Chapter 43 Opener. Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales. Figure 43.2 Population Densities Are Dynamic and Interconnected. Figure 43.3 Life History of the Black-Legged Tick. Figure 52.3 Idealized survivorship curves.
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Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales
Figure 43.2 Population Densities Are Dynamic and Interconnected
Table 43.1 Life Table for the 1978 Cohort of Cactus Ground Finch on Isla Daphne
Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions
Apply the concept p.845 • Births increase and deaths decrease population size • Rancher Jane is thinking of shifting her operation from cattle to American bison but she needs to know well bison will do on her ranch. She buys 50 female bison that are already inseminated and places 10 of them, picked at random, into their own pasture. These 10 females serve as a sample from which Rancher Jane collects demographic data over one year. Use her data to answer the questions that follow.
Apply the concepts p. 845 Questions • Questions: • What is the total number of births and deaths among this sample population? • What are the estimated average per capita birth and death rates (b and d) for the entire bison herd, based on this sample? • What is the estimated per capita growth rate? • Based on these estimates, what is the size of Rancher Jane’s entire bison herd at the end of the year? • What two strategies could Rancher Jane use to expand her bison herd faster?
Concept 43.2 Births Increase and Deaths Decrease Population Size Change in population size depends on the number of births and deaths over a given time. “Birth–death” or BD model of population change:
Concept 43.2 Births Increase and Deaths Decrease Population Size Population growth rate (how fast it is changing):
Concept 43.2 Births Increase and Deaths Decrease Population Size Per capita birth rate (b)—number of offspring an average individual produces Per capita death rate (d)—average individual’s chance of dying Per capita growth rate (r) = (b – d) = average individual’s contribution to total population growth rate
Concept 43.2 Births Increase and Deaths Decrease Population Size If b > d, then r > 0, and the population grows. If b < d, then r < 0, and the population shrinks. If b = d, then r = 0, and population size does not change.
Multiplicatively growing populations have a constant doubling time • Yellow star thistle is a spiny annual plant native to the Mediterranean region. The species is a noxious weed that has invaded several regions of the US. It is unpalatable to livestock (including American bison). Rancher Jane discovers that 1 hectare of the 128 hectare pasture into which she placed her sample cohort of 10 female bison has been invaded by star thistle. A year later she finds the weed has grown to cover 2 hectares.
Questions: • Based on the information above, how many hectares do you predict the star thistle population will cover in 1, 2 and 3 more years if the population is growing additively? How many hectares if the population is growing multiplicatively? • Imagine that Rancher Jane only discovers the star thistle after it has covered 32 hectares of her pasture. How many years does she have until the weed completely covers the pasture if its population is growing additively? Multiplicatively?
Concept 43.4 Populations Grow Multiplicatively, but Not for Long Logistic growth r decreases as the population becomes more crowded; r is density dependent. As the population grows and becomes more crowded, birth rates tend to decrease and death rates tend to increase. When r = 0, the population size stops changing—it reaches an equilibrium size called carrying capacity, or K.
Figure 43.8 Per Capita Growth Rate Decreases with Population Density
Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 1)
Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 2)
Table 52.3 A Hypothetical Example of Logistic Population Growth, Where K=1,000 and rmax=0.05 per Individual per Year
Concept 43.5 Extinction and Recolonization Affect Population Dynamics The BIDE model of popultion growth adds the number of immigrants (I) and emigrants (E) to the BD growth model.
Figure 52.22 Age-structure pyramids for the human population of Kenya (growing at 2.1% per year), the United States (growing at 0.6% per year), and Italy (zero growth) for 1995
Apply the concept p.855 • Extinction risk decreases as population size increases • In a study of subpopulations of Daphnia in Swedish rockpools, ecologist Jan Bengtsson established a number of artificial rockpools that held either 12, 50, or 300 liters of water. He introduced Daphnia and then covered the pools with mesh, which prevented immigration or emigration. Over the subsequent 4 years, Bengtsson returned repeatedly to census numbers of Daphnia, thereby obtaining the average population size in each pool. If a population went extinct, he included only non-zero values for population sizes before extinction in calculating the average. We can calculate the grand average of these time based averages for pools in which the populations went extinct, and for those in which they persisted, to explore how extinction risk changes with pool and population size.
Questions: • Graph the relationship between pool size and probability of extinction, using size as the x-axis. What does your graph show you? • Within each pool size, which populations were at greatest risk of extinction? • How are these two results related to each other? • What might happen if Bengtsson had removed the mesh from the pools after some amount of time?