Summary
Summary Unit 3.5. - Populations and ecosystems
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- WJEC
Notes on key words, energy flows, carrying capacity, nitrogen cycle, sampling methods, succession and more
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Populations and ecosystems Ecosystems are dynamic – they have changing amounts of energy flow, biological cycles,
succession, species composition and their population sizes are constantly changing. They are
subject to change
Population ecology looks at why a population is the size it is
Population numbers fluctuate due to various factors split into biotic and abiotic groups
Population size is dependent on birthrate, death rate, immigration (coming in) and emigration
(leaving)
Population will increase if birth rate + immigration is greater that death rate + emigration
Bacteria are good for population studies because they have a short life cycle so results are
obtained quickly
A generalized graph of population change after initial successful colonisation is split into 4
phases:
1. Lag phase – little cell multiplication and growth (only a few individuals so rate of growth
slow). Period of adaptation and preparation with intense metabolic activity
2. Log phase – exponential growth and rapid cell division as no factors are limiting it
3. Stationary phase – Birth rate is equal to death rate as nutrients deplete and toxins built
up etc
4. Death phase – Death rate is greater than birth rate and there’s sharp decline in
population e.g. food supply is being used up
If a population exceeds its carrying capacity, individuals will die due to lack of resources
Carrying capacity dependent on availability of resources and effect of competition for the
resources
Increase carrying capacity by: increasing food, mates, size of area, disease resistance, remove
competitors
Abiotic factors inc: climatic factors, soil (edaphic) factors, topographic factors, human factors &
catastrophes
Abiotic factors tend to be density independent factors e.g. low light intensity limits plant growth
regardless of number of plants
Abiotic factors vary by season which can cause periodic oscillation in population size – this is
seen in species with a short life cycle compared to the seasons like insects. Species with longer
life cycles don’t change with seasons.
Biotic factors ten to be density dependent factors for example competition will be greater the
larger the population
Interspecific competition is competition for resources between members of a different species
(generally one species out competes the other having a dramatic effect on population). This can
be demonstrated with paramecium
Intraspecific competition is competition for resources between members of the same species.
They have the same niche so compete for exactly the same resources – it has a stabilising
influence on population size.
Population increase -> more intraspecific competition -> population decreases -> less
intraspecific competition -> etc
Populations of predators and prey depend on each other so show cyclical changes
Famously measured for populations of lynx and hares in Canada
, Prey increases -> predator increases -> prey decreases -> predator decreases etc
Similar pattern for parasites and their hosts
Parasite increases -> host decreases -> parasite decreases -> host increases -> etc
In harsh environments, abiotic factors govern who survives because only a few species will have
successfully adapted to the conditions so have less competition whereas in milder
environments, biotic factors govern who survives such as competition
Organisms ecological niche is its role in its food chain and also refers to biotic and abiotic factors
it needs in its habitat
Competitive exclusion principle: Two species cannot coexist in the same habitat if they share the
same niche
Species with narrow niches are called specialists
Specialists can coexist because they’re not competing leading to high diversity. They rely on a
constant supply of their food so are found in abundant stable habitats like the tropics
Species with wide niches are called generalists
Generalists in the same habitat will compete so there is low diversity. They cope with a changing
food supply or changing habitats due to seasons
Density dependent factors generally operate in a large population but density independent
factors operate in small and large population
Density dependent examples: food, shelter, competition, predation, disease
Density independent example: flood, fire, drought, hurricanes, extreme temps
Most abiotic factors are measure with special digital electronic equipment using a sensor or
probe connected to an amplifier or digital display
Lots of quantitative measurements for biotic factors:
1. Abundance – number of organisms in a sample
2. Richness – number of different species in a sample
3. Diversity – Simpsons index
4. Growth – measuring wingspan, mean lengths, number of leaves etc
5. Biomass – measuring the dry mass
Plants best sampled with quadrats as they don’t move
Quadrats let us make quantitative measurements of the abundance of plants either with:
1. Density – Count the number of individuals of a species in a quadrat and divide by the
area of the quadrat
2. Species frequency – Record the number of quadrats in which a species was found e.g.
12/30 = 40%
3. Percent cover – useful when plants are difficult to identify. Percentage area of quadrat
covered by particular species is estimated
Sessile animals (don’t move) and sedentary animals (move slowly) can be sampled with quadrats
like limpets
Other animals move too fast so need other methods:
1. Sweep nets – large fine meshed nets swept back and forth over vegetation catching
insects and invertebrates
2. Kick sampling – D net help on bed of stream facing upstream and mud and stones are
kicked so animals are dislodged and carried downstream
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