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Lecture 18

BIO120H1 Lecture Notes - Lecture 18: Density Dependence, Population Ecology, Asymptote

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Spencer Barrett

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Slide 4:
Population ecology is best at counting individuals, animals. With plants there is ambiguity. Larkspur are
individuals because they are physiologically and genetically distinct. Aspen is only 1 genetic individual,
but many physiological individuals because its roots shoot up trees randomly. Dandelions are
physiologically separate individuals but genetically the same. Population ecology is hard because its hard
to count up all the number of species. This makes the field mostly dominated by theory, and by
mathematical concepts. Density independent models are viewing how populations might grow if there is
no density, if there is no difficulty in finding mates. Density dependent is where population size depends
on the density, slowing down growth as models get bigger because of crowding nd running out of
Slide 6:
We want to prduce a graph that shows how populations change thru time.
Slide 10:
Lada is he iths eeed deaths duig a iteal. If lada is less tha 1, it’s a deeasig
population. If ts eual to oe, it’s a ostat populatio.
Slide 11:
R is a growthrate parameter. If r is positive, pop is growing. If r in negative, pop is decreasing. dN/dt is
the rate of the population that is growing/decresing. dN/dt is the slope of the curve at any moment.
Slide 15:
The method changes by the different organisms you are testing. Geometric models can be used for
tropical bird populations because they are in a seasonless rainforest. Continuous functions can be used
for humans or bacteria.
Slide 16:
Growth curves are showing if births axceed deathd by a certain amounts, and asking if they are a good
odel. It is possile that ith ates a eeed deaths, ut this odel does’t gie a stale old hee
populations are stable through time. Parabolic graphs grow faster and faster and faster.
Slide 19:
This means they are bad models. Although all species can grow exponentially, no species has ever
exploded to infinity. Something must keep lambda around the range of 1, that remains everything in
Slide 20:
One thing that regulates lambda is density dependent regulation where the growth rate of the
population is changed depending on how big it is. Another factor is density independent reduction, ex
an early autmn bitter winter storm can kill the ids that hae’t igated south et.
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