Assignment
Imagine that you are an Ecologist. Your assignment
will be to study the albatross population on an
island. One question you might ask if how the size
of the albatross population has changed over time.
Is the number of albatrosses on the island more than,
less than, or the same as it was 50 years ago? To
answer this question, you must first determine the
current size of the albatross population?
Chapter 20 Section 2 “Studying Populations”
Essential Question: How does ecologist determine the size of a
population?
Scientist use a variety of methods to determine the size of a population. We will go over four of
these methods: direct observation, indirect observation, sampling, and mark and recapture.
Determining Population Size
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Direct Observation- the most obvious way to determine the size of a population
(example, if you were observing crabs in a tide pool, you would simply count them one
by one to determine the total number of crabs).
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Indirect Observation- Figure 6, pg 682, shows mud nests that were built by cliff
swallows. For each nest there is one entrance hole. If you count the number of entrance
holes, you can determine the total number of swallow nest in the area.
Let’s say the average number of swallows per nest is 4 (2 parents & 2 offspring) if a total
of 120 nests were counted, then you can multiply 120 by 4 to determine that there are 480
swallows in the area.
120 * 4= 480
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Sampling- when it is impossible to count the total number of organisms within a
population, because the population is either to large or spread over a wide area, then
scientist makes an estimate.
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Estimate- is an approximation of a number based on reasonable assumptions.
An Example of sampling would be to estimate the size of population by counting the
number of organisms in a small area (sample), and then multiplying to find the number in
a larger area. For more accurate estimates, the sample areas should be typical of the
larger area or population.
For example: Suppose you count 8 birch trees in 100 square meters of a
forest. If the entire forest were 100 times that size, you would multiply your
count by 100 to estimate the total population, or 800 birch trees.
100 * 8= 800
The bowhead population around Alaska has increased since commercial whaling stopped. Alaska
Natives continue to kill small numbers each year. The population off Alaska's coast was about
10,500 animals as of 2001. Researchers from the School of Fisheries and Ocean Sciences have
been studying the whales feeding behavior in the Point Barrow area. In 2008, Canada's
Department of Fisheries and Oceans stated that previous estimates in the Eastern Arctic had
undercounted, with a new estimate of 14,400 animals these larger numbers correspond to prewhaling estimates, indicating this population has fully recovered. However, should climate
change substantially shrink sea ice, they could be threatened by increased shipping traffic.[15]
Mark and Recapture –
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Typically a researcher visits a study area and uses traps to capture organisms alive. Each
organism is marked with a unique identifier (a numbered tag or band), and then they are
released unharmed
first used by Meirion, Naim Salam and Nick Darling in 1894.
Time is allowed to pass for the organisms to return among the unmarked population.
Next, the researcher returns and captures another sample.
Some of the individuals in this second sample will have been marked during the initial
visit and are now known as recaptures.
Other animals captured during the second visit will not have been captured during the
first visit to the study area. These unmarked animals usually are given a tag or band
during the second visit and then are released.
Population size can be estimated from as few as two visits to the study area, the
researcher records the date of each capture, the "capture histories" generated are analyzed
mathematically to estimate population size, survival, or movement.
Changes in Population Size:
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Population can change in size when new members join the population or when
members leave the population.
The main way to join a population is to be born into it birth rate-the number of
births in a population in a certain amount of time.
The main way to leave a population is by dying. The death rate- is the number of
deaths in a population in a certain amount of time.
Population Statement:
If birth rate > death rate, population size increases.
If death rate > birth rate, population size decreases.
When organisms move in and out the size of the population will be affected.
Immigration- means moving in a population
Emigration-means leaving a population