Topic 2 – The Ecosystem

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Topic 2 – The Ecosystem Powered By Docstoc
					Topic 2 – The Ecosystem

       2.3 – Changes
        2.3.1 - 2.3.4
n   2.3.1 – Explain the concepts of limiting factors
    and carrying capacity in the context of
    population growth.

n   2.3.2 – Describe and explain ‘S’ and ‘J’
    population growth curves.
    Population curves should be sketched,
    described, interpreted and constructed from
    given data.
               Population Growth
          Some Facts
n   Nearly 1.6 million people
    join the human population
    each week.
n   84 million people join every
n   In three years the human
    population grows by an
    amount nearly equivalent to
    the entire U.S population.
n   By 2025 the world
    population could exceed 8
              Population Studies
n   The study of any population is an important aspect of

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

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

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

n   Together they affect the rate at
    which population grows, and also
    it’s final size.
    Biotic Factors Affecting Population

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

n   How many abiotic factors can you think of
    that might affect population size?
        Biotic and Abiotic Factors
             Biotic                             Abiotic
1.   Food – both quantity and       1.   Temperature – higher
     quality of food are                 temperatures speed up
     important.                          enzyme-catalyzed reactions
2.   Predators – refer back to           and increase growth.
     predator prey relationships.   2.   Oxygen Availability –
3.   Competitors – other                 affect the rate of energy
     organisms may require the           production by respiration.
     same resources from an         3.   Light Availability – for
     environment.                        photosynthesis and
4.   Parasites – may cause               breeding cycles in animals
     disease and slow down the           and plants.
     growth of an organism.         4.   Toxins and pollutants –
                                         tissue growth may be
   Biotic and Abiotic Factors

All of these things come under the category of
               ‘Limiting Factors’
              Carrying Capacity
n   When a small population grows in a particular
    environment, the environmental resistance is almost

n   This is usually because there is plenty of food and no
    accumulation of poisonous wastes.

n   Look at the graph of population growth.

n   This shows how population growth is eventually
    inhibited by environmental resistance and the
    environment reaches it’s carrying capacity.
              Carrying Capacity
n   The carrying capacity (K) is the maximum
    number of a species that the habitat can hold.

n   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

n   Think of your brine shrimps!?
                   ‘S’ Curves
n   The graph we have just been looking at is an
    example of an ‘S’ curve.

n   This is the type of graph that is almost always
    seen in nature.
n   As the energy resources become more scarce
    the population size levels off at the carrying
    capacity (K).
‘J’ Curves
                     ‘J’ Curves
n   Just as in the ‘S’ curve example, a population
    establishing themselves in a new area will undergo
    rapid exponential growth.

n   This type of growth produces a J shaped growth

n   If the resources of the new habitat were endless then
    the population would continue to increase at this rate.
                     ‘J’ Curves
n   This type of population growth is rarely seen in

n   Initially exponential growth will occur but eventually
    the increase in numbers will not be supported by the

n   Can you think of any examples where ‘J’ curve
    population growth would be extremely desirable.
Is there a Carrying Capacity for Homo
n   ‘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.

n   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.

n   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
n   Complete the activity – The new zoos

2.3.3 – Describe the role of density-dependent
  and density-independent factors, and internal
  and external factors, in the regulation of
     Density Independent Factors
n   The following factors are classed as density-
    independent factors:
n   Drought
n   Freezes
n   Hurricanes
n   Floods
n   Forest Fires

n   These factors exert their effect irrespective of the size
    of the population when the catastrophe struck.
     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
       Density Dependant Factors
n   Intraspecific Competition -
    competition between members of
    the same species.

n   Read the information about the
    gypsy moth.

n   Many rodent populations (e.g.,
    lemmings in the Arctic) also go
    through such boom-and-bust
       Density Dependant Factors
n   Interspecific Competition – this is competition
    between different species for different resources.

n   This can include food, nesting sites, sunlight.

n   This occurs when two species share overlapping
    ecological niches, they may be forced into
    competition for the resource(s) of that niche.
n   2.2.4 – Describe the principles associated with
    survivorship curves including K- and r -
n   “I once ploughed up an old
    field and allowed it to lie
    fallow. In the first season it
    grew a large crop of

n   Ragweed is well adapted to
    exploiting it’s environment
    in a hurry – before
    competitors can become
n   Ragweed’s approach to continued survival is
    through rapid reproduction.

n   We say that they have a high value of ‘r’
n   They are called r-strategists

n   Can you think of any other animals that may
    be r-strategists?
n    In general, r-strategists share a number of
1.   Usually found in disturbed and/or transitory
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
7.   Have efficient means of dispersal to new habitats
n   When a habitat become filled with a diverse
    collection of creatures competing with one another
    for resources, the advantage shifts to K-Strategists

n   K-strategists have a stable population that is close to
n   There is nothing to be gained from a high r.

n   The species will benefit the most by a close
    adaptation to the conditions of the environment.
n    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
7.   Have evolved to become increasingly efficient at
     exploiting an ever-narrower slice of their
                Survivorship Curves
n   The graph shows 4 representative survivorship curves.
            Survivorship Curves
n   Curve A – characteristic of organisms that have low
    mortality until late in life when aging takes its toll.

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

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

n   Curve D – typical of organisms that produce huge
    numbers of offspring accompanied by high rates of
           Survivorship Curves
n   K-strategists usually have survivorship curves
    somewhere between A and C.

n   R-strategists usually have D survivorship

n   The Californian side-blotted lizard
n   2.3.5 – Describe the concept and processes of
    succession in a named habitat.

n   2.3.6 – Explain the changes in energy flows, gross
    and net productivity, diversity and mineral cycling in
    different stages of succession.

n   2.3.7 – Describe factors affecting the nature of climax
            Succession – An intro
n   The gradual process by which the species population
    of a community changes is called ecological

n   A forest following a disturbance such as a fire.

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

n   Early communities modify the physical environment
    causing it to change.
n   This in turn alters the biotic community which further
    alters the physical environment and so on.
     Succession – What happens?
n   Each successive community makes the
    environment more favourable for the
    establishment of new species.

n   A succession (or sere) proceeds in seral
    stages, until the formation of a climax
    community is reached.
            Primary Succession
n   Refers to colonization of regions where there
    is no pre-existing community.

n   Can you think of examples where this would

n   You will be studying glacial moraines in detail
    as well as the succession occurring on bare
n   Community changes on a glacial moraines

n   Study the information on glacial moraines and
    answer the following questions:
     Questions – Glacial Moraines
n    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
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.
    Primary and Secondary Succession
n   Primary Succession – occurs on newly
    formed habitats that have not previously
    supported a community.
n   Examples?

n   Secondary Succession – occurs on sites that
    have previously supported a community of
    some sort.
n   Examples?
Primary Succession – Bare Rock
Bare Rock                      Lichens,                       Grasses and
                              bryophytes                         small
                              and annual                        shrubs

 After 100-200 years
Slower growing                            growing
broadleaf species                         trees e.g.
    e.g. oak                                 Ash
Complex Community

             Example for a Northern Hemisphere lithosere: a succession on bare rock
    In Summary - the 1 Invaders!
n   These are usually fast growing plants that photosynthesize
    well in full sunlight.
n   We call these pioneer species making up the pioneer

n   Examples = lichens, grasses, herbs

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

n   Plants that grow well under full sun are replaced by plants that
    germinate and grow better in deeper shade.
           Secondary Succession
n   This type of succession takes place after a land
    clearance (e.g. from fire or landslide).

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

n   Humans may deflect the natural course of succession
    in these circumstances (e.g. by mowing or farming).

n   This leads to the development of a different climax
    community than would otherwise develop naturally.
          Secondary Succession – Cleared
                                     Open pioneer
   Primary Bare                       community                 Grasses and low
       Earth                        (annual grasses)               growing
Time to develop: Years

                           Young broad                  Scrub: shrubs
                         leaved woodland               and small trees

        mainly oak        150+ = climax community
             Succession Continues
n   As the plant community changes, the soil will also undergo
    changes (abiotic factors will change).

n   Decomposers will join the community as well as animal

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

n   Changing conditions in the present community allows for new
    species to become established (the future community).
n   Succession continues until the climax community is reached.
            Wetland Succession
n   Wetland areas present a special case of
    ecological succession.

n   Wetlands are constantly changing:
     Open water        Plant invasion     Siltation and

• Wetland ecosystem may develop in a variety of ways:
            Wetland Succession
•   In well drained areas, pasture or heath may
    develop as a result of succession from fresh
    water to dry land.

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

n   In special circumstances, a an acid peat bog
    may develop. (may take 5000+ years).
n   Think back to the work on food webs/chains
n   It is often useful to know how much energy is
    passing through a trophic level over a period of time.

n   This is called productivity

n   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.
n   2.2.4 – Define the terms gross productivity, net
    productivity, primary productivity, secondary
    productivity, gross primary productivity and
    net primary productivity.

n   2.2.5 – Calculate the values of gross and net
    productivity from given data
n   The area is normally one square metre and the
    time is usually one year.

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

n   The rate at which producers convert light
    energy into chemical energy is called primary
                    Gross Productivity
n   Gross Productivity (GP) – is the total gain in energy or
    biomass per unit time.
n   This is sometimes shown as GPP – Gross Primary Productivity

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

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

n   The remaining energy is available to the herbivores and is
    known as net primary productivity (NPP)
               Recap of Definitions!
n   Productivity = production per unit time

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

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

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

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

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

n   Net Primary Productivity (NPP) = the gain in energy/biomass per unit time
    remaining after allowing for respiration loses which is passed onto the
      Environmental Productivity
n   Primary productivity varies greatly in different

n   The rate at which plants can convert light energy into
    chemical energy is affected by many factors:
n   Sunlight
n   Water
n   Temperature
n   Amount of nutrients
      Environmental Productivity
n   In natural ecosystems primary productivity
    tends to be highest in tropical regions.

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

n   In the oceans however, the most productive
    areas are in cold regions due to the up-welling
    of water bringing plant nutrients with it.
          Important Calculations
n   We can calculate GPP as follows:
                     GPP = NPP + R

n   We can calculate NPP for both producers and
    consumers as:
         NPP = GPP – energy used in respiration

n   In addition, the equation for consumers only is:
              GP = food eaten – faecal losses
    Calculating Productivity Values
n   Some easy ones to start you off!

n   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?

n   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 ?

n   What is the NPP if the GPP is 2700 kJm-2yr-1 and the energy
    used in respiration is 1850 kJm-2yr-1?
Calculating Productivity Values

  Now for some slightly harder ones!
                  Energy Flow Diagrams
         Energy flow diagrams illustrate energy flow
         through communities and include values for
          respiratory losses and energy flow through
                        the decomposers
Information from energy flow diagrams can be used to
           calculate ecological efficiencies

    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:
              Photosynthetic Efficiency =
        Net production ÷ Light Energy Absorbed
  Use information from the energy flow diagram to:
• 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
Gross Primary Production is the total energy fixed   Photosynthetic
by photoautotrophs during photosynthesis             Efficiency =
Net Primary Production is the energy stored as       3.7 x 104
biomass (gross production – energy lost as heat      ------------ x 100
                                                     172 x 104
in respiration)                                      = 2.15%
                                 NPP =
                             800 kJ m-2year-1
       efficiency = 1.3%

         GPP =
   24 x106 kJ m-2year-1            NPP =                 NPP =
                           69.7 x 103 kJ m-2year-1   200 kJ m-2year-1

114 x 103 kJ m-2year-1
      Finally Back To Succession!
n   The NPP and GPP of any ecosystem is going to
    fluctuate. This is especially the case during each
    seral stage.

n   As ecosystems become more diverse, the overall GPP
    is also going to increase.

n   This is because climax communities are better
    adapted to an efficient rate of utilisation of their
n   They become stable.
                  The Early Stages
n   Gross Productivity = Low

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

n   Net Productivity = High

n   This is due to low respiration rates of the initial producers and
    therefore a lot of energy available to be passed on.

n   This allows the system to grow and biomass to accumulate.
               The Later Stages
n   Gross Productivity = High

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

n   Net Productivity = Low

n   Increased rates of respiration and other energy
    sapping activities by consumers means that NP will
    begin approaching zero.
                   The Climax
n   Succession comes to an end with the establishment of
    a mature, relatively stable community – the climax

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

n   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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