Download Ecology: Organisms and their environment

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Ecology: Organisms and their environment
Are mosquitoes of any use to anyone or anything? If all mosquitoes were killed, would
anything bad happen? You might say they’re just pests, but if you could ask the
opinion of a sunfish, a tadpole, a dragonfly, or a swallow, you’d get a different answer.
For these animals and others, mosquitoes and their larvae are a major food source.
Every organism is connected in some way to many other organisms. In nature,
living organisms interact in a variety of ways. Some relationships are complicated such
as pollination of a flower by a hummingbird, while other are quite simple such as the
carrying of seeds from a plant by a passing animal. In this unit you will learn the basic
principles of ecology including interactions between organisms, community structure,
and populations dynamics. You will also consider how organisms are affected by
physical factors of the environment.
The Beginnings of Ecology
Many people make a hobby out of observing and studying plants and animals
in their natural habitats. Plant enthusiasts explore fields, forests, city parks, and even
vacant lots to learn about the different trees,
and wildflowers that grow there. Plant
identification guides help these people identify
different species of plants and learn about a
plant’s habitat requirements and flowering
times. Casual backyard bird observers may
notice a nest of robins and the behavior
patterns of the male and female. While avid
birdwatchers (people with less than no life)
would certainly be interested in a nest of robins,
their hobby is to identify and observe as many bird species as possible. In order to find
new birds, bird-watchers listen for bird songs and investigate all types of environments
from woodlands to fields to wetlands. The pursuit of bird watching and plant
identification are known as natural history.
The goal of natural history is to find out as much as possible about living things.
Many early natural historians spent their lives studying how various organisms live out
their life cycles and how they depend on one another and on their environments.
Natural historians then began to report their findings in a systematic way.
The branch of biology that developed from natural history is called ecology.
Ecology is the scientific study of interactions between organisms and their
environments. Ecological study uncovers interrelationships between living and
nonliving parts of the world. Ecology combines information from any scientific fields
including chemistry, physics, geology, and other branches of biology.
If you look at the earth you can divide it into several regions, some of which you
already know. The gases that surround the Earth are called the atmosphere, the water
portion of the Earth is called the hydrosphere, and the land part is called the
geosphere. There is a small portion of each of these regions that Ecologists are
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interested in. It is called the biosphere. That is the thin layer of the atmosphere,
hydrosphere, and geosphere that can support life. Ecologists, people who study
ecology, would like to study the whole biosphere but it is not possible since it is way to
big. Instead they try to study the ecosystems that make up the biosphere.
An ecosystem is all the living organisms and all the nonliving things that surround
them. That’s huge if you are studying a natural ecosystem such as the area around
the bog behind the school. Because ecosystems can be incredibly complex,
ecologists often study parts of them. These parts are called communities. A
community is all the organisms that live in an area and interact with one another.
Many communities interacting together make up an ecosystem. Communities in turn
are made up of individual populations. A population is a group of the same species
that live in a certain place at a certain time. For example, 248 leeches that live in the
small pond by Bloomfield Elementary on September 1, 2001. When you are naming a
population you must give the place and the location because the population of
leeches that live in the same small pond on Jan. 1, 2001 will be quite different than on
Sept. 1. It’s important. Populations are made up the same species. A species is a
group of similar looking animals that can successfully interbreed and produce fertile
offspring. The species is the smallest group in an ecosystem.
Now that you know the levels of organization ecologists study, from the huge
biosphere to the single species, you are ready to learn about the interactions between
species and ecosystems.
How Organisms Interact: Species Relationships
Coyotes, roadrunners and other living things interact in a variety of ways. These
interactions allow organisms to get energy and materials necessary for life. These
interactions also help to keep the numbers of organisms stable in populations,
communities and ecosystems. The maintaining of stable numbers is called
homeostasis.
Feeding relationships: How organisms obtain
energy
You may have learned that the role of
an organism plays in its community is it niche.
Feeding relationships between organisms
reflect these niches, or roles in the
community. In a grassland community such
as those found in the American Midwest, the
plants are the producers. Producers are
organisms that use energy from the sun or energy stored in chemical compounds to
make their food. Producers are also called autotrophs. Plants are the most common
land living autotrophs, but there are lots of single-celled organisms that live in the
water that are also producers. All other organisms depend on autotrophs for nutrients
and energy.
Organisms that depend on autotrophs as their source of nutrients and energy
are called heterotrophs. Some heterotrophs, such as grazing, seed-eating, and algaeeating animals, feed directly on autotrophs. The white tailed deer depend on plants
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for their food. Heterotrophs are also known as consumers because they can’t make
their food; they must consume it. A consumer that feeds on plants is called an
herbivore. Herbivores include grazing animals such as rabbits, cows, and
grasshoppers, as well as rodents such as beavers, mice, and squirrels.
Carnivores and scavengers
Some heterotrophs eat other heterotrophs. Animals such as lions that kill and
eat only other animals are carnivores. Some animals do not kill for food; instead, they
eat animals that have already died. Scavengers such as black vultures are animals
that feed on carrion, refuse, and similar dead organisms. Scavengers play a helpful
role in the ecosystem. Imagine for a moment what the environment would be like if
there were no vultures to devour animals killed on the African plains, buzzards to clean
up dead animals along roads, or ants and beetles to remove dead insects and small
animals from sidewalks and basements
Omnivores and decomposers
Humans are an example of yet another type of consumer. The students you see
in your cafeteria at lunchtime eat a variety of both animal and plant materials. They
are omnivores. Raccoons, coyotes, and bears are other examples of omnivores.
A fungus is another type of consumer. Organisms that break down and absorb
nutrients from dead organisms are called decomposers. Decomposers break down
the complex compounds of dead and decaying plants and animals into simpler
molecules that can be absorbed. Many bacteria, some protozoans, and most fungi
carry out this important process of decomposition.
Close relationships for survival
Biologists once assumed that all organisms living in the same environment are in
a never-ending battle for survival. Some interactions are harmful to one species while
beneficial to another. Predators such as lions and insect-eating birds kill and eat other
animals—prey. Predator-prey relationships such as the one between coyotes and
deer do involve fight for survival. But there are also other relationships among
organisms that help maintain survival in which many species. The relationship in which
there is a close and permanent association between organisms of different species is
symbiosis. Symbiosis means, “living together”
There are several kinds of symbiosis. Commensal relationships can occur among
plant species and animal species. Spanish moss is a kind of flowering plant growing on
the branch of a tree. Orchids, ferns, mosses, and other plants are sometimes grown on
the branches of large plants. The larger plants are not harmed,
but the smaller plants benefit from the additional habitat. The
relationship is called a commensal relationship. Commensalism
is a symbiotic relationship in which one species benefits and the
other species is neither harmed nor benefited.
Sometimes, two species of organisms benefit from living in
a close association. A symbiotic relationship in which both
species benefit is called mutualism. An example of mutualism is
the lichen seen to the right. A lichen is actually two organisms,
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a bacteria and a fungus. The bacteria produce food for the fungus and the fungus
give the bacteria a place to live. Mutualism can be also be seen in the relationship
between a type of ant and a species of acacia tree living in subtropical regions of the
world. The ant protects the tree by attacking any herbivore that tries to feed on it.
The ants also clear vegetation away for the trunk of the plant and kill any plant that
begins to grow too close to the acacia. The tree provides nectar and a home for the
ants.
Sometimes, one organism harms another. Have you ever owned a dog or cat
that had been attacked by ticks or fleas? Ticks and fleas are examples of parasites. A
symbiotic relationship in which one organism benefits at the expense of the other is
called parasitism. Parasites have evolved in such a way that they harm, but usually do
not kill, the host. Why would it be a disadvantage for a parasite to kill its host? If the
host dies, the parasite also will die unless it can quickly find another host. Some
parasites live inside other organisms. Examples of parasites that live inside the bodies
of their hosts are tapeworms and roundworms, which live in the intestine of dogs, cats,
and other vertebrates.
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Matter and Energy in Ecosystems
When you pick an apple from a tree and eat it, you are consuming carbon,
nitrogen, and other elements the tree has used to produce the fruit. That apple also
contains energy from the sunlight trapped by the tree’s leaves while the apple was
growing and ripening.
Matter and energy are constantly cycling through stable ecosystems. You have
already learned that feeding relationships and symbiotic relationships describe ways in
which organisms interact. Ecologists study these interactions to make models that
show how matter and energy are transferred through ecosystems.
Food chains: Pathways for matter and energy
The community shown below illustrates an example of a food chain. A food
chain is a simple model that scientist use to show how matter and energy moves
though an ecosystem. Nutrients and energy proceed, from autotrophs to
heterotrophs and eventually to decomposers.
A food chain is typically drawn using arrows to indicate the direction in which
energy is transferred from one organism to the next. One food chain that could be
shown is: grass ⇨ rabbit ⇨ fox. Food chains can consist of three links, but most have
no more than 5 links. This is because the amount of energy left by the fifth link is only a
small portion of what was available at the first link. A portion of the energy lost as heat
at each link. It makes sense, then, that typical food chains are three or four links long.
Trophic levels represent links in the chain
Each organism in a food chain represents a feeding step, or trophic level, in the
passage of energy and materials. A food chain represents only one possible route for
the transfer of matter and energy in an ecosystem. Many other routes exist. As the
picture shows, many different species occupy each level in an ecosystem. In addition,
many different kinds of organisms eat a variety of foods, so a single species may feed
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at several different levels. For example a human can eat a salad and be a primary
consumer (one that eats producers) or can eat a steak and be a secondary
consumer (one that eats primary consumer) or eat a trout and be a tertiary consumer
(one that eats a secondary consumer). The food chain for the trout meal may look
like this algae ⇨ small fish ⇨ trout ⇨ human.
Food Webs
Simple food chains are easy to study, but they cannot show the complicated
relationships that exist among organisms that feed on more than one species. Notice
how the food web of the ecosystem represents a network of interconnected food
chains. In an actual ecosystem, many more plants and animals would be involved in
the food web. Ecologists who are particularly interested in energy flow in an
ecosystem set up experiments with as many organisms in the community as they can.
The model they create, a food web, expresses all the possible feeding relationships at
each level in a community. A food web is a more natural model than a food chain
since most organisms depend on more than one other species for food.
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Energy and trophic levels: Ecological pyramids
How can you show how energy is used in an ecosystem? Ecologists use food
chains and food webs to model the distribution of matter and energy within an
ecosystem. They also use another kind of model, called an ecological pyramid, to
depict energy conversions in an ecosystem. The base of the ecological pyramid
represents the producers, or first trophic level. Higher trophic levels are layered on top
of one another. Examine the ecological pyramid below. Observe that it summarizes
the interactions of matter and
energy at each trophic level. Be
aware that the source of energy
for almost all ecological pyramids
is energy from the sun. Notice the
amount of energy transferred from
one level to another is only 10%.
Why does the grasshopper only
get 10% of the 10,000 kcal of
energy the producers get from
the sun?
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Cycling maintains homeostasis (balance)
Food chains, food webs, and ecological pyramids all show how energy moves
in only one directions through the trophic levels of an ecosystem, and how energy is
lost at each transition from one trophic level to the next. This energy is lost to the
environment in the form of heat generated by the body processes of organisms. Keep
in mind that sunlight is the source of all this energy, so energy is always being
restocked.
Matter, in the form of nutrients, also moves through the organisms at each
trophic level of an ecosystem. But matter cannot be restocked as the energy from
sunlight can. The atoms of carbon, nitrogen, and other elements that make up the
bodies of organisms alive today are the same atoms that have been on Earth since life
began. Matter is constantly recycled. Because of this recycling, some of the atoms in
your body right now could once have been part of a dinosaur. One of these
important materials is water. The cycles of other materials such as carbon and
nitrogen will be taken up later.
The water cycle
Water (H2O) occurs on Earth
as liquid or a solid and in the
atmosphere as a gas. Water
begins its cycle through an
ecosystem when plants
absorb it through their roots.
Animals drink water or get it
indirectly with the food they
consume. As water moves
through the ecosystem,
plants and animals lose it
back to the atmosphere
through respiration.
Organisms also lose water
through excretion. After an
organism dies,
decomposition releases
water back into the
environment.
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Ecology Part 2: Population Biology
Think about large cities of the world such as New York City, Los Angeles, and
Tokyo-- cities populated by millions of people. Millions of people bustle along the
crowded streets, and it seems as if stores, offices, and apartments occupy every last
inch of space. Now compare one of these cities with a rural area like Mercer,
populated by only a few people. Living in a rural area, you wouldn’t experience
traffic jams, crowded streets, or polluted air from automobile fumes. On the other
hand, you probably wouldn’t have your choice of concerts, movies, or sporting events
to attend either. Cities can be exciting places, and according to the most recent
census, the population of many cities is increasing.
Why do populations grow, and how can we predict future population trends?
Can we learn how to deal with problems of larger populations? Many characteristics
of population growth are the same for other organisms as they are for humans. By
studying the principles of population growth in other species, you may be able to
suggest possible answers to these questions.
Population Dynamics
Weeds!! Those pesky little plants are a real
problem to gardeners. You’ve probably
observed a scene like this before. What was
recently a clear, grass-filled field or lawn is now
crowded with hundreds, perhaps thousands, of
bright yellow dandelions. Why do these plants
crop up so quickly and in such large numbers?
Dandelions are a problem to gardeners,
but to population biologists, they are a useful
species because they’re easy to see and study.
By studying organisms such as dandelions, scientists can understand much about
human population growth. Do similarities exist between human population growth
and population growth in other species? In many ways the changes that occur in
human populations are similar to those of other populations. All populations--whether
human, plant or animal--are in a state of constant change.
Changing with the environment
Ecosystems are always changing. Sometimes they change quickly and
dramatically, as with fire or flood. They also
can change slowly. As young saplings grow
into mature trees that shade the ground below
them, shade-loving plants slowly replace
grasses. Changing conditions within an
ecosystem affect the communities of
organisms that live there. When ecologists
study these changes, they discover patterns
that help explain how the ecosystem has
developed. These patterns can be used to
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only to deepen our understanding of the interactions that take place within an
ecosystem, but also to predict what might happen if an ecosystem is disturbed. One
of the things that affect ecosystems that ecologists like to study are limiting factors.
Limiting factors
Why do more people live in middle latitudes than near the North Pole? You
would be correct if you pointed out that there isn’t much to eat and it’s too cold
there! Obtaining food and warmth in the frozen reaches of the Artic is difficult for
humans, but polar bears thrive in this icy environment. Environmental factors, such as
food availability and temperature that affect an organism’s ability to survive in its
environment are limiting factors. A limiting factor is any biotic or abiotic factor that
restricts the numbers, reproduction, or distribution of organisms. An abiotic limiting
factor is any non-living thing that keeps numbers of organisms in check. For example,
a grass population will increase if it has lots of sunshine but will decrease if the area is
experiencing drought conditions. The amount of water is an abiotic factor. A biotic
factor is one that involves something living. For example, the deer population in Maine
is kept in check or is limited by how many hunters get their deer. The hunters are a
biotic factor.
Because all members of a food web are connected, factors that limit one
population in a community may also have an indirect effect on another population.
For example, a lack of water could limit the growth of grass in a grassland, reducing
the number of seeds produced. The population of mice that depend on those seeds
for food will also be reduced. What about hawks that feed on mice? Their numbers
may be reduced too as a result of a decrease in their food supply.
Principles of Population Growth
How and why do populations grow? Population growth is defined as the
change in the size of a population with time. Scientists use many ways to investigate
population growth in organisms. One way involves placing a microorganism, such as
a bacterium or yeast cell, into a tube or bottle of nutrient solution, and observing how
rapidly the population grows. Another interesting method involves introducing a plant
or animal species into a new environment that contains abundant resources, and then
observing the population growth of that species. Through studies such as these,
scientists have identified clear patterns in how and why populations grow.
Hours worked vs. Dollars paid
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Dollars paid
How fast do populations grow?
What’s interesting about the growth
of a population of living organisms is that it
is unlike the growth of some other familiar
things. Think about, for example, the
growth of a weekly paycheck for an afterschool job. Suppose that you are working
for a company that pays you $5 per hour.
You know that if you work for 2 hours, you
will be paid $10; if you work for 4 hours, you
will be paid $20; if you work for 8 hours, you
40
30
20
10
0
0
1
2
3
4
5
6
hours worked
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8
9 10
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will be paid $40; and so on. If you were to plot this rate of increase on a graph, it
would look like this. You can see that the result is a steady, linear increase; that is,
growth occurs in a straight line.
Populations of organisms do not experience this linear growth. Rather, the
resulting graph of a growing population first resembles a J-shaped curve. Whether the
population is one of weeds in a field, of frogs in a
pond, or of humans in a city, the beginning
change is not so great because there aren’t that
many organisms reproducing. Soon, however,
the rate of population growth increases rapidly
because the total number of reproducing
organisms increases. This pattern illustrates the
exponential nature of population growth.
Exponential growth of a population of organisms
occurs when the number of organisms rises at an
ever-increasing rate. Exponential growth, seen in this graph of world population,
results in population explosion.
Limits of the environment
Can a population of organisms grow indefinitely? What prevents the world from
being overrun with all kinds of living things? Through population experiments, scientists
have found that, fortunately, population size does have a limit. Eventually populations
do have limiting factors in their environment, such as food and space. This leveling off
of population size results in an s-shaped growth
curve. The number of organisms of a population that
a particular environment can support over an
indefinite period of time without destroying the
environment is known as its carrying capacity. When
populations are under the carrying capacity of a
particular environment, there will be more births than
deaths until the carrying capacity is reached. If the
population temporarily overshoots the carrying
capacity, there will be more deaths than births until
population levels are once again at the carrying capacity. This up and down trend in
populations can be seen in the graph to the right showing a population of rats
Environmental limits to population growth
Limiting factors, you remember, are biotic or abiotic factors that determine
whether or not an organism can live in a particular environment. Limiting factors also
regulate the size of a population. Limited food supply, extreme temperature, and
even storms can affect population size. Ecologists have identified two kinds of limiting
factors: density-dependent and density-independent factors.
Density dependent factors include disease, competition, and parasites, which
have an increasing effect as the population increases. Disease, for example, spreads
more quickly in a population whose members live close together. Why? In very dense
populations, disease may wipe out an entire population. In crops such as corn or
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soybeans, in which large numbers of the same plant are grown together, a disease will
spread rapidly throughout the whole crop. In less dense populations, fewer individuals
may be affected.
Density independent factors affect all populations, regardless of their density.
Most density-independent factors are abiotic factors such as temperature, storms,
floods, drought, and habitat disruption. An example of a density independent factor is
flood. No matter how many worms are living in a field, they will all drown if the field
floods.
Interactions Among Organisms that Limit Population Size
Population sizes are not limited only by environmental factors, but are also
controlled by various interactions among organisms that share a community;
Predations affects population size
Predation (when one organism hunts, kills, and eats another) of an organisms by
another is important for the proper balance in a community. Remember that energy is
moved through a
community by way of a
food chain and food
webs. Predation makes
sure the flow of energy
continues, but it may
also be a limiting factor
of population size.
Populations of
predators and prey
experience changes in
their numbers over a
period of years. Many
predator-prey
relationships show a cycle of population increases and decreases over time. The rise
and fall of population of snowshoe hare and lynx over an 80-year period is shown
above. The populations of both animals rise and fall almost together. Does this mean
that the lynx over hunted their prey, losing their food supply? Or, did the snowshoe hair
have less food to eat, causing their numbers to drop? While the graph can’t indicate
what factors caused the population changes, it is clear that most populations are
controlled in some way by predators.
Predator-prey relationships are important for the health of natural populations.
Usually, in prey populations, the young, old, or injured members are caught. This
predation keeps the population size within the limits of the available resources.
The effect of competition
Organisms within a population constantly compete for resources. Competition is
when two organisms struggle for a resource that is limited. When population numbers
are low, resources are plentiful. However, as population size increases, competition for
resources such as food, water, and territory can become fierce. Competition is a
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density-dependent factor. When only a few individuals compete for resources, no
problem arises. When population size increases to the point at which demand for
these resources exceeds the supply, the population size decreases.
Effects of crowding and stress
When populations of organisms become crowded, individuals may exhibit stress.
The factors that create stress are not well understood, but the effects have been
documented experimentally in populations of rats and mice. As populations increase
in size, individual animals begin to exhibit a variety of symptoms, including aggression,
decrease in parental care, decrease fertility, and decreased resistance to disease. All
of the symptoms can lead to decrease in population.
Human Population Growth
Does Earth have a carrying capacity for the human population? How many
people can live on Earth? No one knows how many people Earth can support, and it
is presently impossible to tell when the human population will stop growing. However,
an increasing number of scientists suggest that we can’t increase our production of
food enough to keep up with the increase in the population.
Demographic Trends
A good way to predict the future of the human population is to look at past
population trends. For example, are there observable patterns in the growth of
populations? That is, are there any similarities among the population growths of
different countries-similarities that might help scientist predict, and therefore control,
future population disasters? As you have seen, populations tend to increase until the
environment cannot support any additional growth. The study of population growth
characteristics is the subject of demography. Demographers study such population
characteristics as growth rate, age structure, and geographic distribution.
What is the history of population growth for humans? Although local human
population often show ups and downs, the worldwide human population has
increased exponentially over the past several hundred years as shown in the graphs
below. Unlike other organisms, humans are able to change their environment
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somewhat and get rid of competing organisms, increase food production, and
controlling disease organisms.
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Effects of natality and mortality
Two of the rates that determine population whether a population is growing are
natality and mortality. Natality is the birthrate. Mortality is the death rate. In many
industrialized countries, such as the United States, declining morality rates have a
greater effect on total population growth than increasing birthrates. For example, in
the United States, life expectancy (how long the average person will live) increases
almost every year. This means that people your age probably will live slightly longer
than students who are presently in college. Interestingly, although people in the U.S.
are living longer, the fertility rate is decreasing. This is because people are waiting until
their thirties and forties to have children. Childbirth in women in their twenties was
more common just a few decades ago. Today’s families also have fewer children
than they did in previous decades.
What are the birthrates and death rates of some countries around the world?
Look at the table below to see the birthrate, death rate, and fertility rate of some
rapidly growing and slower growing countries.
Birthrate
(per 1000)
Rapidly growing counties
Gaza
50
Iraq
46
Kenya
46
Rwanda
51
Uganda
52
Slowly growing countries
Hungary
12
Germany
12
Italy
10
Sweden
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Death
rate
(per
1000)
Fertility (per Population
woman)
increase
(%)
7
7
7
16
17
7.0
7.3
6.7
8.5
7.4
4.3
3.9
3.8
3.7
3.6
12
12
9
11
1.8
1.5
1.3
2.0
-0.2
0
0.1
0. 2
Fertility is the number of offspring a female produces during her reproductive
years. When fertility rates are high, populations grow more rapidly unless the death
rate is also high. Some countries such as Uganda and Rwanda have high death rates
among children because of disease and malnutrition. However, both countries have
extremely high birthrates, and they are both growing rapidly. Some other countries
such as Sweden and Italy have low death rates, but their birthrates are also low, so
these countries populations grow slowly, if at all.
The birthrate, death rate and fertility rate of a country provide clues to that
country’s rate of population growth. As you can see, different combinations of
birthrate and death rates have different effects on populations.
Mobility effects population size
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Unlike some organisms, humans can move in and out of different communities.
The effects of human migrations can make it difficult for a demographer to make
predictions, but patterns do exist. Movement of individuals into a population is
immigration.
Movement from a population is emigration. Obviously, movement of people
between countries has no effect on total world population, but it does affect national
populations growth rates. Local populations can also feel the effects of a moving
population. Many suburbs of large cities are expanding rapidly. This places stress on
schools, roads, and police and fires services., What other problems result from
suburban growth?
Calculating population growth rates
How can you tell if a population is growing? You have to consider four factors:
how many organisms are born (natality), how many die (mortality), how many come
into the population from somewhere else (immigration) and how many leave the
population for somewhere else (emigration)
Natality and immigration are always positive for the population since they increase the
population. Mortality and emigration are always negative for the population since
they decrease it. You can calculate the rate a population is growing or shrinking by
using the following formula:
Growth rate= Natality - Mortality + Immigration –
Emigration
Since natality, mortality, immigration and emigration are usually give with a time
attached to them, the formula will give you the growth rate per unit of time. For
example, in a population of lemmings, 300 are born each year, 175 die, 100 come to
the population from across the valley and 250 leave for greener pastures. What is the
growth rate of the population?
Growth rate = 300 – 175 + 100 – 250 = - 25 lemmings/year.
Because the growth rate is negative, this means the population is shrinking. You
can figure out what the populations will be in the future if you know what is at some
point in time and the growth rate. For example, to calculate the size of the lemming
population above 8 years from now, you need to know the population is currently 325
lemmings and the following formula:
Future population size = current population + (# time units x
growth rate)
So our lemming population 8 years from now = 325 + (8 x -25) = 325 – 200 = 125
lemmings.
You can calculate what the population was some time in the past using:
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Past population size = current population - (# time units x
growth rate)
Our lemming population 10 years ago = 325 – (10 x -25) =325 –(-250)= 325 + 250 =575
lemmings
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Part 3: Ecosystem Stability and Human Influences
Humans lower the stability of Ecosystems
Ecosystems tend to be stable through time because of the great number of
different types of organisms and interconnecting relationships found in ecosystems.
Usually, the greater the number of types of organisms, the greater the number of links
in a food web. A community that has a more complicated food web is more stable
than a community is with a community with only a few organisms and a simple food
web. A large community is like a web with many threads. If only one of these threads
is broken, the web as a whole may still function. If a disease kills many of the rabbits in
a community, the foxes can eat mice and squirrels until rabbits reproduce or
immigrate into the community. Even though many rabbits have died, the rest of the
ecosystem remains intact. It may be changed, but it is not destroyed. This is the
process of homeostasis. In a community with only rabbits for the foxes to eat, the
entire community would be destroyed if the rabbits all died. One thread in this
ecosystem could unravel the whole thing.
Human activities, however, can change ecosystems greatly. Humans disrupt
homeostatic processes. In doing this, we can have positive effects on other members
of the community, or we can have dramatic negative effects. Farmers, for example,
eliminate many members of a food web when they raise crops to meet human needs.
To feed the human population, farmers need to plant large fields of producers. To use
machinery efficiently, a single crop is planted in a large field. This large field would
provide energy for many birds, insects and animals. These consumers compete with
humans for the crop. Frequently, they are eliminated from the ecosystem along with
undesirable weeds and fungi. If a farmer is to grow crops economically, he or she must
control the food web and reduce the pests and diseases. Often such control involves
widespread spraying of chemicals
on crops, which may have
unplanned side effects.
Humans change ecosystems
for their own benefit. Because
parasitic disease, such as malaria
and African sleeping sickness (both
carried by insects), cause illness or
death, we try to eliminate the
insects that carry them. To
eliminate or to control these pests,
we use poisons called insecticides
to kill insects, herbicides to kill
weeds, and fungicides to kill fungi.
All of these products may be
included in the term biocides.
What effect do these biocides have on the relationships in a community? DDT
serves as a typical example. At one time, marshes along the north shore of Long
Island, New York were sprayed with DDT to control mosquitoes. Later microorganisms
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in the marsh water were found to have 0.04 parts per million (ppm) of DDT in their cells.
This low level was not much to get exited about so they kept spraying. Scientists later
tested minnows, clams and snails that ate the microorganisms and found 10 times as
much in these organisms than in the microorganisms. As scientists checked organisms
higher on the food chain, they found the higher up the chain they got, there was as
much as 10 million times the environmental level found in the bodies of the top level
consumers. It increased 625 times from producers at the bottom of the food chain to
the top consumers.
Because each consumer eats a large number of producers, DDT and other
chemicals become concentrated at upper levels in a food chain. Although the body
of each producer can be broken down and used by the consumer, the DDT in the
producer is not broken down. Each producer contains some DDT. As the consumer
eat lots of these DDT containing producers, the DDT concentrates in the consumer’s
body.
Some pest populations include individuals that are resistant to a biocide. These
individuals vary in their ability to tolerate, detoxify, or avoid the poison. When most of
the pests are killed by a biocide, these few individuals may survive and reproduce.
They pass their biocide resistance to their offspring. Then, when people try to kill these
offspring with the same amount of poison, it has little effect. They must use more or a
stronger biocide. This will work until the organisms become resistant to the stronger
biocide then people will have to use a still stronger biocide.
Humans Threaten the Diversity of Organisms
At the same time that human activities are raising the number of biocideresistant organisms, many potentially important organisms are disappearing. Their
disappearance also is due to human influence and results in a decrease in
biodiversity—the number of different types of organisms that live on earth. Ecosystems
with a large number of different types of organisms are usually quite stable. As our
human population grows and we expand our activities, we occupy more land and
therefore destroy the habitats of many organisms. Often this damage extends to
beneficial and desirable organisms. The smog created by automobiles and industry is
killing many types of trees over a wide area of southern California. The needles of
ponderosa pines, for example, gradually turn brown, and the tops of palm trees have
only small tufts of fronds. When this happens, photosynthesis is drastically reduced.
And the plants die.
Plants and animals in heavily populated areas are not the only ones threatened.
Tropical rain forests are the most diverse ecosystems on earth. They provide habitats
for many different species. A species is a
group of organisms that can only reproduce
successfully with others of the same type.
Unfortunately, deforestation (cutting down
large tracts of trees) for lumber, grazing land,
and other uses is destroying countless species
every day. 2/3 of the world’s species are
located in the tropics and subtropics. If the
current rate of habitat destruction in these
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areas continues, nearly half of the Earth’s species will be extinct (gone forever) or
severely threatened during the next 25 years. To find an extinction rate that is
comparable, we would have to go back to when the dinosaurs became extinct, the
Cretaceous Period, 65 million years ago.
Why does it matter if whooping cranes and pitcher plants become extinct?
One argument against human-caused extinction comes from genetics. If all the crop
plants in a field are genetically similar and one individual plant gets a disease, they will
all get it and be wiped out. Once extinction occurs the genetic material is lost forever.
The second argument against human-caused extinction is related to the instability of
simplified ecosystems. Think of the cornfield ecosystem. If the corn becomes extinct,
the small food web unravels. The third argument against human-caused extinction
comes from research on plants. Deep in the rain forest of South America, a small plant
may be holding the cure for cancer in its leaves. If we destroy its habitat, the cure
may be lost forever.
Humans Also Can Help Conserve Species
We can take many measures to decrease the loss of species diversity. One such
action is the establishment of wilderness areas. Wilderness areas act as one type of
habitat preservation. In these areas, people reduce the effect of human activities on
local organisms through restricted use. In addition, through the efforts of zoos, species
that may be endangered in the wild are able to continue under a watchful breeding
program. In some cases, animals that are bred in zoos are returned to the wild to
restock depleted populations.