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Lecture 5

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University of British Columbia
BIOL 111
Chin Sun

5 Population Potential Learning Outcomes: 1. Identify factors that affect population size. 2. Compare real population growth curves to models 3. Explain what factors affect carrying capacity of an environment. 4. Predict changes in population size due to various abiotic or biotic factors. Readings: Chapter 13: 343-355; Lester Brown interview “World in the Balance” linked in Week 5 Lecture Module Reminders: Midterm Friday Oct 7 from 8:00 to 8:50 in your normal lecture room A population is made up of the Individuals of one species living in a given geographic area at the same time. Populations have certain attributes/characteristics of interest to population ecologists: - the density or the overall size of a population. - the dispersion or spacing (spatial distribution) of individuals in a population - clumped, uniform, or random. - the numbers of individuals in different age classes, or sexes. The spatial distribution of individuals in a population Under what conditions would individuals in a population manifest a clumped distribution? - uniform distribution? The size of a population changes over time due to births, deaths and movements of individuals. These processes are influenced by the interactions of individuals with their environment and with one another. Plot a figure using time as the X axis and Total No as the Y axis; what shape is the line you drew? 1 2 3 4 Time No. of No. born Total No. individuals in No. the population 0 1 0 1 50 1 1 1 2 2 2 2 4 30 3 4 4 8 4 8 10 5 6 1 2 3 4 5 6 Time The growth rate of this population (no. of new individuals added as the percentage of the current no.; column 3 divided by column 2) is constant (in this case, 100%). However, the actual number of individuals added onto the population at each time interval increases steadily - exponential growth A population experiences an exponential growth when the resources are not limited (e.g., when the population size is small). Twelve pairs of rabbits were released in Victoria, Australia in 1859. The population increased to over 120,000 in 6 years. Yet, the present day population sizes of rabbits in Australia remain more or less stable. For this population of yeast cells growing in a culture in a test tube draw a simple plot below of this data and predict the population size at 120 and 135 minutes. Time in Number of minutes cells 15 27 30 932 45 850,000 60 980,000 75 823,000 90 788,000 105 500,000 120 135 QUESTION: What has begun to happen beyond 60 minutes? Natural populations are regulated by density dependent factors, whose strengths and impacts on the population are proportional to the population size (population density).  Shortage of essential resources for survival, growth and reproduction  Risks of infectious diseases  Accumulation of (toxic) wastes excreted by individuals Do you see any commonality among these factors? Due to the effects of these factors, the birth rate begins to decline while the death rate begins to increase. Logistic growth models take into account the effects of density dependent factors on population growth. In the logistic growth model on the left, the number of individuals added to the population begins to decline as the population reaches a certain level (where the arrow pointed). As the population size reaches the carrying capacity of the environment, the population ceases to grow and stays more or less stable. Carrying capacity – the maximum population size that can be supported in an environment indefinitely (without degrading the environment). Figure 13.6 in your textbook Besides density-dependent factors, density-independent factors also affect the size of populations. Examples of density independent factors include severe weather, fire, storms, drought, and other environmental catastrophes. commonality? QUESTION: Which segments of this figure could represent the following scenarios ? 1) extreme climatic event? 2) rapid growth due to ample resources and lack of predators? 3) competition for resources and a gradual reduction of reproductive rate
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