ECOLOGY

Part 2

Population Ecology

Overview:A) THE BIOSPHERE B) POPULATION ECOLOGYC) COMMUNITY ECOLOGYD) ECOSYSTEM ECOLOGYE) THE CARBON & NITROGEN CYCLESF) ECOLOGICAL TECHNIQUESG) SIMPSON’S RECIPROCAL INDEXH) LOCAL ECOLOGYI) BIOLOGICAL DIVERSITY

Ecology is the study of organisms in relation to their environment 

Ecosystem:

a complex, self-sustaining

natural system of which living

organisms are a part, together

with the nonliving

components

Biotic factors include the living component of the 

ecosystem; producers, consumers and decomposers

Edaphic factors – Soil Characteristics

texture pH drainage

Habitat is a specific locality of a place each with an

adapted community of organisms e.g. a rocky shore, a freshwater pool

within each habitat there are populations of individuals which collectively form a community

Question: End-of-Year JUNE, 2011Use your knowledge to discuss the biological significance of the following:

Edaphic and climatic factors play a role in ecosystems.

(5 marks)

Edaphic and climatic factors are chemical or physical abiotic environmental factors which influence the distribution of organisms. Edaphic factors are factors pertaining to the soil e.g. pH, particle size, humidity. Climatic factors include temperature, rainfall, light intensity and wind. The combination of both edaphic and climatic factors determines the environmental conditions of an ecosystem and the type of organisms that can be found. e.g. low rainfall, high light intensity, low humidity in soil can support plants having xerophytic adaptations. Long periods of low temperatures and low light conditions induce plants to enter a period of dormancy and animals to hibernate or migrate. This results in great seasonal changes in the ecosystem.

Community: a group of interacting populations occupying

a given area

Population: a group of individuals of

the same species, all occupying a particular area at the same time

Ecological Species is a set of organisms adapted to a particular set

of resources, called a niche in the environment

Biological Species a group of closely related organisms which are

capable of interbreeding to produce fertile offspring

Define a ‘SPECIES’

POPULATION GROWTH

Population size is the number of individual organisms

present at a given time Numbers can increase, decrease, cycle or remain 

the same

Population Density: is the number of individuals within a

population per unit area

High densities: make it easier to find mates, but increase

competition, and vulnerability to predation

Low densities: make it harder to find mates, but individuals

enjoy plentiful resources and space

FOUR factors determine Population Growth:

POPULATIONPOPULATION

Births/Natality

Deaths

Immigration

Emigration

increases

decreases

(births + immigrants) - (deaths + emigrants)

Population change =

Ignoring immigration and emigration, population growth is primarily affected by birth and death rates.

BIRTH DEATHS

Recruitment: occurs when juvenile organisms survive to

be added to a population, usually a stage whereby the organisms are settled and able to be detected by an observer

is the proportion of offspring that attains sexual maturity in the population

[syllabus definition]

The result of a balance between two major opposing factors:

Biotic potential:factors tending to

increase population

Environmental resistance:

factors tending to decrease population

What determines the ultimate size of any population (omit migration)?

A stable population in nature is the result of the interaction between the biotic potential and environmental resistance.

Increasingresistance

Biotic PotentialReproductive rateAbility to migrate (animals) or disperse (seeds)Ability to invade new habitatsAbility to cope with adverse conditions

Environmental ResistanceLack of food or nutrientsLack of waterLack of suitable habitatAdverse weather conditionsPredatorsDiseaseParasitesCompetitors

Biotic Potential: The maximum rate at which the population

could increase under ideal conditions

Orca whale: low biotic potentialgestation is 13 to 16 months

Salmon: high biotic potential

Factors that influence the Biotic potential:1. The age at which the average individual first reproduces

Factors that influence the Biotic potential:2. The average frequency with which reproduction occurs3. The average number of offspring produced each time

Offspring per mating: 1Age of 1st mating: 6.5 yearsGestation: 15 months

Offspring per mating: about 14Age of 1st mating: 8 monthsGestation: 114 days

Factors that influence the Biotic potential:

4. The reproductive life span of the average organism

5. The average death rate under ideal conditions

Hamster: Offspring per mating – up to 18Age of 1st mating: 21 daysGestation: 21 days

Organisms have reproductive potential beyond that required for replacement of their numbers e.g.

Female insects lay thousands of eggs

and frogs, are not much less!!

The growth rate (r) of a population:

 growth rate = birth rate – death rate

How fast the population grows

A population can grow exponentially in an environment with: bounteous resources no competition 

Environmental resistance includes limits set by the: living

non living environment(e.g. food, space, competition, predation, parasitism)

Environmental resistance can: increase the death rate

Dead fish due to drought.

Fewer offspring produced if

food is lacking.

decrease the birth rate

TWO basic forms of growth curves:

S-shaped (sigmoid) J-shaped

J-shaped Growth Curve

Acclimatisation

Exponential growth

Acclimatisation:an organism needs time to adapt to new conditions as

it is subjected to unusual environmental conditions

J-shaped growth curve:

Exponential growth:Population doubles at each generation

Unrestricted growth

2 4 8 16 321

Which organisms show J-shaped type of growth?

SMALL ones: Bacteria, yeast BUT also large ones [next slide]

The J-shaped curve of exponential growth characterizes some rebounding populations– e.g. the elephant population in Kruger National Park, South

Africa, grew exponentially after hunting was banned

Characteristics of species showing J-shaped growth:

Live in populations that are highly variable Have many offspring Reproduce early

Water flea (Daphnia)Mosquitoes

Theoretical S-shaped growth curve

Acclimatisation

Exponential growth

Logistic growth

Logistic growth Occurs when a population’s growth slows or

stops following a period of exponential growth

Why does this happen?

Lack of resources, lack of space

Theoretical S-shaped growth curve

Point of maximum

growth

Zero growth

The carrying capacity (k):the maximum density the environment can support

Population Oscillations

The population grows exponentially at first, then fluctuates around the carrying capacity.

Characteristics of species showing S-shaped growth:

Live in populations that are at or near equilibrium conditions for long periods

Compete for limited resources

Exponentialgrowth

(biotic potential)

Equilibrium

(Environmental resistance)

Carrying capacity

The growth is driven by biotic potential but levels off owing to environmental resistance.

Population at equilibrium

Population growth with no restraints

J-shaped

S-shaped

Which type of growth curve applies to the World Population?

World Population Growth Through History

Why is it that the world population cannot continue to grow exponentially?

Resources become limiting

Place the following on the graph:Biotic potential; S-curve; J-curve; carrying capacity;

environmental resistance

A

B

C

E

D

Place the following on the graph:

A

BC

E

D

Biotic potential; S-curve; J-curve; carrying capacity; environmental resistance

A: J-curve

Place the following on the graph:

A

BC

E

D

B: Biotic potential

Biotic potential; S-curve; J-curve; carrying capacity; environmental resistance

Place the following on the graph:

A

BC

E

D

Biotic potential; S-curve; J-curve; carrying capacity; environmental resistance

C: environmental resistance

Place the following on the graph:

A

BC

E

D

D: carrying capacity

Biotic potential; S-curve; J-curve; carrying capacity; environmental resistance

Place the following on the graph:

A

BC

E

D

Biotic potential; S-curve; J-curve; carrying capacity; environmental resistance

E: S-curve

Question:Any habitat may be said to have a carrying capacity and this is defined as the maximum population of a given species that can be sustained there. The graph below shows changes in the numbers of wild sheep on a large island during the 100 years following their introduction to the island.

a) From the graph, estimate the carrying capacity of this island for wild sheep. (1)

1.5 million sheep

b) Suggest two factors that may determine the carrying capacity of this island for wild sheep (2)1. Space available2. Amount of food present

c) Comment on the pattern of population change after carrying capacity had been reached. (3)Fluctuates around the carrying capacity. Population increases above carrying capacity if environmental resistance is not harsh and vice versa.

d) Suggest 3 factors that may have influenced the length of time taken for the sheep population to reach carrying capacity. (3)1. Number of female sheep originally present.2. Length of gestation period.3. Number of offspring born per female at each pregnancy.

Make sure you know the following:At which point is the population growth rate:

1. accelerating?2. highest?3. lowest?4. starting to

slow down?

Make sure you know the following:At which point is the population growth rate:

1. accelerating?2. highest?3. lowest?4. starting to

slow down?

ba

c

d

Factors That Limit Population Growth

Availability of matter Energy resources

can limit the number of organisms in a population

Factors Regulating Populations:1. Density-independent factors: affect the same percentage of individuals

regardless of population size

2. Density-dependent factors: affect a greater percentage of individuals in

a population as the number of individuals increases

Examples of Density-Independent Factors:

Density-Independent factors:Natural disastersNatural disasters

Seasonal Cycles Seasonal Cycles [Monsoon][Monsoon]

FIRESFIRES

Density-Independent factors:DeforestationDeforestation

Hurricanes Hurricanes

Density-Independent factors:Human Human ActivityActivity

Polluted River [Indonesia]

Density-Independent factors: Cold weather

An iguana lies dead after falling out of a tree in Davie, Florida. [Feb, 2010]

Density-Independent factors:

The number of organisms affected depends upon the:

1. severity 2. time of occurrence.

1. severity 2. time of occurrence.

How do species respond to such factors?How do species respond to such factors?

Density-independent factors:

They show a characteristic crash in population size.

Salmon die at end of breeding season.

Cause of the ‘bust’ may be:

Insecticide kills insects.

Examples of Density-Dependent Factors:

Density-Dependent Factors:Predation

Competition

Density-Dependent Factors:Disease

Rabbits around a waterhole at the myxomatosis trial enclosure on Wardang Island in 1938. (Australia)

Density-Dependent Factors:Parasitism

"brood parasitism", which is practiced by some birds. These birds lay their eggs in the nests of other birds, tricking the nest owner into caring for parasite’s young.

Density-Dependent Factors:Crowding

[walrus]

Stress

Population growth is:Density-dependent:

[S-shaped][S-shaped]Density-independent:

[J-shaped]

Why is population growth said to be ‘density-dependent’?

Because for a given set of resources, the growth rate depends on the numbers present in

the population

Can populations that do not show boom-and-bust cycles grow exponentially?

YESWhen?

Exponential growth can occur temporarily under special circumstances such as:

elimination of population-controlling factors, such as predators or parasites

increasing the food supply

introduction of foreign or exotic / alien species

Alien populations grow exponentially. Why?

Cane Toads have experienced a massive population explosion in Australia due the lack of

competition.

Individuals invade a new habitat where:1. conditions are favourable 2. competition or predation is scarce

SEP 2013 Paper 2Use your knowledge of biology to explain the following statement. (5 marks)

Invasive alien species are a significant threat to biodiversity.

SEP 2013 Paper 2

Invasive alien species show a J-shaped type of growth curve. Exponential growth leads to rapid increase in population size. Such species have a high biotic potential enabling them to multiply and spread quickly. As alien species use resources at a fast rate, environmental resistance sets in for other species. Space and food become limiting for other species that cannot compete so well as the alien ones. Thus the size of the fundamental niche for such species is reduced leading to a reduction in the carrying capacity of the local populations.

Population StrategiesOpportunist

species(r-selected

species)

Equilibrium species(k-selected

species)

Body size small largeLife span small largeAge at first reproduction

early late

Number of offspring

many few

Parental care little muchFactors limiting population growth

density-independent

density-dependent

Population Strategies

Opportunist Species

(r-selected species)

Equilibrium species(k-selected

species)

Type of environment

highly variable & unpredictable

stable and consistent

Type of organism

opportunists e.g. pests

not so, e.g. trees, man

Dispersal widely & in large numbers

slowly

Defensive strategies

relatively lacking good

Factors Affecting Population Size some factors operate to regulate population

size within certain limits factors act:

intraspecificWithin a Species:

interspecificBetween Species:

Competition Type is:

interspecificinterspecific

intraspecificintraspecific

Semibalanus

Chthamalus

Why is intraspecific competition more intense than an interspecific one? as the needs of the members of the same species

for water, nutrients, shelter, breeding sites and other resources are almost identical

Two regulatory mechanisms which affect birth rate:

territorial behaviour (territoriality)  the physical effects of overcrowding

Adult male musk deer scent gland is used to make musk oil

Urine can be used to mark territory.

What is ‘Territorial Behaviour’?

Grey reef shark

aggressive behaviour to defend an area

Grey reef shark warns intruders by twisting its body, raising its

snout, and dropping its pectoral fins. It finally attacks with

slashing bites that are not part of its normal feeding method. It

can even drive off the 16-foot tiger shark, which can kill a grey reef shark and then swallow it

whole.

Territorial Behaviour occurs in a: wide range of animals, including: certain fish reptiles Birds mammals social insects

fiddler crabs

An animal needs a territory to: to forage, mate, raise the young

What happens to the size of a territory as population grows?

What happens to the size of a territory as population grows?

What happens to those without a territory? Often do not reproduce

Are also easy prey 

Question: MAY, 2007Use your knowledge of biology to explain the evolutionary significance of the following:Territoriality in animals. (5 marks)

An example of intraspecific interaction that limits population size.E.g. birds defend a territory to provide them with a nest and food with minimum expenditure of energy.New members are excluded – die or leave area.It selects in favour of the fittest members

Two regulatory mechanisms which affect birth rate:

territorial behaviour (territoriality)  the physical effects of overcrowding

Overcrowding of small mammals e.g.

mice rats voles Vole

causessocial stress

Social stress results in a number of factors that reduce population size

Social stress reduces population size1. a decrease in the size of the reproductive

organs2. reduced reproductive rate

Rabbit embryo

Social stress reduces population size3. slower growth4. reduced resistance to disease5. reduced parental care6. the young abandon the nest at an early stage,

with consequent reduction in chances for survival

7. cannibalism of young

Interspecific Interactions that affect

Population Size

Predator & Prey Relationships a predator is an animal which preys upon

another and feeds on its prey

one organism which consumes another so could include: herbivores parasites

However, the term can also be used for:

Predator & Prey Populations

are out of phase

Predator-Prey Cycles

Time

Number of individuals

Negative Feedback

Population CyclesSome bird, mammals and insect populations fluctuate with regularity (for example, lemmings show a 4-year cycle, snowshoe hares show a ten-year cycle).

Parasitoids: are usually insects, laying eggs on hosts

which are completely consumed by developing larvae

parasitoid wasp species

Parasitoid wasp pupae emerge from caterpillar

Parasite-host relationships out-of-phase cycles as for predator-prey

Interspecific Competition

Allelopathy is a biological phenomenon by which an organism produces chemicals that influence the

growth, survival, and reproduction of other organisms.

Bare zones

Predation is an important evolutionary force natural selection favors: more effective predators more evasive prey

selective pressures imposed on one another often leads to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations

Arms races : have been recorded in some snails over time:

Snails become more heavily armored prey

Their predators, crabs, develop more massive claws with greater crushing power

Predator & Prey Co-evolve: Co-evolution is the mutual evolution of

predator and prey

Question: [SEP, 2004]Comment on the biological significance of the following observation:Removal of predators may reduce diversity of prey organisms. (2)Predators create a selection pressure, favouring certain phenotypes instead of others. Removal of this pressure, no longer drives populations to diversify.

Essay TitleReview the ways in which populations are controlled in nature.

[DEC, 1983]

Question: End-of-Year SEP, 2011Use your knowledge of biology to describe the significance

of each of the following observations. (5 marks each)

a. Most biological populations tend to show sigmoid growth. b. Populations of predators and prey tend to fluctuate in similar patterns.c. The number of individuals in a population imposes limits on the size of a population.d. The size of a population may be limited by abiotic factors.

END OF SECTIONEND OF SECTION