Biology Edexcel Topic 9 (Ecosystems)

9.1 to 9.8B

Outline the levels of organisation in an ecosystem

Individual, population, community, ecosystem

Define population

All organisms of the same species living with one another in a habitat

Define community

All of the populations of different species living together in a habitat

Define ecosystem

The community of organisms and all the non-living (abiotic) conditions of an area

Organisms within a community are described as being 'interdependent' - what does this mean

Organisms are dependent upon each other so a change in the population of one species could affect other populations with a community

Give some examples of interdependence in a community

Plants depend on pollinators (e.g. bees), herbivores are dependent on plants, and animals are dependent on mates

Describe mutualism

The interaction between two organisms where both benefit as a result of their relationship

Describe parasitism

The interaction between two organisms where only one organism (the parasite) benefits whilst the host does not

What are abiotic factors (give some examples)

The non-living aspects of an ecosystem - e.g. temperature, water availability, light intensity and pollutants

Describe how communities are affected by environmental conditions

Environmental conditions affect the abundance and distribution of organisms within communities (e.g. light intensity affects the rate of photosynthesis in producers which serve as a source of food and shelter for other organisms)

Describe how communities are affected by pollutants

Toxic chemicals bioaccumulate in food chains to deadly concentrations at higher trophic levels, killing tertiary and quaternary consumers, fertilisers can contaminate water sources, causing eutrophication, and air pollution can affect the ability of some plant species to survive

What are biotic factors (give some examples)

The living components of an ecosystem - e.g. competition and predation

Describe how competition affects communities

The presence of competitors affect population distribution and size, and if one species is better adapted to survive, it will outcompete the other species, causing their populations to decline

Describe how predation affects communities

Predation affects prey populations within a community (number of predators decreases, number of prey will increase as fewer are killed) - populations of all other organisms within the food chain will then be affected too

What piece of apparatus is used to measure the abundance and distribution of organisms in an area

A quadrat

What method is used to study the distribution of organisms across a gradient

A belt transect

What does a food chain show

It shows the feeding relationships between organisms and the resultant stages of biomass transfer

Define biomass

The total mass of living material

What are trophic levels

The stages in a food chain

What do arrows in a food chain represent

The direction of biomass transfer

Describe a simple food chain

producer -> primary consumer -> secondary consumer -> tertiary consumer

Why are producers the first trophic level

Producers provide all biomass for the food chain and can make their own food (via photosynthesis) - the rest of the food chain involves the transfer of this biomass

What does a pyramid of biomass represent

It represents the dry mass of living material at each trophic level of a food chain

Why is a pyramid of biomass almost always pyramid shaped

Producers (at bottom) have the greatest biomass as they produce their own, and as you move along the food chain (and up the pyramid), biomass is lost so the bars decrease in length

Why is biomass lost between each trophic level in a food chain

Glucose is immediately used for respiration in plants and respiration generates energy, some parts of the organisms are indigestible and there is some excretion of biomass

Why are there rarely more than four or five trophic levels in a food chain

Above this, there is insufficient biomass and energy to support another population

What is the equation for calculating the efficiency of biomass transfer between trophic levels

Efficiency = (Biomass available after transfer/biomass available before transfer) x 100