Ecology Population & Population Dynamics I

Ecology –Pop_dynanmics I P.1

ECO_POPULATION & POPULATION DYNAMICS I

General Characteristics of population

population consists of all the individuals of the same species found occupying a given habitat and interbreeding with each other, they share a common gene pool.

/ Much of the time, most populations are fairly stable. But once in a while
- a natural disaster can wipe out most of the population, allowing it to recover relatively free of density dependent control (e.g. predation and overcrowding effects)
- or when it colonises a new environment free of competition.

To start from an 'idealized' situation; the growth of a population as it colonizes a new environment free of predators. What happens if you release a few individuals of a species into a new, resource-rich environment (e.g. a few fishes into a lake in which there were no fish previously, or some wild flower seeds onto a mountainside of cooled volcanic ash)?

I) Population growth

A population can start with only a few individuals, or even one, and it can grow to utilize the available re______.

If a small population of a species is intro______into a new area which is suitable for its growth, one sees the beginning of a population exp______. Over time, assuming that resources (food) are available, populations grow at exp______ rates.
Q. What are the assumptions in the above growth pattern. /
In naturally occurring systems (not human-made), f___ availability does not continually increase over time. Biological events (environmental resistance) occur that allow a population to stabilize and be in bal____ with resources / food avail______.
Environmental resistance includes :
· gradual shortage (exhaustion) of f___
· accumulation of w_____ products and
· limiting sh_____ /

A) S-shaped or Sigmoid Growth curve

It is illustrated by the growth of yeast cells in laboratory culture. Yeast, Schizosaccharomyces, with a very short life cycle is used to study the pop______growth in laboratory.
When the number of individuals is plotted against time, the characteristic S-shaped or sigmoid growth curve is obtained. This is a typical growth pattern for a species in a n__ and fav______environment where there is initially no comp_____ nor pre______occurs. /

a) The Three phases of a Sigmoid growth curve

Lag phase (0-4 hrs.)

The initial rate of increase is very s___. During this period, growth, if any, is manifested only in individual yeast cells which increase in s___ and maturity i.e. the cells are adj______ to the new environment.

For sexually reproducing populations, individuals may find it difficult to find a mating partner because the population den____ is low.

Eco-Pop. Dynamics I P.8

Log phase (exponential phase) (4-10 hrs.)

The population increases ra____ because all the cells are alive and are capable of reproducing at a rapid rate because of abundant f___ supply.

Stationary phase (10-18 hrs.)

The population growth rate de______until finally it becomes z___. (i) 10-14 hrs. The growth rate (rate of increase) decreases because of the increase in en______re______resulting from the increase in population density.

These lower the b____ rate but raise the d____ rate so that the growth rate slows down. (ii) 14-18 hrs. Finally when the birth rate e_____ death rate, the population size becomes somewhat stab____, and zero net growth is observed. Now the population is at the eq______level and has reached the max_____ density that can possibly be supported by the environment --- the carrying capacity of the environment.

Organisms invading n___ habitats sometimes show explosive population increases (log phase). In each case dispersal by humans has overcome natural dispersal bar____, without its usual pre_____ , par____ and comp______, the population growth accelerates to utilize the natural resources till it reaches the c______c______of the environment (the ‘stable’ phase) — often causing widespread damage or harm to the nat___ species and the environment. e.g. Introduced rabbits in Australia. The ‘tree killer’ Mikania (the Mile-a-minute Weed) in HK.

b) Carrying capacity

It is the max____ population density that can be supported by the environment. If the environment changes, the carrying c______may also change. Environmental changes may be:
· a physical change such as temperature alteration.
· a chemical change such as removing toxic wastes or more food supply.
Q. What does the barrel and the water represent? /

Read the articles on carrying capacity from:

http://curriculum.calstatela.edu/courses/builders/lessons/less/biomes/carryingcap.html

Q. Give examples of how man altering the carrying capacity of the Earth to sustain the ever-increasing human population.

See how the use of fossil fuel has on the carrying capacity of the Earth for the human species:

http://www.holon.se/folke/kurs/logexp/carrying.shtml

Q. Explain why some argue that we are exchanging ‘fossil fuel for food’.

Q. What would happen to the Human population if fossil fuels are to run out in the near future? Why?

c) Biotic potential and Environmental resistance

Population growth is the increase or decrease in the total number of individuals in a population due to the interplay between biotic potential and environmental resistance.

i) Biotic potential refers to the capacity of the population to rep______ and it depends on several factors:

· bi____ rate (natality) - number of individuals produced per unit time.

· de____ rate (mortality) - number of individuals died per unit time.

· mig______into (immigration) and out (emigration) of the area.

· the average life sp___ of individuals.

· time required for the individual to reach sexual mat_____.

ii) Environmental resistance refers to the biotic and abiotic factors that tend to decrease the fert____ and chance of surv_____ of the individuals thus they li_____ the population growth.

1) Abiotic (non-biological) factors that contribute to limiting a population size include examples such as: w___ (gentle breezes vs. tornados), r____ (drought vs. flood), rain pH (acid?), amount and quality of sun______, clim____, lightn____, f___.
2) Biotic (biological) factors that contribute to limiting a population size include examples such as: Dis_____, Res______availability, Pre______, Com______.
Four variables responsible for changes in population size.

Therefore, Population Change=

(b + I) - (D + E____)

B) J-shaped Growth curve

The S-shaped growth curve is not universal to all populations. Some populations, after reaching the peak instead of maintaining at an equilibrium state, abruptly decrease to very low level by having the majority of the population die at about the s____ time. Giving a J-shaped growth curve.

Populations showing J-shaped growth curves

The populations concerned usually have very sp_____ environmental requirements. When the environment is favourable, they exhibit an unrestrained, rapid and expon_____ growth up to the li____ of the environment.

When the environment becomes unfavourable, such as the exhaustion of food supply resources, accumulation of toxic waste products, adverse physical conditions (photoperiodic shift, frost, drought, etc), the population dies off.

· insect populations which produce only one generation per year or which depends on an annual crop.
· Plankton algae which undergo rapid and tremendous growth (i.e. bloom) at particular seasons in lake and ocean waters. /

II) Survivorship Curves

Within a population, an individual can look forward to a life of a certain length based on the trends of that population. This is called life exp______.

A survivorship curve is plotted on the basis of survivors (say per thousand) against age, or percentage of maximum life span achieved.

Some populations, such as an oyster population, or most fishes, will show a quite different curve from that of present-day man in a developed country.
Oysters produce a vast number of young, most of which die. The individual oyster becomes relatively safe only when it settles. Western man, on the other hand, has a long life expectancy.
Q. The figure shows typical survivorship curves for three types of population. Match each of the following description with one of the curves /

a) Survivorship curve ___

· This curve reflects very high mo_____ rate among the young ones such as during the embryonic, infant and juvenile stages.

· But it is followed by very low mortality rate at the adult stage. It shows that once the organisms survive at the beginning of the life span, they have a good chance of living through the whole life span.

· An oy____ is a good example of this type. Oyster produces many eggs, but most eggs and their larvae die. Only when the larvae successfully attach on a rock, then they have high probability of living to old age.

b) Survivorship curve ___

· Mortality decreases gradually with time and the probability of death is con_____ with age.

· Hydra, gull, robin, most animals and plants have this type of survivorship curve.

c) Survivorship curve ___

· Many la____ mammals including man show this type of curve.

· Mortality is low in the young groups and the possibility of death increases with a___.

· Most individuals can live throughout their physiological life span.

· The population probably has very few, if any, natural en______.

III) Age pyramids

Every population, at any phase in its growth, has an 'age structure'. This is the frequency distribution of ages in it. Populations of constant size may have totally different age-structures, depending on the pattern of de___ with age. The age of death may depend not only on death from old age, but also on the susceptibility of the organism at different stages in its life-history to predation and parasitism.

Age distribution is usually expressed by an age pyramid which shows the number or proportion of individuals in different age groups at any one time. The youngest are at the base and the oldest at the top, males on the left and females on the right.

The growth rate of a population is partially controlled by the distribution of ages in a population. The age distribution can be used to pre____ the future population growth.

Ecologically the individuals in a population can be divided into three groups:

· pre-reproductive (immature),
· reproductive (mature) and
· post-reproductive (old).
There are three typical age pyramids for the expanding, stable and declining populations respectively. /

a) Expanding population ____

The large number of pre-reproductive (young, immature) individuals rapidly grow into the rep______(mature) group.

The resulting larger group of reproductive individuals produce even more young ones than before. The increasing birth rate causes each succeeding generation larger than the preceding one. Such wide-based triangles of rapidly growing populations can be found in South Asia, Africa and Latin America.

b) Stable population____

If the rate of population growth decreases and almost to zero, a stabilized population develops in which the pre-reproductive (young) and reproductive (middle age) groups become more or less e____ in size. The populations of the industrial countries like those in Europe, North America are having such relatively stabilized age structure.

c) Declining population____

It occurs when the birth rate decreases continuously until the proportion of the reproductive (middle age) and post-reproductive (old) groups are larger than the pre-reproductive (young) group. The high mortality leads to the decline of the population.

Q1. You can apply the same principles to age pyramids of human populations. An age pyramid of one country may be different from that of another.
· One is of a South American country (Costa Rica, 1963) with a rapidly growing population;
· one of the UK (1965) an industrial country with a stable population;
· and one of Kuwait (1965), a state on the Arabian Gulf which has grown rapidly in the past few decades, has welcomed substantial immigration from other Arab countries, and is now one of the richest countries in the world due to its oil. /

Say which pyramid represents the population of

a. Costa Rica, b. UK, c. Kuwait.

A country such as Costa Rica (or many countries in South Asia, Africa and South America) with a rapidly expanding population, may eventually change from a pyramid with the weight at the young end to a bell-shaped pyramid indicating a more stable population.

STUDY ITEM

Age structure of a fish population

The plaice (Pleuronectes platessa) is a shallow water flatfish which is an important commercial species in the North Sea. The plaice population has remained fairly stable, except in the war years 1914-1918 and 1939-1945, when fishing was reduced and stocks increased.

The females spawn when they are 5-7 years old, in mid-winter near the coasts. They can lay up to 350 000 fertile eggs each, but this is balanced by an equally high mortality. The eggs float in the plankton until hatching, and the larval plaice are carried about in the currents. At about two months of age they settle out in nursery areas off the sandy coasts of Holland, Denmark, and Germany. They stay there until they are 2-3 years old, when they begin to move away from the coast and towards the middle of the North Sea. They begin to be fished in abundance at 3-5 years old, at a length of 20-30 cm.

The figures in the table show the catch of plaice, per 100 hours of trawling, in the southern North Sea over three seasons. Examine these figures and answer the questions below.
a How can the ages of fish be determined?
b How did the total catch differ from year to year ? /

c Is it justifiable to conclude from these data that the plaice were more abundant in some years than others ?

d Concentrating on the age-classes 4-11 only, calculate for each catch figure the percentage of the year's catch which it represents.

e On the basis of these percentages, calculate the proportion of plaice which survived until the next year, for each of the age-classes surrounded by the rectangle.

f Does the age-distribution change significantly with time ? What will be the age-distribution in ten years time ?

g. Discuss the changes which might occur in the age distribution if

1) fishing intensity increased so that twice as many fish were caught per unit time

2) fishing ceased

3) the mesh size of the nets was greatly reduced.

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