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Population Estimation
There are written up notes for this lecture.
Definitions:
• Closed population - no change in size and composition from births, deaths
and movements during the study.
Open population - Changes in size and composition from births, deaths and
movements during the study.
Getting and understanding an idea of population size is fundamental to all
population studies.
Questions to think about when trying to get a population size:
How many organisms are there?
With plants, it is relatively easy to come up with an idea of the population
estimate as they don’t move like animals do and they can be counted. For
example when thinking about a particular wood or species of tree, you will be
able to count and prepare the trees within that certain area, then multiply up
the density of that particular species/genus/sub-species and make an
estimation of their population for that given area. For animals, it is slightly
more tricky as they are able to move around and migrate.
Do you need an indication of density?
Sometimes an idea of density is needed like knowing how many animals are
there per that unit area as you will need to know whether the environment you
are studying can support a given number of animals when it comes to things
such as grazing or mating, this can then help you understand the maximum
number of animals per unit area.
Is an absolute density needed or will an index do?
Does the above have to be absolute, where it is a precise number and
counting of the species or will an index, where there can be an estimated
number of species within the area, be acceptable for this study.
Will a range answer the question, or is an accurate estimate required.
What is the appropriate method of survey.
• Biologically:
Is it appropriate and possible to capture individual animals?
, • Ethically:
Is it ethically acceptable to remove individual animals, especially if it is an
endangered species. For example, if you have a rare species and your normal
method of capture is to kill the animals using kill traps or using fishing nets, is
it ethically appropriate to do so?
• Statistically:
Thinking about samples when it comes to the study, will one sample be
enough or will multiple samples be needed. Think about how many times you
may need to observe what you are studying.
How do these three points interact with each other?
How much will it cost?
How much will the study cost with regards to money and time, both of these
factors can be limiting. Time can be taken up by doing an extra survey and
potentially not having enough time to correctly go through the analysis side of
the study.
Have you got the money?
What’s the opportunity cost?
Total counts: census:
• Farmers stock records.
• Simple maths - only addition/subtraction.
• Straightforward for large animals by e.g. catch up in small reserves.
• Aerial counting/photos for index of density.
• e.g. marine mammals where many may be at sea (e.g. grey seals Halichoerus
grypus)
Simple methods to count animals would be simple addition or subtraction, like
what farmers do when counting sheep or cattle. This is straightforward and
animals can be cared through tags or spray paint when they have been
counted so losing track of the given species is minimal. Ariel photos and
counting can be used on species that might not be domesticated but within a
herd, for example marine mammals such as grey seals. The main issue with
this is it being extremely time consuming and can only really be used with
large animals in small reserves.
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