| Dept of Biology, Lewis and Clark College | Dr Kenneth Clifton
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Biology
141 Lecture Outline
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Population ecology continued: constraints on population growth and thinking about "open" vs. "closed" populations
I. Models of population growth
A. testing Wednesday's models by comparing their predictions with data.
1. In general, few populations in nature grow exponentially
a. Humans are one example
a "clock" of births and deaths,
this translates into exponential growth
rates of human population growth vary across the globe
what about non-human populations....?
b. populations that are temporarily in very resource-rich environments (short-term, localized)
These examples match the model of exponential growth
B. In contrast to the examples above, most populations have (or appear to have) fairly stable populations.
What has been left out of the models of exponential growth?
Put another way... what unrealistic assumptions did the models contain?
hint: the assumption that birth rates and death rates were constant.
why unrealistic? in different environments, or at different times in the same environment, the amount of resources/indl. can change.
in environments with high resources/indl., we might expect higher birth rates, lower death rates.
in environments with fewer resources/indl., we might expect lower birth rates, higher death rates (more intraspecific competition).
as a population grows, resources/indl. decline, affecting birth and death rates.
population will grow at rate r (r-max) when resources are abundant.
population may not grow at all when resources are scarce.
We can modify the model to make birth and death rates dependent on population density rather than constant.
density-dependence defined and illustrated.
the new model: dN/dt = rN [1-(N/K)]the new term, K, is the carrying capacity of the environment.
prediction of this new model in graphical form.
testing the new model by comparing it to real data
What about human populations?
Does this new model describe all populations?
some populations are regulated, not by competition, but by weather episodes or catastrophes: droughts, storms, fires, etc. The strength of these forces does not depend on population density, so density-independent population regulation.
Summary: what factors are important in determining a population's dynamics?
a. N, birth rate, death rate.
b. whether birth and death rates are constant or density-dependent.
c. K
d. importance of density-independent factors.
II. Estimating the size of real populations.
A. Sampling.
Counting everything
Counting within a known area and proscribed time to establish an estimate of density
Capture-Mark-Recapture (pg 250 in text)
Catch a subset of the population and mark them
Release them and allow for "mixing" of the population
Catch another sample of the population and see how many marked individuals are there
Estimate of the total populations size will be: (# marked originally x size of the second sample) / (# marked in the second sample)
Now: Think a bit more about what this means for open vs closed populations:
In a closed population, there is little or no exchange between populations.
In an open population, there is lots of exchange between populations.
For "closed populations", replenishment is often linked to local resource availability
This, in turn, may be influenced by population size.
When do you expect density dependence and when do you expect density independence in closed populations?
For "open populations", replenishment is related to patterns of production and dispersal.
What influences production?
What influences dispersal?
Limited supply may keep populations well below their carrying capacity.
The implications of "recruitment limitation"
For populations
For communities (more later)
How does longevity (survivorship) influence our thinking about recruitment limitation?
Implications for population persistence and thinking about local "extinctions"
Evidence from studies of coral reef fishes provide data on these topics.
Variation in "planktonic larval duration" (PLD) and biogeography
What else can otoliths tell us?
Bluehead wrasses along the coasts of St Croix.
