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14 Investigating Population Growth Rates T on the sizes of populations ecosystems can support. These limits—the carrying capacities for particular populations—are determined by the resources available and the other species present in the ecosystem. Carrying capacities of an ecosystem change as the resources in the ecosystem change. The graph below shows 25 years of population levels of zebra mussels in a lake. here are limits CjbWZgd[oZWgV bjhhZaheZgb' (%%% '%%% &%%% % % * &% I^bZnZVgh &* '% '* This population of zebra mussels reached its carrying capacity around Year 14. Initially the zebra mussel population grew to a point where the lake could not sustain it, but by Year 14 the population had decreased to a level at which the eco('..H:EJEH<>:Xdad\nH7 system was able to sustain it over an extended period of time. What factors do you ;^\jgZ/('..:XdH7&)T%& 6\ZcYVBZY8dcY.$.#* think might change the carrying capacity for this population? As you learned in the previous activity, many organisms have interdependent relationships. In predator–prey relationships, the size and health of one population is frequently closely tied to the size and health of the other, as you learned in Activity 7, “Energy Flow Through an Ecosystem.” A change in a population level is one indicator that can reflect the health of an ecosystem. A population of a successful invasive species, such as duckweed, will increase rapidly in a new habitat, possibly causing the populations of other, native species to shrink. This can change both population and biodiversity levels. You saw several examples of this in the first few activities of this unit. The population growth rate describes the change in a population over specified intervals of time. A positive growth rate indicates that the population is Anchor covered with zebra mussels. 119 Science & Global Issues/Biology • ECOLOGY growing, while a negative growth rate indicates the population is shrinking. A population that remains the same size has a zero-growth rate. In this activity you will examine how different variables affect population growth rates of a fish population and the carrying capacity of the ecosystem the population lives in. Challenge 00 How do changing variables alter population growth rates and ecosystem carrying capacities? MATERIALS For each pair of students computer with Internet access colored pencils For each student Student Sheet 14.1, “Avril Gulf Seal Population” Student Sheet 14.2, “Investigating Population Growth” sheet of graph paper Procedure 1. With your partner, visit the Science and Global Issues page of the SEPUP website at sepuplhs.org/sgi and go to the population simulation. Use your student sheet to guide you through the simulation. Analysis 1. Describe the shapes of the graphs you created when working with the online simulation. Explain what the shape of the curve indicates about the population. 2. Based on your work in this activity, describe the relationship between birth and death rates and the growth rate of a population. 3. Based on your work in this activity, describe the relationship between carrying capacity and the size of a population. 4. How do the sizes of populations of other species relate to the sustainability of the human population? Think of at least two examples, and describe the relationship of each to the human population. 120 investigating population growth rates • Activity 14 5. Some bacteria reproduce by doubling once every 20 minutes. a. If you started with one bacterium, how many would you have in three hours? b.Describe what would happen if the bacteria continued to reproduce for 24 hours (do not calculate an exact number). c. Suggest at least two limiting factors that will ultimately stop the increase of a bacteria population. 6. Scientists and policy makers often have to work together to set catch limits on fisheries. Imagine that a policy maker proposes to allow the fishing of a certain species to go on at current rates because, she says, “Fishing at current rates will not impact the ecosystem. We are removing fish at a rate lower than the birth rate of their population.” Is this claim valid? What indicators would you look at to support or disprove this claim? Explain your reasoning. 7. Look at the population graph for the fish population shown below. How would you describe the growth rate? Is this population sustainable? 8. Examine the data in the Global Human Population graph provided below. Based on your work in this activity, how would you describe the global human population growth rate? Explain your reasoning. &&!%%% &%!%%% .!%%% -!%%% CjbWZgd[[^h] ,!%%% +!%%% *!%%% )!%%% (!%%% '!%%% &!%%% % &..% &..* '%%% NZVg '%%* '%&% ('..H:EJEH<>:Xdad\nH7 ;^\jgZ/('..:XdH7&)T%( 6\ZcYVBZY8dcY.$.#* 121 Science & Global Issues/Biology • ECOLOGY 9. Some people claim that the human population has passed its carrying capacity. What kind of environmental, social, and economic indicators would you examine to support or argue against this claim? ,#% <adWVa]jbVcedejaVi^dcW^aa^dch +#% *#% )#% (#% '#% &#% % % '%% )%% +%% Key vocabulary ('..H:EJEH<>:Xdad\nH7 ;^\jgZ/('..:XdH7&)T%( carrying capacity 6\ZcYVBZY8dcY.$.#* density-dependent factors density-independent factors 122 -%% &%%% &'%% NZVg &)%% &+%% population population growth rate &-%% '%%%