Download Predation---problems of predators04a

Predation: Life is tougher than it looks. The problems of consumers
I. Stages of predation/consumption
A. The act of foraging
1. Amount of time spent
2. Behavioural modes
a. Sit-and-wait (ambush)
b. Cruising
(1) Speed
(2) Amount of turning
c. Saltatory
(1) Speed between stops
(2) Criteria for stopping
(3) Time spent in a patch after a stop
B. Prey detection
C. Attack given detection
D. Capture given attack
E. Handling after capture
1. Time and energy required]
2. Probability of escape during handling
F. Consumption and net energy gain
G. Refractory period
II. Problems of predators:
A. Prey distribution in time/space is almost always clumped rather than uniform,
therefore one must locate clumps
1. Prey clumps naturally accumulate in places where they are least likely to be
detected.
2. The average detectability of prey goes down in proportion to the amount of
predation on it, even if the prey do not respond behaviorally to the presence of
predators. This effect is in addition to the effects of reduced density.
3. Behavioral responses in prey result in even more marked clumping in time or
space.
a. Shifting activity to times/places where detectability is lower.
b. Reducing the overall level of activity
4. Some plants may manipulate their attractiveness to herbivores that make the
herbivore environment less comfortable and more dangerous, while minimizing
damage to themselves.
a. Producing tender shoots all at once swamps consumption capacity of
predators
b. Clumping susceptible tissues in space produces clumps of herbivores that:
(1) Intensify behavioral interference between herbivores
(2) Attracts predators to herbivore clumps, reducing herbivore survivorship.
B. Anti-detection adaptations
1. Transparency
2. Minimizing movement or limiting activity to times of day where detectability is
low.
C.
D.
E.
F.
III.
A.
B.
3. Concealment within a matrix (e.g. soil, leaves, trees, water)
4. Camouflage (colouration, disruptive colouration)
5. Small size
Anti-capture adaptations
1. Evasive maneuvers or reduced detectability after first attack.
2. Shoaling/flocking
3. Vigilance, cooperative vigilance
4. Warning colouration (aposematic)
Anti consumption adaptations
1. Distastefulness
2. Low nutritional quality
a. Low nutrient concentrations
b. Presence of toxins
c. Indigestibility
(1) e.g. presence of tanins in plants
(2) cellulose, chitin, in plants
d. High handling costs or low net benefit (tooth wear from silica in plants;
shells in nuts, too small or too large prey.
Responses to these adaptations by predators mean that
a. Only a small subset of potential prey tissue is suitable prey
b. The available subset may be exposed to predation only briefly at unfavorable
times of day.
c. We may perceive prey as much more abundant than predator does.
Risks associated with foraging may limit effort
1. Depends on consumer’s own vulnerability to predators and
2. Density of predators.
Conclusions/Summary
The amount of predation that takes place in any situation is determined by
1. The perceptual and locomotory, and strategic capabilities of the predator.
2. The perceptual, locomotory, and strategic capabilities of the prey.
3. The spatial and temporal complexity of the foraging environment.
4. The amount of time and energy the predator is willing to devote to foraging, which
may be determined by:
a. The rewards of foraging per unit time.
b. The risks of foraging per unit time.
c. How (a) and (b) above vary in different local microhabitats and times of day.
d. The predator’s perception of its future prospects for being successful,
relative to those it perceives today.
As a result of the complexity of the interacting determinants:
1. Predators will often be selected to limit the times and habitats in which they
forage to those that are most profitable and least dangerous.
2. Community biomass will be dominated by
a. Prey with high resistance (unresistant prey species will be eliminated)
b. Distributed in habitats where it is least available to predators.
3. Prey will appear to suffer little predatory mortality and predators will appear to
have little major effect on biomass of prey.
4. The power of predator effects will not be revealed unless predators are removed
completely, in which case
a. Unresistant prey species will usually win in competition with resistant prey
species.
b. Species composition and spatial and temporal biomass distribution of the
prey community will be transformed.
5. The indirect effects of the threat of predation may have more influence on the
rate of population growth of a prey population than does the predator’s actual
consumption of prey.
a. Food of prey species in dangerous habitats will be unexploited and abundant
compared with that in habitats where prey are protected from predation.
b. Predators may be deplete available prey and be starving at the same time
that they prey are overcrowded and superabundant in habitats where they
are protected from prey.
c. Predators that never enounter one-another directly, e.g. day-active vs nightactive predators, may have powerful effects on one another by selecting for
prey whose behavioural defenses are effective during both day and night.