Question

The human population of a certain island satisfies the logistic law \((3.58)\) with \(k=0.03, \lambda=3(10)^{-8}\), and time \(t\) measured in years. (a) If the population in 1980 is 200,000, find a formula for the population in future years. (b) According to the formula of part (a), what will be the population in the year \(2000 ?\) (c) What is the limiting value of the population at \(t \rightarrow \infty\) ?

Short answer

In conclusion, the population formula is given by \(P(t) = \frac{0.03(200,000)}{3(10)^{-8) + (0.03 - 3(10)^{-8})e^{-0.03t}}\). For the year 2000, the population will be approximately \(251,835\), and the limiting population value as \(t \rightarrow \infty\) is \(2,000,000\).

Step-by-step solution

Find the Formula for Population

The logistic law is given by the equation: \[ P(t) = \frac{kM}{\lambda + (k - \lambda)e^{-kt}}, \] where \(P(t)\) is the population at time \(t\), \(k\) is the constant, \(\lambda\) is the initial condition, and \(M\) is the initial population. We are given: \(k = 0.03\), \(\lambda = 3 \times 10^{-8}\), and \(M=200,000\). Plugging these values into the logistic law equation: \[ P(t) = \frac{0.03(200,000)}{3(10)^{-8) + (0.03 - 3(10)^{-8})e^{-0.03t}} \]

Evaluate Population at Year 2000

We are given that the population in 1980 is 200,000, and we want to find the population in the year 2000. Since \(t\) is measured in years, we can find the time elapsed between 1980 and 2000 as: \(t = 2000 - 1980 = 20\) years Now we can plug this value of \(t\) into the population formula we found in Step 1: \[ P(20) = \frac{0.03(200,000)}{(3(10)^{-8}) + (0.03 - 3(10)^{-8})e^{-0.03(20)}} \] Evaluate the equation, we get: \[ P(20) \approx 251,835 \]

Find the Limiting Value as \(t\rightarrow\infty\)

Now, we want to find the limiting population value as \(t \rightarrow \infty\). That is, we want to find the value of the formula as the time approaches infinity: \[ \lim_{t \rightarrow \infty} P(t) = \lim_{t \rightarrow \infty} \frac{0.03(200,000)}{3(10)^{-8) + (0.03 - 3(10)^{-8})e^{-0.03t}} \] As \(t \rightarrow \infty\), the term \(e^{-0.03t}\) approaches \(0\). Therefore, the equation becomes: \[ \lim_{t \rightarrow \infty} P(t) = \frac{0.03(200,000)}{3(10)^{-8} + (0.03 - 3(10)^{-8})(0)} \] Simplifying, we get: \[ \lim_{t \rightarrow \infty} P(t) = \frac{0.03(200,000)}{3(10)^{-8}} = 2,000,000 \] So, the limiting value of the population as \(t \rightarrow \infty\) is 2,000,000. In conclusion, the formula for the population in future years is given by: \[ P(t) = \frac{0.03(200,000)}{3(10)^{-8) + (0.03 - 3(10)^{-8})e^{-0.03t}} \] Using this formula, the population in the year 2000 is approximately 251,835, and the limiting population value as \(t \rightarrow \infty\) is 2,000,000.

Key concepts

Differential Equations

In mathematics, differential equations play a crucial role in modeling real-world phenomena. They are equations that relate a function with its derivatives. They are essential for describing how physical quantities change over time or space. In the context of logistic population growth, a differential equation helps us model how a population changes with time under certain constraints.

The logistic differential equation is particular to scenarios where there are limits to growth, like resource availability or space limitations. It differs from simple linear equations by incorporating factors that slow down the growth as the population nears its capacity.

In the logistic equation, usually represented as \( \frac{dP}{dt} = rP\left(1 - \frac{P}{K}\right) \), \( P(t) \) is the population at time \( t \), \( r \) is the intrinsic growth rate, and \( K \) stands for the carrying capacity or the limiting factor of the environment. As the population approaches \( K \), the growth rate decreases, leading to a leveling off or stabilization of the population. This showcases the beauty of differential equations in providing dynamic, time-dependent solutions for complex systems.

Differential equations enable us to transition from continuous change to understanding static solutions, like finding exact population numbers at specific times or predicting maximum sustainable limits.

Exponential Growth

Exponential growth describes a situation where something—typically population or financial investments—grows at a rate proportional to its current value. This results in rapidly increasing values as time progresses. In a biological context, it describes how populations can grow unchecked, assuming unlimited resources and space.

The equation \( P(t) = P_0e^{rt} \) represents this type of growth, where \( P_0 \) is the initial quantity, \( e \) is the base of the natural logarithm, \( r \) is the growth rate, and \( t \) is time. Exponential growth leads to a J-shaped curve when plotted, indicating the explosive potential of populations under ideal conditions.

However, exponential growth is often unsustainable in natural ecosystems because resources are finite. While it provides a useful model for short periods or specific sections of a growth curve, real-world scenarios often involve limiting factors, causing populations to fit a logistic rather than exponential pattern.

Thus, coupling exponential growth understanding with limiting factors gives rise to logistic growth models, where initially rapid growth levels off, offering more realistic predictions for population dynamics over time.

Limiting Population

Limiting population, often referred to as carrying capacity in ecology, is the maximum number of individuals that an environment can sustainably support. This concept is an integral part of logistic population growth modeling, where growth is initially exponential but slows as the population reaches the environmental capacity.

The carrying capacity is determined by various factors, including food availability, living space, water supply, and other essential resources. Once a population reaches this point, it stabilizes because birth rates and death rates balance each other out, or because resources can no longer support a larger population without degrading.

In the logistic model formula \( P(t) = \frac{kM}{\lambda + (k - \lambda)e^{-kt}} \), the denominator adjustment reflects this limiting behavior. As \( t \rightarrow \infty \), the exponential \( e^{-kt} \) term diminishes, and the limiting population value stabilizes at \( kM/\lambda \). This indicates that without external intervention, populations tend towards a sustainable size dictated by the environment.

Understanding limiting factors and carrying capacity is crucial for managing wildlife populations, urban planning, and ensuring sustainable development. It underscores the need to balance growth aspirations with environmental conservation so that ecosystems remain healthy and sustainable.