can vary depending on where it is situated in that particular food
chain especially true of omnivores.
- Omnivores (all-feeders): They sustain themselves by consuming both plant and animal
organisms. E.g. humans, pigs, raccoons, black bear, chicken…
Decomposers
- Connected to all trophic levels. They are also
called detrivores (feeder of disintegrated
matter).
- They Feed on dead organic matter or detritus
(dead plants, animals and excrements).
-They decompose organic matter into inorganic
matter.
- Heterotrophs: They can be eaten by
consumers.
-E.g. all fungi, some bacteria, some worms:
redworms, some insects: sow bugs…
Food web
-Many food chains can make up a food web or
a trophic network.
- Marine food web.
1.2 ECOSYSTEM DYNAMICS
- Matter is transformed into an ecosystem.
- A transfer of energy occurs.
-Material and Energy Flow: when matter and energy is transferred from one trophic level to the
next. It respects the Law of Conservation of Mass where nothing is lost or created, it is simply
transformed. Molecules breakdown and continually combine to form new compounds.
- Chemical recycling: occurs during composting which is a process by which decomposers E.g.
redworms during vermicomposting) They decompose organic matter (from detritus) into
inorganic matter (N, P, K…) so that the inorganic matter becomes available to an ecosystem
(namely plants).
- Matter cycles from one trophic level to the next in an ecosystem
- Fig. 10.9; p.324
Energy Flow
-Energy contained in matter also flows from one trophic level to the next.
- Solar energy is transformed into chemical energy and it is passed in a food chain.
-Energy is lost as it goes from one trophic level to the next, this occurs because organisms
release energy in the form of waste and heat. It is also energy used by organisms to move,
digest, reproduce, grow…
- Unlike matter, energy is not recycled, rather it is lost to the environment. This is why
ecosystems need to continually receive energy from the sun.
1.3 PROMARY PRODICTIVITY IN ECOSYSTEMS
- Biomass: Mass of all plant and animal matter in an ecosystem.
-Primary productivity:
quantity of new biomass made by producers, i.e. the amount of
plant growth that occurs.
-It is import. since it is the amount of energy available to sustain an ecosystem.
-Primary productivity will be affected by:
(1) Amount of solar energy available for photosynthesis.
(2) Amount of water available for photosynthesis.
(3) Quantity of nutrients (N, K, P, C) available.
(4) Temperature .
(2) DISTURBANCES
Definition:
a natural or human driven occurrence that has a negative impact on an
ecosystem thus disturbing its balance, making some resources less available,
and even causing death. E.g. oil spill, drought.
- Disturbances can vary in three ways:
(1) By type: E.g. snowstorms, sandstorms, tornadoes, volcanic eruptions…
(2) In frequency: E.g. a seasonal event such as a river that comes out of its bed and
floods every spring. Another river may flow very rarely, only during
unpredictable torrential rains.
(3) According to seriousness: an oil spill will have a greater impact depending on the
spread of the spill and the volume spilled .
2.1 NATURAL DISTURBANCES
- Although natural, the impact can be very serious.
- E.g. storms, volcanic eruptions, forest fires, heat waves, cold spells, landslides…
2.2 HUMAN DISTURBANCES
-Humans are the main source of disturbances. Human disturbances outnumber natural
disturbances. It can be on an individual or on a larger scale.
- E.g. hydro electrical dam, logging, mining, residential developments, ski hill development...
2.3 ECOLOGICAL SUCCESSION
-Ecological succession is an often lengthy domino effect that an ecosystem undergoes after a
disturbance.
- It is series of different occurrences that succeed one another and can span over hundreds of
years as an ecosystem attempts to recover or reach an equilibrium.
- Often an ecosystem changes forever and new species slowly take over.
2.4 ECOLOGICAL FOOTPRINT
- An ecological footprint refers to the
environmental impact an individual causes.
- More precisely, it refers to the amount of
space (area) an individual requires to fulfill
his/her individual needs.
-the land occupied (our homes) + the land
used to produce goods and services + the land
and the water used to dispose of wastes.
-Our homes alone, just one aspect of
ecological footprint (top layer), is the type that
causes the greatest impact, right image
-Ecological carrying capacity (ECC):
-Ecological carrying capacity (ECC) refers to the Earth’s resources that are available to us without
offsetting its equilibrium . It also refers to the number of individuals, resources in an area can
sustain.
-The current ECC per person is approximately 1.8 hectare or 18,000 m2.
-It is calculated by dividing the total surface of the Earth by the Earth’s population (6.5 billion).
-In Canada our ecological footprint is approximately 7.6 hectares per person which means that if
the planet was made up of Canadians only, we would need over 4.2 times the Earth to fulfill our
needs.
- Fig. 10.17, p.331
(3) ECOTOXICOLOGY
- A branch of science which studies the environmental impact of pollution from substances and0
radiation.
3.1 CONTAMINANTS
-Any substances or radiation that causes harm to the environment (Toxic).
-Four large classes of contaminants:
-1. Inorganic:
-2. Organic:
-3. Microbial:
-4. Radioactive:
Pb (Lead), Hg (Mercury), nitrogen oxides, etc.
PCB’s, pesticides, benzene, etc.
Viruses and harmful bacteria
U (Uranium), Pu (Plutonium), Rn (Radon)
-Contaminant toxicity is dependent on:
(1) Concentration:
the higher the concentration the greater the impact will be. The
maximum concentration thresholds are often determined by the
government. E.g. Pb (Lead) must not exceed 0.01mg/L in drinking
water.
(2) Type of organism that will come in contact with the contaminant:
Some animals are more sensitive than others. E.g. Frogs.
Some contaminants are designed to target a specific group of species, for instance herbicides
target weeds
(3) Length of exposure:
The longer the exposure, the greater the risks. E.g. populations
and lifelong smog exposure.
Toxicity Threshold
- The minimal level of concentration of a contaminant. Above this level, detrimental effects are
produced.
-E.g. The threshold of microcystins (toxins released by cyanobacteria) is set at 16µg
(Microgram)/L for humans. Over this level, exposure can cause nausea, vomiting, etc.
- Lethal dose: The amount of contaminant contained in a single fatal dose. It is established for
contaminants and is used to compare their toxicity.
- LD50 refers to the contaminant dose needed to kill 50% of a population
3.2 THE BIOACCUMULATION & BIOCINCENTRATION OF CONTAMINANTS
- BIOACCUMULATION:
Tendency of some contaminants to build up over time in the
tissues of living organisms. E.g. PCB’s, DDT (a powerful insecticide
now banned in Canada but used in some developing countries.
-BIOCONCENTRATION or BIOAMPLIFICATION:
Tendency of some contaminants to increase in
concentration in organisms of a food chain.
Contaminated producers pass the contaminant
to the primary consumer, then it passes it on
the secondary consumer and so on. The
concentration increases each time.
- Notice the increase in contaminant
concentration within a given area with
Increasing trophic level.
- Toxins are often stored in the animal’s fatty
tissues

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