BIOL 2060 Lecture Notes - Lecture 3: Herbivore, Carrying Capacity, Metapopulation

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6 Dec 2014
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October 6th, 2014
Effect of the Biotic and Abiotic Environment on the Abundance of Organisms
**How can we predict whether a population is going to increase in size, decrease in size, or stay
the same?
Population: a group of organisms of the same species occupying a particular space at a
particular time
There are only 4 parameters that determine what happens to the population size:
oPopulation size = + Natality + Immigration – Mortality – Emigration
All these parameters are affected differently by the age of the organism
oi.g. an older person is more likely to die than a younger one
How can we predict whether a CLOSED population is going to increase in size, decrease in size,
or stay the same?
Closed means that there is no immigration and emigration
Open means that there is immigration and emigration
In real life, this does not happen – only works in theory
Predicting whether a population is going to increase in size, decrease in size, or stay the
same has important real-world applications
oLets us know the best way to manage endangered species
i.g. loggerhead turtle (near extinction)
i.g. trillium (this plant is declining – why is it declining?)
1. Life Histories
Introduction to Life Histories
The life history of a species is determined by the collection of age or stage specific traits
that directly affect an individual’s reproductive success
oMeasuring the number of offspring an organism produced
Question: Which of the following is NOT an example of a life history trait?
a. The age at which an individual reproduces for the first time
b. The number of offspring that an individual produces
c. The size of offspring that an individual produces
d. The number of times that an individual reproduces in its lifetime
e. All of the above are life history traits
a. Collection of traits that collectively affect an individual’s reproductive success
b. Reproducing earlier has a big effect on population growth
c. Size also matters because that affects the probability that an offspring will survive
Important life history traits:
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oNumber of reproductive events per lifetime (one, few, many)
i.g. salmon only reproduce once, then die
oNumber of offspring per reproductive event (“clutch size”)
oAge at first reproduction
Some species reproduce at 1 year of age, others wait (humans)
oRelative length of life history stages (egg or seed, juvenile, adult)
How long you remain a kid, an adult, etc.
oInvestment per offspring (many small versus few large offspring)
oThe probability of dying at different life history stages
In some organisms, very few organisms survive to adult hood, so few
survive to reproductive stage – therefore few are able to reproduce
In others, organisms have high survival rates to adulthood (humans)
This collection of life history traits are sometimes referred to as “life history strategies
oExample: Kenyan giant Lobelias
Tall plants and very abundant, so their life history strategy is good
oExample: Lobelia telekii
Lives for many years
Flowers once, then dies
Produces a large inflorescence (collection of flowers that it makes at one
time) and many seeds per flower
Higher adult mortality
oExample: Lobelia deckenii
Lives for many years
Flowers repeatedly before dying
Produces a small inflorescence and fewer seeds per flower
Lower adult mortality
oBoth life history strategies are successful since both species persist; two solutions
to be successful
Question: Why doesn’t Lobelia deckenii produce as large an inflorescence as Lobelia telekii?
a. There is a trade-off between producing a large inflorescence and surviving to flower gain
a. The species that lives more many years, it would be great if it could make more
flowers, but because resources are limiting, there’s going to be a trade-off
between making a bigger inflorescence and doing something else. In this case, the
species reproduces repeatedly, so if it had a bigger inflorescence, it would use up
more resources and would die.
b. There is a trade-off between producing a large inflorescence and producing many seeds
per flower
a. Could be a possible trade-off, however not for these two species
b. No evidence that this trade-off is affecting the reproductive gain (as one goes up,
the other goes up too)
c. Both A and B
d. None of the above
Trade-offs constrain life history strategies
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oOffspring size and number
If organism has few resources, there’s going to be a trade-off between
producing few big offspring or many small offspring
i.g. humans follow the fewer, big offspring strategy
oCurrent and future reproduction
**All organisms operate within constraints!!
How Does Life History Strategy Affect the Rate of Population Growth?
We use life tables to predict this
2. Life Tables Part I – Survivorship
Life tables can be used to summarize life history traits
Two types of life tables
o1. Cohort Life Table
Follow a cohort of individuals from birth to death
Have to wait until the last individual dies
o2. Static Life Table
Take a cross-section of a population
Easier to collect data, but have to assume that birth and death rates are not
changing over time
Example: A Cohort Life Table for the Sparrows of Mandarte Island
Non-migratory sparrows (live on islands)
oNo immigration or emigration – closed population
oOnly thing that determines population growth is birth and death rates
In nature, there are no corpses
Age in years (x) Observed # of birds alive (nx) Proportion surviving at start
of age interval (lx)
0 115 1.0
1 25 0.217
2 19 0.165
3 12 0.104
4 2 0.017
5 1 0.009
6 0 0.0
oSo if the individual is not there anymore, you are not sure if it died or if it
immigrated
oReason why closed populations are easier to study than open populations
Survivorship (lx): proportion of organisms surviving from the start of the life table to age
x (lx) = nx/no
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