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September 1997
B io Factsheet Number 5
An Idiot's Guide to Populations
April 1997 Number
A population is a group of organisms of one species in a particular location. The location may be defined by natural boundaries (eg. an
island) or defined by the researcher (eg a particular field). We may therefore refer to "the population of frogs in a valley" or "the population
of moths in an orchard".
What Determines Population Size? Table 1. Doubling times for selected species
Changes in population size are caused by:
Species Common Rate of increase Doubling
Births name (individual/day) time
Migration E.coli Bacterium 58.7 17 minutes
Hydra Hydra 0.34 2 days
What determines whether a population increases, decreases or remains R. norvegicus Brown rat 0.0148 46.8 days
constant is the balance between these factors. B. taurus Domestic cow 0.001 1.9 years
A. marina Mangrove 0.00055 3.5 years
Population Growth
N. fusca Southern 0.000075 25.3 years
Neglecting migration, populations will increase if the birth-rate is greater
than the death-rate. Since members of a species of breeding age will normally
replace themselves by a greater number of offspring, populations will tend
to increase unless prevented by unfavourable conditions.
What Stops Growth?
The factors that reduce or stop population growth must act by reducing the
The simplest type of population growth occurs when both birth and death
birth rate and/or increasing the death rate. They are known collectively as
rates are approximately constant (with, of course, the birth rate exceeding
environmental resistance. These factors may be categorised in two ways:
the death rate). An equivalent way of looking at this is to say that the number
Biotic or Abiotic.
of births and deaths is directly proportional to the number of individuals
Density dependent or Density independent
in the population; in other words, if the population size doubled, so would
Abiotic factors include things like climate and weather as well as
the numbers of births and deaths. This leads to exponential growth (see
environmental pollution. Their effect does not have to be negative - unduly
Fig 1)
warm or wet weather may actually favour some species.
Fig 1. Exponential population growth Biotic factors may be due to competition within the species for food and
territory, competition with another species for resources, predation or
parasitism. They may act very indirectly - what happens to a species in
one part of a food web is likely to influence the numbers of many others in
the same web.
Population Numbers
These factors may effectively combine - for example, a change in climate may
reduce plant growth, which will lead to a reduced food supply for herbivores;
this reduced supply may lead to increased competition for resources.
Density Independent factors are those whose effect does not change
proportionately when the population increases or declines. This does not
mean they must always affect the same numbers of the species - it means
they must always affect roughly the same percentage. Abiotic factors will
usually be density independent.
Density Dependent factors, then, change in their proportionate effect as
population numbers change - most will have a much greater proportionate
An important characteristic of populations growing exponentially is that effect on a large population. For example, competition for resources both
they have a constant doubling time. This means that if it takes 2 years for within and between species will not have a noticeable effect if the species
a population to increase from 15 individuals to 30 individuals, in another 2 concerned is sparse and there is plenty of land and food to go round. Common
years it will increase from 30 to 60, and in two years after that it will increase density dependent factors include space, food, intraspecific competition
from 60 to 120. Table 1 shows the rates of natural increase and doubling times (i.e. competition between individuals in the same species), predation and
for selected species. parasitism.
Exponential population growth quickly leads to a massive explosion in
numbers. Exponential growth can only be maintained as long as resources Exam Hint: Be specific in your use of terms such as "exponential
(food, space etc.) are effectively unlimited. This will be the case while the growth". Marks are awarded only for a completely correct use -
exponential growth occurs when there is a constant doubling time,
population is relatively small, or when a new area with no resident
not just if growth is rapid.
competitors is being colonised.

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An Idiot's Guide to Populations BioFactsheet
Density dependent factors, however, usually exert a regulatory effect - they Variable Carrying Capacity
act to reduce the rate of increase as the population grows larger. This is The carrying capacity is determined by the availability of resources. This
usually modelled by considering the death rate to increase proportional to availability is itself subject to the influence of climate and weather, as well
population size (due to disease, predation, parasitism and poor nutrition) as pollution and other man-made influences.…read more

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An Idiot's Guide to Populations BioFactsheet
Population dynamics for two species · The numbers of predators will in general be smaller than those of
The interaction between two or more different species is often the dominant prey,since they are higher in the food chain
influence on the numbers of both. There are two main types of interaction
· The amplitude of the oscilllations will be smaller for predators,
- interspecific competition (i.e. competition between species) and
since adding one extra predator influences prey numbers
predation.…read more

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An Idiot's Guide to Populations BioFactsheet
Fig 5 shows the variation in populations of red grouse and its parasite, the Fig 5. Population cycles for red grouse and nematodes
nematode worm. bird over a 13 year period. The graph shows average
number of parasites per bird, not total parasite numbers; the latter could be 10000
obtained by multiplying the mean number of parasites by the number of
birds. The parasites' eggs pass out of the grouse with faeces.…read more

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An Idiot's Guide to Populations BioFactsheet
Practice Questions 3. The graph illustrates the variation in numbers of hare and
lynx populations over the period 1845 -1935.
1. In an investigation to estimate the population of woodlice in a dead
tree, 100 woodlice were captured and marked, then re-released into
the tree. 4 weeks later, 150 woodlice were captured, of which 15 140 hare
were found to be marked. lynx
Population numbers (000s)
a) Calculate the number of wood-lice in the population.…read more


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