B6.3.1: Ecosystems - OCR A Biology A level A* student notes
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B6.3.1: Ecosystems
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A Level Biology A for OCR Student Book
These notes use information from two textbooks, the internet and extra information from my lessons, and cover each specification point for this topic. They are fully comprehensive and include diagrams etc. Helped me achieve an A* in Biology A level.
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B6.3.1: Ecosystems
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B6: Genetics, Evolution and Ecosystems
B6.3.1: Ecosystems
Ecosystems: a biological community of interacting organisms + their physical environment present in a defined area
[__ boundaries of a particular ecosystem being studied are defined by person/team carrying out the study
Ecosystems range in size
- Can vary dramatically in size eg. tiny bacterial colony, entire biosphere of Earth, a rock pool, a playing
field, a large tree
Ecosystems are dynamic
- Constantly changing - due to living organisms present and environmental conditions
[__ Ecosystems are influenced by both biotic and abiotic factors
BIOTIC factors affecting ecosystems (living factors)
❖ Describes living component of an ecosystem - autotrophs + heterotrophs (plants, animals, fungi, bacteria)
❖ Often refers to the interactions between organisms that are living / have once lived eg.
- Parasitism
- Predation
- Competition - either w/in a pop or between pops
[__ eg. animals compete for food, territory, breeding partners
❖ Presence of different organisms
❖ Size of populations
❖ Waste
ABIOTIC factors affecting ecosystems
★ Describes non-living, chemical + physical factors in the environment eg.
★ Light
- Most plants directly affected by light availability because light is required for photosynthesis
- In general, the greater the availability of light → greater success of plant species
- Plants develop strategies to cope w/ different light intensities eg. larger leaves + photosynthetic pigments
that require less light in areas of low light
★ Temperature
- Greatest effect of temp is on enzymes controlling metabolic reactions
- Plants + ectothermic animals develop more rapidly in warmer temps
- Changes in ecosystem temp eg. due to changing seasons can (depending on the species):
Animal - trigger migration or hibernation
Plant - trigger leaf-fall, dormancy or flowering
★ Water availability
- In most plant + animal pops - lack of water → water stress (if severe → death)
- Plants - lack of water causes wilting + also water is required for P
[__ xerophytes - adaptations to cope w/ water stress
★ Oxygen availability
- Fast-flowing cold water is beneficial in aquatic ecosystems as it cont high concs of oxygen
[__ if water too warm or flow rate too slow → decr in oxygen conc ( → can lead to suffocation of aquatic
organisms)
- Waterlogged soil (airspaces between soil particles filled w/ water) → decor oxygen available for plants
★ [__Edaphic (soil) factors - different soil types have different particle sizes - effects organisms able to
survive in them. 3 main soil types:
1. Clay - fine particles
- Easily waterlogged
- Forms clumps when wet
2. Loam - different-sized particles
- Retains water but doesn’t become waterlogged
Biomass transfers through ecosystems
To include how biomass transfers between trophic levels can be measured
AND the efficiency of biomass transfers between trophic levels
efficiency = biomass transferred / biomass intake x100
AND how human activities can manipulate the transfer of biomass through ecosystems.
Recycling within ecosystems
To include the role of decomposers and the roles of microorganisms in recycling nitrogen within ecosystems
(including Nitrosomonas, Nitrobacter, Azotobacter and Rhizobium)
AND the importance of the carbon cycle to include the role of organisms (decomposition, respiration and
photosynthesis) and physical and chemical effects in the cycling of carbon within ecosystems.
The process of primary succession in the development of an ecosystem
To include succession from pioneer species to a climax community
AND deflected succession
(Ecological) succession: the process of change in the (plant + animal) species present in an ecological community
over time, due to changes in the environment (abiotic factors)
[__ ecosystems are dynamic - this is one of the ways that they change over time
2 types of succession:
1. Primary succession
- Occurs on newly formed or exposed area of land eg. bare rock
- No soil or organic matter present at beginning
- Still taking place - eg. occurs when
- volcanoes erupt → deposit lava → lava cools + solidifies → creates igneous rock
- sand blown by wind / sea deposits it → creates new sand dunes
- silt + mud deposited at river estuaries
- glaciers retreat → deposits rubble + exposes rock
2. Secondary succession
- Occurs on areas of land where soil IS present but cont no plant/animal species eg. bare earth remaining
after forest fire
- In nature we see secondary succession more often than primary succession - existing communities
destroyed by natural phenomena eg. fires, cyclones, dieases OR by anthropogenic (human) activities
Stages of primary succession
- Each stage known as a ‘seral stage’ / ‘sere’ - at each can identify key species that change the abiotic factors
(esp soil) to make it more suitable for subsequent existence of other species
- At each seral stage, organisms better adapted to the current conditions in the ecosystem outcompete many
of the species that were previously present → become the dominant species (most abundant species - by
mass - present in the ecosystem at a given time)
1. Pioneer community
- Organisms known as the ‘pioneer species’ / ‘pioneer colonisers’ (eg. algae, lichen) begin primary
succession by the colonisation of an inhospitable environment
- Pioneer species: organisms specifically adapted to colonise bae, lifeless areas + begin an ecological
community
- Pioneer species arrive as spores or seeds blown by the wind or by the droppings of birds or animals -
deposited
- All pioneer species have abilities that allow them to live in an area that’s potentially inhospitable to other
forms of life
- Adaptations of pioneer species that enable them to colonise a bare environment aka. features of pioneer
species:
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