15.4 ecosystem

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15.4 Population Ecology

A population is a group of individuals of the same species living in the same place at the sametime and interbreeding e.g. aphids on a sycamore tree, a shoal of herring in the sea.

Population growth

1. Population growth is a change in the number of individuals; this is referred to as positivegrowth when the numbers increase and negative growth when they decrease.

2. Population growth at a particular time is determined by

a) birth rate (natality) – the reproductive capacity of the population

b) mortality – the death rate of organisms in the population

c) immigration – the movement of individuals into a population

d) emigration – the movement of individuals out of a

population Therefore,r in pop size during a time interval = (B+I) – (D+E),

where B=births during time interval,D=deaths during time interval,

I=immigration during time interval, andE=emigration during time interval.

If migration (immigration and emigration) is ignored, the change in population sizer N in the

time periodrt, is then expressed as number of births – number of deaths in the time period.r N = B - D

rt

3. Population growth involves three main factors:

a) the biotic potential potential of the population:the maximum rate at which the population can

reproduce, given all the resources it needs.

b) the environmental resistance of the habitat:all the factors that may limit the growth of a population.

Environmental resistance means that populations seldomever achieve their biotic potential.

c) the carrying capacity (k) of the environment:

the maximum population size that a particular environment can sustain, over a relatively long period of time. This occurs when the birth rate balances the death rate.

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Types of population growth curves

1. Exponential (J-shaped) growth curve

This curve is seen when a species colonisesa new area where environmental conditions

are favourable. The birth rate will exceed

the death rate and lead to exponentialgrowth. The population doubles per unittime. An example is seen in alga blooms.

The equation for exponential populationgrowth is d N = r max N , where r max is the

d t max rate of increase

2. The logistic (S-shaped) curve

This curve is typical of species colonizingnew habitats, with resultant environmental

resistance. There is a period of slowgrowth (lag phase) as the species adapts to

the habitat, followed by a period of rapidgrowth (exponential phase) with little

environmental resistance. The graph thenlevels off (stationary phase) as the

population reaches its carrying capacitywhere it encounters environmental

resistance such as food shortage. If a particular factor becomes scarce, it can

limit the growth of the population, which

can then go into decline (decline phase).

3. “Boom and bust” curve

A ‘boom and bust’ curve occurs when a population increases so rapidly during the

exponential growth phase that it overshootsthe carrying capacity. As the environment

cannot support the population, a populationcrash usually follows. Insect pests that

produce many generations in a single year often have this sort of growth curve.

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Survivorship

- is the probability that a given individual in a population or cohort will survive to a particular age or the percentage of an original population that survives to a given age. Plotting the log of

the number of surviving individuals against age, from birth to the maximum age reached by anyindividual, produces a survivorship curve.

The 3 main survivorship curves:

Type I curveA large number of individuals reach their

theoretical maximum age or their physiologically determined life span e.g.

bison and humans. The probability of survdecreases more rapidly with increasing age

Type II curve

An intermediate between types I & III.Populations have a constant mortality ratethroughout their life span i.e. probability o

survival does not change with age resultinga linear decline in survivorship e.g. some

lizards and hydra. Mortality is largelyindependent of age.

Type III curve

Population have high mortality in their earstages of growth, but an individual survivin

beyond that point is likely to live a long timthat is, the probability of survival increases

with increasing age e.g. oysters.

r/K selection theory.

1. In ecology, r/K selection theory relates to the selection of combinations of traits in anorganism that trade off between quantity and quality of offspring.

2. Organisms may focus either upon producing large numbers of offspring which require little parental care, or producing few offspring which require greater parental care.

3. Both strategies are aimed at promoting success in particular environments.

r and K strategies (diff. mechanisms for ensuring a large number of individuals in a

population)

1. Species that reproduce rapidly have a high value of r (the intrinsic rate of increase) and are ]

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called r-species. They have a J-shaped growth curve characterized by exponential growthfollowed by sudden crashes in population size. They are generally opportunistic species and

represent the typical pioneer species of ‘disturbed’ habitats which are unpredictable andrapidly changing. r strategists reproduce early and have many offspring. These offspring are

small, mature rapidly, and receive little or no parental care; their generations are relativelyshort e.g. dandelions, aphids, mice and cockroaches.

2. Species that are relatively slow breeding (low values of r) are called K-species. They tend to

live in habitats that are fairly stable and predictable i.e. they are more typical of later stages insuccessions. They have an S-shaped growth curve and their population is limited in number by

the carrying capacity of the environment, or K. They have delayed reproduction, small broods,few offspring, parental care, long generations and are prone to extinction e.g. whales,

rhinoceroses, coconut palms and most tropical rain forest trees.

3. The two types of species are said to adopt r and K strategies. However, many organisms areneither “pure” r strategists nor “pure” K strategists. Rather, their reproductive strategies lie

somewhere between these two extremes or change from one extreme to the other under certainenvironmental circumstances.

Summary of some of the characteristics of extreme r- and K-type strategists

r strategists

1. Reproduce rapidly (high fecundity,short generation time); therefore high

values of r.

2. Reproduction rate not sensitive to

population density.

3. Investment of energy and materials

spread over many offspring.

4. Population size may temporarilyexceed K.

5. Species not very persistent in a givenarea.

K strategists

1. Reproduce slowly (low fecundity, long

generation time); therefore low value of r.

2. Reproduction rate sensitive to population density, rising rapidly if

density falls.

3. Investment of energy and materialsconcentrated on a few offspring, with parental care in animals.

4. Population size stays close toequilibrium determined by K.

5. Species persistent in a given area.

6. Disperse slowly.7. Reproduction is relatively inexpensive

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6. Disperse widely in large numbers.

7. Reproduction is relatively expensive interms of energy and materials.

8. Small size9. Individuals short-lived

10. Can occupy open ground

11. Habitats short-lived(e.g. ripe fruit for Drosophila larva)

12. Poor competitors (lose easily incompetitive situations)

13. Relatively lacking in defensivestrategies

14. Do not become dominant15. More adaptable to changes in

environment (less specialized)

Examples: bacteria, Paramecium, aphids, flour beetles, annual plants

in terms of energy and materials.

8. Large size.9. Individuals long-lived.

10. Not well adapted to growing in opensites.

11. Habitats stable and long-lived (e.g.

forest for monkeys)12. Good competitors13. Good defence mechanism

14. May become dominant

15. Less resistant to changes inenvironmental conditions (highly

specialized for stable habitat)16. Examples: large tropical butterflies,

condor (large bird of prey), albatross,Man, trees

15.5 Carrying capacity and sustainable development

Population growth slows down due to environmental resistance. Environmental factors that

influence population size may be:

a) Density-dependent factors

1. As the population density (number of organisms in a given space) increases, these factors have

stronger effects, affecting a larger proportion of the population. So the size (or density) of a population affects its growth rate.

2. For e.g., for a fixed food supply, the larger the population gets, the less food there will be for each individual and the slower the growth rate.

3. Density-dependent factors are always biotic. They include food availability, toxic wasteaccumulation, space, disease, competition, predation and parasites.

b) Density-independent factors

1. A population increases until some factors causes a sudden reduction in its size. Its effect is the

same regardless of the size of the population, i.e. it is independent of the population density.

2. For example, a sudden fall in temperature may kill large numbers of organisms regardless of whether the population is large or small at the time.

3. They are usually abiotic and the most important are weather and climate such as rainfall,

temperature and natural catastrophes like drought, fires, floods or storms.

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Management and conservation of ecosystems

Conservation is the protection and preservation of natural resources in the environment. Theimportant renewable natural resources are air, water, soil, wildlife and forests. The non-

renewable resources are fossil fuels and minerals.

The need for conservation.

1. Ensure the protection of plant and animal species and to prevent their extinction.2. Maintain a stable and balanced ecosystem. This prevents disruption of natural cycles

(e.g. water and carbon cycles) and global warming.3. Maintain a large gene pool. Many wild animals and plants possess favourable genes. By

crossbreeding the different varieties of wild plants and animals, we can improve yield,vigour and quality of organisms e.g. better resistance to diseases or drought.

4. Ensure the conservation of marine life, as marine fisheries are a major source of human

food.

5. Ensure the conservation of tropical rainforests, as they are of economic importance. Therainforest houses a large number of animals and plants, especially the tropical rainforest.

Each species has a role to play in maintaining the balance of nature in the ecosystem. Manytropical plants are of great importance as they are sources of raw material for industries,

medicinal drugs, natural insecticides and food.a) Raw materials for industries include rattan, fibres (fabrics from cotton plants and ropes

from coconut husk), rubber (from latex of rubber trees) and oils. b) Nearly 25% of medicinal drugs originated from some plant species in the ainforest.

Quinine (an anti-malarial drug from the bark of Chincona) and morphine (pain-suppressingdrug) are derived from such plants. There are probably many more medicinal drugs in

many plant species waiting to be discovered. Certain poisonous animals in the rainforestmay also contain chemicals that have medicinal value.

c) Pyrethrum, the chemical found in chrysanthemum. Is used in the insecticide industry.d) Maize, rice, pineapple and banana are some examples of food plants that were developed

from rainforest plants.6. Conservation is of scientific value. The study of wildlife provides useful information for

Man’s survival.7. Conservation preserves the natural scenery and wildlife for people to appreciate & relax,

providing one of the aesthetic pleasures in life. It also maintains the natural resources for outdoor recreational activities such as fishing, hiking and skiing.

Conservation measures

Have been carried out indirectly through population control and reducing or eliminating

pollution and directly through conserving natural resources such as:

1. Development of national parks and nature reserves2. Planned land use

3. Legal protection for endangered species4. Commercial farming

5. Breeding in zoos and botanical gardens6. Removal of animals from threatened areas

7. Control of introduced species

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8. Ecological study of threatened habitats9. Pollution control

10. Recycling11. Education

Sustainable development

Forestry – How do we conserve the forests?

1. Prevent indiscriminate cutting down of forests through laws and setting up of forest reserves.

Forests, a major source of oxygen, help to moderate the weather, and provide shade and protection for the soil.

2. Laws ensure that trees are cut down selectively and at a regulated rate protect the remainingforests worldwide. Young trees and seed trees are not felled. New seedlings are planted to

replace those trees that were cut down for timber.3. The forestry department looks after forest reserves and ensures that forest conservation laws

are obeyed. They check the trees regularly and help control insects and diseases that harmthem. Scientists in forestry departments carry out research to improve the quality of forest

trees, and make the forest more productive.

Fishery – How do we conserve fishing grounds?

1. Banning the use of drift nets which indiscriminately trap all forms of sea life2. Using nets with a certain mesh size so that young fish are not caught

3. Regulating the size of ships allowed into fishing rounds4. Limiting the period of fishing in the grounds

5. Banning the harvesting of endangered species; encouraging the raising of these fish inhatcheries and releasing them into fishing grounds where the fish populations are

decreasing.

Agriculture – How do we increase food production?

1. Improved strains of plant and animal species through artificial selection and geneticengineering to increase productivity.

2. Greater use of fertilizers and pesticides.3. Increased mechanization and use of biotechnology to sow, fertilize, harvest, transport and

store crops.4. Changes in farm practices and consequent increase in farm size to accommodate modern

machinery and increase food production.

15.6 Quantitative ecology

Types of estimation

(I) Absolute It is impractical and almost impossible to count accurately every individual of

any species within a habitat. Therefore, sampling techniques are used for estimating population size.

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(ii) Relative The sampling techniques used depend not only on the nature of the habitat but

also on the organism involved. For example, it may be useful to know thenumber of individuals in an animal population whereas percentage cover may

be more relevant for a plant species. Also, quadrats are used in estimating population size of plants and sessile or very slow-moving animals while the

capture-recapture method is for mobile animals that can be tagged or marked .

Sampling methods

1. Capture-recapture/mark-release-recapture(mrr)

1. This method is used to estimate animal populations since they are mobile

and will not stay inside a quadrat to be counted.2. A sample of the animals are caught and counted. Each one is marked and

released back into their habitat and allowed to mix back into their own population. After one or two days, a second sample is taken and counted.

3. The population size is then estimated using the Lincoln Index. P = M x S

R P = the estimate of the population size.

M = the number of animals captured, marked and released in the first sampleS = the number of animals captured in the second sample

R = the number of marked animals recaptured in the second sample

4. When using the Lincoln Index, we make a number of assumptions.a) The marked individuals must redistribute themselves randomly amongst the

unmarked individuals. b) The marks must not come off between marking and recapture.

c) The marks must not affect the behaviour of the animals nor make them morenoticeable to predators.

d) Being caught in the first sample must not increase or decrease the chances of capture in the second sample.

e) Ideally, there should be no movement of individuals into or out of the population during the exercise. Neither should any births or deaths occur in

the population.

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2. Quadrat sampling

1. There are two main types of quadrats: frame quadrats and point quadrats.

2. A frame quadrat is usually a metal, plastic or wooden frame that forms a known area

such as 0.25 m2

or 1 m2. Frame quadrats can be of any regular shape. The most common

are square shaped, but circular and even hexagonal frames can be used as long as the

area is known.3. Sampling with a quadrat may be random or systematic. Random sampling is done by

throwing the quadrat and allowing it to fall while systematic sampling involves placing

the quadrat at regular intervals.4. Frame quadrats can be used to measure population density, percentage cover, or

frequency of occurrence of a species within a selected area. 5. A frame quadrat divided into a 10 x 10 grid with string can be used to estimate

percentage cover. For each species being investigated, the number of squares fullyoccupied, partly occupied, and unoccupied is recorded. Each fully occupied square

represents a 1% cover. From these, the total percentage cover can be calculated.

Estimation of plant density Estimation of percentage cover

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6. If, for example, an area of 1000 m2

is studied and 100 quadrats, each 1 m2

aresampled, it follows that a total of 100 m

2of the area has been sampled. This represents

one-tenth of the total. The total number of individuals of a species in all 100 quadratsmust therefore be multiplied by ten to give an estimate of the total population of that

species in the area.7. A point quadrat ( or pin frame) are usually used to measure percentage cover and is

designed for sampling stationary organisms, usually plants. It is particularly usefulwith transect studies of overgrown habitats where several plant species may overlap.

All species touched by the pin as it descends to the ground are recorded for each of theholes. With a 10-needle point quadrat, each ‘hit’ with a needle represents a 10% cover.

3. Line transect

This method is suitable for sampling plants which grow close

to the ground in areas whichinvolve a change from one habitat

type to another e.g. edge of a pond,field border adjoining hedgerow. If

the transect is too long, record plants touching the tape at regular

intervals e.g. every 1m. Can beused to measure percentage cover.

4. Belt transect

This method is suitable for long transects in the collection

of large amounts of data. All plants within the quadrat arerecorded at regular intervals. Can be used to measure

density and percentage cover of plants.

Advantages and disadvantages of quadrat and transect sampling:

1. A transect is much quicker and more useful in obtaining data for a large, homogenous area. It

will not give reliable information in a heterogenous area.2. Percent cover with a quadrat is slower, but it yields more information than a transect, as every

species in the quadrat can be counted. Small, rare species are less likely to be missed out in aquadrat compared to a transect.

3. Line transects are especially suitable to study how species change across some environmentalgradient e.g. down a seashore, down a hill-slope to a river bank.

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Sampling parameters: Frequency, Density & Coverage

Calculations using quadrat sampling technique (1m x 1m)

1. Density = ______Total number of a species __________

Total number of quadrats x area of each quadrat

2. Relative density = __Density of a species___ x 100%

Total density of all species

3. % species cover = ______Total area of species cover_________ x 100%

Area of 1 quadrat x total number of quadrats

4. Relative species cover = Total area cover of a species_ x 100%

Total area cover of all species

5. Frequency = Total number of quadrats containing a species x 100%

Total number of quadrats

6. Relative frequency = ___Frequency of a species___ x 100%

Total frequency of all species

Calculations using line transect technique (10m)

1. % species cover = Total cross sectional length of a species x 100%

Total length of transect

2. Relative species cover = Total cross sectional length of a species x 100%

Total cross sectional length of all species

3. Frequency = Total number of intervals where the species are found x 100%

Total number of intervals of transect

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Pattern of distribution of organisms in an ecosystem

1. Individuals in a population exhibit different patterns of dispersion (spacing relative to each other).

2. The most common pattern of dispersion is clumped.

Individuals are segregated in patches, usually where

environmental conditions are favourable. Clumping is

also advantageous for predators, allowing them to huntas a group instead of as individuals.

3. A uniform, evenly spaced pattern of dispersion

occurs in some populations. This pattern often results

from strong territorial behaviour, where individuals

show antagonistic social interactions which do not allow

other individuals to be too close to them. This pattern of dispersion is not common.

4. In random dispersion, the position occupied by each

individual is independent of other individuals. This may

occur in a homogenous area where conditions are

favourable throughout the area and organisms can grow

equally well anywhere within the area. This pattern of

dispersin is also not common.

15.4 ecosystem

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