Part of the Local Ecosystems Module Spotlight Biology Preliminary Text Chapter 3 Authors: D. Heffernan, J. Bastina, B. Grieve, K. Humphreys, A. Sartor Science Press 2002 Populations and communities Topic 2: Populations

Populations The 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.au

Populations Single 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

Populations A 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

Populations The 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

Populations Many plants depend on birds or animals to spread their seeds. Plant ‘movement’ may be directly tied to the activities of certain animals dpi.vic.gov.au

Exponential Growth Almost 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 Growth Graphing 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 Growth Exponential 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 Growth J-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 Growth S-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.com

Limits to Growth Population growth may slow down and reach equilibrium any of three main reasons: 1. Drop in birth rate 2. Rise in death rate 3. Drop 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 Growth There are a number of factors that can limit population size which can include: Insufficient sunlight Deficiency in essential nutrients Shortage of prey To much or to little water, heat or humidity These factors can be natural but can also be the result of human activity.

Measuring Populations The 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.com

Measuring Populations A 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 Populations Lets pretend we’re 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= plants. Question: Find the population of saltbush plants in an area square metres if there’s an average of 13 in a quadrat 1m square.

Measuring Populations We can estimate the number of animals by using the capture-recapture method in which you: 1. capture a group of animals 2. tag them 3. release them 4. Capture another group of animals in the same area using the same technique 5. Count the total and the number with tags 6. Calculate the estimate blackwellpublishing.com

Measuring Populations Lets 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 Populations Lets 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 = So we can say that the total population is approximately 314

Measuring Populations The capture-recapture method provides only an estimate because we assume that: 1. Large numbers of individuals being studied did not move into or out of the area during the night and none were eaten 2. All members of the population are equally likely to get caught in the trap 3. No individuals lost their marking or tag

Homework Answer 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. 1. Why do J-shaped population growth curves rarely last long in natural populations? 2. Describe the techniques used to measure the population of a plant and animal species. Give examples in your response. 3. 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.