Population Ecology

Question 1

Why do we care about N

Answer 1

Natural resource management
* Size of fish stocks in the ocean
* Abundance of outbreaking insect pests in forests
* Conservation
* Population declines of species
* Health
* Monitoring populations of viruses or bacteria in
humans
* Understanding and predicting human
population growth
* Basic science question of what limits
population growth

Question 2

Malthus

Answer 2

arguing
that the human
population cannot
grow faster than
food production

Question 3

Paul Ehrlich

Answer 3

arguing that explosive growth in the human
population would have catastrophic social and
environmental consequences

Question 4

goal of most population models

Answer 4

Predict the trajectory of population growth
through time, i.e., N as a function of t
* How many individuals are in the population
now? N t
* (Time advances one step) t  t + 1
* How many individuals are in the population one
step later? N t + 1
* So, the general model is N t + 1 = f ( N t )
* Challenge: choosing simple but realistic
parameters for f

Question 5

what are the time steps?

Answer 5

When using differential equations, time steps are
infinitesimally small: use concept of limits and
calculus; growth is smooth; best suited for species
with continuous reproduction
* When using difference equations, time steps are
discrete units (days, years, etc.): use iterated
recursion equations; growth is stepwise and
bumpy; best suited for episodic reproduction
* Also called “continuous-time” and “discrete-time”
approaches
* Different organisms might be better fit by one or
the other

Question 6

λ

Answer 6

Factor by which a population changes over one time unit. Finite rate of increase. no species has ever maintained lambda > r or lambda < r

Question 7

continuous time

Answer 7

exponential growth, instantaneous, per capita rate of population change
dN/dt = rN

Question 8

r

Answer 8

intrinsic rate of increase

Question 9

When is geometric, when is it expongential

Answer 9

geometric - lambda > 1
exponential - r > 0

Question 10

lambda and r

Answer 10

growth rate (lambda or r) are constants that simply reflect the biology…

Question 11

Density dependent growth

Answer 11

Logistic model - logistic braking model - simplest form of density dependence

Question 12

sigmoid growth curve

Answer 12

N - population size
K - carrying capacity
r - intrinsic growth rate
t - time