Environmental Science Chapter 8: Understanding Populations

Understanding Populations - Big Ideas Population sizes change according to changes in the environment. The size and growth rate of human population has changed drastically over the last 200 years. Those changes have led to profound changes to almost every place on Earth.

Section 8.1: How Populations Change in Size GOALS Describe the 3 main properties of a population Describe exponential population growth Relate how the reproductive behavior of individuals can affect the growth rate of their population Explain how nature regulates population size

The Decline of the Passenger Pigeon How does a species go from having billions to extinct in just one century…The Passenger Pigeon Story. Extinct in the wild since 1900 and last died in the Cincinnati Zoo 1914.

Review: What is a population? Population: all the members of the same species that live in the same place at the same time Field mice living in a corn field, grizzly bears of Yellowstone Park area Wolves in Yellowstone NP

Population Size Why care about the size of the population? How can scientists estimate the population (say of Elephant Seals on a beach in CA)?

Population Size The number of individuals in a population at a given time Sudden and dramatic decreases in population size can indicate an unhealthy population headed toward extinction Ecologists often use sampling techniques to estimate population size. Did You Know? The passenger pigeon was once North America’s most abundant bird. Hunting drove them to extinction in less than 100 years.

Estimating Population How can you estimate the population of trees in this valley? WHY NOT COUNT THEM ALL? WHY IS SAMPLING NEEDED?

Mark and Recapture Common way of estimating population size

Population Density What is population density? What are advantages and disadvantages of high and low density?

Population Density Measure of how crowded a population is Larger organisms generally have lower population densities. Low population density: More space, resources; finding mates can be difficult High population density: Finding mates is easier; tends to be more competition; more infectious disease; more vulnerability to predators

Population Distribution How organisms are arranged within an area: RANDOM CLUMPED UNIFORM Occurs when individuals hold territory or compete for space Plants in a desert Occurs where resources needed are found throughout Wildflowers in a meadow Most Common. Occurs where resources are together Water hole in desert, humans in cities

Many bird species are territorial Many bird species are territorial. In territorial birds species, what kind of population structure would you expect? UNIFORM

Population Distribution Random distribution: Organisms arranged in no particular pattern Uniform distribution: Organisms evenly spaced Clumped distribution: Organisms grouped near resources; most common distribution in nature

What type of distribution? CLUMPED WHY?

Population Growth? Why are there so few whales, but so many bacteria?

Growth Rate Change in the size of a population over a given time period AND Immigration MINUS Emigration

Factors the Determine Population Growth A population’s relative birth and death rates (mortality and natality) affect how it grows births > deaths = population increase deaths > births = population decrease Immigration Emigration

Immigration/Emigration In addition to births and deaths, population growth is affected by immigration and emigration—individuals moving into and out of a population. Migration, seasonal movement into and out of an area, can temporarily change population size.

Population Calculations Net population Change = (births + immigration) – (deaths + emigration) There are 300 people living in a village, 50 children are born, 20 people die, 10 immigrate and 2 emigrate. What is the new population?

Calculating Growth Rate Determined by the following equation: (birthrate + immigration rate) – (death rate + emigration rate) Growing populations have a positive growth rate; shrinking populations have a negative growth rate. Usually expressed in terms of individuals per 1000 Current human growth rate 1.25% down from 2.2% in 1963

Every spring, many species of bird travel north to the Arctic to breed, and in the fall travel south again for the winter. This is an example of _____ MIGRATION

How Populations Grow What resources are available to the population?

Patterns of Growth EXPONENTIAL GROWTH Population increases by a fixed percentage every year. Normally occurs only when small populations are introduced to an area with ideal environmental conditions Rarely lasts long WHAT SHAPE DOES EXPONENTIAL GROWTH LOOK LIKE? Starts slowly, then takes off – “J-Shaped Curve”

Why doesn’t exponential growth last long?

Limiting Factors Limiting Factor Principle: too much or too little of any abiotic factor can limit or prevent growth of a population, even if all other factors are near or above optimum Why are there no citrus fruit growing in NJ? Why do trees grow toward the sky?

What Might Be Some Limiting Factors in the Ocean? may include: Salinity pH Sunlight Dissolved oxygen Temperature

Carrying Capacity Limiting Factors lead to Carrying Capacity: the largest population an environment can sustain What is the maximum amount of wolves that can be sustained in Yellowstone?

Carrying Capacity Limiting resources – used as quickly as provided Competition within population Competition for territory

Carrying Capacity

Logistic Growth Carrying Capacities are NOT fixed

Population (Thousands) Exponential Growth – not controlled, occurs when there are no limiting factors, and little competition Population (Thousands) Black Bear Population in Northern NJ

Logistic Growth – controlled Population (Thousands) Pelican Population off the Western Coast of Florida

Population Growth of Wolves and Moose Living in the Isles Royale of Lake Superior

A population of wolves is reintroduced into Yellowstone National Park A population of wolves is reintroduced into Yellowstone National Park. For the first decade, the wolf population grows exponentially. Then, the population growth slows. The new pattern is known as ___ LOGISTIC GROWTH In the example above, where wolves are reintroduced to Yellowstone National Park, the number of elk and other prey species within the park are _____ Limiting Factors

Biotic Potential A species’ maximum rate at which its population can grow Many factors influence biotic potential, including gestation time and generation time Species with high biotic potential can recover more quickly from population declines than species with low biotic potential

Reproductive Potential The maximum number of offspring that each member of the population can produce in ideal conditions

Biotic Potential Scorpion Fish Mature 3-5 years Release 50,000 to 100,000 eggs Once fertilized take 12-16 days to hatch HIGH BIOTIC POTENTIAL Orangutans Females Mature 10 years Birth to single babies once every eight years LOW BIOTIC POTENTIAL

Population Regulation Population size may be density dependent or density independent.

Density Dependent Deaths occur more quickly in a crowded population than in a sparse population When individuals of a population are densely packed together Limited resources, predation and disease result in higher rates of death in dense populations than in sparse populations

Density Independent Certain proportion of a population may die regardless of the population’s density Affects all populations in a general or uniform way Severe weather and natural disasters are often density independent causes of death

Section 8.1 Population Growth Review Describe the factors that influence population’s growth rate Explain exponential and logistic growth Explain how limiting factors and biotic potential affect population growth Crash Course - Ecology