Population Ecology

1
Population Ecology   I. Attributes II.Distribution
III. Population Growth changes in size through
time IV. Species Interactions V. Dynamics of
Consumer-Resource Interactions
   
 
2
Population Ecology   I. Attributes II.Distribution
III. Population Growth changes in size through
time IV. Species Interactions V. Dynamics of
Consumer-Resource Interactions A. Consumers
(predators, parasites, etc.) can limit
populations 1. Importance
   
 
Predators
Top down
Target Species
Bottom up
Limiting Resources
3
The effects of losing a predator or gaining a
predator can be dramatic
Brown tree snakes on Guam
Accidently introduced between 1946-1953, probably
as a stow-a-way on a ship. Reached peak
densities of 100/ha (ha hectare 100 m x
100 m. Basically, an average of one snake in
every 10 m x 10 m square plot, across the entire
island!!
4
9 bird species have been driven to local or
global extinction on Guam, including the Guam
Flycatcher (picture). Also, three fruit bat
species have been decimated, affecting the plants
they pollinate. This caused a reduction in
vegetational diversity compared to neighboring
islands that lack the snake. Those are some
dramatic top-down effects on the whole
community.
5
Remember the evolutionary dynamics of
predatory-prey relationships, too The Red Queen
and arms race analogies.
These are particularly obvious in pathogen-host
relationships Evolve fast and EVADE the immune
system Influenza Although humans are the
principal reservoir for influenza A virus, it
also infects birds, pigs, and horses. So, even
though we might wipe it out In humans in a
given year, it mutates in other Species and
reinfects us each year with a new strain. Typical
fatality rates are about 0.1.
6
Remember the evolutionary dynamics of
predatory-prey relationships, too The Red Queen
and arms race analogies.
These are particularly obvious in pathogen-host
relationships Evolve fast, EVADE AND DESTROY
the immune system HIV By attacking
T-lymphocytes, the HIV virus destroys a Primary
element of the immune system, leaving the Host
exposed to reproducing HIV virus as well as
other Pathogens that the host would normally be
able to Defend themselves against.
7
Remember the evolutionary dynamics of
predatory-prey relationships, too The Red Queen
and arms race analogies.
These are particularly obvious in pathogen-host
relationships Colonize a NEW HOST SPECIES with
NO immunity COVID-19 There is some linkage to
a market in Huwan, China, where bush meat and
animals were sold. Similar viruses (SARS and
MERS) have closely related species in bats.
Fatality rate is about 2. Ebola and Marburg
Viruses Marburg was from bats ebola was from
bats or other primates. Marburg can be
transmitted as an aerosol or by contact ebola
only by contact. Fatality rate of ebola can
reach 90.
8
Population Ecology   I. Attributes II.Distribution
III. Population Growth changes in size through
time IV. Species Interactions V. Dynamics of
Consumer-Resource Interactions A. Consumers
(predators, parasites, etc.) can limit
populations 1. Importance 2. Examples
   
 
Sea Otter Hunting Populations were overhunted,
Dropping by over 95
Otter pelt sales in London, in thousands
9
Population Ecology   I. Attributes II.Distribution
III. Population Growth changes in size through
time IV. Species Interactions V. Dynamics of
Consumer-Resource Interactions A. Consumers
(predators, parasites, etc.) can limit
populations 1. Importance 2. Examples
   
 
Sea Otter Hunting Otters eat sea
urchins Without top-down regulation, sea
urchin populations exploded.
10
Urchins eat kelp and they grwzed down the kelp
forests off the coast of California, which are
important nursery areas for commercially
important fish species.
   
 
As urchin density increases, kelp beds retreat
(negative displacement) at a rate of meters/month
11
Population Ecology   I. Attributes II.Distribution
III. Population Growth changes in size through
time IV. Species Interactions V. Dynamics of
Consumer-Resource Interactions A. Consumers
(predators, parasites, etc.) can limit
populations 1. Importance 2.
Examples
Opuntia cactus was introduced into Australia for
living fenceposts quickly spread and took over
pastures
12
Cactoblastis catorum introduced from Argentina
95 reduction in 20 years.
13
Unfortunately, it has colonized Florida and is
spreading across southeastern U. S. Spread to
the Southwest and Mexico could put 80 species of
prickly pear cactus at risk. http//www-naweb.iaea
.org/nafa/ipc/public/ipc-cactoblastis-final05.pdf

14
V. Dynamics of Consumer-Resource Interactions A.
Consumers (predators, parasites, etc.) can limit
populations 1. Importance 2.
Examples
Cane toads are native to South and Central
America, and were introduced into Australia in
1935 to consume beetle pests of sugar cane. Now
number over 200 million, and the eat other
species and the poison they secrete kills snakes
and other predators that try to eat
them. https//www.newscientist.com/article/dn14221
-australian-crocs-hit-by-cane-toad-wave-of-death/
Of course, rats, cats, and dogs have been
introduced around the world, with similar effects
on native species.
Dead freshwater crocodile after eating toad
15
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples
   
 
16
A refuge where prey can hide and increase their
population (seasonal, here) Voles and Owls in
Sweden Damped Oscillations Decrease in snow
pack makes voles susceptible to owls all year,
reducing the lag and reducing the amplitude of
oscillations
Winter refuge for voles beneath the snow
17
Measles in London 1948-1968 (before vaccine).
18
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples 2. Reasons - time
lags in specialist predators
When there are lots of prey available, predator
populations will increase. But they cant
convert food into offspring instantaneously
there is a lag, so the peaks in predator
abundance will lag behind the peaks in prey
abundance. They are synchronized, but slightly
out of phase, with the red lynx peaks about a
year after the blue hare peaks.
19
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples 2. Reasons - time
lags in specialist predators - refuges for
prey can increase the lag
In Scandinavia, vole populations had a respite
from predation in winter, hiding beneath snow
pack, and their populations could rebound. Since
1980, with warmer, wetter winters, snow pack is
less, it melts earlier, and spring populations
are much lower due to owl predation.
20
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples 2. Reasons - time
lags in specialist predators - refuges for
prey can increase the lag - patterns in
resistance and susceptibility Remember
predators/pathogens are selected to maximize the
interaction, and they can drive their host
population to extinction. Of course, when this
happens, they go extinct, too. So, selection may
favor less virulent strains that dont kill the
host, especially if the host population is
dispersed and transmission probabilities are low.
We will deal with pathogens more in a minute.

21
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples 2. Reasons 3. Other
Patterns - generalist predators that consume
several prey species can feed on whatever is most
abundant while other prey recover thus, the
predator population can be fairly stable while
each prey species fluctuates.
Different species killed by mountain lions. Elk
(russet), Mule deer (light blue), and
white-tailed deer (grey) are about 55.
22
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern 1. Examples 2. Reasons 3. Other
Patterns - generalist predators that consume
several prey species can feed on whatever is most
abundant while other prey recover thus, the
predator population can be fairly stable while
each prey species fluctuates. - inefficient
predators only harvest the surplus and dont
affect growth rates much.
Efficient Predator kills 80 of prey encountered
Inefficient Predator kills 5 of prey encountered
23
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling Pathogens like
COVID-19
24
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling Pathogens like
COVID-19
Dependent on a. the rate of transmission
(b) b. The contact rate between infected and
susceptible hosts (I x S) c. The infection rate
(I x S)b
25
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling Pathogens like
COVID-19
Dependent on a. the rate of transmission
(b) b. The contact rate between infected and
susceptible hosts (I x S) c. The infection rate
number of new infections (I x S)b d. The
rate of recovery (g). This is the inverse of the
period of contagion. e. Recovery rate number
recovering (I)g So, the reproductive rate of
the disease is the ratio of the number of new
infections to the number of infected people
recovering Ro (I x S)b / (I)g
S(b/g)
26
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling Pathogens like
COVID-19
The reproductive ratio of the pathogen
S(b/g) so a pathogen population will grow (R gt
1, epidemic) if the rate of transmission is
high (infectious) recovery is slow (creating a
long period of contagion) susceptible
individuals are common. In this case, each
infected individual infects more than one new
host and the disease spreads.
27
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like
COVID-19
As a disease proceeds and the number of
susceptible individuals declines, R ? lt 1 and the
epidemic declines. In the simplest version of
this model (single generation), oscillations
dont occur because there is no production of new
susceptible people and immunity after recovery is
permanent. However, if you add the production
of susceptible newborns or susceptible
immigrants, or allow for vertical transmission
from parent to offspring, or add a latency
period, or allow for reinfection (and not
lifetime immunity), oscillations and cyclic
epidemic occur because the number of susceptible
people can increase driving R gt 1.
28
so a pathogen population will grow (R gt 1,
epidemic) if the rate of transmission is
high recovery is slow (creating a long period of
contagion) or susceptible individuals are
common. In this case, each infected individual
infects more than one new host and the disease
spreads.
Cold and Flu Transmission rate low -
34 Recovery slow, favoring spread (Viral
integrity is greatest under low humidity, and
particles remain small (dont take on water and
fall out of the air column). Cool temps reduce
ciliary beating in nasal and respiratory mucosa,
favoring viral reproduction. Both cause higher
transmission rates in winter.
Lowen, et al. 2007. http//www.plospathogens.org/a
rticle/info3Adoi2F10.13712Fjournal.ppat.0030151
29
so a pathogen population will grow (R gt 1,
epidemic) if the rate of transmission is
high recovery is slow (creating a long period of
contagion) or susceptible individuals are
common. In this case, each infected individual
infects more than one new host and the disease
spreads.
Ebola Transmission rate high 75 99
fatality rate short contagious period Deadly
pathogens need a high transmission rate to
persist, or a very dense population (S).
30
so a pathogen population will grow (R gt 1,
epidemic) if the rate of transmission is
high recovery is slow (creating a long period of
contagion) or susceptible individuals are
common. In this case, each infected individual
infects more than one new host and the disease
spreads.
1918 Flu Epidemic (H1N1) Killed 50-100 million
people 3-5 of the global population
Transmission rate was high because of high
density of susceptible hosts (military), and
movement of carriers across Europe (military and
migrating refugees). High density selected for
more aggressive and lethal form
31
Reduce Transmission Rate - stay clean - stay
home Reduce Contagious Period - stay
healthy Reduce of Susceptible Hosts - get
vaccinated
32
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation
Hollings Functional Responses Type I
Constant predation rate Type II Reduced
efficiency at high prey densities due to handling
time limitations and satiation. Type III
reduced efficiency at high and low densities due
to handling time (high density) and search
image/learning issues at low density.
33
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
Consider this figure. It shows the birth and
death (predation) rates of a prey population in
response to its own density. So, there is density
dependence because the birth rate declines as
population size increases. There is also a type
III functional response, because the predation
rate is highest at intermediate densities. What
happens when the birth rate and the death rat are
equal???
Birth rate
Predation Rate
Prey Density (N)
34
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
Consider this figure. It shows the birth and
death (predation) rates of a prey population in
response to its own density. So, there is density
dependence because the birth rate declines as
population size increases. There is also a type
III functional response, because the predation
rate is highest at intermediate densities. What
happens when the birth rate and the death rat are
equal??? Right. EQUILIBRIUM in the Prey
population. This happens at three prey sizes!
Birth rate
Predation Rate
Prey Density (N)
35
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
BUT, these equilibria are not all the same two
are stable, and one is unstable. Consider the
first if the population decreases a little bit
to x, the birth gt death and it increases back
to the equilibrium. Likewise, if it increases to
y, the death gt birth and it drops back to the
equilibrium. Thus, this is a locally stable
equilibrium.
Birth rate
y
X
Predation Rate
Prey Density (N)
36
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
BUT, these equilibria are not all the same two
are stable, and one is unstable. Now, consider
the second equilibrium. Here, if the size drops
to y, death gt birth and the population drops
down to the first equilibrium density. Likewise,
if the population bounces up to x, Birth gt death
and it continues to increase to the third
equilibrium. So, the second equilibrium is
unstable. Any deviation from that exact
population size will cause it to either collapse
to the lower equilibrium or explode to the higher
equilibrium.
Birth rate
y
X
Predation Rate
Prey Density (N)
37
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
BUT, these equilibria are not all the same two
are stable, and one is unstable. The third
equilibrium is locally stable a slight decrease
in size to x means B gt D and the pop grows back
to the equilibrium, etc..
Birth rate
Predation Rate
y
X
Prey Density (N)
38
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States
BUT, these equilibria are not all the same two
are stable, and one is unstable. The third
equilibrium is locally stable a slight decrease
in size to x means B gt D and the pop grows back
to the equilibrium, etc.. SO, we can explain
eruptions or population explosions as a
consequence of a population exceeding the point
(middle equilibrium) at which it can be
controlled by a predator. If it can exceed that
density, it explodes to the third equilibrium
where it is near its carrying capacity and
limited more by its own population size.
Birth rate
Predation Rate
y
X
Prey Density (N)
39
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators
Top Predator
mesopredators
Mesopredators are in the middle (meso) of the
food chain. They are both PREY and COMPETITORS of
top predators. Prey are eaten by all the
predators.
prey
40
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators
Top Predator
mesopredators
The top predator may keep the mesopredators in
check, reducing their effects on the prey
species. If the top predaotr is removed and the
mesopredators increase, the prey may suffer MORE
predation than when the top predator was present.
prey
41
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators 4. Maximum Sustainable
Yield
We are predators, taking individuals out of other
populations. If we want to do this
sustainably, we might take out just the number
that the population is adding, based on its size
and growth rate.
42
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators 4. Maximum Sustainable
Yield
We are predators, taking individuals out of other
populations. If we want to do this
sustainably, we might take out just the number
that the population is adding, based on its size
and growth rate. At low and high densities, the
growth rate the slope of the curve - is low.
So, we cant take many individuals sustainably,
because the population isnt adding many (net).
43
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators 4. Maximum Sustainable
Yield
But at intermediate densities, the population is
growing rapidly, adding lots of individuals/time
interval. So, THIS is the density at which we
would have MAXIMUM SUSTAINABLE YIELD, being able
to take the MOST individuals from the population,
and having them replaced. However, this is at
HALF the Carrying Capacityso we are reducing the
population dramatically, to start, and this might
have important effects on the food web.
44
V. Dynamics of Consumer-Resource Interactions A.
Predators can limit the growth of prey
populations B. Oscillations are a Common
Pattern C. Modeling pathogens like COVID-19 D.
Complexities 1. Density Dependent
Predation 2. Alternate Stable States 3.
Mesopredators 4. Maximum Sustainable
Yield
Plus, we dont take them randomly. We use nets
or guns, and take the largest individuals. These
are the ones with the highest reproductive rates,
and thus we can have a negative effect on
subsequent population growth, and even act as a
selective pressure, selecting for small size and
earlier reproduction.