Populations and Communities Flashcards
Define population
a group of individuals of the same species which occupy a particular habitat
Give an example of a population in closed conditions
growth of bacteria or yeast in a nutrient medium in a beaker. This will produce the characteristic pattern of population growth.
Describe the characteristic pattern of population growth
- lag phase
- exponential phase
- stationery phases
- decline phase
describe and explain the lag phase in population growth
- Population numbers remain relatively constant or may even decline
- This is the time taken for nutrients to be assimilated, as well as the production of new offspring (eg egg production, egg/larvae development, gestation period in mammals)
describe and explain the exponential/log phase in population growth
- Population numbers increase by the same factor each time unit
- Each new member of a population has the reproductive capacity to generate more individuals (eg one bacterium divides to form 2, 2 form 4, etc).
- In this phase, growth is rapid and there is little competition since there are sufficient resources
- The numbers increase by the intrinsic rate of natural increase (r), as the population is exhibiting its biotic potential
describe and explain the stationary phase in population growth
- Population numbers remain relatively stable
- Increased numbers in the population result in environmental resistance.
- The result of environmental resistance is a reduction in birth rate and increase in death rate - the outcome being the population remaining constant.
- The population is said to have reached its carrying capacity (K)
- Most organisms will stay in this phase and not reach the decline phase.
describe and explain the decline phase in population growth
- Population numbers drop rapidly
- The reduced availability of resources as well as an increase in toxic waste materials results in birth rate falling to zero, and the death rate increasing.
- This results in a steep decline (crash) in population numbers.
define biotic potential
the reproductive capacity of the population under optimum conditions.
define carrying capacity
the maximum number of organisms the environment can support
name some factors of environmental resistance
- accumulation of waste
- lack of resources eg nutrients
- increased competition (in the case of population curves it is intraspecific competition)
what conditions need to be in place to see a J-shaped population growth curve?
the absence of any limiting factors/environmntal resistance to growth
what does a J-shaped curve represent?
the biotic potential of a population
Give some examples of where a J-shaped growth curve could be seen
- yeast in an open (continuous) culture where waste and dead cells are constantly removed, and fresh medium is constantly added
- a population of mammals where they are protected from their natural predators
Describe why protoctists often display J-shaped population growth patterns
- characteristic of many protoctistan populations (eg algae)
- very rapid growth in spring as there is an abundant nutrient availability in the water increase in temp and light levels, and relatively few herbivores (eg zooplankton).
- Population may crash in midsummer with no stationary phase, as the nutrient supply becomes exhausted, but herbivores also increase in number and accumulate waste.
Describe ways to encourage J-shaped curves
- Extra resources (eg nutrients) provided
- Larger volume of medium (if yeast/bacteria culture), or more space - provide more resources eg sunlight and will dilute waste/toxins produced
- Removal of waste products
how does temperature influence population fluctuations?
- Temperature isn’t a resource, but it will determine the metabolic rate in organisms, and so the rate at which they develop.
- In lab populations it can be demonstrated that the rate of increase (during exponential phase), will rise at higher temperatures, but a higher temperature will not influence the size of the maximum population (in the stationary phase). This is determined by the resources such as available nutrients.
- eg. a warm spring will produce rapid increases in insect populations, which will then benefit the growth of populations of insectivorous birds.
Name the factors affecting population growth
- birth rates
- immigration
- death rates
- emigration
Give the equation to estimate change in population size
(birth rate + immigration) - (death rate + emigration)
describe the equation to represent a population in equilibrium
birth rate + immigration = death rate + emigration
Give some examples of the factors affecting population size at work
- For bacteria cultured in a laboratory, migration is not an issue, so the balance is dependent on births and deaths
- The rapid increase in the populations of migratory birds in spring and summer shows the seasonal effect on population size, dependent on a combination of births and migration.
- Many species of animals give birth in spring or early summer, so there are large populations at these times of year, when temperatures are suitable, and resources are plentiful.
what do survivorship curves show?
the percentage of individuals (of a particular species) surviving over a year or period of years
how can population sizes change from year to year?
predators, changes in food supply, or abiotic factors eg severe temp changes.
from where does r- and k-selection theory originate?
work on island biogeography by MacArthur and Wilson
Describe some characteristics of r-selected species
- individuals grow very quickly and are short lived
- population size increases very rapidly under ideal conditions
- Numbers may decline very rapidly when conditions are less favourable and exhibit “boom and bust” growth patterns
- Are occasionally referred to as ‘opportunistic’
- small body size
- reproduce rapidly with many offspring and little parental care
- able to disperse rapidly and colonise new habitats
- low competitive ability - unlikely to become dominant
- not specialised so adaptable to change and evolve in an environment (eg antibiotic resistance)
- often inhabit unstable or short lived habitats, eg weeds
why are r-selected species described as such?
Have an r strategy because of the prominence of the intrinsic rate of natural increase (r)
emphasis on reproduction and colonisation of new areas rather than survival.
why are k-selected species described as such?
population size remains ver close to carrying capacity (k)
emphasis on survival and dominance rather than colonisation and expansion of population
describe some typical characteristics of K-selected species
- Stable populations and are often described as “equilibrium”
- In stable environments, k-selection predominates as the ability to compete successfully for limited resources is crucial
- Due to their lower ability to migrate, they are more prone to extinction following an environmental disaster
- larger body size
- long life cycle - long period of parental care, and long period of maturation before the organism can reproduce
- low dispersal ability - colonisation of new habitats less frequent
- high competitive ability/dominance
- tends to be highly specialised so therefore more vulnerable to environmental change
give some examples of r-selected species
bacteria, protoctista, insects, annual plants/weeds
give some examples of K-selected species
oak trees, elephants, humans, chimpanzees
describe a typical population growth curve of an r-selected species
population size fluctuates rapidly above and below the carrying capacity
describe a typical population growth curve of a K-selected species
gradually increases to carrying capacity, where it remains with small fluctuations above and below
describe and explain a -/- population interaction
competition
- In most natural habitats there is a limit to the availability of food, space and water. If there is not enough of one resource for all members of the population, competition for that resource will occur.
- The effect of the competition is to reduce the growth rate of individuals, and their reproductive capacity to a lower level hat they could achieve if there was no competition - influences carrying capacity.
define intraspecific competition and provide an example
- Competition between members of the same species
- Will become more severe as the population increases and resources become limiting.
- Eg. Bacteria being grown in a flask, redwood trees growing to an extreme height to compete for light.
define interspecific competition and provide an example
- Competition between members of different species
- 2 different species compete for the same resource in the same or overlapping niche.
- Success of one species over another is directly related to the adaptions of each of the species to the conditions that prevail at the time (eg. temp, humidity, etc)
- Eg. 2 species of herbivores in the same field
describe the characteristics of competition
- Both species do less well when competing for the same resource
- One species is eventually eliminated from the habitat
- The winner may utilise the resource more efficiently and so be more successful, or it may enable the winner to compete more effectively (eg. Aquatic plants with air sacs can float above plants without air sacs, and so absorb more available light)
- The outcome of the competition may also be determined by the environmental conditions eg one species of flour beetle will outcompete the other if it is warm and humid, but the other species will prevail if it is cold and dry.
define the competitive exclusion principle
a situation where, due to the severity of the competition for resources, one species is eliminated. This can only happen when there is an overlap between niches, as no two species can occupy the identical ecological niche
name some +/- population interactions
grazing, parasitism, predation
describe grazing as a population interaction
The grazer will feed on another organism without killing it outright, but potentially causing it harm
eg. Herbivores such as cows or capybaras, grazing on grasses - herbivore benefits by getting nutrients and energy, while the grass loses photosynthetic ability as its leaves are eaten.
describe parasitism as a population interaction
- Parasites live on or in a host organism, feeding upon or within it. The parasite benefits, whilst over time the host suffers.
- It is to the parasite’s advantage not to kill the host, as if the host was killed it would result in the parasite being required to find a new host for it to survive.
- Often the host provides the ideal conditions for reproduction and survival.
- Parasites which are introduced to a new habitat can cause a devastating effect on their host populations eg Dutch elm disease, caused by a fungus wiped out nearly all dutch elm trees in Britain.
give some examples of parasitic population interactions
- Animals - fleas, lice human tapeworm, malarial parasite, parasitic wasps
- Other parasites - common tar spot fungus (infects sycamore leaves), mistletoe (infects trees across north-west Europe - hangs in dense spheres from the tree, can photosynthesise to produce its own carbohydrate, but penetrates the tree to absorb water and minerals as it has no roots to reach the ground.)
describe predation as a population interaction
- the predator is set to gain food from the hunting and killing of prey
- If there are large numbers of prey, the predators have more food available and their numbers will increase, while the prey then decrease, resulting in a subsequent decrease in the numbers of the predator.
Describe the typical features of a predator/prey population graph
- The predator/prey interactions have oscillating growth curves, with alternate peaks and troughs.
- The predator peaks and troughs are similar in length, but lag behind the prey peaks and troughs. The time lag depends upon the rate and time involved for the predators to produce offspring.
- The numbers of predators is normally significantly lower than the number of prey individuals at equivalent points in the cycle.
- In instances where the predator has more than one type of prey, the relationships aren’t as obvious as the predator has more than one prey species and so the curve has a smoother patter, with less of a correlation between any one prey and its predator.
give some examples of predator-prey relationships
snowshoe hair and canadian lynx (lynx’s only type of prey)
owls and mice
lions and zebras
grasshoppers and leaves
name and describe a +/+ population interaction
Mutualism
- Each species benefits off of the presence of the other.
- These relationships have often evolved to the extent where at least one of the species can’t survive on its own.
Describe some examples of mutualistic relationships
- Oxpecker eats the bugs/parasites off a zebra or rhino - oxpecker gets food, zebra/rhino gets less parasites
- Bacteria in cattle intestines produce cellulase that breaks down cellulose. In return for this enzyme provision, the cow provides a constant supply of food materials and a stable environment for the bacteria.
- Lichens are an obligate mutualistic relationship - compound organisms consisting of highly modified fungi with green algae among hyphae - fungi provide the supporting framework and water and nutrients (as well as protection from desiccation), algae photosynthesise providing carbohydrates and organic compounds to the fungi
- Nitrogen-fixing bacteria eg Rhizobium live in root nodules of legumes eg peas and beans - bacteria benefit from carbohydrates from the plant, plants gain nitrogen containing compounds from the bacteria.
Describe some features that will often differ between parasites and predators
- Parasite is generally smaller
- Parasite keeps host organism alive for longer - benefits off it while it is alive, while predator often kills quickly
- Predators often have multiple prey in their lifetime - parasites will often have one (or a smaller amount)
Give some examples of micropredators
- Vampire bats are smaller than their host and will take blood from multiple cows
- Aphids feed on plants
- Mosquitoes feed on animal blood
- As they don’t kill their host, these are often thought of now as parasites
Define biological control
the use of natural enemies to eradicate populations of pests
define pest
a species that competes with or adversely affects a valuable/commercial population of crop species (or animals), causing economic damage.
Describe some features of biological control
- It doesn’t aim to eradicate a pest, but to reduce its numbers to a level where they don’t cause major economic damage, and to maintain the predator’s food source.
- In effective biological control, the introduced predator integrates naturally into the ecosystem, building a sustainable population and therefore doesn’t need to be continually introduced.
- Natural enemies of insect pests include predators, parasitoids, and pathogens.
define parasitoids
species whose immature stage develops on or within a single insect host, ultimately killing the host. Most have a very narrow host range. Many species of wasp and some flies are parasitoids
Define pathogens
isease causing organisms including bacteria, fungi and viruses. They kill or debilitate their host and are relatively specific to certain insect groups.
define antagonists
Biological control agents of plant diseases. Biological control agents of weeds include herbivores and plant pathogens.
Describe some methods of biological control
- Introducing a predator eg ladybirds controlling aphids in orange groves
- Introducing a herbivore eg South American moth to control prickly pear cactus in Australia
- Introducing a parasite eg wasp to eat tomato plants
- Introduce sterile males - reducing mating and so pest numbers
- Using pheromones - sex hormones attract the pests, which are then destroyed.
What are the drawbacks of using broad spectrum pesticides (BSPs)?
- BSPs may not work particularly well against the pest and they can develop resistance
- Many kill beneficial organisms including some natural enemies of the pest - the pest can experience a pest resurgence (number drastically increase due to the elimination of a natural predator).
- can reduce biodiversity
- expensive as needs frequent reapplications
- can be harmful to human health and to agriculture (eg farm animals)
Give some advantages of biological control
- Only pest species is targeted
- No negative effect on biodiversity (as with BSP)
- Much reduced chance of pest developing resistance to biological control agent
- As biological control reproduces, no need to reapplication (self-perpetuating)
- Can be relatively cheep - saves money on continued use of pesticides
- Doesn’t leave chemical residues in the environment - reducing ecological harm and preventing bioaccumulation in food chains.
give some problems with biological control
- Doesn’t get rid of pest completely
- Predator must only control pest population and not other similar organisms
- Must be able to survive and reproduce in the given conditions
- Predator may become a pest itself
- Controlling one pest may allow another to fill its niche and make things worse
- Not appropriate for pests of stored products like grains it would be contaminated with the dead remains of the pest and control agent.
- May take a long time to reduce the pest population.
define community
- A community consists of all the populations of different species which live in the same place at the same time, and interact with each other
- a community can be thought of as the biotic component of an ecosystem, and involving interactions between autotrophic and heterotrophic competition
define ecosystem
- An ecosystem is a natural unit consisting of all plants, animals, and micro-organisms (biotic factors), in an area functioning together with all the physical (abiotic) factors of the environment.
- In an ecosystem many species are interdependent upon each other.
- Eg. The rock type and climate will determine the type of community that will develop and the different species present will interact in areas such as energy flow, nutrient and gas exchange.
define succession
- Succession is a directional non-seasonal cumulative change in the types of species that occupy a given area through time.
- It involved the processes of colonisation, establishment and extinction, which act on the participating plant species.
what can seres be recognised by?
the collection of species that dominate at that point in the succession
In what conditions does primary succession occur in?
previously uncolonised, newly formed barren areas
Describe some environments where primary succession will occur
sand dune formation due to wind, lake being created by subsiding land, lava fields produced after a volcanic eruption
describe the stages of primary succession
- Pioneer plants arrive. These are often r-selected species demonstrating high levels of dispersal eg lichens.
- The colonisers modify the abiotic environment during subsequent cycles of growth, reproduction, death, and decomposition (eg. The development of humus)
- The altered abiotic environment can now support more advanced species, as it becomes less hostile, as the soil forms and the growth of plants provides shelter for other organisms. (small annual plants, perennial herbs, grasses)
- The process continues allowing larger plants to thrive, so the height and biomass of vegetation increases. (grasses, shrubs, shade-intolerant trees)
- The climax community is formed as the species present (k-selected) develop a stable environment. (shade-tolerant trees)
Name the typical trends that occur as succession develops
- Soil depth increases
- Soil fertility increases
- Species diversity increases (this may decrease at maturity)
- Plant biomass tends to increase
- Greater number of niches develop, and communities become increasingly complex
- Increase in the number of food chains and webs.
Define biotic climax
a community where the stable end stage of succession is maintained by biotic factors
(eg. Shading trees, trampling by heavy animals)
define climax community
the stable end stage of succession which is in equilibrium with the environment.
Describe typical climax communities in Britain
In most of lowland Britain, this is mixed broadleaf deciduous forests, dominated by eg. oak. In harsh upland areas, this can be moorland.
define climatic climax
the culminating stage of plant succession for a given environment, the vegetation has reached a highly stable condition as long as the environment remains unchanged.
Describe why sand dunes are an example of primary succession
Dunes inland are mature dunes at a climax community - covered in various species of well-developed plants
Dunes closer to the sea are still forming and most exposed to extreme environmental conditions - reduced diversity with only well-adapted species such as marram grass being able to survive.
Describe the conditions required fro secondary succession to occur
following the destruction of an existing ecosystem (eg gorse fire)
Describe why secondary succession tends to be much more rapid than primary succession
- Development of the community doesn’t begin with lichen, and tends to be much more rapid as there is already soil present to support larger species of plant
- Some plant species, their roots, or their seeds can remain in the soil and regenerate/germinate when abiotic conditions become favourable again
- Many soil organisms eg nitrifying bacteria, or detritivores are also usually present in the soil, so the climax community is reached in a faster time frame
what is a haemocytometer?
a specifically designed microscope slide that allows cells to be counted.
How can a haemocytometer be useful for counting populations of cells?
- The glass of the haemocytometer has been accurately etched to create a number of grid lines. As well as this, the level of the etched glass is slightly lower than the cover slip (usually 0.1mm).
- Due to the fact that we know the dimensions of the grid and the depth of the liquid (growth medium containing a cellular population), we can calculate the volume of liquid held in the squares, and so work out the number of cells per unit volume
What is the depth of a haemocytometer?
0.1mm
What is the area, volume and conversion factor to 1mm^3 of a type-A square in a haemocytometer?
area = 1mm2
volume = 0.1mm3
conversion factor to 1mm3 = 10
What is the area, volume and conversion factor to 1mm^3 of a type-B square in a haemocytometer?
area = 0.4mm2
volume = 0.004mm3
conversion factor to 1mm3 = 250
What is the area, volume and conversion factor to 1mm^3 of a type-C square in a haemocytometer?
area = 0.0025mm2
volume = 0.00025mm3
conversion factor to 1mm3 = 4000
Describe the practical technique in using a haemocytometer
- Clean both the haemocytometer and the cover slip with lens tissue and alcohol before use (don’t use normal tissue as it can scratch the slide)
- Place the cover slip on top of the haemocytometer before loading
- Ensure any samples used are well mixed to ensure counts of cells are representative (swirl flask)
- Place the tip of the pipette in one of the grooves of the haemocytometer and carefully add a small sample
- Don’t ‘overload’ the haemocytometer - can cause the cover slip to float above the normal level and so cause the liquid to have a greater depth.
- Take care when focusing the microscope - focus by racking down not up.
- Pick an appropriate square type to count - there should be 10-20 cells in the square type chosen
- If even the type C square has too many cells to count, you may need to complete serial dilutions (1 part cell suspension to 9 parts isotonic buffer) and then multiply any raw count to the dilution factor
- Only count cells that are inside the grid and toughing the top and left sides (the north west rule)
Describe the procedure for a mark release recapture experiment
- A large sample of the species under study is captured using an appropriate technique (eg. Pitfall trap for insects, sweep net for species in long vegetation, humane traps for small animals).
- Each individual in the first sample is marked in an appropriate way (must not make organism more obvious to predators, must be permanent and non toxic), the number in the first sample is recorded.
- Marked individuals are released back into their original habitat
- After appropriate time (dependent on size and life cycle of the species studies) to allow the marked organisms to mix well with the rest of the population, a second sample is captured of similar size to the first sample
- Total number in second sample and the number of marked individuals are recorded
- Lincoln index is applied to estimate population size
what is the lincoln index?
population size = (number collected in first capture x number collected in second capture) / number of marked in second capture
what are the assumptions made in the mark release recapture method?
- No emigration or immigration (closed population) - can be avoided by sampling in environments where there are clear boundaries to the species habitat
- No significant changes to the population size due to births or deaths
- Trapping process don’t affect the animal in any way - should allow the organism to be trapped again in the same way (ie don’t traumatised organisms as they will avoid the area/traps in the future)
- Sufficient time is allowed for mixing the first sample back into the general population of the species.
Give the practical technique for counting the size of a population of yeast cells using a haemocytometer
- Obtain 3 flasks of different sizes (50cm3, 150cm3 and 500cm3)
- Pour 50ml of apple juice into each flask and add one drop of yeast suspension into each
- Cover each flask with muslin to keep out the dust while allowing for free passage of air
- Leave the flasks in a warm place (eg. Incubator set between 25-30ºC for 3-4 days)
- Thoroughly clean haemocytometer slide and cover slip with lens paper and ethanol
- Thoroughly mix the contents of each flask by swirling the flask gently before removing a sample
- Load the haemocytometer slide using a separate pipette for each of the three flasks to prevent cross contamination
- Estimate the concentration of cells using a haemoytometer
- If cell density is too difficult to count use a dilution of 1cm3 yeast to 9cm3 buffer
what is a streak plate?
A streak plate involves the progressive dilution of an inoculum of bacteria or yeast over the surface of solidified agar medium in a petri dish. The result is that some of the colonies on the plate grow well separated from each other.
What is the purpose of creating a streak plate?
- This technique is used to check the purity of cultures that are being maintained over a long period of time - regular sampling and streaking will show any contamination by other microbes.
- It can also be used by expert practitioners to start new maintained cultures by picking off an appropriate isolated colony of an identifiable species with a sterile loop and growing the cells in a sterile nutrient broth.
Describe the method of creating a streak plate
- Loosen the cap of the bottle containing the inoculum
- Hold an inoculation loop in your right hand
- Flame the loop and allow it to cool
- Lift the bottle containing the inoculum with your left hand
- Remove the cap of the bottle with the little finger of your right hand
- Flame the neck of the bottle
- Insert the loop into the culture broth and withdraw. At all times hold the loop as still as possible
- Flame the neck of the bottle again
- Replace the cap of the bottle using the little finger of your right hand. Place the bottle on the bench
- Partially lift the lid of the petri dish containing the solid medium
- Hold the charged loop parallel to the surface of the agar. Smear the inoculum backwards and forwards across a small area of the medium
- Remove the loop and close the petri dish
- Flame the loop again and allow it to cool
- Turn the disk through 90º anticlockwise
- With the cooled loop, streak the plate from area A across the surface of the agar in three or four parallel lines (area B). Make sure that a small amount of the culture is carried over
- Remove the loop and close the petri dish
- Flame the loop again and allow it too cool. Turn the disk 90º anticlockwise and streak the loop across the surface of the agar from B in three or four parallel lines (area C)
- Remove the loop and close the petri dish
- Flame the loop and allow to cool. Turn the disk 90º anticlockwise and streak the loop from C to the centre of the plate (D)
- Remove the loop and close the petri dish. Flame the loop again
- Tape the plate closed and incubate at 20-25ºC for 2-3 days in an inverted position
What is a pour plate?
- In a pour plate, a small amount of inoculum from a broth culture is added by pipette to the centre of a Petri dish
- Cooled, but still molten, agar medium in a test tube or bottle is poured into the petri dish.
- The dish is then rotated gently to ensure that the culture and medium are thoroughly mixed and the medium covers the plate evenly
describe what pour plates allow you to observe
- Pour plates allow microorganisms, to grow both on the surface and within the medium.
- Most of the colonies grow within the medium and are small in size and may be confluent.
- The few colonies that grow on the surface are of the same size and appearance of those on the streak plate
describe the method for preparation of a pour plate.
- Loosen the cap of the bottle containing the inoculum
- Remove the sterile Pasteur pipette from its container, attach the teat and hold in your right hand
- Lift the bottle containing the inoculum in your left hand
- Remove the cap with the little finger of your right hand
- Flame the neck of the bottle
- Squeeze the teat bulb of the pipette very slightly. Put the pipette into the bottle and draw up the required volume of the culture (one squeeze approx 0.5ml). Do not squeeze the teat bulb of the pipette after it is in the broth as this could cause air bubbles and possibly aerosols
- Remove the pipette and flame the neck of the bottle again. Replace the cap.
- Place the bottle on the bench
- Lift the lid of the petri dish slightly with your right hand and insert the pipette into the petri dish. Gently release the required volume of the inoculum onto the centre of the dish. Replace the lid.
- Put the pipette into a discard pot of disinfectant.
- Collect a bottle of sterile molten agar from the water bath
- Hold the bottle in your right hand. Remove the cap with the little finger of your left hand.
- Flame the neck of the bottle
- Lift the lid of the petri dish slightly with the left hand and for the sterile molten agar into the petri dish. Replace the lid.
- Flame the neck of the bottle and replace the cap.
- Move the dish gently to mix the culture and the medium thoroughly and to ensure that the medium covers the plate evenly. Rotate the dish until the medium and inoculum are well mixed and cover the base of the dish
- Allow the plate to solidify.
