Ecosystems

Question 1

What is meant by a population?

Answer 1

The number of organisms of the same species living within an area

Question 2

What is meant by a community?

Answer 2

All of the populations of species living in an area

Question 3

What is meant by a habitat?

Answer 3

An environment where an organism naturally lives

Question 4

What is meant by an ecosystem?

Answer 4

• An area with many biotic and abiotic factors interacting with eachother
• They can be large or small

Question 5

How would you investigate the population size of an organism in two different areas?

Answer 5

• Measure the total area of the habitats (e.g. fields)
• Use a random number generator to find two different locations, one in each habitat
• Place two 1m² quadrats in both of these locations
• Count the number of plants of the given species found inside each quadrat, and multiply it by the total number of m² of the habitat to find the approximate population size in that habitat
• Compare the results for the two habitats
• The biodiversity in two different areas can be measured by counting the number of species in each quadrat and working out the percentage cover of each organism by measuring what percentage of squares (out of the 100) in the quadrat are occupied by a certain organism

Question 6

What is meant by biodiversity?

Answer 6

• The range and variety of species in an ecosystem
• It takes into account the number of different species, but not the population size
• However it does take into account the distribution - the more evenly distributed the populations of each organism in an ecosystem are, the higher the biodiversity

Question 7

How do biotic factors affect the population size and distribution of organisms?

Answer 7

• Higher numbers of prey mean more food is available and organisms can survive and reproduce for longer
• Higher numbers of predators means organisms may die before having the chance to reproduce
• If there are multiple organisms competing for the same resources (e.g. prey), the better adapted organism will get the resource more and survive longer while the other one may die before reproducing

Question 8

How do abiotic factors affect the population size and distribution of organisms?

Answer 8

• Higher light intensity, carbon dioxide levels and wind speed (because it increases the rate of transpiration and therefore movement of water) will increase the rate of photosynthesis, increasing the population of plants
• Plants and animals require the correct temperature, levels of moisture, and levels of oxygen for survival
• Plants also rely on soil pH and mineral content

Question 9

What are the six different trophic levels and what do each of them do?

Answer 9

• Producers - They produce their own nutrients from the sun
• Primary consumers - They feed on producers
• Secondary consumers - They feed on primary consumers
• Tertiary consumers - They feed on secondary consumers
• Quaternary consumers - They feed on tertiary consumers
• Decomposers - They feed on dead organisms or undigested waste through extracellular enzymes

Animals can be at different trophic levels in an ecosystem at the same time

Question 10

What is a food chain?

Answer 10

They show the transfer of energy from one organism to the next through the consumption of eachother

Question 11

What is a food web?

Answer 11

• A network of interconnected food chains
• It is a more realistic representation than a food chain because organisms rarely rely on just one other

Question 12

What is a pyramid of numbers?

Answer 12

• A pyramid showing the number of each organism at each trophic level
• It is usually a pyramid shape though not always as 1 tree can feed 500 ladybugs (for example)

Question 13

What is a pyramid of biomass?

Answer 13

• A pyramid showing the total biomass of the organisms at each trophic level (excluding the water levels)
• They will always be pyramid shaped as it is impossible for a larger mass to feed on a smaller mass
• They are much better at representing the amount of each organism and therefore the interdependence

Question 14

What is a pyramid of energy transfer?

Answer 14

• They illustrate the amount of energy contained within the biomass of the organisms within a trophic level
• They will also always be pyramids as large amounts of energy are lost as you ascend the trophic levels

Question 15

How is energy transferred along a food chain?

Answer 15

• Light energy from the sun is converted into chemical energy by producers, some of which is used for the plant to grow
• Primary consumers eat the producers and secondary consumers and tertiary consumers then eat eachother, using some of the energy they derive from the biomass of their food to grow
• Eventually, once the tertiary/quaternary consumers die, most of the energy is transferred to the environment - therefore this process is non-cyclical

The non-cyclicity is in contrast to the chemical elements of organisms which are recycled to form new organisms such as helping plants in soil grow (by decomposers etc.)

Question 16

On average, what percentage of energy is transferred to the next trophic level in a food chain?

Answer 16

10%

Question 17

Why is most of the energy lost from trophic level to trophic level?

Answer 17

• Not all of the light energy plants receive from the sun is used to make new cells which can be eaten
• The primary consumer then does not consume all of the comestible cells in the producer - and not all of the energy they do consume goes into making their own comestible cells as much is made into bones etc. which are not eaten and much is not digested (faeces)
• Lots of the consumed energy is also used up while the organisms are alive
• This process repeats up until the final organism in the food chain

• The fact that so much energy is lost explains why food chains are rarely longer than 5 organisms
• A lot of the uneaten material is consumed by decomposers
• Biomass is also lost at each stage

Question 18

What are the ways in which organisms lose energy while they are alive?

Answer 18

• Waste products - urine (urea and water), faeces (undigested food) and carbon dioxide for animals - oxygen, carbon dioxide and water for plants
• Metabolic reactions
• Movement
• Heat energy (from respiration for plants and animals - mammals must maintain homeostasis)

Question 19

What are the stages of the carbon cycle?

Answer 19

• Carbon is taken out of the atmosphere in the form of carbon dioxide and used in photosynthesis by plants, turning it to glucose
• This glucose passed on to animals by them feeding on plants, and passed on to other animals through feeding on eachother
• Some is returned to the atmosphere in the form of carbon dioxide through the respiration of all organisms
• Otherwise, if organisms die and no decomposers are present, their mass will be converted to fossil fuels over millions of years
• These fossil fuels can then be combusted, releasing the carbon in the form of carbon dioxide into the atmosphere again
• The carbon in undigested or dead organic matter which is not fossilised is released by decomposers

• Carbon is needed for life to exist as it makes up a large amount of living matter
• Carbon is a finite resource so needs to be recycled to allow new life to grow

Question 20

What are the stages of the nitrogen cycle?

Answer 20

• N₂ is very stable and cannot be absorbed from the air
• However nitrogen fixing bacteria in the soil can take nitrogen gas and turn it into nitrates in the soil
• Alternatively, lightning can split the nitrogen atoms apart, where they will then turn into nitrous oxides, dissolve into rainwater and leach as nitrates into the soil
• Plants can then absorb these nitrates and turn them into proteins
• Animals eat plants (or other animals) and derive nitrogen from them for their own use
• When animals or plants die (or egest), the organic matter decays and turns into ammonium compounds which are put into the soil by decomposers
• Ammonium cannot be absorbed by plants, but through nitrification, nitrifying bacteria in the soil will turn ammonium compounds into nitrites then nitrates which can be absorbed by plants
• However, another type of bacteria in the soil called denitrifying bacteria can take some of these nitrates and convert them into N₂ gas, releasing it back into the atmosphere

• Nitrogen, which is finite, is needed by life as it is a fundamental part of proteins
• Farmers will not want denitrifying bateria as they want nitrates in their soil