Topic 2 – The Ecosystem 2.6 – Changes and some 2.5 Function

2.6.1 - 2.6.4

And 2.5.5 and 2.5.6

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Specifications

2.6.1 – Explain the concepts of limiting factors and carrying capacity in the context of population growth.

2.6.2 – Describe and explain ‘S’ and ‘J’ population growth curves.

Population curves should be sketched, described, interpreted and constructed from given data.

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Population Growth

Some Facts • Nearly 1.6 million people

join the human population each week.

• 84 million people join every year.

• In three years the human population grows by an amount nearly equivalent to the entire U.S population.

• By 2025 the world population could exceed 8 billion

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Population Studies

• The study of any population is an important aspect of science.

• Studies on both human populations and smaller ecosystem populations are carried out in depth.

• We are going to concentrate on population control of ecosystems but these theories can also be applied to human populations.

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Population Size

• By taking samples and counting the numbers of organisms in a particular habitat, ecologists can study the affects of any factor on the size of a population.

• The factors affecting a population size may be biotic or abiotic.

• Together they affect the rate at which population grows, and also it’s final size.

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Biotic Factors Affecting Population Size

• How many biotic factors can you think of that might affect population size?

• How many abiotic factors can you think of that might affect population size?

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Biotic and Abiotic Factors

Biotic 1. Food – both quantity

and quality of food are important.

2. Predators – refer back to predator prey relationships.

3. Competitors – other organisms may require the same resources from an environment.

4. Parasites – may cause disease and slow down the growth of an organism.

Abiotic 1. Temperature – higher

temperatures speed up enzyme-catalyzed reactions and increase growth.

2. Oxygen Availability – affect the rate of energy production by respiration.

3. Light Availability – for photosynthesis and breeding cycles in animals and plants.

4. Toxins and pollutants – tissue growth may be reduced.

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Biotic and Abiotic Factors

All of these things come under the category of ‘Limiting Factors’

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Carrying Capacity

• When a small population grows in a particular environment, the environmental resistance is almost non-existent.

• This is usually because there is plenty of food and no

accumulation of poisonous wastes.

• Look at the graph of population growth. • This shows how population growth is eventually

inhibited by environmental resistance and the environment reaches it’s carrying capacity.

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Carrying Capacity

• The carrying capacity (K) is the maximum number of a species that the habitat can hold.

• Once the carrying capacity is reached, unless the environmental resistance is changed, e.g. by a new disease, the size of the population will only fluctuate slightly.

• Think of your brine shrimps!?

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‘S’ Curves

• The graph we have just been looking at is an example of an ‘S’ curve.

• This is the type of graph that is almost always seen in nature.

• As the energy resources become more scarce the population size levels off at the carrying capacity (K).

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‘J’ Curves

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‘J’ Curves

• Just as in the ‘S’ curve example, a population establishing themselves in a new area will undergo rapid exponential growth.

• This type of growth produces a J shaped growth curve.

• If the resources of the new habitat were endless then the population would continue to increase at this rate.

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‘J’ Curves

• This type of population growth is rarely seen in nature.

• Initially exponential growth will occur but eventually the increase in numbers will not be supported by the environment.

• Can you think of any examples where ‘J’ curve population growth would be extremely desirable.

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Is there a Carrying Capacity for Homo sapiens?

• ‘As we have seen, the human population growth curve is

currently following an exponential curve or a "J-shape”. Common sense tells us that such growth cannot continue - otherwise within a few hundred years every square foot of the Earth's surface would be taken up by a human.

• Furthermore, experience with other species tells us that,

ultimately, resource limitations and/or habitat degradation will force the human population curves to approach an upper limit - the carrying capacity, often symbolized as " K" by ecologists.

• It is very natural to ask the linked questions - does humanity

have a carrying capacity and, if so, what is it - and when will we reach or overshoot this

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Activity

• Complete the activity – The new zoos

2.6.3 – Describe the role of density-dependent and density-independent factors, and internal and external factors, in the regulation of populations.

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Density Independent Factors

• The following factors are classed as density-independent factors:

• Drought • Freezes • Hurricanes • Floods • Forest Fires

• These factors exert their effect irrespective of

the size of the population when the catastrophe struck.

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Density Independent Factors

This graph shows the decline in the

population of one of Darwin's finches

(Geospiza fortis) on Daphne Major, a

tiny (100-acre) member of the

Galapagos Islands.

The decline (from 1400 to 200

individuals) occurred because of a

severe drought that reduced the

quantity of seeds on which this

species feeds.

The drought ended in 1978, but even

with ample food once again available

the finch population recovered only

slowly.

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Density Dependant Factors

• Intraspecific Competition - competition between members of the same species.

• Read the information about the gypsy moth.

• Many rodent populations (e.g., lemmings in the Arctic) also go through such boom-and-bust cycles.

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Density Dependant Factors

• Interspecific Competition – this is competition between different species for different resources.

• This can include food, nesting sites, sunlight.

• This occurs when two species share overlapping ecological niches, they may be forced into competition for the resource(s) of that niche.

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Specifications

• 2.6.4 – Describe the principles associated with survivorship curves including K- and r -strategists.

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R-Strategists

• “I once ploughed up an old field and allowed it to lie fallow. In the first season it grew a large crop of ragweed.”

• Ragweed is well adapted to exploiting it’s environment in a hurry – before competitors can become established!

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R-Strategists

• Ragweed’s approach to continued survival is through rapid reproduction.

• We say that they have a high value of ‘r’

• They are called r-strategists

• Can you think of any other animals that may be r-strategists?

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R-Strategists

• In general, r-strategists share a number of features:

1. Usually found in disturbed and/or transitory habitats

2. Have short life spans 3. Begin breeding early in life 4. Have short gestation times 5. Produce large numbers of offspring 6. Take little care of their offspring (infant

mortality large) 7. Have efficient means of dispersal to new

habitats

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K-Strategists

• When a habitat become filled with a diverse collection of creatures competing with one another for resources, the advantage shifts to K-Strategists

• K-strategists have a stable population that is close to K.

• There is nothing to be gained from a high r.

• The species will benefit the most by a close adaptation to the conditions of the environment.

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K-Strategists

• K-strategists share these qualities: 1. Found in a stable habitat 2. Long life spans 3. Begin breeding later in life 4. Long gestation times 5. Produce small numbers of offspring 6. Take good care of their young – infant

mortality low 7. Have evolved to become increasingly efficient

at exploiting an ever-narrower slice of their environment.

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Survivorship Curves

• The graph shows 4 representative survivorship curves.

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Survivorship Curves • Curve A – characteristic of organisms that have low

mortality until late in life when aging takes its toll.

• Curve B – typical of populations in which factors such as starvation and disease inhibit the effects of aging and infant mortality is high.

• Curve C – a theoretical curve for an organism whereby the chance of death is equal at all stages

• Curve D – typical of organisms that produce huge numbers of offspring accompanied by high rates of mortality.

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Survivorship Curves

• K-strategists usually have survivorship curves somewhere between A and C.

• R-strategists usually have D survivorship curves.

• The Californian side-blotted lizard

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Specifications

• 2.6.5 – Describe the concept and processes of succession in a named habitat.

• 2.6.6 – Explain the changes in energy flows, gross and net productivity, diversity and mineral cycling in different stages of succession.

• 2.6.7 – Describe factors affecting the nature of climax communities.

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Succession – An intro • The gradual process by which the species population

of a community changes is called ecological succession.

• A forest following a disturbance such as a fire.

• Succession takes places as a result of complex interactions of biotic and abiotic factors.

• Early communities modify the physical environment causing it to change.

• This in turn alters the biotic community which further alters the physical environment and so on.

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Succession – What happens?

• Each successive community makes the environment more favourable for the establishment of new species.

• A succession (or sere) proceeds in seral stages, until the formation of a climax community is reached.

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Primary Succession

• Refers to colonization of regions where there is no pre-existing community.

• Can you think of examples where this would occur?

• You will be studying glacial moraines in detail as well as the succession occurring on bare rock.

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Succession

• Community changes on a glacial moraines

• Study the information on glacial moraines and answer the following questions: 5

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Questions – Glacial Moraines

• During succession there is a change in species composition of a community. There are also changes in species diversity, stability of the ecosystem, and in gross and net production until a climax community is reached.

1. Explain what is meant by a climax community. 2. Explain each of the following changes which occur

during succession: a) Species diversity increases b) Gross production increases c) Stability of the ecosystem increases 3. Give two reasons why farmland in the UK does not

reach a climax community.

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Primary and Secondary Succession

• Primary Succession – occurs on newly formed habitats that have not previously supported a community.

• Examples?

• Secondary Succession – occurs on sites that have previously supported a community of some sort.

• Examples?

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Primary Succession – Bare Rock

Bare Rock Lichens,

bryophytes

and annual

herbs

Mosses,

Grasses and

small

shrubs

Fast

growing

trees e.g.

Ash

Slower growing

broadleaf species

e.g. oak

Complex Community

After 100-200 years

Example for a Northern Hemisphere lithosere: a succession on bare rock

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In Summary - the 1st Invaders!

• These are usually fast growing plants that photosynthesize well in full sunlight.

• We call these pioneer species making up the pioneer community

• Examples = lichens, grasses, herbs

• As these species begin to grow well, they produce shade. Their own seedlings grow more poorly than shade-adapted plants.

• Plants that grow well under full sun are replaced by plants that germinate and grow better in deeper shade.

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Secondary Succession

• This type of succession takes place after a land clearance (e.g. from fire or landslide).

• These events do not involve loss of the soil. • Secondary succession therefore occurs more rapidly

than primary succession. • Humans may deflect the natural course of succession

in these circumstances (e.g. by mowing or farming). • This leads to the development of a different climax

community than would otherwise develop naturally.

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Secondary Succession – Cleared Land

Primary Bare

Earth

Open pioneer

community

(annual grasses)

Grasses and low

growing

perennials

Scrub: shrubs

and small trees

Young broad

leaved woodland

Mature

woodland:

mainly oak

Time to develop: Years 1-2

3-5

16-30 31-150

150+ = climax community

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Succession Continues

• As the plant community changes, the soil will also undergo changes (abiotic factors will change).

• Decomposers will join the community as well as animal species.

• Animal species have a profound affect on the plant species occurring within a habitat.

• Changing conditions in the present community allows for new species to become established (the future community).

• Succession continues until the climax community is reached.

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Wetland Succession

• Wetland areas present a special case of ecological succession.

• Wetlands are constantly changing:

Open water Plant invasion Siltation and

Infilling

• Wetland ecosystem may develop in a variety of ways:

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Wetland Succession

• In well drained areas, pasture or heath may develop as a result of succession from fresh water to dry land.

• In non-acidic, poorly drained areas, a swamp will eventually develop into a fen.

• In special circumstances, a an acid peat bog may develop. (may take 5000+ years).

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Productivity

• Think back to the work on food webs/chains

• It is often useful to know how much energy is passing through a trophic level over a period of time.

• This is called productivity

• Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in an area, over a certain period of time.

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Specifications

• 2.5.5 – Define the terms gross productivity, net productivity, primary productivity, secondary productivity, gross primary productivity and net primary productivity.

• 2.5.6 – Define and Calculate the values of gross and net productivity from given data

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Productivity

• The area is normally one square metre and the time is usually one year.

• It is therefore measured in units of kilojoules per square metre per year (kJm-2year-1)

• The rate at which producers convert light energy into chemical energy is called primary productivity.

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Gross Productivity

• Gross Productivity (GP) – is the total gain in energy or biomass per unit time.

• This is sometimes shown as GPP – Gross Primary Productivity

• It is related to the total amount of chemical energy incorporated into the producers.

• The producers use some of this energy during respiration and energy needs which is eventually lost to the environment as heat.

• The remaining energy is available to the herbivores and is known as net primary productivity (NPP)

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Recap of Definitions! • Productivity = production per unit time

• Primary Productivity = The rate at which energy/biomass is formed through

photosynthesis

• Secondary Productivity = The rate at which primary material is synthesised into animal tissue per unit area in a given time.

• Gross Productivity (GP) = the total gain in energy/biomass per unit time.

• Gross Primary Productivity (GPP) = the total gain in energy of the producers.

• Net Productivity (NP) = the gain in energy/biomass per unit time remaining after allowing for respiration (R) loses.

• Net Primary Productivity (NPP) = the gain in energy/biomass per unit time remaining after allowing for respiration loses which is passed onto the herbivores.

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Environmental Productivity

• Primary productivity varies greatly in different environments.

• The rate at which plants can convert light energy into chemical energy is affected by many factors:

• Sunlight

• Water

• Temperature

• Amount of nutrients

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Environmental Productivity

• In natural ecosystems primary productivity tends to be highest in tropical regions.

• This is due to good light levels and high temperatures in the tropics.

• In the oceans however, the most productive areas are in cold regions due to the up-welling of water bringing plant nutrients with it.

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Important Calculations

• We can calculate GPP as follows:

GPP = NPP + R

• We can calculate NPP for both producers and consumers as:

NPP = GPP – energy used in respiration

• In addition, the equation for consumers only is:

GP = food eaten – faecal losses

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Calculating Productivity Values

• Some easy ones to start you off!

• What is the % energy from sunlight that is fixed as GPP if the total energy from the sun in 3 x 106 and the gross primary productivity = 2.8 x 104?

• What is the GPP of an ecosystem if the NPP is 1660 kJm-

2yr-1 and the energy lost during respiration is 573 kJm-2yr-

1 ?

• What is the NPP if the GPP is 2700 kJm-2yr-1 and the energy used in respiration is 1850 kJm-2yr-1?

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Calculating Productivity Values

Now for some slightly harder ones!

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Energy Flow Diagrams

Information from energy flow diagrams can be used to calculate ecological efficiencies

Photosynthetic Efficiency = Net production ÷ Light Energy Absorbed

Ecological Efficiency is the net production of new biomass at each trophic level as a percentage of the

total energy flowing through that trophic level

Therefore, for photoautotrophs, photosynthetic efficiency is determined as:

Energy flow diagrams illustrate energy flow through communities and include values for respiratory losses and energy flow through

the decomposers

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• Explain the meaning of the term Gross Primary Production

• Explain the meaning of the term Net Primary Production

• Calculate the Photosynthetic Efficiency of the phytoplankton

Use information from the energy flow diagram to:

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Gross Primary Production is the total energy fixed by photoautotrophs during photosynthesis

Net Primary Production is the energy stored as biomass (gross production – energy lost as heat in respiration)

Photosynthetic Efficiency = 3.7 x 104

------------ x 100 172 x 104

= 2.15%

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GPP =

24 x106 kJ m-2year-1

Photosynthetic

efficiency = 1.3%

NPP =

800 kJ m-2year-1

NPP =

200 kJ m-2year-1

NPP =

69.7 x 103 kJ m-2year-1

114 x 103 kJ m-2year-1

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Finally Back To Succession!

• The NPP and GPP of any ecosystem is going to fluctuate. This is especially the case during each seral stage.

• As ecosystems become more diverse, the overall

GPP is also going to increase.

• This is because climax communities are better adapted to an efficient rate of utilisation of their resources.

• They become stable.

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The Early Stages

• Gross Productivity = Low

• This is due to the initial conditions and the relatively low density of producers.

• Net Productivity = High • This is due to low respiration rates of the initial

producers and therefore a lot of energy available to be passed on.

• This allows the system to grow and biomass to accumulate.

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The Later Stages

• Gross Productivity = High

• This is due to an increase in the consumer community who can synthesise a lot of energy from the food they eat.

• Net Productivity = Low

• Increased rates of respiration and other energy sapping activities by consumers means that NP will begin approaching zero.

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The Climax

• Succession comes to an end with the establishment of a mature, relatively stable community – the climax

• Climax communities are more stable that the seral stages that preceded them.

• Ultimately, the climate will be responsible for affecting the nature of the climax community unless human or other factors maintain an equilibrium at a sub-climax community.

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