Chapter 15

Section 1

How Populations Grow

Objectives

Distinguish among the 3 patterns of dispersion in a population

Contrast exponential growth and logistic growth

Differentiate R-strategists from K-strategists

Population

Consists of all the individuals of a species that live together in one place at one time – Populations tend to grow: multiple offspring– Limits resources limit population

Ex. Population of walleye in Maple Lake

Population

Demography

Statistical study of all populations

Study the composition of a population and try to predict how the size will change

Ex. Growth of the population of Canada in the next 10 years

Population Size

Number of individuals in a population

Can affect the populations ability to survive

Ex. 40 moose in Glacier National Park

Population Density

Number of individuals that live in a given area If individuals of a population are few and spaced widely apart, they may seldom encounter one another, making reproduction rareEx. If 20 walleye live by the dock of my cottage

Population Density

Dispersion

The way the individuals of the population are arranged in space

3 patterns:– Random Distribution: location of each individual is

self determined or determined by chance– Even Distribution: located at regular intervals– Clumped Distribution: individuals are bunched

together in clusters

Dispersion

Population Model

Hypothetical population that attempts to exhibit the key characteristics of a real population

By making a change and observing the outcomes, demographers can predict what might occur in a real population

Growth Rate

Population grows when more individuals are born than die

A simple population model describes the rate of population growth as the difference between the birth and death rates

Ex. Find the rate of population growth where there are 360 births and 250 deaths in a year– 360 - 250 = 110 (population is increasing by 110

individuals/year)

Exponential Growth Curve

Curve in which the rate of population growth stays the same, as a result the population size increases steadily

Population size vs. time, J shaped

Exponential Growth Curve

N = size of the current population

r = rate of growth

K = carrying capacity, population size that an environment can sustain

Density Dependent Factors

Resources are density dependent factors

The rate at which they become depleted depends upon the population density of the population that uses them

Ex. Population of 200 seagulls on lake Erie vs. Population of 50 seagulls on lake Erie – Which will use up resources more quickly?

Logistic Model

Population model that takes into account the declining resources available to a population

Exponential growth is limited by a density dependent factor

Assumes birth and death rates vary

Logistic Model

Logistic Model

When population is below carrying capacity, growth rate is rapidAs population approaches carrying capacity death rates rise, birth rates slow– Result: rate of growth slows

Population stops growing when birth and death rates are equal If population exceeds carrying capacity, deaths will increase and outnumber births until population falls to carrying capacity

Population Growth Models

Simple model (part one): calculating the population growth rate – r (rate of growth) = birthrate - death rate

The rate of population growth equals the rate of births minus the rate of deaths

Population Growth Models

Simple model (part 2): exponential growth curve – Delta N (change in population) = rN

Once r has been determined for a population (part 1) the number of individuals that will be added to a population as it grows is equal to the rate of growth multiplied by the number of individuals in the current population (N)

Population Growth Models

More realistic model: logistic model– Delta N = rN (K-N)/K

Population size calculations often need to be adjusted by the number of members of the population at carrying capacity (K)

Density Independent Factors

Environmental conditions

Weather and climate

Ex. Mosquito populations increase in the summer while the weather is warm, but decrease in the winter

R-strategist

Strategy means pattern of livingR-strategist: grow exponentially when environmental conditions allow them to reproduce Results in temporarily large populations When environmental conditions worsen population size drops quickly Usually have short life span, reproduce early, and have many offspring, offspring are small and mature rapidly on their own

R-strategist & K-strategist

K-Stategists

K-strategists: population density is usually near carrying capacity

Characterized by long life span, few young, slow maturing process, reproduction late in life, provide care of young, live in stable environments

R vs. K Strategist

Ex. R-strategist– Insects like mosquitoes

Ex. K-strategist– Whales, tigers

Review

1. Identify the pattern of dispersion of fans attending a basketball game as random, even, or clumped. Explain your answer.

2. Differentiate a logistic growth pattern from an exponential growth pattern.

3. Describe why an R-strategist might be better suited for an unpredictable environment than a K-strategist.

Answers

1. The pattern of dispersion of fans at a basketball game could be described as clumped. This could be because usually friends and family sit together, making a clumped pattern.

2. A logistic growth pattern is a population model that takes into account the declining resources available to a population. Whereas a exponential growth pattern is a curve in which the rate of population growth stays the same, as a result the population size increases steadily.

Answers

3. An R- strategist may be better suited for an unpredictable environment because they are able to reproduce exponentially when environmental conditions are favorable. They would be able to build up a large population and be able to handle an unpredictable environment when their population size would drop.