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The MacArthur-Wilson theory of biogeography: Consider an island separated from the mainland, which contains a pool of potential colonizer species. The MacArthur-Wilson theory of biogeography \({ }^{2}\) hypothesizes that some species from the mainland will migrate to the island but that increasing competition on the island will lead to species extinction. It further hypothesizes that both the rate of migration and the rate of extinction of species are exponential functions, and that an equilibrium occurs when the rate of extinction matches the rate of immigration. This equilibrium point is thought to be the point at which immigration and extinction stabilize. Suppose that, for a certain island near the mainland, the rate of immigration of new species is given by $$ I=4.2 \times 0.93^{t} \text { species per year } $$ and that the rate of species extinction on the island is given by $$ E=1.5 \times 1.1^{t} \text { species per year } . $$ According to the MacArthur-Wilson theory, how long will be required for stabilization to occur, and what are the immigration and extinction rates at that time?

Short Answer

Expert verified
Stabilization occurs at \( t \approx 15.2 \) years with rates of immigration and extinction both roughly 0.611 species per year.

Step by step solution

01

Define the Problem

We need to find the time \( t \) when the rate of immigration \( I \) equals the rate of extinction \( E \), as stabilization is when these rates balance each other.
02

Set the Equations Equal

Set the given equations for immigration and extinction equal to each other to find the stabilization point where \( I = E \): \[ 4.2 \times 0.93^{t} = 1.5 \times 1.1^{t} \]
03

Solve for \( t \)

To solve for \( t \), we rearrange the equation:\[ \frac{0.93^t}{1.1^t} = \frac{1.5}{4.2} \]Taking the natural logarithm of both sides gives us:\[ t \ln(0.93) - t \ln(1.1) = \ln\left(\frac{1.5}{4.2}\right) \]This simplifies to:\[ t (\ln(0.93) - \ln(1.1)) = \ln\left(\frac{1.5}{4.2}\right) \] Finally, solve for \( t \) by dividing:\[ t = \frac{\ln\left(\frac{1.5}{4.2}\right)}{\ln(0.93) - \ln(1.1)} \]
04

Calculate \( t \)

Compute the numerical value of \( t \) using the above formula. First, calculate natural logs: \[ \ln(0.93) \approx -0.07257069283 \] \[ \ln(1.1) \approx 0.09531017981 \] \[ \ln\left(\frac{1.5}{4.2}\right) \approx -1.02961941718 \] Substitute these into the equation for \( t \): \[ t = \frac{-1.02961941718}{-0.07257069283 - 0.09531017981} \approx 15.207 \]
05

Calculate Immigration and Extinction Rates

Substitute \( t = 15.207 \) back into the equations for \( I \) and \( E \) to find the rates at stabilization. For immigration: \[ I = 4.2 \times 0.93^{15.207} \approx 0.611 \text{ species per year} \] For extinction: \[ E = 1.5 \times 1.1^{15.207} \approx 0.611 \text{ species per year} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Exponential Functions
Exponential functions are crucial in many scientific and mathematical analyses. They describe processes that change at rates proportional to their current value. Understanding exponential functions helps us model phenomena that grow or decay continuously, such as populations, currencies, and even data transmission. These functions can be identified by equations that have a constant term raised to the power of a variable, often represented as \( f(t) = a imes b^t \).
In the context of biogeography, the MacArthur-Wilson theory uses exponential functions to describe rates of species immigration and extinction. The immigration function, for example, is given by \( I = 4.2 \times 0.93^t \), indicating a decrease in new species arrivals over time. Conversely, the extinction function, \( E = 1.5 \times 1.1^t \), illustrates an increase in species extinction as time progresses. This showcases how exponential functions can model complex ecological dynamics effectively.
Key characteristics of exponential functions include:
  • The base \( b \), which determines the growth (\( b > 1 \)) or decay (\( b < 1 \)) rate.
  • The coefficient \( a \), representing the initial amount or rate.
  • The variable \( t \), usually signifying time or another sequential measure.
These aspects render exponential functions powerful tools for understanding various natural and human-made systems.
Data Analysis
Data analysis involves extracting insights and recognizing patterns from raw data. This approach is an essential component of scientific research and everyday decision-making. By applying data analysis techniques, we can draw meaningful conclusions that guide predictions or validate hypotheses.
In exploring the MacArthur-Wilson theory, data analysis aids in calculating stabilization points, like determining the time when immigration equals extinction. For this problem, critical data analysis steps included:
  • Setting equal the immigration and extinction functions: \( 4.2 \times 0.93^t = 1.5 \times 1.1^t \).
  • Using logarithms to simplify complex equations for easier computation.
  • Solving for \( t \) to find when species rates stabilize.
By following such analytical steps, one can uncover significant stabilization insights, which is vital to understanding the dynamic balance within ecosystems.
Data analysis transforms numeric expressions into actionable insights. Whether by graphing the data for visual inspection or solving equations analytically, the process offers a structured way to interpret and forecast biological patterns.
Mathematical Modeling
Mathematical modeling is a technique that uses mathematical structures and representations to simulate real-world processes. Through equations, inequalities, and functions, models help us analyze complex systems by distilling them into simpler components.
In biogeography, the MacArthur-Wilson theory employs mathematical modeling to understand how species migrate and face extinction. Immigration and extinction functions, like those given in our exercise, represent the core components of this model. By setting the two formulas equal, we model the point of equilibrium where these processes balance each other.
Mathematical modeling involves several steps:
  • Identifying and defining relevant variables and parameters.
  • Setting logical relationships between them, often involving equations.
  • Solving these equations to simulate realistic scenarios or predict future events.
This systematic approach allows scientists and researchers to evaluate scenarios such as how long it takes for species rates to stabilize on an island.
The beauty of mathematical modeling lies in its versatility. With it, we can not only comprehend biological phenomena but also apply these principles to other fields like economics, engineering, and social sciences. As seen in the exercise, it equips us with the necessary tools to project real-world behavior through disciplined mathematical frameworks.

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Most popular questions from this chapter

The pH scale: Acidity of a solution is determined by the concentration \(H\) of hydrogen ions in the solution (measured in moles per liter of solution). Chemists use the negative of the logarithm of the concentration of hydrogen ions to define the \(\mathrm{pH}\) scale: $$ \mathrm{pH}=-\log H . $$ Lower pH values indicate a more acidic solution. a. Normal rain has a pH value of 5.6. Rain in the eastern United States often has a pH level of \(3.8\). How much more acidic is this than normal rain? b. If the \(\mathrm{pH}\) of water in a lake falls below a value of 5 , fish often fail to reproduce. How much more acidic is this than normal water with a \(\mathrm{pH}\) of \(5.6 ?\)

Radioactive decay: A scientist is studying the amount of a radioactive substance present over a period of time. A plot of the logarithm of the amount shows a linear pattern. What type of function should the scientist use to model the original data?

Dispersion models: Animal populations move about and disperse. A number of models for this dispersion have been proposed, and many of them involve the logarithm. For example, in \(1965 \mathrm{O}\). H. Paris \(^{30}\) released a large number of pill bugs and after 12 hours recorded the number \(n\) of individuals that could be found within \(r\) meters from the point of release. He reported that the most satisfactory model for this dispersion was $$ n=-0.772+0.297 \log r+\frac{6.991}{r} $$ a. Make a graph of \(n\) against \(r\) for the circle around the release point with radius 15 meters. b. How many pill bugs were to be found within 2 meters from the release point? c. How far from the release point would you expect to find only a single individual?

Population growth: A population of animals is growing exponentially, and an ecologist has made the following table of the population size, in thousands, at the start of the given year. $$ \begin{array}{|c|c|} \hline \text { Year } & \begin{array}{c} \text { Population } \\ \text { in thousands } \end{array} \\ \hline 2003 & 5.25 \\ \hline 2004 & 5.51 \\ \hline 2005 & 5.79 \\ \hline 2006 & 6.04 \\ \hline 2007 & 6.38 \\ \hline 2008 & 6.70 \\ \hline \end{array} $$ Looking over the table, the ecologist realizes that one of the entries for population size is in error. Which entry is it, and what is the correct population? (Round the ratios to two decimal places.)

Stochastic population growth: Many populations are appropriately modeled by an exponential function, at least for a limited period of time. But there are many factors contributing to the growth of any population, and many of them depend on chance. There are a number of ways to produce stochastic models. We consider one that is an illustration of a simple Monte Carlo method. We want to model a population that is initially 500 and grows at an average rate of 2% per year. To do this we make a table of values for population according to the following procedure. The first entry in the table is for time t = 0, and it records the initial value 500. To get the entry corresponding to t = 1, we roll a die and change the population according to the face that appears, using the following rule. $$ \begin{array}{|c|c|} \hline \begin{array}{c} \text { Face } \\ \text { appearing } \end{array} & \begin{array}{c} \text { Population } \\ \text { change } \end{array} \\ \hline 1 & \text { Down 2\% } \\ \hline 2 & \text { Down } 1 \% \\ \hline 3 & \text { No change } \\ \hline 4 & \text { Up 2\% } \\ \hline 5 & \text { Up 4\% } \\ \hline 6 & \text { Up 9\% } \\ \hline \end{array} $$ To get the entry corresponding to \(t=2\), we roll a die and change the population from \(t=1\), again using the above rule. This procedure is then followed for \(t=3\), and so on. a. Using this procedure, make a table recording the population values for years 0 through 10 . b. Plot the data points from your table and the exponential model on the same screen. Comment on the level of agreement.

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