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Lecture

lecture note 22 for BGYB50

by

Department
Biological Sciences
Course Code
BIOC50H3
Professor
Herbert Kronzucker

Page:
of 5
LECTURE 22:
- Predation can be seen as an extreme form of competition, and the distinction between
competition and predation is often blurry
- Predation drives the flow of energy in ecosystems, and as such has far-reaching effects
in ecology and evolution
-The interactions between predators and their prey are among the most fascinating in all
of biology, with a bewildering array of adaptations in both: the interaction is so
intimate” and intense that the evolutionary refinement of predators and their prey is
often tightly linked in coevolution (the complementary evolution of closely associated
species: an evolutionary advance in the predator triggers an adaptive response in the prey;
the prey tends to be a “step” ahead: this is seen as a consequence of the Life Dinner
Principle, recognizing that only dinner is at stake for the predator, but life is at stake for
the prey!)
- Predator adaptations for prey detection and recognition are manifold and include the
utilization and refinement of:
- sight (e.g. eagles with two fovea centralis focal points in the eyeball; night
vision in viper snakes includes the perception by special head sensors of
infrared radiation emitted as body heat by potential prey; sharks can “see”
electrically, using the organ known as the Ampullae of Lorenzini),
- sound (e.g. echolocation in bats: emission of high-frequency ultrasonic
soundwaves to locate insect prey as small as 0.1 mm; directional changes
within less than 1 degree of arc and frequency differences between ears of as
little as 0.0001 kHz can be distinguished by some bats; the middle ear has a
special muscle which temporarily disengages the sound transmission bones
some 6 ms prior to sound emission; this avoids self-destruction of the auditory
system),
- smell (e.g. the vomeronasal organ of snakes allows them to detect individual
molecules sampled by the tongue, which is then placed into olfactorily
sensitive pits in the roof of the mouth; many dogs can detect odors in
concentrations of a few parts per billion ! drug sniffing, detection of minute
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quantities of volatile chemicals, as are emitted prior to the onset of epileptic
seizures in humans; tickssmell butyric acid in the sweat of homeotherms),
- touch (e.g. web-building spiders detect vibrations through their legs; to
determine the location of an insect caught in the web, the spider sequentially
plucks the individual strands of the web, and vibration dampening gives away
the location of the prey),
- locomotion, i.e. the ability to outrun, outfly, outswim, outcrawl
- Crypsis, in combination with chemical or mechanical means of warfare, is used
effectively by many predators as well as by prey. Some examples discussed in class
include:
- stonefish: the most venomous fish on earth; looks like an encrusted rock as it
waits for small fish or shrimp to swim by (prey are sucked into its mouth in
less than 0.015 s!); spines on its back can release jets of poison from bulging,
and rechargeable, venom glands: result: excrutiating pain and paralysis
- box jellyfish (“The Box of Death”): Chironex fleckeri, kills more people than
sharks, crocodiles and stonefish combined; due to its perfect translucency it is
virtually invisible in open water; up to 60 tentacles, reaching 5 m in length
and armed with 5 billion stinging cells {nematocysts}; these are triggered into
action by chemicals exuded from the skin of fish or humans; there is enough
poison in one animal to kill 100 humans!
- There are predatorial plants as well, some catch insects on sticky surfaces (e.g. Drosera
spec.), others have gliding surfaces to entrap animals as large as mice (e.g. Nepenthes
spec.); digestive enzymes are exuded which “dissolve” the captured prey
- Predators are often very much smaller than their prey (e.g. ticks, bacteria, viruses)
- Prey tacticsin the face” of predation include:
- locomotion, of course
- protective outer surfaces (e.g. shells of turtles and mussels, silicon
exoskeleton of diatoms, tree bark)
- safety in numbers (e.g. societalalarm systems in prairie dogs, schools of
fish, flocks of birds, herds of buffalo)
- fighting back (e.g. moose antlers, “poison spray” in skunks or darkling
beetles)
- being “unappetizing” or poisonous (e.g. skin poisons in dart frogs, lionfish
poison, “Zombie poison” {tetrodotoxin} in pufferfish); very often, such
poisons or distasteful chemicals areadvertised” to potential predators with
striking colour displays, called aposematism, and since many poisonous or
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distasteful animalsmimic” one other in that strategy, there is a special term
for it: Müllerian mimicry; this is different from Batesian mimicry, where
warning displays or behaviours are used by harmless organisms to imitate
poisonous or distasteful ones, e.g. mayfly imitating scorpion posture
- Microorganisms fight back chemically as well: Penicillium fungi exude a chemical
which causes the explosion of potentially predatory bacteria in their vicinity by inhibiting
cell-wall synthesis in those bacteria (penicillin)
- Defense from predation (herbivory, parasitism) in the plant kingdom most often
involves the production of secondary chemicals (e.g. phenolics, terpenoids, cyanide-
releasing compounds; also:plant aspirin” in willows and poplars)
- Many plant and microbial defense products are medicinally active (e.g. penicillin, garlic
extracts, the anticancer compound taxol from yew plants, the antileukemia drug
vincristine from the rosy periwinkle), and more than 50% of all prescription drugs
contain some such product (a 30x109 US$/year industry)
- Predator-prey relationships are often characterized by linked oscillation cycles
(increased prey abundance facilitates increased predator abundance, increased predator
abundance then reduces prey abundance, whereupon predator numbers decline etc.)
- Lotka and Volterra provided a simple mathematical model to illustrate those predator-
prey oscillations (you can consult the textbook on this, or the description below; the
textbook uses the interaction between hosts and parasites to describe the equations the
same essential idea):
- The Lotka-Volterra predation model makes the following assumptions:
(a) For the change of number of individuals over time in the predator population we can
write:
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