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Predator-prey
By definition, are the following ‘predator-prey’ interactions?
o parasite and host – What are the differences between parasite -host
interactions and more ‘traditional predator – prey interactions?
o parasitoid and host
o plant and herbivore Is this more like parasite - host interactions or
more ‘traditional predator – prey interactions?
Co-evolution very intense in this interaction because improved adaptation of
one species will directly affect other, resulting in sophisticated
adaptations:
e.g. in prey
-visual adaptations such as countershading, cryptic coloration,
patterns, flash coloration
-chemical such as toxins, alarm pheromones in fish, chemical signals
from predators (length of spines increase in prey rotifer
Brachionus in the presence of fitrates from the predacous rotifer
Asplanchna).
-swamp predators such as timing of reproduction (e.g cicadids), living
in groups (passenger pigeons)
Models of Predation
Lotka and Volterra proposed independently:
dx1/ dt = r1x 1 - Px1x2
where
x1 = density (or population size) of the prey
P = coefficient of predation
x1x2 = probility of encounter between predator and prey
dx2/ dt =
P2x1x2 - d2x2
P2 = coefficient of expressing effectiveness of the predator
d2 = mortality rate of the predator
What do these equations assume about factors that affect growth rate
of prey (i.e. don’t they consider)?
What do these equations assume about factors that affect reproductive
rate of predators (i.e. don’t they consider)?
On outcome- Ossillation of predator and prey (when prey high and
predator low, predator increases.....)
Very simplistic- single species ignoring refuges, emigration....
Experiments in Predation
Gauss, using the ciliates Paramecium and Didinium, could not create a real
system that would mimic predator-prey cycles predicted by LotkaVolterra.
However, the system could be maintained either by occasional reintroduction of prey or by providing a refuge in form of bottom
sediment. In other words, open system required to maintain predatorprey.
How important is predation in ecological systems?
Example: Brooks and Dodson - size selective fish eliminate large-bodied
zooplankton species.
Predatory-Prey Coexistance (Why don't predators drive prey extinct?)
1. Spatial refuges (dispersal of predator completely restricted, no time
component necessary) Gause's ciliates (above) and 2 spp. of flour
beetles using 1 mm capillary tubing(Crombie, 1945).
2. Temporal-spatial refuges (dispersal of predator slowed by distance,
time component necessary). e.g. Huffaker (1958) used 2 spp of mites
(predator Typhlodromus occidentalis and prey Eotetranychus
sexmaculatus) on oranges and added barriers to dispersal (rubber balls,
paper, vasoline). In moderately complex environment (20 orange
surfaces and 20 foodless surface) predator drove prey extinct and
pred. went extinct. In complex environment (252-orange systen with
only 1/20 exposed, oscillations developed. Prey were able to colonize
oranges in a hop-skip-and-jump fashion and keep one step ahead
of e predator. Once a patch cleared of prey, predators moved on a
patches were recolonized. (Similar to fugitive species concept but
employees two spp. rather than 1 species and physical disturbance). An
interesting speculation made in this paper is that monocultural farming
practices would be more susceptible to pests.
Such a system may even be non-equilibrium. Non-equilibrium systems
have no mathematically stable points, predator existence always leads
to extinction of prey and itself on a small-scale. In contrast, models
like density-dependent growth (N greater effect as approaches K) or
Lotka-Volterra predation (predator and prey a function of one another)
have some sort of negative feedback. Coexistence can occur because
"for even very simple components in reasonably small numbers, high
levels of connectedness lead to astronomically long delays in
reaching equilibrium" (in this case large-scale extinction). e.g.:
100 light bulbs - on or off
For each bulb in a second, on->off is 0.5 (local extinction)
For each bulb in a second, off->on is 0.5 only if connected bulb on,
but 0.0 if connected bulb off (will not go extinct if migration
occurs)
Starting with all bulbs on:
If no bulbs are connected (no migration), all off in 1 s (global
extinction)
If all bulbs are connected (migration to any patch possible, all off
in 1022 years (universe is 1010).
if each bulb connected to only 10 other bulbs, all off in 17 minutes.
Caswell (1978) develops a such a model for predator and 3 prey
species that compete - no extinct after 1000 generations
(extinction of 1 sp in 80 without predator).
Bottom line: systems that are connected to other systems can
maintain proccess longer even though there are no processes that
should lead to persistence. Perhaps by conceptualizing systems as
closed (percieved to be easier) ecologists spend to much time
developing mathematical situations where equilibriums are
necessary.
3. Multiple prey species (again, by simplifying the system, the
problem becomes harder).
Key to understanding: Predators do not always respond to prey with
same intensity
Prey taken is a function of prey density
Numerical response - easy to understand. More prey mean greater
energy available for predator reproduction. More predators to take
more prey as modelled by Lotka-Volterra. Response is long-term
and a function of predator generation time. At some point, levels
off because some other factor limits predator population.
Functional response - the number of prey taken per existing predator
(i.e. number of predators do not change) increase. At some point,
levels off because individuals become satiated.
Type I - linear - simple a function of increased rates of
encounter due to higher prey densities.
(Type II - rapid inc. and slower inc. as satiation approched -prey handling time limits rate of consumption when prey
numbers get higher
-predators attract one another and interfer with each
others feeding)
Type III - slow inc. at low prey, rapid a some point. Text book
refers to threshold of security, search image, and prey
switching seperately. All inter-related. At low density of
prey, prey ignored (either searching delayed or another
species sought), at some density prey sought more intently
(search image developed, other search images turned off),
resulting in switch in prey species. Switching point will
depend not purely on relative densities, but also relative
palatability, catch effort, etc. This is the process that can
explain coexistence under multiple prey species system. No
prey species is eliminated completely because predators can
afford to ignore prey at low density and turn to other prey
species of higher density (without other prey species,
predators would be forced to feed on prey until eliminated).
The "refuge" is not spatial, but purely temporal and related
to density.
Will be important when we consider the interaction of
competition and predation.
Question on when to switch can also be extended to other
questions such as when to move on, etc. Can this be
predicted from theory:
Optimal Foraging
Example in text of Robin foraging. How much time should a predator
devote to an area? to a prey type of a certain nutritional quality and
escape ability? Natural selection should result in a strategy that
maximizes energy gain.
Two components:
Optimal diet - type of prey both in terms size and quality (often
indicated by palatability). If items taken out of proportion to
their availability, then optimal food suggested. However,
availability difficult to measure in real environment (frequency of
occurrence will not equal availability if less accessible...). Text
example of sunfish feeding on Daphnia. Will feed on any size they
can get when prey density is low, but selective for large individuals
when prey density is high.
Optimal foraging efficiencies - prey density; predator should
concentrate activity in patches of highest density and leave those
patches once profitability falls below the average of the entire
area. Again, easy to demonstrate in lab, but in field several
condition may complicate (e.g. distance between patches could
result in significant energy costs or risk of predation, food quality
and size may differ between patches...)
Other types of "predation"
Cannibalism - usually occassional (often in otherwise herbivous species)
and associated with harsh conditions
Actually interference intraspecific competition with the added benefit of
added nutrition. So if such behavior eliminates competitors and
delivers high-quality prey, why isn't cannibalism more common?
1) According to text, probably is. ("moral significance attached")
2) Difficulty in recognizing kin and future mates
3) Size and escape ability too closely matched between would-be
predator and prey (when it does occur, often occurs on more
vunerable individuals such as young or runts)
Herbivore-Plant
Poorer quality food relative to carnivore, so choice based on nutrition
often more important then availability. Choice both between plant
species and within a plant (young leaves tend to be more nutritious).
Often more like parasitism in terms of partial consumption of prey
(plant). In such cases mortality is not outright, though plant may be
weaken. However, moderate grazing can stimulate biomass production,
especially in grasses where new tissue from the meristem is close to
the ground and not grazed. Fruit is adaptation of a plant to encourage
grazing for seed dispersal (predation grading into mutualism)
Prey (plant) defense can also be quite sophisticated
-Trees can transfer chemical deterents to damaged leaves (one
insect predator counters this by ringing an area to prevent
transfer)
-Individual trees differed in defensive chemicals and the scale
insect pest were adapted to attack individual trees (one tree
was maladaptive for colonization of another tree).
Intrapopulation diversity of trees prevented the pest species
for evolving consistently effective methods of attack (Edmunds
and Alstad 1978 from Connell 1980)
Actual act of heribivory and predation relatively rare events. How can
these be studied.
Example of not only addressing this problem but also studying underwater
systems:
Saturation diving (most aquatic biology not so romantic) NOAA
-allowed study below surf zone minizing physical disturbance
-Saturation diving eliminates N problem (explain). 1 Atm/32 ft. Mixed
air required deeper.
-Lab set up and saftey.
-decompression - higher pressure must be slower brought down to
surface pressure.
Ways to looking at herbivory
1) Photograph
-deits of sea urchins (Tripneustes) and conch: Are foods taken in
same proportion as encountered or are diets mixed based on
nutritional content. Several spp. of algae and seagrass
(research suggest limited consumption due to toxins such as
sulphated phenolic acid)
2) Acuostic sampling (eliminates enclosure problem)
3) Damage
-Small portion of blades removed indicating no disease, physical, or
large herbiv.
-2% of tissue consumed
-Distribution of damage random
Indicates that seagrass blades in open areas are not easily
grazed, resulting from some combination of feeding
interference by predators or chemical deterents in
seagrass.
Parasitism - prey not killed out right, usually smaller. Perhaps more
important than most ecologist are willing to recognize (has traditionally
been a seperate field). However, recognize that parasites are common
though often go unnoticed due to their small size (two spp. of forehead
mites in H. sapien)
Mutualism
may also be more common than previously believed. More subtle than
predation, more possible mechanisms than competition. May be faculative
or obligatory, direct or indirect. "Evidence suggests that mutualism is
more a reciprocal exploitation than a cooperative effort."
Bottom line: Lots of work still to be done in ecology as parasitism and
mutualism ingregrated into community dynamics. In addition, lots of
possible interactions (even between a single species pair) community
dynamics are complex. Are there generalizations to be made?