Study guide

IB HL Biology Unit 4 Ecology Notes

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Course
IB HL Biology

Institution
Bayview Secondary School

IB Higher Level (HL) Biology notes from unit 4 - Ecology. I achieved a level 7 in this course using these high-yield, in-depth notes. They include course content from the Oxford IB HL Biology textbook, Cambridge HL Biology textbook, Bioninja and the classroom. These notes go well in-depth into the...

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4.1 Species, Communities and Ecosystems
Friday, August 2, 2019 3:31 PM

Definitions
• Species: Group of organisms that can potentially interbreed to produce fertile, viable
offspring.
○ The offspring themselves need to be fertile: able to reproduce again.
○ When 2 different species produce offspring through cross-breeding, the hybrids
are reproductively sterile(e.g. liger, mule etc.)
• Population: group of organisms of the same species living in the same area at the
same time.
○ Organisms that can interbreed to produce fertile offspring but live in different
areas: same species, different populations.
○ Reason for population classification: Organisms of the same species but different
populations are separated as they are unlikely to interbreed.
▪ I.e. different populations of the same species indicates that they are unlikely
to interbreed.
• Community: Group of populations living together and interacting with each other
within a given area.
• Habitat: The environment in which a species normally lives(location of the organism).
• Ecosystem: A community and it's abiotic environment
○ Abiotic environment: The non-living physical and chemical components of an
ecosystem: sun, wind, water etc.
• Ecology: Study of the relationships between living organisms or between living
organisms and their environment.

Modes of Nutrition
• Definition: Ways in which living organisms obtain chemical energy.
• 3 types: autotrophs, heterotrophs and Mixotrophs
1. Autotrophs: Producers
• Definition: Organisms which synthesize organic molecules from simple inorganic
substances obtained from the abiotic environment(e.g. carbon, nitrogen etc.)
2. 2 types;
a. Photoautotroph(most common)
▪ Photosynthesis: organic compounds are made using energy derived from
the sun
b. Chemoautotroph
▪ Chemosynthesis: organic compounds are made using energy derived from
the oxidation of chemicals
3. Heterotrophs: Consumers
• Definition: Organisms which obtain organic molecules from other organisms
○ They may obtain some inorganic substances from the environment but principally
obtain their carbon and nitrogen from the organic molecules produced by
autotrophs
• 3 types
1. Consumers: ingest organic matter which is living or recently killed.
1. Herbivores: consumers that feed principally on plant matter.
2. Carnivores: consumers that feed principally on animal matter
3. Omnivores: consumers that have a principle diet composed of both plant
and animal matter.
4. Scavengers: Feed on dead and decaying carcasses(e.x. hyenas, vultures etc.)
2. Detritivores: ingest nutrients from non-living, organic sources
□ Detritus: dead, particular organic matter(e.g. decaying organic material

Test 6 - Ecology Page 1

, □ Detritus: dead, particular organic matter(e.g. decaying organic material
and fecal matter).
□ Humus: decaying leaf litter intermixed within the topsoil
▪ Example: earthworms, woodlice, snails, crabs and millipedes.
3. Saprotrophs: Live on(or in) and feed on non-living organic matter by secreting
digestive enzymes and absorbing the products of digestion.
▪ Difference from detritivores: Saprotrophs do not ingest food but rather use
enzymatic secretion to facilitate external digestion.
□ I.e. digestion takes place outside the body for saprotrophs.
▪ E.g. decomposers: bacteria and fungi

3. Mixotrophs
○ Use both forms of nutrition, depending on availability
▪ Usually unicellular organisms
□ E.g. Euglena gracilis: possess chlorophyll for
photosynthesis(autotrophic) but may also feed on
detritus(heterotrophic).

Nutrient Cycling
• Nutrients: molecular materials required by an organism(carbon, nitrogen etc.)
• Reason for nutrient cycling: Supply of inorganic nutrients on Earth is finite and thus
chemical elements must be constantly recycled after use.
○ Reason for finite nutrients: Law of conservation of mass: new elements cannot be
created.
• Process of nutrient cycling
1. Autotrophs obtain inorganic nutrients from the abiotic environment and convert
into organic compounds
2. Heterotrophs ingest these organic compounds and use them for growth and
respiration, releasing inorganic byproducts.
3. Upon death, saprotrophs decompose the remains and free inorganic materials into
the soil.
4. The return of inorganic nutrients to the soil ensures the continual supply of raw
materials for autotrophs.

Test 6 - Ecology Page 2

, `
Mesocosms
Sustainability of an ecosystem
• Properties of ecosystems
1. Contains the interactions between biotic components(communities) and abiotic
components(the habitat).
2. Self-contained(not influenced by factors outside of the biotic and abiotic
components).
3. Have the capacity to be self-sustaining over long periods of time.
• 3 main components required for sustainability in an ecosystem
1. Energy availability
▪ In most communities, this is the availability of sunlight
2. Nutrient availability:
▪ Determined by the availability of saprotrophic decomposers, who recycle
inorganic nutrients
3. Recycling of harmful waste
▪ Determined by the availability of certain bacteria, that can detoxify the
harmful waste byproducts
□ e.g. denitrifying bacteria such as Nitrosomonas
Mesocosms
• Definition: enclosed environments that allow a small part of a natural environment to
be observed under controlled conditions
• Composition: Consists of a small, transparent container(e.g. glass or plastic) in which
selected plants or animals are kept and observed.
Making a self-sustaining Terrarium
1. Building a verdant foundation
1. Add a bottom layer of pebbles, gravel or sand: used for drainage
▪ Smaller vessel = thinner rock layer
2. Add a second layer of activated charcoal: prevents mold and aerates soil
3. Spread a thin cover of sphagnum moss(or organic coffee filter) to create a barrier
between the lower layers and soil.
4. Place a pre-moistened growing medium(i.e. potting mix)
2. Select the right plants:
○ Choose plants that are both slow growing and thrive in a bit of humidity(e.g.
most ferns, club moss etc.)

Test 6 - Ecology Page 3

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