Population (Lily) Activity-1

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Pennsylvania State University, Berks *

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419

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Biology

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Dec 6, 2023

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Population Growth Exercise 1: Exponential Growth of Populations Which one of the graph lines in this graph does your lily pad graph most resemble? Highlight or put a star or somehow identify that graph line. The graph you have constructed represents exponential growth . It is called a " J-curve ". Exponential growth occurs when there are no limits to the size to which a population can grow. Food, water, and living space are some of the resources individuals need to live healthfully and to reproduce to their maximum potential. When such resources are unlimited, populations may undergo exponential growth. Some characteristics of exponential growth curves are slow initial growth and then a rapid, dramatic population "explosion" after several generations. There are several factors affecting the shape of the growth curve.  reproductive rate (number of new organisms produced compared with number of deaths)  limits on resources: food, water, living space 4. Would there be a maximum number of lily pads that can grow in your pond (your table top)? Explain. 5. Resources and space are typically limited in ecosystems. Exponential growth generally occurs only when the population is either a) very small relative to available resources or b) very aggressive in taking resources away from other populations. What type of interaction would the latter (situation b) represent? Exercise 2: Can populations grow forever? 1. Examine the graph shown below. What do you notice about the graph? How is it different from the graph of lily pads you saw earlier? 2. What do you suppose could explain this type of population growth?

As you may have predicted, there is a maximum number of lily pads that can grow in your pond. This is called the carrying capacity . There are several factors which limit the carrying capacity of any environment. Some of them are:  climate (temperature, etc)  food and water availability  salinity  sunlight  physical space  disease  predators 3. Which one of the above was the limiting factor in the lily population simulation? What about in a real life situation? The figure above shows what happens to a population when it reaches the carrying capacity of the surrounding environment. This is called an " S curve " because it is roughly shaped like an "S". Label the graph as an S curve then identify the point in the growth where carrying capacity is reached. 4. What is the carrying capacity of the population recorded in the above figure (about how many organisms of this species can be supported in this ecosystem)? 5. Take a look at the figure to the right. What do you notice is occurring to the population? What happens at point "A"? What happens at point "B"? What could explain such a population growth pattern? How is it different from the previous graph? If there is a sudden change which affects the amount of available resources (for instance, a drought or a frost) a population which is growing exponentially may experience a dramatic decrease in size. This is called a population crash . Some populations experience cycles of exponential growth followed by crashes. This pattern is called boom and crash and is illustrated in the above figure. 6. Sometimes a population drops to zero when it crashes. An example is the Hohokam tribe in central Arizona, which lived there more than 2,000 years ago. Experts believe that the population was at one time over 1 million people, but somehow the entire Hohokam culture vanished. Can you think about another population that we learned about which had a similar fate? Explain.

7. If a factor that stops a population’s growth is influenced by the population’s density, then it is called a density- dependent limiting factor . Density-dependent limiting factors come into play when a population reaches a certain number of organisms. For example, when a population reaches a certain size, there won’t be enough resources (food, shelter, water) for all of the organisms. This could cause the population to stop growing when it reaches its ______________?________________ If the population’s density does not influence whether or not the factor stops the population’s growth, then it is called a density-independent limiting factor . These include natural disasters, temperature, sunlight, and the activities of humans in the environment. Natural disasters such as tornadoes, floods, and fires will stop a population from growing no matter how many organisms are living in a certain area. The same goes for the temperature of an area and the amount of sunlight an area receives. Human activities that alter the environment will also decrease the amount of organisms in a population, no matter the size of the population. 8. Imagine that the squirrel population near campus is growing rapidly. a) List possible density-independent factors that could stop the squirrels’ population’s growth. b) List possible density-dependent factors that would limit the squirrels’ population’s growth. Exercise 3: How Do Populations Affect Each Other? 1. Examine the graph showing the populations of lynx (predator) and snowshoe hares (prey) throughout the years. What do you notice is happening with each population? What do you think could be a scientific explanation for the observed data? What type of interaction is taking place? 2. In the early 1900's, wild rabbits were taken from England to Australia to be used for hunting. Since then, rabbits have multiplied exponentially and have severely affected the ecosystem where they live. The total damage done by wild rabbits in Australia is estimated at $600 million dollars. The loss of vegetation from rabbit grazing threatens the survival of native birds, mammals, and insects that rely on plants for food and shelter. Wild rabbits compete with livestock for available pasture and kill young trees and shrubs. The holes they dig contribute to soil erosion by removing vegetation and disturbing soil. a. Why do you suppose this problem did NOT occur in England or even here in the U.S. where the rabbit population remains at a relatively constant size? b. In what ways are the rabbits in Australia similar to the lily pads mentioned in Exercise 1? c. How did the introduction of the rabbits in Australia impact the food web?

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