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Introduction to Ecology


Ecology is the study of organisms
and their interactions with their
environment.
The environment includes 2 types of
factors:

Biotic factors – the living parts of the
environment


Plants, animals, bacteria, fungi, protists
Abiotic factors – the nonliving parts of the
environment
 Water, oxygen, light, temperature, etc.
Levels of Organization

Ecologists can look at the world in levels of
increasing complexity:
 Population – a group of organisms of the same
species that live together in a certain area


Community – a group of organisms of different
species that live together in a certain area


Example:
Biome – a group of ecosystems that have similar
climates and communities


Example:
Ecosystem – all the living and nonliving things in
a certain area


Example:
Example:
Biosphere – the entire portion of the planet that
supports life
Population Ecology

A population is a
group of
individuals of the
same species that
live in the same
area
Population Ecology: Vocabulary

Density:



The number of individuals per unit
area/volume
Example: 47 elephants/km2
Dispersion:

The pattern of spacing among
individuals in a population
Clumped
 Uniform
 Random

uniform distribution
•results from intense competition or
antagonism between individuals where
environmental conditions are fairly uniform.
Random distribution
• occurs when there is no competition,
antagonism, or tendency to aggregate where
environmental conditions are fairly uniform
• It is rare for all these conditions to be met.
Clumping
• is the most common distribution
• environmental conditions are seldom uniform
• reproductive patterns favor clumping
• animal behavior patterns often lead to
congregation.
Factors That Influence Population Size

There are 3 major factors that influence
population size:
1. the number of births
2. the number of deaths
3. the number of individuals that enter or
leave a population
- immigration:
individuals entering an existing population
- emigration:
individuals leaving an existing population
Limiting Factors

factors that limit the size of a populations

Density-dependent limiting factors

effect intensifies as the population increases


Ex’s. Intraspecific competition (food & space),
Predation
Density-independent limiting factors

The occurrence and severity are unrelated to
population size

Ex’s. Climate, Disease, Pollution
Estimating Population Size




The mark-recapture method can be
used to estimate the size of a
population
Capture, mark, release
Recapture and count
Equation:
N = Number marked x Total catch 2nd time
Number of marked recaptures
Patterns of Population Growth

Exponential Growth:
 Occurs in ideal
conditions with
unlimited resources
 Characterized by a Jshaped curve
 Example:
 1 bacterium
(reproducing every
20 minutes) could
produce enough
bacteria to form a
1-foot layer over
the entire surface
of the Earth


Exponential growth
cannot continue
indefinitely
It is characteristic of
populations who are
entering a new
environment OR
those whose
numbers are
rebounding from a
catastrophic events
Logistic Growth




Pattern of population
growth which takes
into account the
effect of population
density on
population growth
Takes carrying
capacity into account
Occurs when
resources become
more scarce
Characterized by an
S-shaped curve
Patterns of Population Growth

Carrying capacity:
 The maximum number of individuals that a
particular environment can support over a long
period of time
 Symbolized as K
 Determined by such limiting factors as crowding
and food resources
 The point at which a graph “levels off” is the
environment’s carrying capacity for that
particular species
• Where the curve levels off, births and deaths
are in balance and the population has zero
population growth.
• This occurs because environmental limitations
become increasingly effective in slowing
population growth as the population density
rises.
• When the density approaches the carrying
capacity, the limitation becomes severe.
•Most populations oscillate around the carrying
capacity. When a population goes above the
carrying capacity, the environment can not
sustain it the population will fall. If it falls
below the carrying capacity, then resources are
plentiful and population will increase.
•The populations of many small short-lived animals,
or those living in variable environments, go through
a period of exponential growth, followed by a sudden
crash (boom and bust curve).
•The crash occurs before the populations reach the
carrying capacity; it is due to a density-independent
limitation such as weather or other physical
environmental factors.
Populations can be K-selected
or r-selected
• K-selected populations (equilibrial
populations) live near or at the carrying
capacity
• r-selected populations (opportunistic
populations) live in environments where
the population densities fluctuate or
where they face little competition
Characteristic
-Maturation time
-Lifespan
-growth curve
-Death rate in young
-Investment of
time in offspring
-Number of offspring
per reductive episode
-# reproductions
per lifetime
-Timing of first
reproduction
-Habitat
r-selected
populations
short
short
boom-bust
high
none
K-selected
populations
long
long
logistics curve
low
some
many
few
one
several
early in life
unstable
late in life
stable
Some populations show regular fluctuations
in density in a cyclic way.
Mammals
-Small herbivores (lemmings) 3-5 year
-Large herbivores (snowshoe hares)9-11
year cycle
Hypotheses for fluctuations
-Stress from high densities may alter
hormone balance and reduce fertility
or may cause physiological changes
-High density may cause changes in the
immune system,
•Oscillation in the population of the hare preceded
changes in the lynx density. This was once considered
to be interrelated but the hare shows such oscillations
on islands where the lynx is absent. The lynx
population oscillation may be the result of hare
population but not the other way around.
Demography
• the study of factors that affect birth and death rate.
Age structure
-Every age group has characteristic birth and death
rate
a. Birth rate (fecundity) is greatest for those of
intermediate age
b. Death rate is often greatest for the very young
and very old
c. A population with more older, non-reproductive
individuals,grows slower than a population with
a larger percentage of young, reproducing
individuals.
• Sex ratio is the # males vs. # females.
• It is not always 50:50. If the male
mates with more than one female, then he
may have a harem of females e.g. elk. If
the species form monogamous pairs, the
ratio is more 50:50.
• In addition to the birth rate and death
rate, the potential life-span, the average life
expectancy, and the average age of
reproduction are important determinants of
the makeup of a population.
Diagrams
•A major factor in the variation of the growth rates among
countries is the variation in their age structure.
•The relatively uniform age distribution in Italy, for example, is
due to the fact that the growth is rate is stable. For every birth
there is a death on the average. It has been this way for some
time. This keeps the population at large from growing. The
size of each age class from pre-reproductive through early
post-reproductive is roughly equal.
•In the U.S. the death rate is closely equal to the birth rate,
however this is recent. Because of that, the U.S. has a
disproportionate percentage of pre-reproductive and younger
reproductive individuals. The U.S. would reach equilibrium like
Italy in about 25 years, however, the high rate of immigration
will keep it like the population distribution as it is shown.
•In Kenya the age distribution is heavily weighted toward the
bottom, with larger percentages of younger age classes in the
bottom.
survivorship
curves
•Determining the mortality rates for the various age
groups in the population gives a survivorship curve.
•type I curve - all the organisms live to old age and
die quickly
•type II curve - constant mortality rate at all ages
•type III curve - mortality among the young is very
high, but those who survive the early stages tend to
live for a long time. In nature, high mortality among
the young is the rule.
Human Population Growth
•So far, the history of
human population growth
has shown exponential
growth. Humans have
“changed” many of their
limiting factors
(agriculture, medicine,
eliminate interspecific
competition). But that
cannot continue forever!
Human Population Growth

Implications of exponential human population
growth:





Lack
Lack
Lack
Lack
of
of
of
of
food supplies
space
natural resources (metals, fossil fuels, etc)
sites for waste disposal
Ecologists cannot agree on a carrying capacity
for Earth


Are we going to reach carrying capacity through
individual choices and/or government programs?
OR
Is Earth’s population going to “level off” as a result
of mass deaths?