Variation in population size

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Emily

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a population is all the organisms of one species in a habitat
populations of different species in a habitat make up a community
population size is the total number of organisms of one species in a habitat:
this number changes over time bc of the effect of various factors
the max stable population size of a species that an ecosystem can support is called the carrying capacity:
varies as a result of both biotic and abiotic factors
abiotic factors and population size:
the population size of any species varies due to abiotic factors e.g. the amount of light, water or space available, or the temp or the chemical composition of their surroundings
when abiotic conditions are ideal for a species, organisms grow more quickly and reproduce successfully
e.g. when the temperature of a mammal's surroundings is the ideal temperature for metabolic reactions to take place, they don't have to use up as much energy maintaining their body temp - means more energy can be used for growth and reproduction - population size will increase
when abiotic conditions are not ideal for a species, organisms can't grow as fast or reproduce as successfully
e.g. when the temp of a mammal's surroundings is higher or lower than their optimum body temp, they have to use a lot of energy to maintain the right body temp - this means less energy will be available for growth and reproduction - population size will decrease
population size can also vary bc of biotic factors:
interspecific competition
intraspecific competition
predation
interspecific competition:
when organisms of different species compete with each other for the same resources
this can mean that the resources available to both populations are reduced e.g. if they share the same source of food, there will be less available to both of them
means that both populations will be limited by a lower amount of food
have less energy for growth and reproduction - population sizes will be lower for both species
if 2 species are competing but one is better adapted to its surroundings than the other, the less well-adapted species is likely to be out-competed - won't be able to exist alongside the better adapted species
e.g. Grey squirrels were introduced to the UK - now compete with the native red squirrels for same food sources and habitats
as they share the same food source - less available for both of them
so in areas where both red and grey squirrels live both populations are smaller than they would have been if there was only one species there
since introduction of the grey squirrels - native red has disappeared from large areas - grey squirrel has better chance of survival - larger and can store more fat over the winter - can also eat a wider range of food
intraspecific competition:
when organisms of the same species compete with each other for the same resources
can cause a cyclical change in population size around the ecosystem's carrying capacity - where the population grows, shrinks, grows again...
the is bc the population of a species increases when resources are plentiful
as the population increases - more organisms competing for the same amount of food and space
eventually these resources become limiting
if the population grows beyond the carrying capacity there will not be enough resources for all the organisms and the population will begin to decline
a smaller population then means that there is less competition for space and food - better for growth and reproduction - so the population starts to grow again
cyclical pattern continues
e.g.
lots of resources available - population of rabbits grew
population grew so large that the resources became limiting - carrying capacity of the ecosystem was exceeded - not enough resources - rabbit population fell
a smaller population of rabbits (below carrying capacity) meant there was less competition - population of rabbits began to grow again
Predation:
where an organism (the predator) kills and eats another organism (the prey)
e.g. lions predate on buffalo
the population sizes of predators and prey are interlinked - as the population of one changes it causes the other population to change
as the prey population increases - there is more food for predators - the predator population grows
as the predator population increases more prey is eaten so the prey population then begins to fall
less food for the predators - population decreases and so on
e.g.
the lynx population grew after the snowshoe hare population increased - bc there was more food available for the lynx
greater numbers of lynx ate a lot of snowshoe hares, so the population of hares fell
reduced snowshoe hare numbers meant there was less food for the lynx - population fell
predator-prey relationships are usually more complicated than this though bc there are other factors involved, like availability of food for the prey e.g. its thought that the population of snowshoe hare initially begins to decline bc there is too many of them for the amount of food available - then accelerated by predation from the lynx
investigating the population growth of bacteria:
with enough food and space, the size of a population of microorganisms e.g. bacteria will grow at a steady rate
this can be investigated experimentally by growing bacteria in a liquid broth - liquid containing the nutrients the bacteria need to grow
a liquid broth containing bacteria can be called a broth culture
when light is passed through a sample of broth culture, some of it is scattered bc bacteria are present - reduces the amount of light passing through the culture
a machine called a spectrophotometer can measure the amount of light passing through a sample of the culture and produce an absorbance value
the more bacteria present in a culture, the less light will pass through to be detected by the spectrophotometer
producing a higher absorbance value
so a broth culture sample with a high absorbance has a high number of bacteria present and vice versa
the beginning of the experiment:
most light passes straight through, so a low absorbance value is recorded
a few hours later:
less light passes through, so a high absorbance value is recorded
if you plot a graph of absorbance against time - get an exponential graph - shows the bacteria doubling in number at regular intervals
as the absorbance is proportional to the no. of bacteria in a sample - can convert the figures and draw a graph that shows how the population of bacteria changes over time
however the no. of bacteria present will increase hugely over time - hard to draw a scale on y-axis that can cover all values measured
to get around this problem - take a logarithm of the no. of bacteria present at each point
these log values will be much smaller than the no. of bacteria t each point
these log values will be much smaller than the number of bacteria so they'll be easier to draw a scale for
if you plot the log of the number of bacteria against time - straight line
interpreting data values from a logarithmic scale:
if you see microbial growth plotted on a graph with the log10 number of bacteria on the y-axis possible to work out how many cells are present in the culture at any given time by finding the inverse long/ antilog
logarithmic graph paper can also be used to plot microbial data on a logarithmic scale:
paper allows you to plot the actual number of bacteria rather than the log10 values
means you can read the number of bacteria present in the culture directly off the y-axis
increments on the y-axis are not evenly spread out
logarithmic graph paper has one axis with a logarithmic scale - the first 10 divisions on the paper gradually get closer togther until the 10th division when everything resets again
pattern repeats every 10 divisions
if you work out the number of cells present at 2 time points on a growth curve you can use them to work out the rate of growth during hat time period
log10 10,000 means how many 10s need to be multiplied together to get 10,000
4 - 10 x 10 x 10 x 10 = 10, 000
if the experiment continues for long enough, the number of bacteria eventually levels off, then declines as the bacteria used up all the nutrients in the culture
when bacteria are growing exponentially you can work out how many bacteria are present in a population using the formula:
number of cells = initial no. of cells x 2^n
where n is the number of divisions
e.g. if 300 bacteria grow exponentially for 7 divisions:
300 x 7^2 = 384000 cells
spectrophotometers can also output a value for the percentage of light transmitted e.g. a sample of clear liquid will have a very high percentage transmission bc most of the light will pass through it
a broth culture containing lots of bacteria will appear cloudy (turbid)