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Chapter 52. Population Ecology. Population Ecology. Population ecology is the study of the populations and their interactions with the environment. It also explores how the environment influences these populations in terms of size, age structure, and distribution. Population Ecology.
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Chapter 52 Population Ecology travismulthaupt.com
Population Ecology • Population ecology is the study of the populations and their interactions with the environment. • It also explores how the environment influences these populations in terms of size, age structure, and distribution. travismulthaupt.com
Population Ecology • Ecologists usually begin an investigation of a population by defining appropriate parameters such as density and dispersion. travismulthaupt.com
Density • How many individuals live within a given area. • To determine the density of individuals, it is possible to count all of the organisms within a given area, but it is not likely. travismulthaupt.com
Dispersion • Dispersion is the spacing patterns among individuals within the boundaries of a population. travismulthaupt.com
Sampling Techniques • Usually a wide variety of them are used. • Scientists can count all individuals in a given area. • They can do this in a number of different spots. • Then, average all of the numbers together to make educated estimates about the population density. travismulthaupt.com
Sampling Techniques • Scientists also employ the mark and recapture method. • Animals are captured, marked, and released. • The animals can then be tracked or captured at a later date. • Density and distribution can be studied. travismulthaupt.com
Sampling Techniques • These methods are okay, but sometimes the data becomes unreliable because the organisms you are studying behave differently during study. travismulthaupt.com
Population Density • The density is always changing. • Birth, death, immigration, and emigration are ways a population changes. travismulthaupt.com
Dispersal Patterns • There are varying dispersal patterns of organisms within a population’s geographic range. • These variations in local populations are extremely important to ecologists. travismulthaupt.com
Three Common Patterns of Dispersal: • 1. Clumped • 2. Uniform • 3. Random travismulthaupt.com
1. Clumpled • 1. Organisms are in uniform patches travismulthaupt.com
2. Uniform • Organisms are evenly spaced. travismulthaupt.com
3. Random • Organisms exhibit unpredictable spacing patterns. They could be grouped together, or there could be an uneven distribution pattern. travismulthaupt.com
Demography • Demography is the study of the vital characteristics of a population. • For example: • Ecological needs • Spacing of individuals • Interactions of individuals within a population. travismulthaupt.com
Demographers • These are people who study populations. • They develop life tables to determine the survival pattern of a population. • The use a cohort--a group of individuals of the same age that are followed from birth to death. travismulthaupt.com
Life Tables • These are difficult to build and maintain. • It is easier to graphically depict a life table--a survivorship curve. travismulthaupt.com
Survivorship Curves • These typically involve 1000 individuals from a population. • The numbers are obtained by multiplying the surviving population by 1000 each year. • Plotting these numbers vs. age indicates the death rate (or life expectancy). travismulthaupt.com
Survivorship Curves • There are three types of survivorship curves: • 1. Type I • 2. Type II • 3. Type III travismulthaupt.com
1. Type I • Type I curves start flat indicating a low death rate for early and middle life. • They decline sharply as individuals get older indicating a high death rate. travismulthaupt.com
2. Type II • Type II curves exhibit relatively constant death rates from birth to death. travismulthaupt.com
3. Type III • Type III curves see death rates very high in the beginning but as the animals grow and mature the death rates level off. • Example: animals that produce many young and provide little or no care for them. travismulthaupt.com
Life Histories • Life histories are products of natural selection. • The traits that affect an organism’s schedule of reproduction and survival comprise its life history. travismulthaupt.com
Life Histories • There are three basic variables that life histories entail: • 1. When reproduction begins. • 2. How often the organism reproduces. • 3. How many offspring are produced during a reproductive cycle. • For the most part, life histories are the product of evolutionary outcomes because most animals don’t choose when to reproduce. travismulthaupt.com
Reproductive Modes • In general, there are 2 reproductive modes that are followed: • 1. Big bang reproduction--semelparity. • 2. Repeated reproduction--iteroparity. travismulthaupt.com
Reproductive Modes • The evolutionary events that favor these are determined by the environment. travismulthaupt.com
Semelparity • This is a “one and done” scheme for reproduction. • The organism takes a big chance. • Favored when the survival rate of the offspring is low. • Occurs when an organism lives in a highly variable or highly unpredictable environment. • Examples: • Salmon and agave plants. travismulthaupt.com
Iteroparity • This is repeated reproduction. Organisms continually give rise to offspring throughout their lives. • Iteroparity is favored when environments are more stable. travismulthaupt.com
Energy Constraints • Time, energy, and nutrients cannot be used for one thing as well as something else. • This is the tradeoff that prevents producing a large number of offspring very frequently. travismulthaupt.com
Population Growth • Unchecked population growth is considered exponential. • There are mechanisms that prevent exponential population growth. • This can be estimated using mathematical equations to describe the per capita growth rate. travismulthaupt.com
Population Growth • It essentially boils down to the rate being equal to the number of births minus the number of deaths. • r = b-m • r>0 the population is increasing • r<0 the population is decreasing • r=0 no change travismulthaupt.com
Population Growth • Because resources are limited populations cannot grow exponentially forever. • Ecologists try to identify the carrying capacity of an environment, K. • K is the maximum population size an environment can support. travismulthaupt.com
Population Growth • To account for changes in the environment, scientists have created a logistic growth model to explain how populations vary in size. travismulthaupt.com
Population Growth • The exponential growth model is used as a starting point. • We add information about the environment that acts to reduce the per capita rate of increase. • If K is the maximum, K-N is the number of individuals the environment can accommodate. • N is the population size. travismulthaupt.com
Population Growth • (K-N)/K is the fraction of the carrying capacity available for population growth. • Multiplying by the maximum rate of increase of the population, rmax, allows us to modify the growth rate of the population as its size increases. • rmax N (K-N)/K travismulthaupt.com
Population Growth • When N = K, the population stops growing. • The logistic model will produce an S-shaped (sigmoid) growth curve when population size is plotted over time. travismulthaupt.com
Population Growth • New individuals are added at the highest rate at intermediate population sizes. • This is when the breeding population is of substantial size and space and resources are abundant. • As N approaches K, the population size slows. travismulthaupt.com
Population Growth • The logistic model. • This incorporates the idea that every individual added to the population has the same negative effect on population growth. • This is not true. travismulthaupt.com
Population Growth • Certain populations exhibit the Allee effect. • This describes a situation where individuals may have a difficult time surviving and reproducing when the population gets too small. • The logistic model fits few, if any, real populations. • It serves as a good starting point. travismulthaupt.com
Population Growth • There are two general questions that are asked when studying population growth: • 1. What environmental factors stop a population from growing? • 2. Why do some populations show radical size fluctuations while others stay stable? travismulthaupt.com
Population Growth • To understand the answers to these questions, we have to: • Examine the birth and death rates • Immigration and emigration • How these factors affect population density. travismulthaupt.com
Population Growth • If immigration and emigration are equal, then birth and death rates will affect population size. travismulthaupt.com
Population Growth • A birth rate or death rate that does not change with population density is said to be density independent. • A death rate that rises when population density rises is said to be density dependent--an example of negative feedback--it halts population growth. travismulthaupt.com
