Ecology  Part 1
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Ecology Part 1. Ecology is the study of the interaction between organisms and their environment. There are three higher levels of organization: 1. Populations - groups of individuals belonging to the same species.

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Ecology Part 1

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Ecology Part 1

Ecology is the study of the interaction between organisms and their environment. There are three higher levels of organization:

1. Populations- groups of individuals belonging to the same species.

2. Communities- units composed of all the populations living in a given area.

3.Ecosystems- the sum total of the communities and their physical environment considered together.

4. Biosphere- The various ecosystems are linked to one another by biological, chemical, and physical processes. The entire earth is a true ecosystem

Population organization and growth

Ecologist often need to count how many individuals are in a population or the density of a population (# individuals per unit area).

  • Counting Populations

  • Count the total # of individuals, easy if the organisms are large and area is not too large.

  • Divide the area into # of quadrants and count the number of individuals in several of quadrants and then estimate the entire area.

3. Mark-and-recapture technique. A limited number of individual (e.g. 20) are captured at random and marked with a dye or tag and then are released back into the environment. At a later time a second group of animals is capture at random from the population and the percentage of marked individuals determined. Now if 10% of the animals in this second group is recaptured, then the original 20 represented 10% of the population and the population then is 200.


c1= % of Recapture

C = # Captured

P = Population

30 turtles are captured in 1 km2, they are marked and released back into the wild. Two weeks later 30 more turtles are caught. 6 had the marking of the original population. Based on this information, what is the best estimation of the turtle population in the area?





The distribution of individuals within an area may be uniform, random, or clumped.

Uniform Distributions

Are relatively rare and occur only where environmental conditions are fairly uniform. A uniform distribution results from intense competition or antagonism between individuals.

Random Distributions

Occurs when there is no competition, antagonism, or tendency to aggregate. The conditions are uniform. It is rare for all these conditions in the environment to be met.

In an environment that has environmental factors that remain constant, a particular species of trees excrete a toxin that prevents any other tree from becoming established in a given radius from the tree. What sort of population distribution would these trees would be most likely have?





Clumping is the most common distribution because environmental conditions are seldom uniform, reproductive patterns favor clumping, and animal behavior patterns often lead to congregation. The optimum density for population growth and survival is often an intermediate one; undercrowding can be as harmful as overcrowding.

The observation that members of a population are uniformly distributed suggests that

A)the size of the area occupied by the population is increasing.

B)resources are distributed unevenly.

C)the members of the population are competing for access to a resource.

D)the members of the population are neither attracted to nor repelled by one another.

E)the density of the population is low.

How population size is regulated

The regulation of population density in organisms with boom-and-bust curves is primarily due to the work of density-independent limitations. The maximum density achieved before the decline is primarily a function of the organisms' high reproductive rates. Since these organisms have evolved high intrinsic rates, they are called high r-max species.

Population Growth

All organisms have the potential for explosive growth; under ideal conditions their growth curve would be exponential.

Ex. If a pair of house flies were allowed to reproduce starting in April and all were able to survived long enough to produce a full complement and so on for a number of generations that by August there would be 2 x 10^20 offspring by August. This would cover all of the land on earth 10 cm. Elephants would cover the planet in only a couple of centuries.

The equation for a population growth is

DN/Dt=(b-d)N, where

DN= change in population

Dt= change in time

N= number of individuals in the population

(b-d)=the birth rate minus the death rate called intrinsic rate of increase or r

Another to write the above equation is

Nt+1 - Nt=(b-d)N

where t is defining two separate times.

  • If the birth rate is greater than the death rate, the population grows (b-d)>0.

  • If the death rate is greater then the birth rate, the population decreases (b-d)<0.

  • If the birth rate equals the death rate, then there is no growth, b-d=0.

  • This change in the population is called the intrinsic rate of increase or r. When the birth rate is at its maximum and the death rate is minimum then the population grows at population grows at its fastest rate. The is termed r max. Now the equations can be written as follows:

  • DN/Dt=rN or Nt+1 - Nt=rN

The maximum intrinsic rate of increase (r max) varies with different species. It is greater for house flies than it is for elephants

In an ideal population, growth is not only a function of r-max but also N. N becomes large with each generation and that means that DN/Dt also becomes larger with each successive generation. This means that not only is the population growing but that the rate of increase is also accelerating.

When the birth rate of an organism is at its maximum and the death rate is at its minimum, the population is reproducing

A) at its steady state

B) at r-max

C)at K or carrying capacity

D)at a deceleration rate

Exponential growth can not continue forever. The exponential growth of many real populations begins to level off as the density approaches the carrying capacity(K)of the environment. The carrying capacity of a population is the maximum density of a population that the environment can support over a sustained period without damage to the environment.

Intrinsic rate of increase (r) then will be high with lower densities and decrease with higher population densities until the population levels out reaching a carrying capacity (K). Such a growth curve is called an S-shaped or logistic growth curve and results from a changing ratio between births and deaths. In the beginning the rate of increase is accelerating but later as the population increases the rate of increase decreases even though the birth rate is greater than the death rate. At some point in time equilibrium is reached or the carrying capacity and the birth rate equals the death rate.

In a population that is growing as described by the logistic growth model,

A) the number of individuals added per unit time is greatest when N is close to zero. [The key equation is dn/dt = rmax N(K-N/K).]

B) the per capita growth rate (r) increases as N approaches K.

C) population growth is zero when N equals K.

D) the population grows exponentially when K is small.

E)the birth rate (b) approaches zero as N approaches K.

After the curve has leveled 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. A density dependent limitation, (K-N)/K, is one whose density is determined by the density of the very population it helps limit. The equation for the logistic growth curve is


This inserts the density dependent limiting factor.

At low population densities, population growth is exponential; the rate of increase reaches its maximum at the inflection point of the curve. When the population exceeds the carrying capacity, DN/Dt (i.e. Dbirths per year) becomes negative, and the population decreases.

Growth of a population with rmax of 1.0, K of 100, and an initial size N of 4. It assumes that all individuals in the previous generation survives. In the beginning and N is small, the density limiting factor is very close to 1.

The growth curve for a population is shown. Which of the following is false?

A)N is becoming larger each successive generation

B)r-max is positive

C)this is exponential growth

D)growth rate is accelerating

The inflection point is where the growth moves from accelerating to decelerating.

This point is the maximum sustainable yield

If man wanted to harvest an organism, then the population should be kept at the maximum sustainable yield and not at the carrying capacity

Which part of the graph demonstrates exponential growth?





E) E

Most populations oscillate around the carrying capacity. When a population goes above the carrying capacity, theenvironment 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. The operation of such a limitation does not depend of the density of the organisms.

A population that is exhibiting this sort of is least likely to be regulated by

A)shortage of food


C)density-dependent factors


If a fish farmer wanted to harvest his fish so that it recovered at the maximum rate, where should population be





E) E

If rmax is high, it is possible to have a wildly fluctuating population size that is independent of the environment and still have density dependent limiting factors.

Where is r=0?





E) E

Demography is the study of factors that affect birth and death rate.

Age structure and sex ratio

-Many populations have overlapping generations where individuals of more than one generation coexist. (Exceptions include species in which all the adults reproduce at the same time and then die e.g. annual plants and many insects). This phenomenon produces an age structure for the population = relative numbers of individuals of each age group.

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.

Generation time = the average span of time between birth and the birth of their offspring.

a. Correlation with body size.

b. A shorter generation time usually results in faster population growth providing birth rate is greater than death rate with all other factors being equal.

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.

Determining the mortality rates for the various age groups in the population gives a survivorship curve. A type I curve is one in which all the organisms live to old age and die quickly; a type II curve shows a constant mortality rate at all ages; and a type III curve is typical of populations where the 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.

Organisms which produce many young are most likely to have which mortality curve?

A)I or II





Which mortality curve indicates that the probability of death is equal at any age?






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 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.

The survivorship curve in India is shifted toward curve #2 with higher infant mortality and a large average family size. This will have larger population distribution for the younger generations.

The human population doubled every 1600 years until the last thousand years.

Life History = An organism's scheduled of reproduction and death.

Factors include trade offs between survival, frequency of reproduction, investment of parental care and number of offspring per reproductive episode.

Diversity in life history

a. Some animals hatch in one type of biome, migrate to another where they mature and then return to the initial biome to reproduce. e.g. salmon

b. Animals hatch and mature in a single habitat, then have small reproductive efforts each year for several years. e.g. lizards

c. Life histories often vary with environmental factors such a latitude. e.g. Tropical birds lay fewer eggs than those in higher latitudes because days are longer giving more time to gather food raise more offspring. This pattern is found to be true in mammals, lizards and insects

d. Life histories can be a trade-off between factors like fecundity and mortality. e.g. Albatrosses only average one surviving offspring every five years but adult birds only have a 5% chance of dying whereas tree sparrows have a 50% chance of dying between breeding seasons but produce 6 surviving offspring per year. Delayed maturation and high parental investment in offspring tend to be correlated with low fecundity and low mortality.

The relationship between fecundity and mortality between 14 species of birds. An increase in mortality is correlated with an increase in fecundity.

This graph indicates that

A) the longer a bird lives the more eggs it will

B)larger birds lay larger eggs

C)larger bird lay fewer eggs

D)birds that are more likely to die will lay more eggs in a given year.

Population Regulation

Populations are regulated by density-dependent factors and density independent factors either separately or in combination. The importance of these factors differs among species and their specific circumstances.

I. Density independent factors regulating population are unrelated to population density.

-Density-independent factors affect the same percentage of individuals regardless of the size of the population

-Weather, climate and natural disasters such as freezes, seasonal changes, hurricanes and fires are examples

-The severity and time of occurrence is the determining factor on what proportion of the population is affected

-In some populations these effects routinely control population size before density-dependent factors become important

e.g. Eucalyptus tree died in Oakland after an unexpected freeze '72. The trees are returning until another freeze. The corn has died as a result of a drought and this did not depend on the density of the corn's population.

Populations of Thrips grow rapidly during spring in flowers that provide food and shelter. Before the population reaches carrying capacity, the insects' numbers are drastically reduced during the dry Australian summer in December. A few individuals remain in the surviving flowers, and these allow the population grow to resume when conditions are favorable.

The regulation of population density in organisms with density-independent factors exhibit with boom-and-bust curves. The population never reaches the carrying capacity K. The maximum density achieved before the decline is primarily a function of the organisms' high reproductive rates. Since these organisms have evolved high intrinsic rates, they are called high-rmax species.

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.

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