Biology: POPULATION ECOLOGY
Table of Contents
Ecology | Population Growth | Altering Population Growth | Range and Density |
Ecology
the individual cell, tissue, and organism. There are levels of organization
above the individual organism that will be the subject of this unit.
Individual organisms are grouped into populations, which in turn form
communities, which form ecosystems. Ecosystems make up the biosphere,
which includes all life on Earth. If there is life on other planets,
will we need another level of organization?
The sum of all living things taken in conjunction with their environment.
In essence, where life occurs, from the upper reaches of the atmosphere
to the top few meters of soil, to the bottoms of the oceans. We divide
the earth into
and biosphere (life).
The relationships of a smaller groups of organisms with each other and
their environment. Scientists often speak of the interrelatedness of
living things. Since, according to Darwin's theory, organisms adapt
to their environment, they must also adapt to other organisms in that
environment. We can discuss the flow of energy through an ecosystem
from photosynthetic autotrophs to herbivores to carnivores.
The relationships between groups of different species. For example,
the desert communities consist of rabbits, coyotes, snakes, birds, mice
and such plants as sahuaro cactus (Carnegia gigantea), Ocotillo, creosote
bush, etc. Community structure can be disturbed by such things as fire,
human activity, and over-population.
Groups of similar individuals who tend to mate and produce viable, fertile
offspring. We often find species described not by their reproduction
(a biological species) but rather by their form (anatomical or form
species).
Groups of similar individuals who tend to mate with each other in a
limited geographic area. This can be as simple as a field of flowers,
which is separated from another field by a hill or other area where
none of these flowers occur.
One or more cells characterized by a unique arrangement of DNA "information".
These can be unicellular or multicellular. The multicellular individual
exhibits specialization of cell types and division of labor into tissues,
organs, and organ systems.
(in multicellular organisms). A group of cells, tissues, and organs
that perform a specific major function. For example: the
in circulation of blood.
(in multicellular organisms). A group of cells or tissues performing
an overall function. For example: the
an organ that pumps blood within the cardiovascular system.
(in multicellular organisms). A group of cells performing a specific
function. For example
found in the heart and its unique contraction properties aid the heart's
functioning as a pump. .
The fundamental unit of living things. Each cell has some sort of hereditary
material (either DNA or more rarely RNA), energy acquiring chemicals,
structures, etc. Living things, by definition, must have the metabolic
chemicals plus a nucleic acid hereditary information molecule.
A subunit of a cell, an organelle is involved in a specific subcellular
function, for example the
site of protein synthesis) or
site of ATP generation in eukaryotes).
atoms, and subatomic particles: The fundamental functional levels of
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
is thus possible to study biology at many levels, from collections of
organisms (communities), to the inner workings of a cell (
the study how organisms interact with each other and their physical
environment. These interactions are often quite complex. Human activity
frequently disturbs living systems and affects these interactions. Ecological
predictions are, of a consequence, often more general than we would
like.
Population Growth |
geographic area. The study of factors that affect growth, stability,
and decline of populations is population dynamics. All populations undergo
three distinct phases of their life cycle:
- growth
- stability
- decline
of individuals able to exploit them. Reproduction is rapid, and death
rates are low, producing a net increase in the population size.
stability is often proceeded by a "crash" since the growing
population eventually outstrips its available resources. Stability is
usually the longest phase of a population's life cycle.
is the decrease in the number of individuals in a population, and eventually
leads to population
Factors Influencing Population Growth
potential to expand at an exponential rate, since reproduction is generally
a multiplicative process. Two of the most basic factors that affect
the rate of population growth are the
and the death rate. The intrinsic rate of increase is the birth rate minus the death
rate.
as a J-curve) occurs when there is no limit to population size. The Logistic curve
(also known as an S-curve) shows the effect of a limiting factor (in
this case the carrying capacity of the environment). Image from
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
Population Growth Potential Is Related to Life History
affects the rate of population increase.
to the age of sexual maturity, age of death, and other events in that
individual's lifetime that influence reproductive traits. Some organisms
grow fast, reproduce quickly, and have abundant offspring each reproductive
cycle. Other organisms grow slowly, reproduce at a late age, and have
few offspring per cycle. Most organisms are intermediate to these two
extremes.
II, and III); b, c, and d) three real populations. Note that the real
curves approximate one of the three hypotheticals. Images from Purves et al.,
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
structure refers to the relative proportion of individuals in each age
group of a population. Populations with more individuals aged at or
before reproductive age have a pyramid-shaped age structure graph, and
can expand rapidly as the young mature and breed. Stable populations
have relatively the same numbers in each of the age classes.
Comparison of the population age structuire
in the United States and Mexico. Note the deographic bulge in the Mexican
population. The effects of this buldge will be felt for generations.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
aged and began to have children of their own this created a secondary
bulge termed Generation X. What happens when the Generation X members
begin to have their own children? Image from Purves et al.,
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
populations are in a growth phase. Since evolving about 200,000 years
ago, our species has proliferated and spread over the Earth. Beginning
in 1650, the slow population increases of our species exponentially
increased. New technologies for hunting and farming have enabled this
expansion. It took 1800 years to reach a total population of 1 billion,
but only 130 years to reach 2 billion, and a mere 45 years to reach
4 billion.
technological advances, factors influencing population growth will eventually
limit expansion of human population. These will involve limitation of
physical and biological resources as world population increased to over
six billion in 1999. The 1987 population was estimated at a puny 5 billion.
Human population growth over the past 10,000
years. Note the effects of worldwide disease (the Black death) and technological
advances on the populatiuon size. Images from Purves et al.,
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
Populations Transition Between Growth and Stability
interactions with other physical and biological factors (including other
species). After an initial period of exponential growth, a population will encounter
a limiting factor that will cause the exponential growth to stop.
population enters a slower growth phase and may eventually stabilize
at a fairly constant population size within some range of fluctuation.
model fits the
The carrying capacity is the point where population size levels off.
Relationship between carrying capacity (K)
and the population density over time. Image from Purves et al.,
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
Several Basic Controls Govern Population Size
categories of factors are commonly used: physical environment and biological
environment. Three subdivisions of the biological environment are
predation, and symbiosis.
environment factors include food, shelter, water supply, space availability,
and (for plants) soil and light. One of these factors may severely limit population size, even if the
others are not as constrained. The Law of the Minimum states that population growth is limited by the resource in the shortest
supply.
biological role played by a species in the environment is called a
Organisms/populations in competition have a
a scarce resource for which they compete.
between two species when competition is so intense that one species
completely eliminates the second species from an area. In nature this is rather rare. While owls and foxes may compete for a common food source, there are
alternate sources of food available. Niche overlap is said to be minimal.
its food source with a competing species.
when the competing species is no longer present and its constraint on
the winner's population size is removed.
are in a competition situation one species will win, the other will
lose (bottom graph). Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission..
kill and consume other organisms.
on animals, herbivores consume plants. Predators usually limit the prey population, although
in extreme cases they can drive the prey to extinction. There are three
major reasons why predators rarely kill and eat all the prey:
- Prey species often evolve protective mechanisms
such as camouflage, poisons, spines, or large size to deter predation. - Prey species often have refuges where the predators
cannot reach
them. - Often the predator will switch its prey as
the prey species becomes lower in abundance: prey switching.
a 40-year span. Note the effects of declines in the wolf population
in the late 1960s and again in the early 1980s on the moose population.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/)
and WH Freeman (http://www.whfreeman.com/),
used with permission.
has come to include all species interactions besides predation and competition.
a symbiosis where both parties benefit, for example algae (zooxanthellae)
inside reef-building coral.
a symbiosis where one species benefits while harming the other.
act more slowly than predators and often do not kill their host.
a symbiosis where one species benefits and the other is neither harmed
nor gains a benefit: Spanish moss on trees, barnacles on crab shells.
is a symbiosis where members of one population inhibit the growth of
another while being unaffected themselves.
The Real World Has a Complex Interaction of Population Controls
have the combined effects of many controls simultaneously playing roles
in determining population size. If two beetle species interact in the laboratory, one result occurs; if a third species
is introduced, a different outcome develops. The latter situation is more like nature, and changes in one population
may have a domino effect on others.
factors, if either, is more important in controlling population growth:
physical or biological? Physical factors may play a dominant role, and are called density
independent regulation, since population density is not a factor
other extreme has biological factors dominant, and is referred to as
density dependent regulation, since population density is a factor.
It seems likely that one or the other extreme may dominate in some environments,
with most environments having a combination control.
Population Decline and Extinction
extinction is the loss of all individuals in a population.
extinction occurs when all members of a species and its component populations
go extinct. Scientists estimate that 99% of all species that ever existed are
now extinct. The ultimate cause of decline and extinction is environmental
change. Changes in one of the physical factors of the environment may cause
the decline and extinction; likewise the fossil record indicates that
some extinctions are caused by migration of a competitor.
declines in human population happen periodically in response to an infectious
disease. Bubonic plague infections killed half of Europe's population between
1346 and 1350, later plagues until 1700 killed one quarter of the European
populace. Smallpox and other diseases decimated indigenous populations
in North and South America.
Human Impact
Human populations have continued to increase,
due to use of technology that has disrupted natural populati
Destabilization of populations leads to possible outcomes:
- population growth as previous limits are removed
- population decline as new limits are imposed
Agriculture and animal domestication are examples
of population increase of favored organisms. In England alone more than 300,000 cats are put to sleep per year,
yet before their domestication, the wild cat ancestors were rare and
probably occupied only a small area in the Middle East.
Pollution
into the air and water. Many lakes often have nitrogen and phosphorous as limiting nutrients
for aquatic and terrestrial plants. Runoff from agricultural fertilizers increases these nutrients, leading
to runaway plant growth, or eutrophication. Increased plant populations
eventually lead to increased bacterial populations that reduce oxygen
levels in the water, causing fish and other organisms to suffocate.
Pesticides and Competition
a population explosion in that species competitor. Pesticides sprayed
on wheat fields often result in a secondary pest outbreak as more-tolerant-to-pesticide
species expand once less tolerant competitors are removed.
Removal of Predators
allowing the prey population to increase. Elimination of wolves and panthers have led to increase in their natural
prey: deer. There are more deer estimated in the United States than
there were when Europeans arrived. Large deer populations often cause over grazing that in turn leads
to starvation of the deer.
Introduction of New Species
is perhaps the greatest single factor to affect natural populations. More than 1500 exotic insect species and more than 25 families of
alien fish have been introduced into North America; in excess of 3000
plant species have also been introduced. The majority of accidental introductions may fail, however, once an
introduced species becomes established, its population growth is explosive.
Kudzu, a plant introduced to the American south from Japan, has taken
over large areas of the countryside.
Kudzu covering a building (left) and clos
of the flowers and leaves (right). Images from http://www.alltel.net/~janthony/kudzu/
, photographs by Jack Anthony, used with permission.
Growth
the extinction of species at thousands of times the natural rate.
by alteration of a population's environment in a harmful way.
color: #0000FF;">Habitat disruption
of the physical environment of a species, for example cutting a forest
or draining wetlands. Habitat disruption in currently the leading cause of extinction.
environment occur in three ways.
An exotic species is introduced into an area where it may have no predfators
to control its population size, or where it can gratly out compete native
organisms. Examples include zebra mussels introduced into Lake Erie, and lake trout released into Yellowstone
Lake where they are threatening the native cutthroat trout populations.
at killing the prey, the prey population may decline or go extinct.
Examples today include big game hunting, which has in many places reduced
the predator (or in this case prey) population. In human prehistory we may have caused the extinction of the
mammoths and mastodons due to increased human hunting skill.
species can cause migration or extinction of any species that depends
largely or solely on that species as a food source.
Populations
Have a Minimum Viable Size
of individuals survive, the population size may become too small for
the species to continue. Small populations may have breeding problems. They are susceptible to random environmental fluctuations and
than are larger populations. The chance of extinction increases exponentially
with decreasing population size.
population size that can avoid extinction by the two reasons listed
above. If no severe environmental fluxes develop for a long enough time,
a small population will recover. The MVP depends heavily on reproductive rates of the species.
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- Tracking the genetic effects of global warming: Drosophila
and other model systems
This online article looks at the spread and microevolution withing the
common fruit fly, Drosphilia and possible relationahips of this speciation
and global warming. A bit technical in a few places, but an interesting
read. - Kudzu, the Vine
Learn about this oriental import that has taken over large parts of
the souther United States. I remeber my first views of kudzu when I
moved to central Georgia...green vines covering trees, houses, phone
lines. - Invaders
Learn about many of the invaders that have already caused economic harm
and ecological damage to the areas around New Orleans, Louisiana, USA. - USGS Scientists Find New Population of Asian Swamp
Eels in South Florida
This news release (3/00) from the US Geological Survey details a population
of asian eels possibly invading the Everglades. - Salvinia molesta
The water hyacinth seems a harmless enough plant, but it has choked
waterways all over the southern USA, Learn about this pesky species
at this U.S. Geological Survey page.
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purposes is heartily encouraged.
Email: nolvyhindarto@gmail.com
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