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1 SPECIES, COMMUNITIES & ECOSYSTEMS
Species and Populations
- Species
= Groups of organisms with similar characteristic, that can potentially interbreed to produce fertile
offspring


EXCEPTIONS:
 ASEXUALLY reproducing organisms e
...
bacteria
 HYBRIDS e
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mules, zeedonks, ligers
 INFERTILE organisms




INTERBREEDING = when 2 members of the same species mate & produce offspring
CROSS-BREEDING = when members of different species breed together
 Offspring are nearly always infertile

- Populations
= A group of organisms of the same species who live in the same area at the same time
-

Populations may be isolated from eachother in a number of ways = may lead to
REPRODUCTIVE ISOLLATION which may lead to the formation of a new species



POPULATION DENSITY = number of individuals of a species per unit area

The Population Growth Curve

A
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TRANSITION PHASE
= Phase of slowing yet still high growth rate against time/generations (I+N>E+M)
 WHY? Resources become scarce & competition increases
C
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g
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g
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g
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g
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g
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g
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g
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g
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g
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) are released into the
abiotic environment
3) Remaining energy is lost from the system as HEAT
4) AUTOTROPHS convert the simple molecules into complex molecules = re-enter a food chain

Sustainability of Ecosystems
SUSTAINABLE = when something can continue indefinitely


3 key requirements must be met for an ecosystem to be sustainable:
1) Nutrient availability
2) Detoxification of waste products
3) Energy availability



Why ecosystems are usually sustainable:
 Continued energy supply as light from the sun (energy can’t be recycled)
 Nutrients can be recycled indefinitely = no lack of the chemical elements on which life is
based
 Waste products of 1 species are usually exploited as a resource by another species
 E
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NH4+ ions released by decomposers are absorbed & used for an energy
resource by Nitrosomonas bacteria in soil = this toxic ion doesn’t accumulate

Mesocosms (practical)
= Small experimental areas that are set up as ecological experiments

- Bottle gardens


PHOTOSYNTHESIS:
 Sealed space creates a completely self-sufficient ecosystem in which plants can survive
by using photosynthesis to recycle all the nutrients available
 Only external input needed to keep the plant going is light (provides it with the energy
it needs to create its own food and continue to grow)
 Sunlight falling on the plant leaves is absorbed by proteins containing chlorophyll
 Some of that light energy gets stored in the form of ATP molecules
 Rest is used for removing electrons from the water, which is absorbed from the soil
through the roots - these “free ” electrons get used up in chemical reactions, which
convert carbon dioxide into carbohydrates, hence releasing oxygen
...

 During night time, when there is no sunlight for photosynthesis, the plant uses cellular
respiration for its basic functions by breaking down the stored nutrients
...


Fieldwork – associations between species
- Quadrat sampling
PROCEDURE:
1) Base line marked out along the edge of the habitat using a measuring tape (must extend all the
way along the edge of the habitat)
2) Random number obtained using either a table or random number generator
3) 1st random number used to determine a distance along the measuring tape (x-coordinate)
4) 2nd random number used to determine a distance out across the habitat at right angles to the
tape (y-coordinate)
5) Quadrat placed at distances determined by the 2 random numbers

- Testing for an association between species
-

Positive associations = if 2 species occur in the same parts of a habitat, they will tend to be
found in the same quadrat (can also be –ve associations or the distribution of 2 species can be
independent)

-

Observed frequencies = those collected during the investigation
Expected frequencies = those which would be predicted if the null hypothesis was true



NULL HYPOTHESIS = that there is no statistically significant difference between observed and
expected ratios and they are essentially the same
...

ALTERNATE HYPOTHESIS = that there is a statistically significant difference between
observed and expected ratios
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

- The Chi-squared test
= Used to test the null & alternate hypotheses
- Only valid if all the expected frequencies are 5+ and the sample was taken at random from the
population
 Method for chi-squared test:
1) Draw a contingency table of observed frequencies (numbers of quadrats containing or not
containing the 2 species)
 Calculate row & column totals (adding row or
column totals should give the same grand total in
lower right cell)

2) Calculate the expected frequencies, assuming independent distribution:
 Expected frequency = (row total X column total) / grand total
3) Calculate the number of degrees of freedom:
 Degrees of freedom = (m-1)(n-1) – where m & n are the number of rows & columns
4) Find critical region for chi-squared from a table of chi-squared values, using the degrees of
freedom calculated & a significance level (p) of 0
...

 Critical region = any value of chi-squared larger than the value in the table
5) Calculate chi-squared:
(𝑓 −𝑓 )2
 𝑋2 = ∑ 0 𝑓 𝑒
𝑒

Where 𝑓0 is the observed frequency, 𝑓𝑒 is the expected frequency

6) Compare the calculated value of chi-squared with the critical region
 If value is in critical region, there is evidence at the 5% level for an association between
the 2 species – we can reject hypothesis H0
 If value is not in critical region, because is equal or below the value obtained from the
table of chi-squared values, H0 is not rejected – there is not evidence at the 5% level for
an association between the 2 species

- Statistical significance


SIGNIFICANCE LEVEL = the cut-off point for the probability of rejecting the null hypothesis
when in fact it was true
 Level of 5% is minimum acceptable significance level in published research

-

Examples of what the significance level represents:
 If there is a DIFFERENCE BETWEEN THE MEAN RESULTS FOR THE 2 TREATMENTS IN
AN EXPERIMENT, a statistical test will show whether the difference is significant at the
5% level
 If is, less than 5% probability of such a large difference between the sample
means arising by chance = statistically significant evidence that they differ
 If TESTING FOR AN ASSOCIATION BETWEEN 2 SPECIES, chi-squared test shows
whether there is a less than 5% probability of the difference between the observed &
expected results being as large as it is without the species being either +vely or –vely
associated

-

Results of biological research being displayed on a bar chart:
 2 different letters (a & b) = mean results have a statistically significant difference
 2 of the same letters (a & a) = any difference isn’t statistically significant

4
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G
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e
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consumers at different trophic levels)

Sunlight & Ecosystems
- Obtaining energy as sunlight



AUTOTROPHS/PRODUCERS (e
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plants, eukaryotic algae & cyanobacteria) – by
photosynthesis
HETEROTROPHS (e
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consumers, Saprotrophs & Detritivores) – are indirectly dependent
on light energy – use carbon compounds in food as a source of this (almost all energy in the
carbon compounds will originally have been harvested by PS in producers)

Energy Conversion
1)
2)
3)
4)

Producers absorb sunlight using CHLOROPHYLL & other photosynthetic pigments
This converts light energy to chemical energy
This energy is used to make carbohydrates, lipids & all other carbon compounds in producers
Producers release energy from their carbon compounds by CELL RESPIRATION & then use it
for cell activities – energy released in this way is usually lost to environment as waste HEAT
5) Largest amount of carbon compounds in producers remains in cells & tissues of producers
6) Energy in these carbon compounds is available to heterotrophs

Respiration & Energy Release


Cell activities needed by living organisms that require energy:
1) Synthesising large molecules (anabolic) e
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DNA, RNA & proteins
2) Pumping molecules or ions across membranes by ACTIVE TRANSPORT
3) Moving things around inside the cell e
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chromosomes or vesicles, or in muscle cells the
protein fibres that cause muscle contraction
 ATP provides energy for these activities

- ATP
-

Produced by all cells via cell respiration
How it is made:
1) Carbon compounds e
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carbs & lipids are OXIDISED (reactions are EXO)
2) Energy released is used in ENDO reactions to make ATP
Why ATP is needed instead of just using chemical energy from glucose etc:
 Chemical energy in carbon compounds is not immediately usable by the cell, but the CE
in ATP can be used directly for many different activities
Why energy transformations are never 100% efficient (2nd law of thermodynamics)
1) Not all energy from oxidation of carbon compounds in cell respiration is transferred to ATP
2) Remainder is converted to heat
3) Some heat also produced when ATP is used in cell activities (e
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muscles warming up when
they contract)

Heat Energy in Ecosystems


Energy conversions that living organisms can perform:
1) Light energy – chemical energy (in PS)
2) Chemical energy – kinetic energy (in muscle contraction)
3) Chemical energy – electrical energy (in nerve cells)
4) Chemical energy – heat energy (in heat-generating adipose tissue)
 Can’t convert HE into any other form of energy

Heat Losses from Ecosystems
-

Heat from cell respiration = living organisms get warmer
 Useful in making cold-blooded animals more active
 Birds & mammals increase rate of heat generation if necessary to maintain constant
body temperature



Why heat produced in living organisms is eventually lost to the abiotic environment:
 LAWS OF THERMODYNAMICS = heat passes from hotter to cooler bodies
 Therefore heat from living organisms is radiated into the (cooler) atmosphere

- Explaining the length of food chains
= Rarely more than 4 or 5 stages in a food chain
- WHY? – Energy flow along food chains & the energy losses that occur between trophic levels
means that the amount of energy remaining after 4-5 stages wouldn’t be enough to support
another trophic level

Energy Losses & Ecosystems


BIOMASS = total mass of a group of organisms, incl
...

 Biomass has energy because carbon compounds have chemical energy
 The energy added to biomass by each successive trophic level is less (e
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in secondary
consumers, the amount of energy is always less per year per m2 of ecosystem than in
primary consumers)

- Loss of energy between trophic levels


SUNLIGHT - PRODUCERS:
 Most of the available energy from the sun is lost before photosynthesis occur e
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1
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Conversion to heat
3
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Misses chloroplast/chlorophyll
 Process of photosynthesis is highly inefficient due to the multiple reactions involved =
energy is wasted here creating triose-phosphate (TP)
 Formation of complex molecules e
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cellulose, starch and sugars (glucose, fructose,
sucrose etc
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g
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g
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Growth is the exception; this represents the
proportion of energy stored in tissues and available to the next trophic level
 Mammals and birds maintain a constant body temperature so have significant
losses of heat to the environment

-

Biomass also diminishes along food chains:
 Loss of CO2 & H2O from respiration
 Loss from the food chain of uneaten or undigested parts of organisms

Pyramids of Energy

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Units: kJ m-2 yr-1
Length of each bar should be proportional to the amount of energy that it shows (to scale)
Height of each bar should be the same



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