Chapter 48: Populations and Communities

Section 1: Population Growth

Population Growth

• In order to study relationships between organisms, ecologists need to know how groups of organisms change over time

• Ecologists study populations

• A group of organisms that all belong to the same species and that live in a given area

Exponential Growth: A Baby Boom

• Almost any organism provided with ideal conditions for growth and reproduction will experience a rapid increase in its population

• The larger the population gets, the faster it grows

• If nothing stops the population from growing, it will continue to expand faster and faster

• Exponential growth curve

Exponential Growth Curve

Exponential Growth: A Baby Boom

• Exponential growth does not continue in natural populations for long

• Most offspring of plants and animals do not survive long enough to reproduce

• WHY?

Logistic Growth: A Step Closer to Reality

• The population growth history of a particular species is a bit more complicated than simple exponential growth

• Most populations go through a number of growth phases

• Can be shown on a logistic growth curve

Logistic Growth: A Step Closer to Reality

• Suppose a few animals are introduced into a new environment

• At first their numbers will begin to grow slowly

• Soon, however, the population will begin to grow very rapidly

• Few animals are dying and a great many are being produced

Logistic Growth: A Step Closer to Reality

• Exponential growth does not continue for long

• A population grows more when more organisms are produced in a given period of time than die during the same period

• Birthrate is greater than deathrate

• Population growth may slow down because either the birthrate decreases or because the deathrate increases or both

Logistic Growth: A Step Closer to Reality

• When the birthrate and deathrate are the same, population growth will stop

• During a steady state, the average growth rate is zero

• Rises and falls of populations average out around a certain population size

Logistic Growth: A Step Closer to Reality

• If you were to draw a horizontal line through the middle of the steady state region, that line will tell you how big the population is in steady state• Carrying capacity

• Once a population reaches the carrying capacity of its environment, certain factors keep the population from growing any further• Lack of food, overcrowding, and competition

among the individuals in the population

Chapter 48: Populations and Communities

Section 2: Factors That Control Population Growth

Factors That Control Population Growth

• The growth of individuals can be controlled by limiting factors

• Both plant and animal populations can be controlled by several factors

• No single species has ever threatened to overpopulate the entire planet

Density-Dependent Limiting Factor

• When factors that control population size operate more strongly on large populations than on small ones, they are called density-dependent limiting factors• Usually operate only when a population is large and

crowded• Do not affect small, widely scattered populations

much• Density-dependent limiting factors include

competition, predation, parasitism, and crowding

Competition

• When populations become crowded, both plants and animals compete, or struggle, with one another for food, water, space, sunlight, and other essentials of life

• Competition between members of different yet similar species is a major force behind evolutionary change• No two organisms can occupy the same niche in the

same place at the same time• When two species compete, both find themselves

under pressure from natural selection to change in ways that decrease their competition• Ties ecology and evolution together

Predation

• Just about every species serves as food for some other species

• Predators and prey coexist over long periods of time• Have become accustomed to each other’s strengths

and weaknesses• Prey have evolved defenses against predators

• Poisonous chemicals, shells, camouflage• Predators have evolved counter defenses

• Stronger teeth and jaws, powerful digestive enzymes, extra keen eyesight

Predation

• Typically, at some point the prey population grows so large that prey are numerous and easy to find

• With such a large and available food supply to feast upon, there may soon be almost as many predators as prey• This situation cannot last because each predator

needs many prey to satisfy its energy needs• Predator-prey relationships are important in

controlling natural populations

Parasitism

• Parasites live off their hosts, weakening them and causing disease

• Like predators, parasites work most effectively if hosts are present in large numbers

• Parasitism works as a density-dependent limiting factor on population growth

Crowding and Stress

• Most animals have a built-in behavioral need for a certain amount of space• Room to hunt• Nesting• Territory

• The number of suitable territories regulates population size in a density-dependent manner

• Certain species fight among themselves if they are overcrowded• Creates stress

Density-Independent Limiting Factors

• Many species show boom-and-bust growth curves• Populations grow exponentially for some time

and then suddenly crash• Insects that feed on plant buds and leaves

can be washed out by a rainstorm• They may also be harmed by long hot

periods of dry weather• Frosts, too, can cause sudden drops in insect

populations

Density-Independent Limiting Factors

–For these species, storms, cold weather, dry weather, or other natural occurrences can nearly wipe out the population

• Happen regardless of how large or small the population is

• Density independent limiting factors

Human Population Growth

• Human populations tend to increase in size with time• For a long period of time the human population grew

slowly• Then, about 500 years ago, the world’s human population

grew exponentially• Today, population growth in the US and parts of Europe

has slowed down• Most of the world’s people do not live in these countries

• Instead, they live in China, India, and part of Africa and Latin America – places where populations are still growing very rapidly

Chapter 48: Populations and Communities

Section 3: Interactions Within and Between Communities

Interactions Within and Between Communities

• After populations, the next larger biological units are communities

• A community consists of all the populations of organisms living in a given area

• Populations in communities interact with one another in many ways

Interactions Within and Between Communities

• Example:• Plant species compete for water, nutrients,

and sunlight. At the same time, some plants have evolved defenses against herbivores. Herbivores compete with one another for food and space. These herbivores may have evolved counter defenses against the plant species. While this is going on, carnivores are hunting the herbivores.

Symbiosis

• There are several relationships that play an important role in nature

• Symbiosis

• “Living together”

–Parasitism

–Commensalism

–Mutualism

Parasitism

• One species benefits and the other is harmed

• Example:

• Tapeworm feeding off of nutrients in the digestive tract of an organism

• http://media2.foxnews.com/112008/worm_tumor_700.wmv

Commensalism

• One member benefits and the other is not harmed• Example:

• Shrimp live within the stinging tentacles of sea anemones

• The shrimp are not affected by the anemone’s poison

• As a result, the shrimp are protected from predators that cannot tolerate the anemone’s stings

• Anemones are not harmed by shrimp living on them, but they are not helped either

Mutualism

• Two species live together in such a way that both species benefit

• Example:• Clownfish benefit from living within the

stinging tentacles of the sea anemone in the same way shrimp do

• However, clownfish also help the anemones by chasing away several species of anemone-eating fish

Interactions Among Ecosystems

• Not only do populations and communities interact, ecosystems also interact with one another in many ways

• Nearly every ecosystem is connected, either directly or indirectly, with other ecosystems