Lecture notes
Populations in ecosystems - A-Level Biology notes
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- Populations in ecosystems
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- AQA
Populations in ecosystems - A-Level Biology notes
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Chapter 19: Populations in ecosystems19.1 Populations and ecosystems
Ecology the study of inter-relationships between organisms and their environment. The environment includes
both abiotic (non-living) factors, like temperature and rainfall, and biotic (living) factors, like competition
and predation. Ex. lake or pond
Population a group of individuals of one species that occupy the same habitat at the same time and are potentially
able to interbreed.
Carrying capacity An ecosystem supports a certain size of a population of a species
Community All the populations of different species living and interacting in a specific place at the same time
Habitat The place where an organism normally lives and is characterised by physical conditions and the other
types of organisms present. Within an ecosystem, there may be many habitats.
Microhabitats Within each habitat there are smaller units, each with their own microclimate. Ex. the mud at the bottom
of the stream may be the microhabitat for worms.
Niche A place where an organism lives and what it does there. It includes all the biotic and abiotic conditions to
which an organism is adapted to survive, reproduce and maintain a viable population
Competitive No two species occupy exactly the same niche; some species may appear very similar, but their nesting
exclusion principle habits, behavioural traits or environmental preferences may be different.
Within an ecosystem there are 2 major processes to consider:
1. The flow of energy through the system The size of a population can vary as a result of:
2. The cycling of elements within the system - The effect of abiotic factors
Within each ecosystem, there are a number of species. Each species - Interaction between organisms
is made up of a group of individuals that make up a population.
19.2 Variation in population size
A graph with a log scale is used when a microorganism multiply rapidly over a short period of time.
There are a number of factors that determine the size of a population. Initially a population may grow quickly due to
there being no limiting factors, however there are many abiotic and biotic factors that will affect the size and rate of
population growth later.
The abiotic factors that affect population growth are:
1. Temperature - each species has a different optimum temperature that it is best able to survive at, the further
away from this the fewer individuals that are able survive.
2. Light - this is a basic necessity of light, with the rate of photosynthesis increasing as light intensity increases.
3. pH - this can have an impact on the action of enzymes with each enzyme having an optimum pH that it can work
at. Where the appropriate pH exists there is a larger proportion of organisms.
4. Water and humidity - in instances where water is scarce only small populations of adapted species will exist.
Humidity affects transportation in plants and therefore only those that are adapted to environments where
transpiration is high will survive.
19.3 Competition – The greater the availability of resources, the larger the population
1. Interspecific competition: members of different species compete with one another for resources
- One population will increase in size while the other will diminish (competitive exclusion principle)
- This occurs most often when different species occupy the same niche Ex. red and grey squirrels in the UK
2. Intraspecific competition members of the same species compete for resources , like food, water, mates, shelter,
minerals and light.
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