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.