IB Biology Sub-topic C4.1 Notes

Populations

After successful competition, a species can establish their niche in a habitat and begin a population. Remember that this is an interacting group of organisms of the same species living in an area. You are expected to understand how a population in a habitat behaves in response to external pressures.

For this to be possible, population numbers need to be monitored. A population can be monitored and aspects of it measured to provide information about that population. Population composition, population dynamics and population distribution and abundance are all such attributes that can be studied.

This is mainly performed by random sampling, which is useful to provide insight of the population dynamics. Such insights may reveal information about the composition, distribution, density or more of that population. It is often neither feasible nor practical to count all members of a population as there are often too many individuals and time is not infinite. Thus, sampling is a means of circumnavigating these setbacks to get a good insight about the population in question. Precisely how well the sample represents the population, is a question of understanding the significance of the extent of error.

Sampling can provide information about:

1. Population size - how many members of that population reside in a specific area. Counting some individuals in a stated area and extrapolating to approximate the entire population size is a way to estimate this.
2. Population distribution - how a given population is distributed, and whether it is  stable or fluctuates. This is useful when investigating the geographical range of species.
3. Age distribution - the abundance of different age groups, including infants, adults, and elderly organisms. This can be used to determine breeding programs for endangered species for example.

Note that estimating population size is not without challenges. Populations may be unevenly distributed over vast areas, they may be mobile, they may be hidden; In any given area there may be fewer or more individuals than in any other area. Population statistics must take this into account when estimating population size. It is assumed that any such estimate will come with a margin of error as not all the population has been measured.

For this reason, sample statistics will always be somewhat of a departure from any precise population parameters. The difference between these values is the sampling error. However, as we rarely know the actual population parameters, the sampling error is estimated using a confidence interval.

Methods of estimating population size

While random sampling will hold true for any estimation protocol, different organisms are present in different numbers and move around differently, requiring different estimation methods. The two main methods you need to be aware of are:

1. Random quadrat sampling - mainly used for sessile species. During this, a series of random placements of a quadrat frame of known dimensions, across an area, are used to estimate the abundance or diversity of organisms. Quadrat sampling can also be used to test for the association between two species. For this:
   1. Enter the observed values (O) into the contingency table.
   2. Calculate the expected value (E) and enter into the contingency table.

   3. Calculate (O-E) and use it to find the value of chi squared (χ²):

      $\chi^{2} = \Sigma \frac{(O-E)^{2}}{E}$

   4. Use the χ² table to determine the critical χ² value. For this, the degrees of freedom are always 1, and the significance α is typically 0.05, unless specified otherwise.

2. Capture-mark-release - mainly used for motile species. A sample of animals is first randomly captured, marked, and released back into the wild. After a period of time, a second capture occurs, which may capture previously caught organisms. Population size can be determined using the Lincoln index.

   $N = \frac{MS}{R}$

   Here, N is the population size estimate, M is the marked individuals released, S is the second capture size, and R is the number of marked animals recaptured.