[Notes] - The Three Main Types of Population Dispersion

The Three Main Types of Population Dispersion

Population dispersion is the observation of where individuals are found in a habitat. How individuals "disperse" themselves. There are three main types of dispersion: clumped, uniform and random.

Clumped Dispersion

Is the tendency for populations to be found in tight clusters, dispersed across a large landscape. In between these population hubs, very few to no individuals are usually found. This sort of a dispersion can be caused by a number of factors. Some species cluster together for protection, while others group around natural resources necessary to their survival.

Uniform Dispersion

Is the tendency for populations to be found evenly distributed about their habitat. This is generally caused by a species ability to survive anywhere in their habitat - they use the resources found immediately around them, and spread out as to use all of the available resources.

Random Dispersion

Is the tendency for populations to be found randomly about their habitat. In immobile species, this is usually caused by their ability to live anywhere in a given habitat, except, they are limited to growing wherever they are first set root (which is usually caused randomly, from spores drifting in the wind to seeds falling and tumbling on the ground). In motile populations, individuals are able to move about their habitat, so that at any given instance, they can be found anywhere about their environment.

How Are Populations Sampled?

Using Quadrats to Calculate Populations

Using quadrats is a very simple way of accurately calculating immobile populations. The procedure is very simple - the number of individuals in a measured area (the area being a tiny fraction of the habitat of the species being counted) is counted, then averaged to get a population density. The total area of the species habitat is also figured out, then by multiplying the area of the habitat by the average density of individuals in an area, the total population can be derived.

For Example

Counting the number of blades of grass in a 200 square meter field would be extremely difficult and time consuming. So instead, several samples are taken from quadrats taken from around the field. If the average density of blades of grass found in these quadrants is 10,000 blades of grass per meter squared, then by multiplying that by the number of square meters in the field (200), the an approximate total population can be calculated (2 million blades of grass).

# individuals / sample area = density
density * total area = total population
10,000 * 200 = 2,000,000

Limitations of Using Quadrats

Quadrats give accurate results when the density of individuals does not change much from one location to another in the species' habitat. If one area of the habitat can sustain twice the number of individuals per unit of area than another area in the habitat, then it can drastically bias the results. Avoiding this is relatively simple though. By dividing the habitat into smaller areas with similar populations densities, then figuring out the size of the populations in those areas, and combining them, a more accurate model of the population of the total area can be calculated.

The Capture-Mark-Release-Recapture Method of Calculating Populations

Calculating the population size of motile creatures can be extremely difficult. As they are able to move, one cannot section off their habitat into a number of quadrats, and then count the number in each quadrat, as the animals being counted may simply move out of the quadrat. Furthermore, it can be very difficult to accurately gauge how many individuals are being counted. One would have to use some sort of distinct marking to make sure not to count certain individuals twice.

The Method to this Madness

A very clever method of counting motile populations has been devised. It is known as the Capture-Mark-Release-Recapture method. The procedure is very simple: capture a number of individuals of species who's population is being calculated, mark them so that each they can be later identified as separate from those that have not been captured, then release them. After waiting enough time to allow the marked sample to disperse back into its habitat, another sample of the same species is captured. By calculating the percentage of marked individuals in the second capture group, and knowing the total number of individuals that were marked, it is relatively simple to approximate the total number of individuals in a population.

For Example

To figure out the number of elephants in India, a research team captures, marks, and releases 500 elephants. Several weeks later, after allowing the marked elephants to recirculate into the unmarked population, another 500 are captured. If 50 of the newly captured elephants are marked (10% of the second sample), this suggests that around 10% of the total elephants in India were previously captured and marked. Since the first sample was of 500 elephants, and that is only 10% of the population, it is reasonable to expect that the total population of the elephants in India is close to 5000.

total marked / total individuals = sample marked / sample total
500 / total = 50 / 500
500 * 500 / 50 = total
total = 5000

Drawbacks & Limitations

The Mark-Recapture method (as it is often abbreviated) of calculating population sizes, although powerful, is only accurate under certain assumptions - that the population size of both marked and unmarked individuals stays the same, and that the second sample accurately portrays the ratio of marked to unmarked individuals. This means that to stay accurate there must be no immigration, emigration, deaths, births or shed markings, and that marking does not increase the ease of further captures. While many factors cannot be isolated, two things can be done to best avoid bias: waiting only enough time between captures so that the marked individuals can evenly disperse, and properly designing the marking method.

Avoiding Bias

Properly designed markings are crucial. They must be secure enough to not fall off or wear out. The markings must be obvious enough so that they are not missed or confused with other possible markings while counting the recapture sample, but must be discreet enough so that the marked individual is allowed to re-enter its natural habitat. The marking process and the mark itself must not change the likelihood of recapturing marked individuals (for example, some mouse populations used to be captured with bait. Some mice would become "trap shy" once captured, understanding that the food was a bait, others would become "trap friendly" once captured, returning multiple times to secure a free meal.) Lastly, the markings must not damage the creature's ability to survive. Although these requirements may seem difficult to fulfill, properly designed markings can greatly decrease the possibility of error in the counting.

Properly timing the recapture is also necessary. Enough time must be allowed so that the marked sample can disperse into the unmarked population, but, with more time, the probability of births, deaths, emigration and immigration increases. Calculating the best time is a tricky business of minimizing the amount of population change, while maximizing the amount of time allowing marked individuals to disperse.