OCR A LEVEL BIOLOGY - 6.5 & 6.6: Ecosystems, Populations + Sustainability

What is an ecosystem?

A community of living organisms (biotic factors) interacting with the non-living physical environment (abiotic factors)

What is a habitat?

The place where an organism lives

What is a population?

All of the organisms of one species in a habitat

What is a producer?

An organism that produces organic molecules using sunlight energy, e.g. plants

Materials in the environment are converted into glucose, a carbohydrate

What is a consumer?

An organism that eats other organisms e.g. animals/birds

What is a decomposer?

An organism that breaks down dead/undigested organic material, e.g. bacteria

What is a trophic level?

A stage in a food chain occupied by a particular group of organisms, e.g. producers are the first trophic level in a food chain

What are abiotic vs biotic factors?

Biotic → living features of an organism

Abiotic → non-living features of an ecosystem

What does it mean by an ecosystem being dynamic?

Constantly changing

What are 7 abiotic factors?

• Temperature

• Light

• Water availability

• O2 concentration

• CO2 concentration

• pH

• Mineral ions

What are 4 biotic factors?

• Competition

• Predation

• Disease

• Food availability

What are 3 examples of ecosystems?

• Rock pools

• Playing field

• Large tree

Describe the rock pool ecosystem in terms of both abiotic + biotic factors

Biotic factors

• seaweed - food source for consumers, e.g. limpets that graze on this producer

  • intense competition for food (e.g. seaweed) - limits number of organisms present in a small rock pool ecosystem

Abiotic factors

• rock pools - heavily influenced by tides

  • high tide → completely submerged by the ocean so experience similar abiotic factors (e.g. pH/salinity/temp, etc.)

  • low tide - more extreme abiotic conditions (e.g. higher salinity + temperatures) → only some organisms can tolerate their conditions

Describe the playing field ecosystem

Biotic factors

• producers include grass + other plants, e.g. daises, clover + dandelions → large amount of these plants attract a large number of organisms that use them as a food source, e.g. rabbits/caterpillars

Abiotic factors

• rainfall + sunlight affect growth of producers in ecosystem

  • very wet conditions → soil may become waterlogged - plants struggle to grow

    • poor plant growth - decreases number of consumers the ecosystem can support

Describe the large tree ecosystem

Biotic factors

• Insects, e.g. caterpillars can use the leaves of a tree as a source of food

  • if all leaves on a tree are consumed entirely → tree growth is slowed + death comes to tree

Abiotic factors

• Drought conditions (e.g. prolonged periods of very low rainfall) → negatively impacts tree growth → whole tree/part of tree can die

What is the main route by which energy enters an ecosystem? What is another less common way?

Photosynthesis

Bacteria uses chemicals from deep sea vents as an energy source

What is biomass?

Mass of living materials, e.g. mass of plant material

What are energy transfers also known as?

Biomass transfers

How is energy transferred through an ecosystem?

1. Energy enters (typically through photosynthesis)

2. Plants convert sunlight energy into a form that can be used by other organisms → biomass

3. Energy is transferred through living organisms when organisms eat other organisms, e.g. producers → primary consumers → secondary → tertiary

4. Energy locked up in things that cannot be eaten (e.g. bones/faeces) is recycled back into ecosystem by decomposers

What do food chains + web show?

Energy transfer through an ecosystem

What are food chains/webs?

Food chains - simple lines of energy transfer

Food webs - lots of food chains in an ecosystem + how they overlap

What are 3 differences between plant + algae?

• Algae can be unicellular/multicellular organisms

• Algae live underwater whilst most plants live on land

• Algae have no leaves, stems/roots

How do consumers obtain the nutrients they need from other organisms?

Digestion

What is a byproduct of photosynthesis?

Oxygen - released back into the atmosphere

What is an example of a food chain?

Acacia tree

Giraffe

Lion

What is interdependence?

The way in which living organisms depend on each other to survive, grow + reproduce

What causes fluctuations in population sizes in food webs?

Fluctuations in population sizes of other populations

What is bioaccumulation?

When levels of chemicals build up in an organism as they are transferred along a food chain

What is succession?

Process by which an ecosystem changes over time

What are 2 types of succession?

• Primary Succession = happens on land that has been newly formed/exposed, e.g. volcano erupted to form a new rock surface/sea level dropped

• Secondary Succession = occurs on land that has been cleared of all the plants, but where the soil remains, e.g. after a forest fire/where a forest has been cut down by humans

What is the process of succession?

1. Primary succession begins

   1. Seeds + spores are blown by the wind + begin to grow

   2. Pioneer species colonise the area + grow because they are specialised to cope with harsh conditions

   3. Pioneer species change the abiotic conditions as they die + microorganism decompose the dead organic material

   4. Basic soil is formed → forming less hostile conditions (e.g. helping to retain water so new organisms can move in + grow)

   5. New organisms die + decompose → soil becomes deeper + richer in minerals

   6. Larger plants can grow

   7. More plants → create more habitats → more animals

2. Secondary succession

   1. Already a soil layer → pioneers species= larger plants → secondary occurs at a later stage

3. Species diversity (num of diff species + abundance of each species) increases

4. Amount of biomass increases → plants at later stages are denser + larger

5. Climax community - ecosystem is supporting the largest + most complex community of plants + animals → steady state

What are pioneer species?

First species to colonise an area

What is dead organic material also known as?

Humus

What are the conditions like at the start of primary succession?

• Harsh (e.g. no soil to retain water)

• Only pioneer species grow because they are specialised to cope with the harsh conditions →E.g. marram grass - sand dunes, deep roots to get water + can tolerate the salty environment

What is an example of primary succession?

Bare rock to woodland

• Pioneer species colonise the rocks e.g. lichens grown on + break down rocks, releasing minerals

• Lichens die + are decomposed helping to form a thin soil - thickens as more organic material is formed - so other species e.g. mosses can grow

• Larger plants that require water → can move in as soil deepens, e.g. grasses +flowering plants → soil continues to deepen as larger plants die + decomposed

• Shrubs, ferns + small trees begin to grow → out-compete the grasses + smaller plants to become the dominant species. Diversity increases.

• The soil is finally deep + rich enough in nutrients to support large trees → these become the dominant species + the climax community is formed

What results in different climax communities being formed? Example - ?

Different ecosystems

E.g. temperate climate → plenty of available water, mild temperatures + not much change between seasons → deep soils develop _ parge trees grwo