Download 3. Ecosystems Booklet [A2]

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Ecosystems
Learning objectives:
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Define the term ecosystem;
State that ecosystems are dynamic systems;
Define the terms biotic factor and abiotic factor, using named examples;
Define the terms producer, consumer, decomposer and trophic level;
Describe how energy is transferred through ecosystems;
Outline how energy transfers between trophic levels can be measured;
Discuss the efficiency of energy transfers between trophic levels;
Explain how human activities can manipulate the flow of energy through ecosystems;
Describe one example of primary succession resulting in a climax community;
Describe how the distribution and abundance of organisms can be measured, using line
transects, belt transects, quadrats and point quadrats;
 Describe the role of decomposers in the decomposition of organic material;
 Describe how microorganisms recycle nitrogen within ecosystems (only Nitrosomonas,
Nitrobacter and Rhizobium need to be identified by name);
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
ecosystem
habitat
population
community
biotic factors
abiotic factors
producer
Definition
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Key term
consumer
decomposer
trophic level
food web
pyramid of biomass
pyramid of energy
productivity
megajoule (MJ)
primary productivity
net primary productivity
succession
pioneer community
climax community
saprotrophs
nitrogen-fixing bacteria
mutualistic
Definition
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Ecosystems
The concept of the ecosystem was developed to describe the way groups of organisms are
predictably found together in their physical environment. A community comprises all the organisms
within an ecosystem. Both physical (abiotic) and biotic factors affect the organisms in a community,
influencing their distribution and their survival, growth and reproduction.
The biosphere
The biosphere containing all the Earth’s living organisms amounts to a narrow belt around the Earth
extending from the bottom of the oceans to the upper atmosphere. Broad scale life-zones or
biomes are evident within the biosphere, characterised according to the predominant vegetation.
Within these biomes, ecosystems form natural units comprising the non-living, physical
environment (the soil, atmosphere, and water) and the community (all the populations of different
species living and interacting in a particular area).
Community biotic factors
 producers
 consumers
 detritivores
 decomposers
Interact in the community as: competitors, parasites, pathogens, symbionts, predators, herbivores.
Physical environment
Atmosphere
 wind speed and direction
 humidity
 light intensity and quality
 precipitation
 air temperature
Soil
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nutrient availability
soil moisture and pH
composition
temperature
Water
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dissolved nutrients
pH and salinity
dissolved oxygen
temperature
Physical factors and gradients
Gradients in abiotic factors are found in almost every environment; they influence habitats and
microclimates, and determine patterns of species distribution.
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Energy in ecosystems
An ecosystem is a natural unit of living (biotic) components, together with all the non-living (abiotic)
components with which they interact. Two processes central to ecosystem function are energy flow
and chemical cycling. The mitochondria of eukaryotic cells use the organic products of
photosynthesis as fuel for cellular respiration. Respiration generates ATP; an energy currency for
cellular work:
 static biomass locks up some chemical energy
 growth and repair of tissues
 muscle contraction and flagella movement
 active transport processes e.g. ion pumps
 production of macromolecules e.g. proteins
Cellular work generates heat which is lost from the system. The waste products of cellular
respiration are used as the raw materials for photosynthesis. Chemical elements such as nitrogen,
phosphorus, and carbon are cycled between the biotic and abiotic components of the ecosystem.
Energy, unlike matter, cannot be recycled. Ecosystems must receive a constant input of new energy
from an outside source. In most cases, this is the Sun.
Food chains and food webs
Every ecosystem has a trophic structure: a hierarchy of feeding relationships that determines the
pathways for energy flow and nutrient cycling. Species are divided into trophic levels on the basis of
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their sources of nutrition. The first trophic level (producers), ultimately supports all other levels.
The consumers are those that rely on producers for their energy. Consumers are ranked according
to the trophic level they occupy (first order, second order, etc.). the sequence of organisms, each of
which is a source of food for the next, is called a food chain. Food chains commonly have four links
but seldom more than six. Those organisms whose food is obtained through the same number of
links belong to the same trophic level. Note that some consumers (particularly ‘top’ carnivores and
omnivores) may feed at several different trophic levels, and many primary consumers eat many
plant species. The different food chains in an ecosystem therefore tend to form complex webs of
interactions (food webs).
The way living things obtain their energy can be classified into two categories. The group upon
which all others depend is called producers or autotrophs. They are organisms able to manufacture
their food from simple inorganic substances. The consumers or heterotrophs (comprising the
herbivores, carnivores, omnivores, decomposers and detritivores), feed on the autotrophs or other
heterotrophs to obtain their energy. The energy flow into and out of each trophic level in a food
chain can be identified and represented diagrammatically using arrows of different sizes. The sizes
of the arrows represent different amounts of energy lost from that particular trophic level.
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Primary productivity
The energy entering ecosystems is fixed by producers in photosynthesis. The rate of photosynthesis
is dependent on factors such as temperature and the amount of light, water, and nutrients. The
total energy fixed by producers in photosynthesis is referred to as the gross primary production
(GPP) and is usually expressed as Joules (or kJ)m-2. However, a portion of this energy is required by
the plant for respiration. Subtracting respiration from GPP gives the net primary production (NPP).
The rate of biomass production, or net primary productivity, is the biomass produced per area per
unit time.
Measuring productivity
Primary productivity of an ecosystem depends on a number of interrelated factors (light intensity,
nutrients, temperature, water and mineral supplies), making its calculation extremely difficult.
Globally, the least productive ecosystems are those that are limited by heat energy and water. The
most productive ecosystems are systems with high temperatures, plenty of water and non-limiting
supplies of soil nitrogen. The primary productivity of oceans is lower than that of terrestrial
ecosystems because the water reflects (or absorbs) much of the light energy before it reaches and is
utilised by producers.
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Leaf area index (LAI) – a measure of the total leaf area of a given plant;
Harvestable dry biomass – used for commercial purposes, it is the dry mass of crop available
for sale or use;
Relative growth rate (R) – the gain in mass of plant tissue per unit time;
R=
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increase in dry mass in unit time
original dry mass of the plant
Net assimilation rate (NAR) – the increase in plant mass per unit of leaf area per unit time;
essentially it is the balance between carbon gain from photosynthesis and carbon loss from
respiration.
NAR = increase in dry mass in unit time
leaf area
The flow of energy through an ecosystem can be measured and analysed. It provides some idea as
to the energy trapped and passed on at each trophic level. Each trophic level in a food chain or web
contains a certain amount of biomass: the dry mass of all organic matter contained in its organisms.
Energy stored in biomass is transferred from one trophic level to another (by eating, defaecation
etc.), with some being lost as low-grade heat energy to the environment in each transfer. Three
definitions are useful:
 Gross primary production: the total of organic material produced by plants (including that
lost to respiration).
 Net primary production: the amount of biomass that is available to consumers at
subsequent trophic levels.
 Secondary production: the amount of biomass at higher trophic levels (consumer
production). Production figures are sometimes expressed as rates (productivity).
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The percentage of energy transferred from one trophic level to the next varies between 5% and
20% and is called the ecological efficiency (efficiency of energy transfer). An average figure of 10%
is often used. The path of energy flow in an ecosystem depends on its characteristics. In a tropical
forest ecosystem, most of the primary production enters the detrital and decomposer food chains.
However, in an ocean ecosystem or an intensively grazed pasture more than half the primary
production may enter the grazing food chain.
Ecological succession
Ecological succession is the process by which communities in a particular area change over time.
Succession takes place 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. Each successive community
makes the environment more favourable for the establishment of new species.
A succession (or sere) proceeds in seral stage, until the formation of a climax community, which is
stable until further disturbance. Early successional communities are characterised by a lower
species diversity, a simple structure, and broad niches. In contrast, climax communities are
complex, with a large number of species interactions, narrow niches, and high species diversity.
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Past community – some species in the past community were outcompeted, and/or did not
tolerate altered abiotic conditions.
Present community – the present community modifies such abiotic factors as: light
intensity, wind speed, air temperature, soil composition, light quality, wind direction, soil
water, and humidity.
Future community – changing conditions in the present community will allow new species
to become established. These will make up the future community.
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Primary succession
Primary succession refers to colonisation of regions where there is no pre-existing community.
Examples include regions where the previous community has been extinguished by a volcanic
eruption (such as the Indonesian island of Krakatau which erupted in 1883), newly formed glacial
moraines, or newly formed volcanic islands (as when Surtsey appeared off Iceland in 1963). The
sequence of colonisation described below is typical of a Northern hemisphere lithosere; a
succession on bare rock. This sequence is not necessarily the same as that occurring on another
substrate, such as volcanic ash, which allows the earlier establishment of grasses.
Bare rock → lichens, bryophytes, and annual herbs → grasses and small shrubs → fast growing trees
e.g. rowan → slower growing broadleaf species (ash and oak) [climax community] after 100-200
years
Secondary succession in cleared land
(150+ years for mature woodland to develop again)
A secondary succession takes place after a land clearance e.g. from fire or landslide. Such events do
not involve loss of the soil and so tend to be more rapid than primary succession, although the time
scale depends on the species involved and on climatic and edaphic (soil) factors. Humans may
deflect the natural course of succession e.g. by mowing, and the climax community that results will
differ from the natural community. A climax community arising from a deflected succession is called
a plagioclimax.
Primarily bare earth → open pioneer community (annual grasses) [1-2 years] → grasses and low
growing perennials [3-5 years] → scrub: shrubs and small trees [16-30 years] → young broad-leaved
woodland [31-150 years] → mature woodland mainly oak [150+ years]
Gap regeneration cycle in a rainforest
(500-700 years)
Large canopy trees have a profound effect on the make-up a rainforest community, reducing light
penetration and impeding the growth of saplings. When a large tree falls, it opens a hole in the
canopy that lets in sunlight. Saplings then compete to fill the gap.
Canopy tree removed → gap created by fall of large tree is colonised by tree ferns → growth of
subcanopy trees suppresses tree ferns; seedlings of canopy trees grow beneath the subcanopy →
rapid growth of young canopy species to occupy the gap → mature trees develop to form climax
community of rainforest
Wetland succession
Wetland areas present a special case of ecological succession. Wetlands are constantly changing as
plant invasion of open water leads to siltation and infilling. This process is accelerated by
eutrophication. In well drained areas, pasture or heath may develop as a result of succession from
freshwater to dry land. When the soil conditions remain non-acid and poorly drained, a swamp will
eventually develop into a seasonally dry fen. In special circumstances an acid peat bog may
develop. The domes of peat that develop produce a hummocky landscape with a unique biota.
Wetland peat ecosystems may take more than 5000 years to form but are easily destroyed by
excavation and lowering of the water table.
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An open body of water, with time, becomes silted up and is invaded by aquatic plants.
Emergent macrophyte species colonise the accumulating sediments, driving floating plants
towards the remaining deeper water.
The increasing density of rooted emergent, submerged, and floating macrophytes
encourages further sedimentation by slowing water flows and adding organic matter to the
accumulating silt.
The resulting swamp is characterised by dense growths of emergent macrophytes and
permanent (although not necessarily deep) standing water. As sediment continues to
accumulate the swamp surface may, in some cases, dry off in summer.
In colder climates, low evaporation rates, high rainfall, and invasion by Sphagnum moss
leads to development of a peat bog; a low pH, nutrient poor environment where acidtolerant plants such as sundew replace swamp species.
Shoreline zonation
Zonation refers to the division of an ecosystem into distinct zones that experience similar abiotic
conditions. In a more global sense, differences in latitude and altitude create distinctive zones of
vegetation type, or biomes. Zonation is particularly clear on a rocky seashore, where assemblages
of different species form a banding pattern approximately parallel to the waterline. This effect is
marked in temperate regions where the prevailing weather comes from the same general direction.
Exposed shores show the clearest zonation. On sheltered rocky shores there is considerable species
overlap and it is only on the upper shore that distinct zones are evident. Rocky shores exist where
wave action prevents the deposition of much sediment. The rock forms a stable platform for the
secure attachment of organisms such as large seaweeds and barnacles. Sandy shores are less stable
than rocky shores and the organisms found there are adapted to the more mobile substrate.
The zonation of species distribution according to an environmental gradient is well shown on rocky
shorelines. In Britain, exposed rocky shores occur along much of the western coasts. Variations in
low and high tide affect zonation, and in areas with little tidal variation, zonation is restricted. High
on the shore, some organisms may be submerged only at spring high tide. Low on the shore, others
may be exposed only at spring low tide. There is a gradation in extent of exposure and the physical
conditions associated with this. Zonation patterns generally reflect the vertical movement of
seawater. Sheer rocks can show marked zonation as a result of tidal changes with little or no
horizontal shift in species distribution.
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Decomposers and recycling
Living things need nitrogen to make proteins and nucleic acids. The diagram above shows how
nitrogen atoms are cycled between the biotic and abiotic components of an ecosystem. Bacteria are
involved in ammonification, nitrogen fixation, nitrification and denitrification.
Nitrogen fixation – can occur when lightning strikes or through the Haber process. However,
these processes only account for about 10% of nitrogen fixation around the world. Nitrogenfixing bacteria account for the rest. Many of these live freely in the soil and fix nitrogen gas,
which is in the air within soil, using it to manufacture amino acids.
Nitrification – happens when chemoautotrophic bacteria in the soil absorb ammonium ions
and oxidise them to nitrites or by oxidising nitrites to nitrates.
Denitrification – other bacteria convert nitrates back to nitrogen gas. When the bacteria
involved are growing under anaerobic conditions such as in waterlogged soils, they use
nitrates as a source of oxygen for their respiration and produce nitrogen gas and nitrous
oxide.
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1. Total plant growth within an ecosystem depends on the light intensity,
temperature and the supply of water and inorganic minerals to the ecosystem.
Table 3.1 shows the net primary production by plants in four different
ecosystems.
(a) Discuss possible reasons for the differences in net primary production in
these ecosystems.
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(b) To calculate the net primary production figures in Table 3.1 in kJm-2year-1, it
is necessary to measure the energy content of the primary producers.
Outline how the energy content, in kJ, of a primary producer such as grass
can be measured in the laboratory.
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(c) The efficiency with which consumers convert the food they eat into their
own biomass is generally low.
Table 3.2 compares the energy egested, absorbed and respired in four types
of animals.
Complete Table 3.2 to show the percentage of energy consumed that is
converted into biomass in the perch and the cow.
You may use the space below for your working.
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(d) Describe and explain, using the data from table 3.2, how the trophic level of
a mammal affects the percentage of its food energy that it is able to convert
to biomass.
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(e) Using the data from Table 3.2 and your knowledge of energy flow through
food chains, suggest which of these four animals could be farmed to provide
the maximum amount of food energy in kJm-2year-1 for humans.
Explain the reasons for your choice.
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2. Knowledge of the nitrogen cycle can be used to make decisions about
management of farmland. A farmer uses her grass meadow to raise sheep. In a
separate field she grows cabbages.
Fig. 1.1 shows part of the nitrogen cycle. The four boxes on the bottom line of
the diagram refer to substances in the soil.
(a) Briefly describe the steps that must occur for plant protein to be converted
to animal protein in the farmer’s sheep, as shown by arrow A on Fig. 1.1.
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(b) List the processes which contribute to B in the meadow where sheep are
raised.
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(c) Name the bacteria that carry out processes C and D, and explain the
significance of these bacteria for the growth of plants.
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(d) Use the letters on Fig. 1.1 to explain why the soil nitrate concentration will
decrease in the cabbage field if it is used to grow repeated crops of
cabbages year after year.
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(e) The farmer does not wish to use inorganic fertiliser to replace the nitrate in
the soil of the cabbage field. She wishes to make use of process F.
Suggest a crop she could plant that would allow process F to occur and
explain how this would add nitrate to the soil.
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3. Two-spot ladybirds, Adalia bipunctata, show a colour polymorphism. They are
normally red with two black spots. However, melanic individuals occur which
are black with two red spots.
A student investigated the proportion of these colour forms in the ladybird
population along a transect going up a hill near his school.
(i) Suggest a suitable technique by which the student might have collected his
samples of ladybirds along this transect.
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(ii) The student’s teacher suggested he should make several transects up the
hill rather than just one transect.
Explain why this is good experimental design.
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(iii) The student’s results are shown in Table 7.1.
Suggest a method of processing this data to make comparisons between
the frequency of the red form and black form of ladybird at the different
altitudes more valid.
Explain why your method is an improvement.
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(iv) Evaluate whether the student was correct to conclude as follows:
“My data showed a positive correlation between increasing altitude and the
frequency of the black form of the ladybird. I therefore concluded that high
altitude causes the black form to survive better.”
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(v) The black, melanic, form of the ladybird is caused by an allele (B) that is
dominant.
The red form of the ladybird is therefore homozygous recessive at this locus
(bb).
State what is meant by the term recessive.
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(vi) The data in Table 7.1 give the total number of the red form of ladybird
found as 296, and the total number of the black form of ladybird as 50.
The Hardy-Weinberg principle states that:
p+q=1
p2 + 2pq + q2 = 1
Use the Hardy-Weinberg principle and the figures given above to calculate
the frequency of the dominant allele, p, and the recessive allele, q, in the
two-spot ladybird population.
Show each step in your working. Give your answers to 2 decimal places.
p = ____________________
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4. Organisms do not live in isolation, but interact with other organisms and with
their physical environment.
(a) State the word used to describe:
the study of the interactions between organisms and their environment
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the physical (non-living) factors in the environment
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a physical area that includes all the organisms present and their interactions
with each other and with the physical environment
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(b) State and describe two types of ecological interaction that can occur
between different species in a habitat.
As part of each description, you should name the two species involved in
your chosen example.
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5. The distribution and abundance of plants in a habitat can show how a physical
factor varies across the habitat.
Describe how you would measure the distribution and abundance of plants
over a distance of 100 metres.
In your answer, you should make clear the sequence of procedures you would
follow
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6. Peat bogs are large areas of waterlogged land that support a specialised
community of plants.
Peat bogs take thousands of years to form.
Fig. 5.1 lists the main stages in the formation of a peat bog.
(a) Name the process summarised in Fig. 5.1 that changes a lake community
into a peat bog community.
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(b) Using Fig. 5.1, list two abiotic factors that play a role in determining what
species of plant can grow in an area.
1 ____________________________________________________________
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(c) Most of the minerals in a peat bog are held within the living plants at all
times, not in the soil.
 Plants like bog cotton and bog asphodel recycle the minerals they
contain.
 The leaves of these plants turn orange as the chlorophyll within them
is broken down.
 Minerals such as magnesium ions are transported from the leaves to
the plants’ roots for storage.
Describe one similarity and two differences in mineral recycling in a peat
bog and in a deciduous forest.
similarity ______________________________________________________
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differences ____________________________________________________
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(d) In Ireland in 2002, two well preserved Iron Age human bodies were found in
peat bogs. Despite having been dead for over two thousand years, the
bodies had not decomposed. They still had skin, hair and muscle.
Suggest why these bodies had not decomposed.
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(e) Suggest two reasons why the large scale removal of peat from bogs for use
in gardens is discouraged by conservation groups.
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7. Great tits, Parus major, are birds that form male-female pairs. The male of each
pair then establishes an area of territory, which he defends against other great
tits by singing and threat displays.
The birds build a nest within the territory in which the eggs are laid and young
chicks are reared. Weasels, Mustela rivalis, are predators which eat eggs and
young chicks.
Fig. 6.1. shows how the territory size of great tits affects the risk of nest
predation by weasels.
(a) Describe the relationship shown in Fig. 6.1.
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(b) Suggest and explain what effect weasels may have on the population size of
the great tit.
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8. The ochre starfish, Pisaster ochraceus, is a starfish that lives on rocky intertidal
shores. It is the top predator in its habitat.
Fig. 6.2 shows part of the food web for this starfish.
An experiment was carried out in which all the starfish were removed from am
8m x 2m area of the shore. In an equivalent area of the same size, the starfish
were not removed.
The population sizes of the other organisms in the food web were monitored at
intervals. It was found that in the area in which starfish were removed:
 chitons and limpets disappeared
 anemones, sponges and nudibranchs decreased in abundance
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(a) explain why two areas of the same size were monitored.
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(b) Using Fig. 6.2., explain why the chitons and limpets disappeared in the area
from which starfish were removed.
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(c) Using Fig. 6.2., suggest the sequence of events that led to the decrease in
abundance in nudibranchs in the area from which starfish were removed.
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9. Earthworms are abundant in fertile soil where they play an important role in
the transfer of energy in the ecosystem. An example of a food chain involving
earthworms is shown in Fig. 8.1.
(a) Define the following terms:
producer ______________________________________________________
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consumer _____________________________________________________
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trophic level ___________________________________________________
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(b) One way of measuring the abundance of earthworms is as follows:
 place quadrat frames of known area onto the ground
 pour a chemical solution onto the soil to cause the earthworms to
come up to the surface
 wait and then count the earthworms
Researchers used this technique in 2004 and 2006 to compare the
abundance of earth worms in four areas of soil:
 soil underneath buckthorn plants
 soil underneath honeysuckle plants
 bare soil after the removal of buckthorn plants
 bare soil after the removal of honeysuckle plants
The results are shown in Fig. 8.2.
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A2 BIOLOGY
Suggest two variables which the researchers should have controlled in
order to make the results comparable.
1 ____________________________________________________________
_____________________________________________________________
2 ____________________________________________________________
___________________________________________________________ [2]
(c) Evaluate, with reference to the error bars in Fig. 8.2, whether the data show
a valid difference in the abundance of earthworms between the ‘soil
underneath honeysuckle’ and ‘soil with honeysuckle removed’ sites for July
2004.
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___________________________________________________________ [2]
(d) Ecosystems can be described as dynamic.
State two pieces of evidence from Fig. 8.2. that show that the ecosystem is
dynamic.
1 ____________________________________________________________
_____________________________________________________________
2 ____________________________________________________________
___________________________________________________________ [2]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Populations and sustainability
Learning objectives:
 Explain the significance of limiting factors in determining the final size of a population;
 Explain the meaning of the term carrying capacity;
 Describe predator-prey relationships and their possible effects on the population sizes of
both the predator and the prey;
 Explain, with examples, the terms interspecific and intraspecific competition;
 Distinguish between the terms conservation and preservation;
 Explain how the management of an ecosystem can provide resources in a sustainable way,
with reference to timber production in a temperate country;
 Explain that conservation is a dynamic process involving management and reclamation;
 Discuss the economic, social and ethical reasons for conservation of biological resources;
 Outline, with examples, the effects of human activities on the animal and plant populations
in the Galapagos Islands;
Key definitions:
Compile a glossary by writing your own definitions for the following key terms related to the
learning objectives above.
Key term
carrying capacity
competition
intraspecific
interspecific
coppicing
conservation
biodiversity
Definition
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Populations
Populations have a number of attributes that may be of interest. Usually, biologists wish to
determine population size (the total number of organisms in the population). It is also useful to
know the population density (the number of organisms per unit area). The density of a population
is often a reflection of the carrying capacity of the environment, i.e. how many organisms a
particular environment can support. Populations also have structure; particular ratios of different
ages and sexes. These data enable us to determine whether the population is declining or increasing
in size. We can also look at the distribution of organisms within their environment and so
determine what particular aspects of the habitat are favoured over others.
One way to retrieve information from populations is to sample them. Sampling involves collecting
data about features of the population from samples of that population (since populations are
usually too large to examine in total). Sampling can be done directly through a number of sampling
methods or indirectly e.g. monitoring calls, looking for droppings or other signs.
Density and distribution
Distribution and density are two interrelated properties of populations. Population density is the
number of individuals per unit area (for land organisms) or volume (for aquatic organisms). Careful
observation and precise mapping can determine the distribution patterns for a species. The three
basic distribution patterns are: random, clumped and uniform.
Random distributions: occur when the spacing between individuals is irregular. The presence
of one individual does not directly affect the location of any other individual. Random
distributions are uncommon in animals but are often seen in plants.
Clumped distribution: occur when individuals are grouped in patches (sometimes around a
resource). The presence of one individual increases the probability of finding another close
by. Such distributions occur in herding and highly social species.
Uniform distribution: occur when individuals are evenly spaced within the area. The
presence of one individual decreases the probability of finding another individual very close
by.
Predator-prey interactions
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Some mammals, particularly in highly seasonal environments, exhibit regular cycles in their
population numbers. Snowshoe hares in Canada exhibit such a cycle of population fluctuation that
has a periodicity of 9-11 years. Populations of lynx in the area show a similar periodicity. Contrary to
early suggestions that the lynx controlled the size of the hare population, it is now known that the
fluctuations in the hare population are governed by other factors, most probably the availability of
palatable grasses. The fluctuations in the lynx numbers however, do appear to be the result of
fluctuations in the numbers of hares (their principal food item). This is true of most vertebrate
predator-prey systems: predators do not usually control prey populations, which tend to be
regulated by other factors such as food availability and climatic factors. Most predators have more
than one prey species, although one species may be preferred.
Characteristically, when one prey species becomes scarce, a predator will ‘switch’ to another
available prey item. Where one prey species is the principle food item and there is limited
opportunity for prey switching, fluctuations in the prey population may closely govern predator
cycles.
Competition and niche size
Niche size is affected by competition. The effect on niche size will vary depending on whether the
competition is weak, moderate or intense, and whether it is between members of the same species
(intraspecific) or between different species (interspecific).
Moderate interspecific competition: the tolerance range represents the potential
(fundamental) niche a species could exploit. The actual or realised niche of a species is
narrower than this because of competition. Niches of closely related species may overlap at
the extremes, resulting in competition for resources in the zones of overlap.
Intense interspecific competition: when the competition from one or more closely related
species becomes intense, there is selection for a more limited niche. This severe competition
prevents a species from exploiting potential resources in the more extreme parts of its
tolerance range. As a result, niche breadth decreases (the niche becomes narrower).
Intense intraspecific competition: competition is most severe between individuals of the
same species, because their resource requirements are usually identical. When intraspecific
competition is intense, individuals are forced to exploit resources in the extremes of their
tolerance range. This leads to expansion of the realised niche to less preferred areas.
Overlap in resource use between competing species
From the concept of the niche arose the idea that two species with the same niche requirements
could not coexist, because they would compete for the same resources, and one would exclude the
other. This is known as Gause’s ‘competitive exclusion principle’. If two species compete for some
of the same resources e.g. food items of a particular size, their resource use curves will overlap.
Within the zone of overlap competition between the two species will be intense.
Interspecific competition
In naturally occurring populations, direct competition between different species (interspecific
competition) is usually less intense than intraspecific competition because coexisting species have
evolved slight differences in their realised niches, even though their fundamental niches may
overlap (a phenomenon termed niche differentiation). However, when two species with very
similar niche requirements are brought into direct competition through the introduction of a
foreign species, one usually benefits at the expense of the other.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
The inability of two species with the same described niche to coexist is referred to as the
competitive exclusion principle. In Britain, introduction of the larger, more aggressive, grey squirrel
in 1876 has contributed to a contraction in range of the native red squirrel, and on the Scottish
coast, this phenomenon has been well documented in barnacle species.
The introduction of ecologically aggressive species is often implicated in the displacement or decline
of native species, although there may be more than one contributing factor. Displacement of native
species by introduced ones is more likely if the introduced competitor is also adaptable and hardy.
It can be difficult to provide evidence of decline in a species as a direct result of competition, but it
is often inferred if the range of the native species contracts and that of the introduced competitor
shows a corresponding increase.
The European red squirrel, Sciurus vulgaris, was the only squirrel
species in Britain until the introduction of the American grey
squirrel, Sciurus carolinesis, in 1876.
In 44 years since the 1940 distribution survey, the more adaptable
grey squirrel has displaced populations of the native red squirrels
over much of the British Isles, particularly in the South. Whereas
the red squirrels once occupied both coniferous and broad-leafed
woodland, they are now almost solely restricted to coniferous
forest and are completely absent from much of their former range.
Intraspecific competition
Some of the most intense competition occurs between individuals of the same species (intraspecific
competition). Most populations have the capacity to grow rapidly, but their numbers cannot
increase indefinitely because environmental resources are finite. Every ecosystem has a carrying
capacity (K), defined as the number of individuals in a population that the environment can support.
Intraspecific competition for resources increases with increasing population size and, at carrying
capacity, it reduces the per capita growth rate to zero. When the demand for a particular resource
e.g. food, water, nesting sites, nutrients or light exceeds supply, that resource becomes a limiting
factor.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Populations respond to resource limitation by reducing their population growth rate e.g. through
lower birth rates or higher mortality. The response of individuals to limited resources varies
depending on the organism. In many invertebrates and some vertebrates such as frogs, individuals
reduce their growth rate and mature at a smaller size. In some vertebrates, territoriality spaces
individuals apart so that only those with adequate resources can breed. When resources are very
limited, the number of available territories will decline.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
Humans and the Galapagos
Charles Darwin’s visit to the Galapagos Islands in 1835 provided the stimulus for his theory of
natural selection. The island’s isolation and small population sizes provide optimal conditions for
rapid evolutionary change. The Galapagos form part of one of the best-conserved tropical
archipelagos and have high numbers of native species. These include the famous Darwin’s finches,
giant tortoises and land iguanas. For these reasons, the United Nations allocated them World
Heritage Site status in 1978.
Unfortunately, 50% of vertebrate species and 25% of plant species on the islands are recognised as
endangered, and conservation issues on the Galapagos reflect similar issues in the rest of the world.
The population of the Galapagos has grown in response to a developing tourist trade, an expanding
demand for marine products like sea cucumbers and lobster, and economic problems in mainland
Ecuador in the 1990s. in 1980, the population was about 5000, and 4000 tourists visited the islands
each year. In 2005, the population was 28 000 and 100 000 tourists visited the islands.
This dramatic increase in population size has placed huge demands on water, energy and sanitation
services, which the authorities are struggling to meet. More waste and pollution have been
produced, and the demand for oil has increased. An oil spill in 2001 had an adverse effect on marine
and coastal ecosystems. Increased pollution, building, and conversion of land for agriculture have
caused destruction and fragmentation of habitats. Forests of Scalesia trees and shrubs (a genus
unique to the islands) have been almost eradicated on Santa Cruz and San Cristobal to make way for
agricultural land.
Any conservation plan for the Galapagos Islands has to take the concerns of local people seriously,
and plan for their economic livelihood. Finding a balance between environmental, economic and
social concerns is essential for conservation to be successful.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
1. Sarawak is an area of tropical rainforest in south-east Asia. Logging has been
allowed in 60% of the forest.
A study was carried out into the effects of logging on the diversity of mammal
species living in the forest. An area of rainforest was sampled before logging,
immediately after logging and then again two years and four years after
logging.
Before logging began, there were 29 mammal species and four years after
logging there were 26 mammal species.
Table 5.1 shows the population densities of six groups of mammals before and
after logging. Where numbers were too small to measure the density, the
species was recorded as ‘present’.
(a) Marbled cats and otters are carnivores, while squirrels, shrews and deer are
herbivores. Use the information provided to choose the best word(s) or
terms to complete the following passage:
The rainforest ____________________ is a dynamic set of interactions
between populations of organisms and the abiotic environment. Energy
flows from ____________________, such as trees, to
____________________ consumers, such as squirrels, and on to consumers
such as cats and otters and higher ____________________. The activities of
decomposers contribute to the energy lost from the
____________________ component of the rainforest but decomposers
allow ____________________ to be recycled.
[6]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Table 5.1 shows that the number of small squirrels increases initially, but
then decreases.
Explain, using your knowledge of factors affecting population growth, why
the small squirrel population in this rainforest does not increase in size
indefinitely.
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(c) Describe, using the information provided, how species richness and species
evenness change in the rainforest by comparing the situation before logging
and four years after logging.
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___________________________________________________________ [2]
(d) Suggest why marbled cats and oriental small-clawed otters became extinct
in this area but other mammals did not.
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___________________________________________________________ [1]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(e) Outline three reasons for conserving biological resources, such as the
rainforest in Sarawak.
1 ____________________________________________________________
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2 ____________________________________________________________
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3 ____________________________________________________________
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(f) Timber is produced sustainably in the United Kingdom.
Describe and explain the benefits of two management practices used in
sustainable timber production in a temperate country.
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2. The sheep on a farm belong to a rare breed called Greyface Dartmoor. The
Rare Breeds Survival Trust (RBST) gives advice on looking after these sheep and
keep records to monitor the breeding of these sheep, in order to maintain a
healthy population.
(a) Why is the continued existence of rare breeds of farm animals desirable?
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
______________________________________________________________
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(b) North Ronaldsay sheep are listed as ‘endangered’ by the Rare Breeds
Survival Trust. These sheep were raised on a small Scottish Island where
they were kept along the seashore for most of the year. The sheep
developed an unusual metabolism that allowed them to survive by eating
seaweed. They are, however, susceptible to copper poisoning when fed on
grass.
State the two essential steps that must have occurred for a breed to
develop a distinctive metabolism, such as the ability to eat mainly seaweed.
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(c) Suggest what particular problems make the North Ronaldsay breed one of
the most endangered sheep breeds in the United Kingdom.
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___________________________________________________________ [2]
3. Wading birds (waders) are birds that feed in shallow water. Table 4.1 shows
changes in the population size of four species of wader in two areas of the
Western Isles off the coast of Scotland.
 Area 1 is an area that has remained free of hedgehogs.
 Area 2 is an area where four hedgehogs were introduced from the
mainland in 1974. Since then, they have established a large population.
Hedgehogs eat the eggs of ground-nesting birds like waders.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(a) Calculate the percentage decrease in the number of breeding pairs of snipe
in area 2 between 1983 and 2000.
Show your working.
Answer = ____________________ % [2]
(b) Use the data in Table 4.1 to describe and explain the effect of the
introduction of hedgehogs on the number of breeding pairs of waders in
area 2.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(c) Suggest two factors that might have allowed a large population of
hedgehogs to increase from just four individuals in area 2.
Explain how each factor has led to an increase in the hedgehog population.
1 ____________________________________________________________
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2 ____________________________________________________________
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(d) Three suggested methods to reduce the effect of hedgehogs on the
numbers of waders in area 2 were considered. These were:
 trapping and moving hedgehogs to the mainland
 trapping hedgehogs and keeping them in captivity indefinitely
 trapping of hedgehogs followed by humane killing
The third method was judged to be the most effective and likely to succeed
in reducing hedgehog numbers.
Comment on the ethical issues involved in making this decision.
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
4. The Hardy-Weinberg principle can be used to predict the expected frequencies
of albino and non-albino alleles in a population. However, this principle can
only be applied to populations which fulfil all of the following criteria:
 sexually reproducing organisms
 diploid organisms
 large populations
 randomly-mating populations
The tiger, an endangered species of mammal, is undergoing a worldwide
captive breeding programme in zoos.
Suggest why the Hardy-Weinberg principle cannot be used to predict the
expected frequencies of albino and non-albino alleles in the worldwide zoo
population of tigers.
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_____________________________________________________________ [2]
5. A change in allele frequencies in a population is described as an evolutionary
change.
List two factors that might cause allele frequencies to change from generation
to generation in a population that meets the Hardy-Weinberg criteria.
________________________________________________________________
_____________________________________________________________ [2]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
6. The Galapagos Islands are 600 miles away from the nearest land mass, South
America. They consist of 15 main islands, 3 smaller islands, and 107 rocks and
islets. This collection of islands is home to many endemic species of animals
and plants. This means that these species are found nowhere else in the world.
(a) Explain, using scientific terms, why a collection of small islands remote from
the mainland provides optimal conditions for speciation.
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___________________________________________________________ [2]
(b) In 1978, the United Nations (UN) declared the Galapagos Islands a World
Heritage Site. This led to a rise in the resident human population and the
number of visitors to the Islands.
Table 2.1 shows how the number of people living on and visiting the
Galapagos Islands changed between 1980 and 2005.
Calculate the percentage increase in the number of visitors to the Galapagos
Islands between 1980 and 2005.
Show your working. Give your answer to the nearest whole number.
Answer = ____________________ % [2]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(c) Outline the main ways in which increased human presence and activity have
put endemic species on the Galapagos Islands, and in the sea around them,
at risk of extinction.
In your answer, you should link the ecological pressures imposed by human activity
to examples of Galapagos Island species that have been affected
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
________________________________________________________________
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_____________________________________________________________ [7]
(d) In 2007, the United Nations (UN) put the Galapagos Islands on its Red List of
endangered sites. The Galapagos government’s response to this action
included making new laws and placing restrictions on human activity,
issuing eviction orders and culling introduced species of animals.
Suggest one economic and one ethical problem that might have arisen from
this 2007 UN decision.
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___________________________________________________________ [2]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
7. Fig. 4.1 shows some notes that a gardener pinned to his notice board to
remind him of jobs to do.
Each is based on a different biological principle.
(a) Match the notes, A to H, with the biological principles on which they are
based.
Write the correct letter next to the description of each principle.
Biological principle
Letter
artificial selection
______
predator-prey interaction
______
apical dominance
______
nitrogen fixation
______
reproductive cloning
______
positive chemotaxis
______
decomposition
______
use of plant hormones
______
[8]
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(b) Four other procedures associated with growing or storing crops are
described in Table 4.1 below.
Name a biological process that is slowed down or stopped by each
procedure.
8. Many species of insects have evolved resistance to chemical insecticides.
Three different patterns of resistance in insect species R, S and T are shown in
Fig. 6.1.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
(a) Complete the table below with the letter(s), R, S and T, to indicate which
species show a continuous pattern of variation and which species show a
discontinuous pattern.
(b) A student noted a number of statements on his revision card that referred
to the patterns of resistance shown in species R, S and T in Fig. 6.1.
Complete Table 6.1 below, by selecting the correct numbered statement(s)
that explain the genetic basis of each pattern of resistance for each species.
SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
You may select a number more than once.
(c) Dog fleas are small parasitic insects that live in the fur of dogs and feed on
their blood. Dogs are routinely treated with sprays or powders to kill fleas.
A vet believes that dog fleas may have become resistant to a popular fleakiller product.
He asks an A-level work experience student to plan an experiment to test
this hypothesis.
The student needs to sample fleas from dogs visiting the surgery and also
fleas from long grass in fields visited by dog-walkers. The fleas then need to
be treated for resistance to the flea-killer.
Describe the methods the students could use to:
 collect both samples of fleas
 find out the proportion of fleas that are resistant
 process the data
In your answer, you should describe the methods for collection, testing and data
processing in a logical series of steps
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SACKVILLE SCIENCE DEPARTMENT
A2 BIOLOGY
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_____________________________________________________________ [7]