population dynamics Flashcards
what are the four patterns of population growth which can be experiences at different points in time
1) populations are dynamic entities
2) individuals can move from one population to another
3) population size can change from one time period to another
4) populations show a wide range of growth patterns
what is exponential growth
when a population increases by a constant proportion at each point in time
when can exponential growth occur
- when conditions are favourable
- when a species reaches a new geographic area
what could occur if conditions are favourable in a new area
the population may grow exponetially until density dependant factors regulate numbers
what is logistic growth
when some populations reach a stable size that changes little over time, such populations increase in size by a small amount but then fluctuate around the carrying capacity
what does K represent in the logistic equation
the population size for which birth and death rates are equal
K represents carrying capacity
what is K assumed to be in the logistic equation
assumed to be constant
births and deaths are the same over time at any given density
what is wrong with the logistic equation assuming that K remains constant
births and deaths do vary over time so K is meant to fluctuate
K should be the density where birth and death rates intersect
what are population cycles
where some populations have alternating periods of high and low abundance at regular intervals, such seen in predation
give an example of delayed density dependance
if predator pop is small to begin the prey pop will increase and as a result predator levels increase but with a time lag and vise versa
how does the risk of extinction vary depending on population size
increases greatly in smaller populations
give an example of how smaller populations are more vulnerable to extinction than larger ones
- studies of bird populations in channel islands showed 39% of pops with fewer than 10 breeding pairs went extinct
vs - no extinctions in populations with over 1000 breeding pairs
jones and diamond 1979
what is genetic drift
where chance events influence which alleles are passed onto the next generation which causes allele frequencies to change at random from one gen to the next in small populations
why are smaller populations more vulnerable to genetic drift
1) loss of genetic variability reduces response to future changes
2) can cause harmful alleles to occur at high frequencies
3) small pops show high frequency of inbreeding
what can inbreeding result in
increases the frequency of homozygotes including those that have two copies of harmful alleles which reproduce reproductive success
define demographic stochasticity
the chance events related to the survival and reproduction of individuals
give an example of demographic stochasticity
in a pop of 10 individuals, a storm wipes out 6
the 40% survival rate may be lower than the rate predicted on average of that species therefore increasing chances of pop extinctin
how does demographic stochasticity affect extinction
populatin-level birth and death rates are constant within a given year but actual fates of individuals differ due to probabilty causing pop to drop
what are allele affects
when pop growth rate decreases as pop density decrease as individuals have difficulty finding mates at low pop densities
what is environmental stochasticity
unpredictable changes in the enviro
how is environmental stochasticity involved in extinction
changes in birth or death rates that occur from year to year beacuse of random changes in enviro conditions
what is meant by a metapopulatin structure
sets of spatially isolated populations are linked by dispersal and characterised by repeated extinctions and colonisations
what assumptions were made by levins when using the equation describing extinxtin and colonisatin of habitat patches
1) there are an infinite number of identical habitat patches
2) all patches have and equal chance of being colonised
3) all patches have an equal chance of extinction
4) once colonised its pop increases to carrying capacity more rapidly that rates of colonistino or extinction
how do real metapopulatins violate levins model’s assumptions
patches vary in size and ease of colonisation so extinction and colonisation rates can vary
the rates can be influences by non-random enviro factors
what did pulliam 1988 say about megapopulations which opposed levins model
in reality a mosaic of sources and sink habitats characterise metapopulatins
