Topic 2: Populations

Part of the Local Ecosystems ModuleSpotlight Biology Preliminary Text Chapter 3 Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. SartorScience Press 2002Populations and communitiesTopic 2: PopulationsPopulationsThe word population has a very specific meaning to ecologists and geneticists. A population is a group of individuals of a single species in a given area at the same time that interact with each other and most importantly interbreed.

sydney.edu.auPopulationsSingle populations may either cover large areas or be restricted to small ones. This is dependent on the organisms ability to find and mate with one another.museumvictoria.com.au

PopulationsA strip of land just a few metres wide between two adjacent pond may form a barrier that completely separates individuals in each of the two populations. pureflorida.blogspot.com

PopulationsThe area covered by a population is determined by a number of factors. For example, many plants depend on insect pollinators. The distance over which the plants can exchange genetic material may be controlled by the movement of certain insects. iconiclandsinformedfarmers.com

PopulationsMany plants depend on birds or animals to spread their seeds. Plant movement may be directly tied to the activities of certain animalsdpi.vic.gov.au

Exponential GrowthAlmost any population of organisms that has plenty of food and is free of predators and disease will begin to grow rapidly. As long as local conditions remain favourable, the size of a population will grow.ddmrb.org.au

Exponential GrowthGraphing the size of a population over time produces a curve that rises slowly at first and then accelerates rapidly as the population doubles from 2 to 4 to 8 to 16, 32, 64, 128 and so on. This is known as exponential growth: the larger the population is the faster is grown. digitalc3.com

Exponential GrowthExponential growth curves are sometimes called J-shaped growth curves. The rate at which a population grows is determined by the birth and death rates of individuals.

See Handout Spotlight Biology Prelim Text Page 68.dreamstime.com

Limits to GrowthJ-shaped curves rarely last long in natural populations. It often peaks as competition for resources increases with the increase in number of organisms. If a laboratory population of bacteria is cultivated with a limited amount of space and food supply, its growth can be graphed with an S-shaped curve. modelkinetix.com

Limits to GrowthS-Shaped curves: At the top of this curve, population growth is zero. New individuals are being born but others are dying at the same rate. Therefore the total number of individuals remains constant and the populations is in equilibrium.modelkinetix.comLimits to GrowthPopulation growth may slow down and reach equilibrium any of three main reasons:Drop in birth rateRise in death rateDrop in birth rate and rise in death rate at the same time

There is zero population growth (ZPG) when the birth and death rates balance each other. Ecologists say that a population in equilibrium contains as many individuals as its particular environment can support for extended periods of time. The population has reached the carrying capacity of the environment for that species.Limits to GrowthThere are a number of factors that can limit population size which can include:Insufficient sunlightDeficiency in essential nutrientsShortage of preyTo much or to little water, heat or humidity

These factors can be natural but can also be the result of human activity.Measuring PopulationsThe most accurate way to find the size of a population is to count every member of it! However this is very difficult, if not impossible. We often use quadrats to estimate the number of plants and the capture-recapture method to estimate the number of animals.

true-wildlife.blogspot.comMeasuring PopulationsA quadrat is a square used to mark off a specific area. You can count the number of individuals within that area. You can then estimate the number of individuals in a large area by multiplying the number per square by the total number of squares in a large area. Confused?? true-wildlife.blogspot.com

Measuring PopulationsLets pretend were measuring the amount of grass plants on the school oval. We could make a quadrant that is 1 metre square. We count 100 grass plants in quadrat. To get a more accurate estimate you would get an average of say 5 quadrats.

Lets say the oval as 1000 square metres. We then to the maths: 100x1000=100000 plants.

Question: Find the population of saltbush plants in an area 12000 square metres if theres an average of 13 in a quadrat 1m square. Measuring PopulationsWe can estimate the number of animals by using the capture-recapture method in which you:capture a group of animals tag themrelease them Capture another group of animals in the same area using the same techniqueCount the total and the number with tags Calculate the estimate

blackwellpublishing.comMeasuring PopulationsLets say you want to count the number of ground beetles in the park. -First you would set some pit traps (a cup set into the ground) early in the morning.-At the same time the following morning you would count the number of beetles you caught (M1) and mark each with a bright coloured paint and then release them. csiro.au

Measuring Populations-You would then return the next morning and count the number of beetles caught again (N) and the number of beetles with paint on them (M2). You would then use the formula to calculate and average estimated population:

Total Population = original number tagged (M1) x total recaptured (N) number tagged on recapture (M2)

Measuring PopulationsLets look at an example:-50 animals are caught and tagged and released. Later, 220 animals are caught and it is noted that 35 of these animals are tagged. What would be the total population estimate?

Total Population= 50 220 35 = 314.28

So we can say that the total population is approximately 314

Measuring PopulationsThe capture-recapture method provides only an estimate because we assume that:Large numbers of individuals being studied did not move into or out of the area during the night and none were eatenAll members of the population are equally likely to get caught in the trapNo individuals lost their marking or tagHomeworkAnswer the following questions in your notebooks. Come to class prepared to discuss next lesson. Remember you will also be marked as attempting/not attempting these.Why do J-shaped population growth curves rarely last long in natural populations?Describe the techniques used to measure the population of a plant and animal species. Give examples in your response.A biologist catches 100 deer in a forest, tags them and releases them back into the forest. A year later he catches 90 deer, of which 12 have tags. Estimate the total population of deer.