Lecture 11 Populations Flashcards
Population
A group of interbreeding individuals found within a given area at a given time
Individual: unitary species
Zygote formed through sexual reproduction grows in to a genetically unique organism e.g. fish, humans and many insects known as ‘genets’
modular species
Asexual/vegetative reproduction
Zygote develops into a module that asexually produces further similar modules - a ‘genet’ produces a ‘ramet’ e.g. some plants, sponges, bryozoans and corals
Largest organism in the world
Aspen grove in Utah covering 0.43km squared each stem could be considered a ramet and therefore individual for purpose of defining a population
Describe a population : spacial distribution
Spacial distribution - location of individuals within a population stationary (sessile) vary (migratory)
Mixing through flow of individuals/gene flow may be uniform - equally distributed or random or aggregated (clumped in groups)
Describe a population: gene flow
Less frequent movement of individuals or reproduction between parts of a population defines local or sub-populations
Describe a population: metapopulations
Linked throughout/across landscape by immi/emmigration
Population density
No. Of individuals in area covered
Population structure
Not all individuals the same, vary in: spacial location/origin, age,sex or size
Can categorise individuals by age and no. In each category (age structured pop.) E.g. 0-1, 1-2, 2-3 etc.
Often more logical to categorise according to stage of development (stage structured pop.) Categories: juvenile, sub adult, adult or pre reproductive, reproductive, post reproductive
Life tables
Used to understand pop. Struct change over time
2 approaches
1) snapshot of info about individuals of diff ages at one time
2) follow survivorship of a cohort of individuals over time
E.g. Acorn barnacle native to Pacific, now invading intertidal zones in Europe and Argentina (adults sessile larvae free living plankton)
Survivorship/fecundity
Survivorship - no. of individuals surviving to an age out of total
Fecundity - no. of offspring per surviving individual of certain age
Survivorship curves
Plot survivorship over time to produce survivorship curve X axis age and y acid no. of survivors (log scale)
eg. Acorn barnacle have high no. Larvae, most die, those that survive settle on rocks and become sessile adults with high survivorship
Other species have high survivorship at early ages e.g. mountain sheep and humans
A third type of curve represents constant survivorship
Life history strategies
the way individuals within/among species allocate resources to growth, repro, and survival based on genetic and enviro factors
Limited resources result in tradeoffs
Can be r selected - usually pest/weed species : high ability to reproduce i.e. high fertility rate, small body size, early onset of maturity, short time of generation, and the capability to disperse off the progeny widely. Often reproduce once a large quantity of offspring. Periods of exponential growth followed by periodic/seasonal decline. Tolerate unpredictable conditions
Or k selected - reproductive strategies tend towards heavy investment in each offspring, are more common in long-lived organisms, with a longer period of maturation to adulthood, heavy parental care and nurturing, often a period of teaching the young, and with fierce protection of the babies by the parents. Reproduce more than once producing few offspring. Slowly rising pop that stabilises at carrying capacity. thrive in stable enviro e.g. humans
Population dynamics
Births & deaths, immigration and emmigration affect pop. dynamics
Pop change :
Nt+1 = Nt+Bt-Dt+It- Et
No. of individuals in pop at time t+1=
(Nt) No. If individuals at t +
(Bt) no. Of births between t and t+1 -
(Dt) no. Of deaths between t and t+1 +
(It) no. Of immigrants between t and t+1-
(Et) no. Of emigrants
In a closed pop I=E=0 - no individuals leave or arrive
Why use pop. Dynamics?
To manage and conserve species, understand birth/death rate find out risk of extinction and estimate sustainable harvest rates e.g. timber trees and fishing
Modelling pop change
In closed system Delta N = B-D
Express B & D as per capita rates for pop ( N) at time (0)
B= bN0 = birth per capita
D= dN0 = death per capita
So B-D is the diff between birth and death rate aka the per capita birth rate (r)
Population growth rate
Change in pop written as
Delta N = rN0
When b>d r>0 pop increase
b<d r<0 pop decrease
b=d r=0 pop no change
r= per capita birth rate
Modelling populations: exponential growth model
All pop. Have potential for exponential growth if resources unlimited, no migration and no mortality
N(t) =N(0)e^rt
(r - per capita growth rate doesn’t change)
If r=0.5 in year 1 (t=1) then N1 will be 100xe^(1x0.5) = 1.65
Modelling populations: logistics growth
Most pop do not grow indefinitely growth slows until b~d and per capita growth rate r~0
dN/dt= r max (k-N/k)N
or dN/dt=r max(1-N/k)N
k= carrying capacity: no. Of individuals that any particular environment can sustain indefinitely
r max = pop growth rate when pop v. Small
Growth is fastest when pop size is k/2 (r is highest)
E.g. k=100 N=99 r max = 1
dN/dt= 1(1/100)99= 0.99
So r*99=0.99
r= 0.01 positive growth
Factors limiting population growth
K - carrying capacity, can be determined by density dependent factors
Limiting resources
Intraspecific competition e.g. for food
Predators may be attracted to prey dense areas so death rate rises
Pathogens - may spread more easily in dense pop leading to rise in death rate
Population summary
Can be defined by spatial distribution, no. And density of individuals and age structure
Life tables summarise demographic changes and survivorship over time
Survivorship curves link to life history strategies
Pop dynamics can be modelled w/exponential and logistic growth models (latter accounts for carrying capacity k)
Can be impacted by density dependent/independent factors
