Community Ecology (Who's who and who's not)
Predation (Consumer-Resource Interactions)
Models of Predation
Lotka and Volterra proposed independently:
dt = r1N1
density (or population size) of the prey
= coefficient of predation
probility of encounter between predator and prey
dt = P2N1N2
density (or population size) of the predator
coefficient of expressing effectiveness of the predator
mortality rate of the predator
What do these equations assume about
that affect growth rate of prey (i.e. do they not consider)?
What do these equations assume about
factors that affect reproductive rate of predators (i.e.
models have also been developed.
Ossillation of predator and prey (when prey high and predator low,
modeling an open system or a closed system? Is this
equilibrium or non-equilibrium coexistence? What is the 'feedback'
that maintains predator-prey coexistence? Are cyclic population
changes observed in nature?
Experiments in Predation
using the ciliates Paramecium
and Didinium, could not create a real system that would
predator-prey cycles predicted by Lotka-Volterra.
However, the system could be maintained
by occasional re-introduction of prey or by providing a refuge in form
of bottom sediment. In other words, an open system
to maintain predator-prey.
How important is predation in
Effects on morphology of prey:
Brooks and Dodson - size selective fish
eliminate large-bodied zooplankton species.
Effects on evolution of predators and their
importance of predation is clearly demonstrated in the sophisticated
and prey adaptions. Co-evolution is very intense in these
because improved adaptation of one species will directly affect other,
resulting in adaptations that influence the ability of predators to
Examples of predator-prey adaptations
Chemical interactions such as toxic
responses of prey to substances released by predators.
'Swamping' predators including
the timing of reproduction and movement.
For example, how
might length of spines change in
the prey rotifer Brachionus in
the presence of fitrates from the
If competition is prevalent, then
predators drive prey extinct?
Based on the theory of natural
would predators evolve strategies to 'manage' prey populations in such
a way as to prevent their elimination?
- Spatial refuges
achieved in Gause's ciliates by adding sediment that acted as a
In another experimental system using flour
beetle, 1 mm capillary tubing provided refuge for prey resulting
in coexistence (Crombie, 1945).
- Temporal-spatial refuges
(1958) used 2 species of mites (predator Typhlodromus occidentalis
and prey Eotetranychus sexmaculatus) on oranges and added
to dispersal (rubber balls, paper, vasoline).
- In a moderately complex
surfaces) predator drove prey extinct and predators then went extinct.
- In a complex environment
with only 1/20 exposed of orange surface exposed, oscillations
were able to colonize oranges in a hop-skip-and-jump fashion and keep
step ahead of the predator.
models can produce
and coexistence over long periods of time.
(Non-equilibrium systems have no
stable points, e.g. predator existence always leads to extinction of
and itself on a small-scale. In contrast, models like the
predation model where predator and prey have negative feedbacks that
lead to coexistance)
is the 'feedback' that maintains predator-prey coexistence?
Coexistence in a non-equilibrium
occur because "for even very simple components in reasonably small
high levels of connectedness lead to astronomically long delays..."
- 100 light
bulbs - on or off
- For each bulb in a
turning from on to off is 0.5 (local extinction)
- For each bulb in a
turning from off to on is 0.5 only if a connected bulb is
on (will not go extinct if migration
occurs), but is 0.0 if connected
bulbs are off
(1978) develops a such a model
for predator and 3 prey species that compete - no species went extinct
even after 1000 generations.
Bottom line: systems that are
other systems can maintain proccess longer even though there are no
that should lead to persistence. Perhaps by conceptualizing systems as
closed (percieved to be easier) ecologists spend to much time
mathematical situations where equilibriums are necessary. What
are the implications of this for the study of landscape ecology?
- Multiple prey species
Key to understanding: Predators do
always respond to prey with same intensity
Prey taken is a function of prey
- Numerical response -
size increases (why?) as prey
- Functional response -
number of prey
taken per existing predator (i.e. number of predators do not change)
At some point, levels off because individuals become satiated.
Which of the above predator
to prey density could explain predator prey coexistence in a system
multiple prey species, and how?
What factors should determine when
prey density) should a preadator switch?
What happens if one species of prey is removed from a system where
predator switchs to different prey depending on their denisity?
Other types of "predation"
definition, are the following
- Cannibalism - is not common
is often associated with harsh conditions.
could also be considered as interference
intraspecific competition with the added benefit of added nutrition. So
if such a behavior eliminates competitors and delivers high-quality
why isn't cannibalism more
common in nature?
- parasite and host: What
the differences between parasite -
interactions and 'typical' predator - prey interactions?
How is the "habitat" of parasites different than that of a 'typical' predator and what then are the
challenges for a parasite?
Some generalizations concerning
- Highly sophisticated despite
tendency to think
of parasites as degenerate. In fact, parasites display some of the most
and specialized adaptation. Life cycles are often complex
several vectors that may include reservoir and intermediate hosts
- Highly specific and important
to ecology of hosts "An average species has 16 parasites at home,
only about three of those make it over. After it arrives, it picks
another four, but on average, it has less,"
- Overlooked in many fields of
biology and medicine,
but are extremely diverse (may outnumber the number of free-living
4 to 1).
- Plant and herbivore: Is
more like parasite - host interactions or 'typical' predator -
nutritional value is often more important in herbivore food
relative to choice based on availability. Why?
between plant species and
a plant (young leaves tend to be more nutritious).
herbivore-plant interactions always
the general definition of predator-prey interaction?
Moderate grazing can stimulate
production, especially in grasses where new tissue from the meristem is
close to the ground and not grazed.
Why is fruit so tasty, nutritious, and
Prey (plant) defense
can also be quite sophisticated:
- Trees can transfer chemical
leaves (one insect predator counters this by ringing an area to prevent
- Individual trees of the same
in defensive chemicals making it difficult for scale insect pest to
consistently effective methods of attack (Edmunds and Alstad 1978 from
act of heribivory and predation
be relatively rare events. How can these be studied? (And how do we
organism that live in environments that are less hospitable to humans?)
Bottom line on species
interactions: There are lots of possible interactions
multiple species with a community and in most communities there are
Given this complexity, are there generalizations that can be made in
study of whole communities?