Chapter 6 - Populations and Communities Flashcards
What is a population?
A group of organisms of the same species living in a particular area.
How can population numbers vary?
Population numbers can remain relatively stable or can grow or decline over time.
What factors influence population growth?
Birth rate
Death rate
Immigration
Emigration
What is one of the most studied examples of population growth?
The growth of bacteria (or yeast) in nutrient medium in closed conditions, for example, in a beaker or conical flask.
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions, for example, in a beaker or conical flask. Draw a graph showing the characteristic pattern of population growth typically seen in these circumstances.
Textbook page 89
What units is population growth measured in?
Population growth (or decline) is measured as the change in the number of organisms
How does the population of bacteria in a nutrient medium in closed conditions increase?
Bacteria will divide (split) to produce two new daughter cells.
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions. How many distinct phases can the pattern of growth be divided into?
Four
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions. What are the four distinct phases that the pattern of growth can be divided into?
Lag phase
Exponential (or log growth) phase
Stationary (or stable) phase
Decline phase
What is the lag phase?
• In this phase there will be a very slow increase in number (numbers may even decrease for a time). This is a stage when nutrient assimilation takes place - this may involve the bacteria activating genes and producing the appropriate enzymes to metabolise a particular food substrate.
What is the exponential (log) phase?
- The bacteria divide exponentially.
- There is no restriction to growth (for example, abundant resources present and insignificant waste accumulation) and the bacteria can divide to produce new bacteria at the maximum rate.
- A bacterium can divide to produce two new bacteria as often as once every 20 minutes and consequently the increase in numbers can be exponential.
What is the stationary (stable) phase?
- In this stage food supplies may begin to become limiting so the number of new individuals produced falls.
- Waste products and toxins may have also accumulated to a level that restricts growth.
- During the stationary phase the ‘birth’ and death rates approach equilibrium.
What is the decline phase?
- The death rate exceeds the birth rate and the population declines, sometimes very rapidly in a population ‘crash’.
- In the bacterial population discussed as our example, this can be due to the accumulation of toxic waste and/or the nutrient supply running out.
What stages comprise the sigmoidal (S-shape) growth curve of a population?
The lag phase
The exponential phase
The stationary growth phase
When does the sigmoidal (S-shape) growth curve comprising the lag, exponential and stationary phases of growth apply to the population of a species?
In particular circumstances, such as when colonising a new area.
The populations of different species all share the same characteristic growth phases and principles with reference to population growth. However, different factors can contribute to particular parts of the growth curve for different species. Give some examples of these factors.
For example, the lag phase can be due to the time taken for egg or larval production or the gestation period or even the time to grow and reach sexual maturity.
Why do most natural populations remain in the stationary phase?
Most natural populations are most likely to remain in the stable phase rather than progress into the decline phase - a phase typically associated with ‘closed’ conditions.
What scale can be used when representing the change in numbers of microbes over time?
A logarithmic scale
Why must a logarithmic scale be used when representing the change in numbers of microbes over time in certain circumstances?
As the increase can be over several orders of magnitude.
How is the change in numbers of microbes over time responded on a graph?
- Typically the time (on the x-axis) is scaled as normal but number of microbes (on the y-axis) has a log scale.
- Special ‘semi-log’ graph paper can be used for this purpose.
Outline of semi-log graph on textbook page 90
How can a normal graph be distinguished from a semi-log graph?
Exponential growth is seen as a straight line only when using a log scale on the y-axis
Define the term ‘biotic potential’
The maximum rate of growth of the population as seen in the exponential phase reflects the population’s biotic potential - the reproductive potential (rate) of a population under optimum environmental conditions with unlimited resources.
Define the term ‘environmental resistance’
- The environmental restrictions on population growth, for example, as evident in the stationary phase, create what is described as an environmental resistance.
- Environmental resistance is the restriction by the environment on the population reaching its maximum growth rate and its biotic potential.
Why does environmental resistance occur?
- Environmental resistance can be due to many factors including nutrient shortage or accumulation of waste but also climate, competition from other organisms, predation and disease.
- The factors that influence populations can be grouped into two main categories.
- Abiotic factors are factors in the chemical or physical environment and are loosely referred to as non-living; examples include water, nutrient, light and oxygen availability.
- Biotic factors are the effects of other organisms whether the same or other species, for example, food supply, competition, predation.
Define the term ‘carrying capacity’
- The carrying capacity is the maximum number of a population that the ecosystem can support.
- The carrying capacity is very much determined by the amount of resources available.
- In our example of bacterial growth, if extra resources were provided - for example, a larger volume of medium which would have additional nutrients (and which would also dilute the waste/toxins produced) - there would be a higher carrying capacity.
The characteristic flattening out in the stationary phase and rapid fall of the decline phase in the number of bacteria in closed conditions is due to what?
- The resources being non-renewable.
- The nutrients that were there at the start of the investigation were not replaced (as well as the waste not being removed).
Give an example of a population which utilises renewable resources
If resources are renewable, as in a broadleaved, deciduous woodland where tress shed their leaves each year and provide food for earthworms, the earthworm population tends to remain in a stationary or stable phase.
Describe the shape of the growth of an algal (planktonic protoctistan) population over the course of a year
J-shaped growth curve
Very rapid period of growth followed by a population crash (no stationary phase)
Draw a graph showing the growth of an algal (planktonic protoctistan) population over the course of a year
Textbook page 91
What growth curve is characteristic of many protoctistan species?
A J-shaped growth curve
Explain the shape of the growth curve for an algal (planktonic protoctistan) population over the course of a year
• Graph shows that there is a very rapid period of growth in spring as there is:
- abundant nutrient availability in the water
- both temperature and light levels are increasing
- relatively few herbivores in the water at this time of year.
• However, in mid-summer the population often ‘crashes’ (with no stationary phase) and rapidly falls largely because:
- the nutrient supply becomes exhausted
- herbivores (zooplankton) increase in number
- wastes accumulate
In this example, resource availability is again a key determinant of the growth pattern.
Describe the shape of the human population growth curve
The human population growth curve is a J-shaped curve with very rapid and increasing growth over the last 200 years. The big difference with the algal populations is that, as yet, there has been no crash.
What is competition?
Competition is when different organisms are competing for the same resource which is in limiting supply.
Why is competition important?
Competition is an important factor in providing environmental resistance and influencing the carrying capacity.
What are the two types of competition?
Intraspecific
Interspecific
What is intraspecific competition?
Members of the same species compete for the same resource which is in limiting supply
What is interspecific competition?
Members of different species compete for the same resource which is in limiting supply
Draw two population growth graphs summarising the investigation involving interspecific competition carried out by Russian zoologist G.F. Gause.
Textbook page 92
What can be deduced from G.F. Gause’s investigation involving interspecific competition?
- Gause investigated competition between populations of two species of Paramecium.
- When cultured separately in the laboratory and fed on bacteria the P. aurelia reached a higher final density compared to P. caudatum, as shown in diagram A on textbook page 92.
- However, when cultured together in the same conditions, the P. aurelia population increased at only a slightly reduced rate to when grown separately.
- However, the P. caudatum was eliminated as a consequence of being losers in the competition for the food resource.
- Clearly, the smaller P. aurelia was better adapted for utilising the food resources available.
- This example highlights a point made during the AS course, in that no two species occupy the identical ecological niche.
- When this happens one species loses out as a consequence of the competitive exclusion principle.
Describe the shape of a predator-prey interaction growth graph
Oscillating growth curves are produced as a consequence of predator-prey interactions with alternating peaks and troughs.
Draw a predator-prey interaction growth graph
Textbook page 93
Describe what will happen to prey and predator numbers if prey numbers increase
- If there are large numbers of prey there will be more food available for predators so their numbers will increase.
- In due course the increased numbers of predators will cause the numbers of prey to decrease, which in turn will cause the number of predators to decrease and so on.
What are some of the common features of predator-prey interaction growth curves?
- The predator peaks and troughs lag behind the prey peaks and troughs - the time lag depends on a number of features including the rate and time involved in which the predators can produce offspring.
- Although lagging behind, the length of the predator cycle is usually similar to the length of the prey cycle.
- The number of predators is normally significantly lower than the number of prey individuals at equivalent points on the cycle.
Why are theoretical predator-prey interaction growth curves rarely seen when experimental data is displayed on a graph in real life?
- Predator-prey relationships following the typical oscillating pattern are only obvious when the predator relies on one particular prey.
- In reality, most predators have more than one prey species, so the growth curves often have a smoother pattern and there is less of a correlation between any one prey and any one predator.
What do population growth curves indicate about the number of individuals making up a population?
That the number of individuals making up a population fluctuates over time.
What factors determine the change in size of a population?
Birth rate
Death rate
Any migration that take places
Form an equation equal to ‘population growth’
Population growth = (births – deaths) + (immigration – emigration)
Population growth = (births – deaths) + (immigration – emigration)
How can the above formula be simplified so as to be applied to a population of bacteria cultured in a laboratory?
With bacteria cultured in a laboratory, migration is not a factor so the population growth (whether increasing or decreasing) is dependent on the balance between the ‘births’ and deaths.
What factors contribute to the rapid increase in the populations of migratory bird species throughout the spring and summer months?
A combination of both immigration and a high birth rate.
The rapid increase in the populations of migratory bird species throughout spring and summer is a combination of both immigration and a high birth rate.
What feature does the above statement highlight about populations in general?
In many species there is a very obvious seasonal effect on population size.
Describe the seasonal effect on population size
- Thousands of migrant birds may be all too obvious during spring and summer but in contrast there may be no local population during winter.
- For most species the seasonal effect on population size is not a migration effect but is determined by the balance between births and deaths.
- Many species of animals give birth in spring or early summer, so there are large populations at this stage, when temperatures are high and food resources are plentiful.
How can seasonal and yearly effects on population size be represented graphically?
- Seasonal effects can often be represented by survivorship curves which show the percentage of individuals surviving over a year as the seasons progress.
- Survivorship curves can also be used to show the number of individuals of a particular species surviving over a period of years.
Give an example of when a survivorship curve may be used to illustrate the change in population size over many years
Survivorship curves can be drawn for tawny owls over their first five years of life - the percentage survival values represent the number of chicks from one particular year that survive over successive years as they age.
Graph on page 94 of the textbook.
For what reasons may a population size change from year to year?
This can be for many reasons including being part of a normal predator-prey cycle, changes in food supply or abiotic factors such as a colder winter.
What factors influences the population dynamic of a species as evident in their population growth curve?
- The characteristics of the species itself.
* Most species can be broadly grouped into either r- or K-selected species.
What are r-selected species?
- r-selected species tend to be ‘opportunistic’ and grow both very quickly as individuals and increase the population number very rapidly when conditions are ideal.
- Their numbers also decline very rapidly when conditions are less favourable.
- They tend to exhibit ‘boom and bust’ patterns of growth as the emphasis is on reproduction and the colonisation of new areas rather than survival.
Give some examples of r-selected species
Bacteria
Protoctistans
Annual plants (many species of weeds)
What are K-selected species?
- K-selected species have more stable populations and the population size usually remains at or close to the carrying capacity for the species.
- In K-selected species, the emphasis is more on survival and dominance rather than expanding the population or colonising new areas.
Give some examples of K-selected species
K-selected species include many large mammals such as humans and many species of trees.
List some of the typical features displayed by r-selected species
- Small body size
- Short lived
- Reproduce rapidly with usually many offspring
- Very little parental care
- Able to disperse rapidly and colonise new habitats
- Population size (density) very variable
- Low competitive ability - unlikely to become dominant
- Not specialised so adaptable to change in environment - can evolve rapidly, for example, antibiotic resistance in bacteria
- Often inhabit unstable or short lived habitats, for example, weeds colonising a ploughed field
List some of the typical features displayed by K-selected species
- Large body size
- Long life cycle - usually a number of years before mature and able to produce offspring
- Few offspring
- Large amount of parental care - high investment in young - few young produced so important that they have high chance of survival
- Low dispersal ability - colonisation of new habitats less frequent
- Population size (density) more constant
- High competitive ability - may be a dominant species in the ecosystem
- Tend to be highly specialised so less resistant to environment change - prone to becoming endangered or exciting in changing environment, for example, polar bears and global warming.
- Typically occur in stable habitats that remain relatively undisturbed for many years, for example, oak tress in a forest
The characteristics of the species itself influences the population dynamic. Most species can be broadly grouped into either r- or K-selected species.
What does ‘r’ designate?
- r is the designation for the intrinsic rate of natural increase (the biotic potential).
- r-strategists have short life cycles and reproduce very rapidly and therefore have a high value for r - they approach the biotic potential.
The characteristics of the species itself influences the population dynamic. Most species can be broadly grouped into either r- or K-selected species.
What does ‘K’ designate?
- In population growth curves K represents the carrying capacity.
- Consequently K-strategists have population sizes that remain close to the carrying capacity (K).
Give some examples of +/– interactions
Predator-prey interactions
Herbivores grazing on plants
Parisitism
What is a +/– population interaction?
The populations of two different species interact and influence each other, with one species gaining and the other losing out.
Why is parasitism described as a +/– population interaction?
As the parasite gains at the expense of the host.
Why is grazing described as a +/– population interaction?
As one species (eg the cow) gains and another (eg the grass) loses.
Define the term ‘parasite’
An organism that lives in or on another organism (the host) benefitting from it and causing it harm over an extended period of time.
How do parasites differ from predators?
The differences between a predator-prey relationship and a parasite-host relationship can often be very subtle and difficult to distinguish with no clear demarcation.
However, as a general rule, parasites differ from predators in four ways:
• The parasite lives in or on the host.
• The parasite causes harm to the host over an extended period of time.
• The parasite is usually smaller than the host.
• The parasite seldom kills the host (or if it does, it is a very slow process).
Give some examples of parasites
Animal kingdom:
The flea
The human tapeworm
The malarial parasite Plasmodium that is transferred between humans by female mosquitoes
Plant kingdom:
The common tar spot fungus is a fungal parasite that infects sycamore leaves
Mistletoe is a plant parasite that infects tress across much of north-west Europe
Comment on the amount of harm that parasites can cause
The amount of harm that parasites cause varies considerably - normally a flea is a little more than an irritant but malaria is often fatal.
Why is mistletoe described as a parasite?
• Mistletoe is an example of a plant parasite that infects tress across much of north-west Europe.
- Mistletoe has evergreen leaves and forms dense spheres that hang from the host tree.
- The mistletoe can photosynthesise to produce its own carbohydrate but produces special structures to penetrate the tree and absorb water and minerals for its needs - it has no roots as such that reach the ground.
What is a –/– population interaction?
The populations of two different species interact and influence each other, where both species suffer
Give an example of a –/– population interaction
Competition
Why is competition referred to as a –/– population interaction?
- If we revisit the growth curves of Paramecium aurelia and P. caudatum we can observe that when the two species are grown together, neither species grows as well as it does when growing on its own.
- The graphs also show another common feature of competition, the loser is often eliminated by competitive exclusion.
- Although the example of competition described above shows that the species with better competitive ability can lead to the elimination of another species, in nature there is as much competition among members of the same species (intraspecific competition) as there is among different species (interspecific competition).
- For example, oak seedlings growing around an oak tree are competing with each other (and with the parent tree).
- Over time, fewer and fewer individuals survive due to competition for resources such as space, light, water and minerals.
What is a +/+ population interaction?
The populations of two different species interact and influence each other, where both species benefit.
Give an example of a +/+ population interaction
Mutualism
Comment on the complexity of mutualistic relationships
Mutualistic relationships can be complex and often have evolved to the extent where at least one of the species cannot survive on its own.
State some examples of mutualistic relationships
- Lichens
- Nitrogen-fixing bacteria in the root nodules of leguminous plants
- Cellulose digesting bacteria and/or protoctistans in the gut of herbivores
Explain why lichens are an example of a mutualistic relationship
- Lichens are an example of an obligate mutualistic relationship between fungi and green algae.
- The fungi provide the supporting framework and absorb water and minerals (as well as sheltering the algae and protecting them from desiccation), and the algae photosynthesis thus providing carbohydrates and other organic compounds, some of which are available to the fungi.
Explain why nitrogen-fixing bacteria in the root nodules of leguminous plants is an example of a mutualistic relationship
- Mutualistic nitrogen-fixing bacteria live in the nodules of the roots of legumes such as peas and beans.
- The nitrogen-fixing bacteria benefit through gaining carbohydrate from the plants and the plants benefit through gaining amino acids (or other nitrogen-containing compounds) from the bacteria.
Explain why cellulose digestion in many herbivores is an example of a mutualistic relationship
- Cellulose digestion in many herbivores is a consequence of mutualism between the herbivores and bacteria and/or protoctistans in the gut of the herbivore.
- In cows and other ruminants the bacteria possess the cellulose required to hydrolyse cellulose.
- The cows benefit as they can utilise a very abundant source of food not accessible to so many other animal species, and their mutualistic partners gain a ready supply of food and are maintained at a relatively high and constant temperature, ensuring rapid metabolic activity.
Give an example of when humans deliberately manipulate the relationship between other species for their own benefit
The control of pest species
Why is biological control used to control pest species as opposed to other alternative methods?
Due to the harm caused by the use of chemical pesticides, the biological control of pest species is seen as an attractive alternative.
Define ‘pest species’
A species that damages a valuable/commercial crop species, causing economic damage.
Define ‘biological control’
Biological control involves deliberately introducing an organism that will target and cause harm to the pest. This can be a predator, a competitor, a parasitic or pathogenic organism.
How does biological control benefit the environment?
- Biological control will benefit the environment by reducing the need for chemical pesticides and the harm that they cause but are also used because ‘broad-spectrum’ pesticides (such as insecticides and herbicides) may not work particularly well.
- Broad-spectrum pesticides may kill many beneficial organisms, including many natural enemies of the pest.
Draw a predator-prey interaction growth curve illustrating what can happen if a broad-spectrum insecticide is used to target the pest species (the prey)
Textbook page 97
What can happen if a broad-spectrum insecticide is used to target a population of pest insects?
- Pest resurgence can occur following the application of a broad-spectrum insecticide, in that the numbers of the pest species increase rapidly due to the elimination of the natural predator.
- In pest resurgence after the use of insecticides, the numbers of pests can rise to well above what it was before the insecticide was applied.
Draw a graph showing how effective biological control can reduce pest numbers below the threshold of economic damage
Textbook page 98
What is the key factor that makes biological control effective?
With effective biological control the introduced predator integrates naturally into the ecosystem, building a sustainable population and therefore does not need to be continually re-introduced.
What are the advantages of effective biological control?
- With effective biological control the introduced predator integrates naturally into the ecosystem, building a sustainable population and therefore does not need to be continually re-introduced.
- There is no chemical damage to the environment with the risk of significant ecological harm and bioaccumulation in food chains.
- Biological control targets only the pest species - there is reduced collateral damage affecting other organisms.
- The development of resistance by the pests is unlikely.
- Pest resurgence is unlikely.
- Biological control, if successful, needs little additional action and saves money on the continued use of pesticides.
How does insecticide resistance arise in pest populations?
- Insect populations treated with insecticides frequently develop resistance to the pesticide and it ceases to become effective.
- Before the pesticide is applied, a small number of individuals in a population will have resistance due to mutation.
- However, when the pesticide is used, the non-resistant individuals are killed leaving only the resistant individuals (that survive and reproduce) resulting in the population becoming resistant.
What are the limitations of biological control?
Biological control has its limitations though and it is not always successful. These limitations (potential disadvantages) include:
- The pest is unlikely to be totally eliminated. However, with effective biological control, its density will be reduced to below the threshold for economic damage.
- Biological control will only work well if the biological control species can adapt well and thrive in the ecosystem into which it is introduced. Often an ‘unnatural’ crop ecosystem not found in the wild can be hostile to the introduced biological control species.
- It is important that the introduced control species does not outcompete native species (cause harm to non-target species).
Community definition
The sum total of all the populations (species) in a particular area (habitat or ecosystem).
More complete definition = The biotic component of an ecosystem involving interaction between the autotrophic and heterotrophic populations present.
Ecosystem definition
The community plus its physical environment - it consists of both biotic and abiotic components.
Comment briefly on the relationship between different species within an ecosystem
In an ecosystem many of the species are interdependent on each other.
What factor(s) determine the type of community that can develop within an ecosystem?
The abiotic environment, such as rock the and climate, determines the type of community that can develop.
In what fundamental ways will different species present interact within an ecosystem?
Different species present will interact in areas such as energy flow, nutrient and gas exchange.
Comment on the changes that occur to ecosystems over time
Ecosystems are constantly changing and succession is the term used to describe the changes over time in ecosystems.
Succession definition
- Succession is the term used to describe the changes over time in ecosystems.
- It is important to note that succession involves changes to both the community (the species present) and the abiotic environment due to the ongoing interaction between these two components.
Where and when does primary succession occur?
Primary succession occurs on newly formed, barren substrates that have not been previously colonised.
Give some examples of where primary succession can occur
- Lava fields produced after a volcanic eruption.
- The sudden appearance of a volcanic island.
- The exposed rock at the base of a disused quarry.
What key requirement must be met in order for primary succession to occur?
The exposed land (rock) provides a very harsh and hostile environment for life. There must be no soil present to support plants.
What is the first stage in succession?
The appearance of colonising pioneer species.
Give an example of a pioneer species
- Pioneer species such as lichens are able to survive the hostile conditions.
- The lichens can grow on the bare rock, tolerating desiccation.
Describe the sequence of events that occur during primary succession
- The first stage in the succession is usually the appearance of colonising pioneer species.
- Pioneer species such as lichens are able to survive the hostile conditions.
- The lichens can grow on the bare rock, tolerating desiccation.
- Over time the lichens begin to degrade the rock and help promote the weathering (aided by the climate, for example, frost action) that forms the embryonic soil.
- Initially the soil will accumulate in cracks in the rock.
- As the lichens die and decompose, the ‘soil’ will develop to a stage where it can support mosses.
Annotate the image of a rock possessing lichens and mosses on its surface
Textbook page 99
What impact do the communities present at each sere have on the abiotic environment that surrounds them?
The communities present at each particular stage of the succession help to modify the abiotic environment thus creating the abiotic conditions necessary for the next stage.
The communities present at each particular stage of the succession help to modify the abiotic environment thus creating the abiotic conditions necessary for the next stage.
Give an example of this.
For example, the decomposing lichens and mosses form the embryonic ‘soil’, increasing mineral availability.
Comment on the nature of the colonising plant species that succeed lichens and mosses in the early stages of succession
The colonising plant species that succeed lichens and mosses in the early stages of succession are usually short lived r-strategists that have excellent dispersal mechanisms allowing hem to rapidly colonise new areas.
Annotate the image on page 100 of the textbook
Textbook page 100
Comment on the changes in certain features of the environment over time as primary succession progresses
Typically with time, as the succession develops, soil depth and fertility increase, so does the number of different plant species (although plant biodiversity often decreases again as the succession approaches maturity). The plant biomass also tends to increase as succession progresses.
Draw some graphs showing changes in certain features in the environment over time as primary succession progresses
Textbook page 100
Features:
Soil depth
Soil fertility
Plant diversity
Plant biomass
What name is given to each stage in succession?
A sere
What is the climax community?
- Eventually after a number of seres, the climax community develops.
- The climax community is the stable end stage of a succession which is in equilibrium with the environment.
Describe the types of climax community found in the UK
- In most of lowland Britain the climax community is mixed broadleaf deciduous forest dominated by oak and other common species.
- In harsher upland environments, moorland is often the climax community.
Draw an annotated diagram showing the environment during the first seral stage (sere) of primary succession in lowland Britain
Textbook page 101
Draw an annotated diagram showing the environment during the second seral stage (sere) of primary succession in lowland Britain
Textbook page 101
Draw an annotated diagram showing the environment during the third seral stage (sere) of primary succession in lowland Britain
Textbook page 101
Draw an annotated diagram showing the environment during the fourth seral stage (sere) of primary succession in lowland Britain
Textbook page 101
Draw an annotated diagram showing the environment during the fifth and final seral stage (sere) of primary succession in lowland Britain
Textbook page 101
If the composition of the climax community is determined by the climate it is called …
A climatic climax
What is a climatic climax?
If the composition of the climax community is determined by the climate it is called a climatic climax.
If the composition of the climax community is determined by biotic factors, such as grazing, it is called …
A biotic climax
What is a biotic climax?
If the composition of the climax community is determined by biotic factors, such as grazing, it is called a biotic climax.
Why is primary succession more unpredictable compared with secondary succession?
As the actual succession that will take place depends on a number of factors, including climatic and biotic factors.
What features do all primary successions tend to have in common?
- They are predictable in pattern - pioneer species will always be the initial colonisers and a similar climax community will develop in the same conditions.
- The abiotic environment becomes less hostile as soil forms and the growth of plants provides shelter for the organisms in the later stages.
- The height and biomass of vegetation increases.
- Communities become increasingly complex, with more complex food webs, as a greater number of niches are provided for animals.
- There is increased biodiversity (at least until mid-succession).
- Communities in later stages of the succession are usually more stable than in earlier seres.
Case study - a sand dune succession
Comment on the seral community found in most successions at any one time
- At any one time most succession will be at a particular stage or sere.
- If you visit a mature mixed deciduous woodland you will be in a climax community - a community that used to dominate most of Britain and Ireland but is much rarer now.
- You have to image in what the pioneer stage was like many years ago as this type of succession will take several hundred years to develop.
Case study - a sand dune succession
Why are sand dunes a particularly interesting type of succession to investigate?
- As they can demonstrate all the different seral stages (seres) at the same time.
- A sand dune ecosystem is continually developing as sand blown by the wind or carried by the force of the sea is forming new dunes close to the high tide mark.
- As dune renewal takes place at the hostile interface with the sea, there is a progression of increasingly older dunes further inland as conditions become more benign and the dune system becomes more stable and mature.
See textbook page 102: Young dune at edge of shore
Case study - a sand dune succession
Describe the appearance of the first seral stage of a sand dune succession
- In the young dunes at the shore edge, marram grass is the principle pioneer species and coloniser.
- The marram grass is highly specialised in a number of ways - it is a Xerophyte and able to survive in the sand which is unable to effectively retain the (usually abundant) rain that falls in these ecosystems.
- The roots of the marram grass can rapidly penetrate through the sand and have a very important role in binding the sand together and stabilising the dunes.
See textbook page 102: Roots of marram grass bind the sand particles together
Case study - a sand dune succession
Describe the appearance of the second seral stage of a sand dune succession
- Just behind these young developing dunes, small areas of ‘grassland’ called dune slacks have a degree of protection and are rich in mosses and many other ground-hugging plant species such as birdsfoot trefoil and thyme.
- Animal species such as snails are also becoming more common as the food webs are becoming increasingly complex.
- Typically, these areas have the greatest biodiversity in the sand dune system.
- The continuing cycle of the growth and decomposition of the plants allows a thin soil to develop.
See textbook page 102: Dune slack immediately behind the young dunes
Case study - a sand dune succession
Describe the appearance of the third seral stage of a sand dune succession
- Further inland, as the dunes are more mature, the ground is stable enough and the soil developed enough to allow shrubs such as heather to dominate.
- The increasing height of the plant community shades out many of the plants that dominated the ground cover in the previous sere.
- Not surprisingly, the biodiversity begins to fall although there is a significant increase in biomass at this stage.
See textbook page 103: Mature dune with heather and marram
Case study - a sand dune succession
Describe the appearance of the fourth seral stage of a sand dune succession
- This trend continues as we move inland and reach the old dunes - dunes that have been formed for hundreds of years.
- The old dunes are even more stable and are colonised by bracken and gorse.
- These species allow very little light to penetrate to ground level.
- Few species other than mosses are common under the bracken and gorse.
See textbook page 103: Old dunes with bracken and gorse
Case study - a sand dune succession
Describe the appearance of the fifth and final seral stage of a sand dune succession
• Further inland again, the heather and gorse community is often replaced by woodland.
See textbook page 103: Dune system merging into woodland
Why, in reality, do most successions not show the relatively slow sequence from bare land - usually rock but sand in the case of a sand dune succession - to climax community?
- In reality, most successions that take place do not show the relatively slow sequence from bare land - usually rock but sand in the case of a sand dune succession - to climax community.
- Instead, they are successions that take place when the normal primary succession is interrupted or the climax community is damaged or destroyed due to, for example, fire, flooding, wind damage or through human interference (for example, woodland clearance or ploughing land).
- This type of succession is called secondary succession.
What is secondary succession?
Secondary succession is succession that takes place when the normal primary succession is interrupted or the climax community is damaged or destroyed due to, for example, fire, flooding, wind damage or through human interference (for example, woodland clearance or ploughing land).
How does secondary succession differ from primary succession?
- Secondary succession does not usually begin with the typical pioneer species, such as lichen, as the soil is already formed and will contain the seeds of many species as well as other plant parts that can rapidly regenerate, for example, roots.
- Many other soil organisms, such as nitrifying bacteria and detritivores, are also present in the soil.
- As a consequence of all these factors, secondary succession is invariably much quicker than primary succession, with the climax community being reached in a much shorter time.
Describe a common secondary succession in Britain
- A common secondary succession in Britain is the succession that takes place after woodland is cleared.
- Although some of the cleared land is often used for urban development, or for farming activity, some frequently remains untouched and has the opportunity to revert to climax community through secondary succession.
- The sequence of seres generally represents the sequence at the latter end of a primary succession but there are often some differences compared to the typical primary succession.
How are some species particularly adapted to the ecological niches associated with cleared woodland?
They can either regenerate very rapidly from seed banks in the soil or colonise from surrounding areas.
Name a species common in the early stages of a woodland secondary succession following clearance
Foxglove
Annotate the photograph on page 104 of the textbook showing secondary succession following woodland destruction by a severe storm
Textbook page 104
Practical work - The haemocytometer
What is a haemocytometer?
- The haemocytometer is an instrument for counting cell numbers (density).
- As its name suggests it was originally designed for counting blood cells, but can also be used to count yeast or any type of cells that are large enough to be seen under the microscope.
- The haemocytometer resembles a modified microscope slide with a grid (or grids) containing squares of known size.
- The design enables the central area (counting platform) containing the grid to be slightly lower by a fixed distance (0.1 mm) than the coverslip.
- This ensures that the squares in the grid represent not only a known area, but the liquid above them has a known volume.
Practical work - The haemocytometer
How can a yeast population be cultured in order to carry out an investigation?
If carrying out an investigation on yeast populations in the laboratory, the yeast can be cultured in a conical flask containing glucose solution.
Draw a diagram showing the top view of a haemocytometer
Textbook page 105
Draw a diagram showing the side view of a haemocytometer
Textbook page 105
Draw a diagram of an enlarged grid on the counting platform of a haemocytometer
Textbook page 105
Calculate the volume enclosed for each of the different sizes of squares in the grid of a haemocytometer
Textbook page 105
Practical work - The haemocytometer
How can cell numbers (density) be calculated from a cell suspension in the laboratory?
- A suspension containing the cells to be counted should be placed on the grid on the haemocytometer.
- Depending on the density of the cells, a type-A, type-B or type-C square should be used.
- If there are very few cells in the samples the type-A square should be used (as many type-B and most type-C squares will be empty).
- If there are so many cells in the sample that it would take too long to count all the cells in a type-B square, then a type-C square should be used.
- When actually counting the cells in an investigation, many of the cells may lie over the grid lines of the square being counted.
- In this situation it is very important that cells are not counted twice or are not counted.
- Therefore one approach is to follow the ‘north-west’ rule.
- Cells touching (or lying on) the outer grid line at the top (north) and left (west) side of the square should be counted, but those touching or overlying the grid lines on the right hand side (east) and the bottom (south) should not be counted.
Practical work - The haemocytometer
Worked example - Page 106 of textbook
Calculate the number of cells/mm^3 in the suspension shown in the type-B square below.
Number of cells in the type-B square = 22 (using the north-west rule)
Volume of square = (0.04 mm^2 x 0.1 mm) = 0.004 mm^3
22 x 250 = 5500/mm^3
For clarity and scale purposes the type-A square is not shown. In this diagram it is better to count the number of cells in the type-B square as there are too few cells in the type-C squares. For increased reliability, a good rule is to use the largest sized square that it is realistic to count.
What are some important points to be remembered when using the haemocytometer?
When carrying out investigations using the haemocytometer it is important to:
• Mix the cell (yeast) suspension thoroughly before taking a sample for counting.
• Sample from the same depth in the flask.
• If sampling from the ‘natural’ environment, for example, sampling phytoplankton from a lake, the retrieval of different samples should also be taken from the same depth and if taken over a period of time may need to be taken at the same time of day.
• Avoid getting the yeast suspension onto the top if the coverslip or into the grooves of the haemocytometer.
• It is important to carry out an appropriate number of replicates for reliability.
When calculating the density of cells in some investigations there may be too many cells to be able to count them as discrete entities, even when using a type-A cell. How can we work around this?
- If so, serial dilution will be necessary.
- Serial dilution involves diluting the suspension, usually by a factor of 10.
- For example, a 1 cm^3 of the original suspension could be added by pipette into 9 cm^3 of isotonic buffer and then the new solution examined under the microscope.
- If this first dilution is still too concentrated, the dilution process should be repeated and so on.
- However, it is very important that when calculating the number of cells / mm^3 the dilution factor is taken into account.
Why is the haemocytometer a good tool for measuring increase in yeast density during the lag and exponential phases of population growth?
- When investigating the growth curve if yeast (or other single-celled organisms) it will usually be impossible to distinguish between living and dead cells without further analytical work.
- For this reason, the haemocytometer is a good tool for measuring increase in yeast density during the lag and exponential phases but less good for estimating numbers during the stationary and decline phases (as the counting of dead cells will suggest the number may not be falling) unless additional work is done to distinguish between living and dead cells.
Practical work - Estimating the size of an animal population using a capture-recapture technique
Why can population sizes of animals be more difficult to estimate than plants?
As animals move and many species spend considerable time in places where they are difficult to locate or observe, such as burrows.
What is the capture-recapture technique also referred to as?
The mark-release-recapture technique
Describe the principle of the capture-recapture technique
- The principle of the capture-recapture technique is that a number of animals are trapped/caught and counted, marked and then released.
- After an appropriate time, a number of the target species are caught in a second sampling process.
- At this stage the recaptured animals should contain a number of marked animals (ie caught on both occasions).
- The following formula (the Lincoln index or Peterson estimate) is used to estimate the population size.
Estimated population size = S1 x S2 / r
What do the different variables in the Lincoln index for population estimation correspond to?
S1 = number caught in the first sample S2 = number caught in the second sample r = number recaptured (ie caught in both samples)
Textbook page 107
What logical assumptions can be concluded about the population size given the number of recaptured animals is known?
- If the population is very small it is logical to assume that the second sample will contain a very high number of marked animals.
- If the population is large it is probable that only a very small proportion of the recaptured animals in the second sample will be marked.
Describe the detailed procedure for the capture-recapture technique
- A large sample of the species is caught or trapped using an appropriate technique (for example, pitfall trap for beetles, sweep net for grasshoppers).
- The caught animals are marked in a way that will last over the sampling period (the marking is permanent or semi-permanent using, for example, waterproof ink, correction fluid or bird ringing).
- The marking should be done in a way that does not harm the animal or make it any more likely to be predated than other non-marked animals. This can be done by marking the animals on their underside, which is out of site to most predators.
- The marked animals are released. There must be sufficient time to allow the animals caught in the first sample to mix throughout the overall population.
- The population is then re-sampled using the same trapping process as before and the population size estimated using the Lincoln index formula.
Describe some of the assumptions made by this method of population estimation
- There are no significant gains or losses through immigration or emigration. This can be avoided through carrying out the sampling in a discrete area where mixing with other populations is less likely (for example, sampling the beetles in one wood rather than just part of the wood).
- There are no significant gains or losses through births and deaths respectively.
- The trapping process (or subsequent marking) does not affect the animal in any way (for example, making it more wary of the trapping mechanism and reducing its possibility of being trapped in the re-sample or being more likely to be predated).
- The marked animals have mixed throughout the population by the time of the re-sampling period.
The mark-recapture technique works particularly well for small, mobile animals that are easily trapped such as beetles and grasshoppers.
