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Predation
Great White Shark and Fur Seal
Predator-Prey Interactions
Food Preferences
• A preference exists if the proportion of a type
of food is higher in the animal’s diet than it is
in its proportion in the environment
• Ranked preference – predator preferentially
eats prey which is most valuable (usually in
terms of calories)
• Balanced preference – predator eats prey
items that provide integral parts of a balanced
and mixed diet (usually in terms of nutrients
and vitamins)
Pied Wagtail
Caribou feeding in winter
Preferences
• Fixed preference – predator eats certain items
regardless of what else is available – can be
fixed in terms of species consumed or energy
consumed
Edible mussel – Mytilus edulis
Shore crab and edible mussels
Backswimmers
Asellus aquaticus – prey for
backswimmers
Guppy as predator
Guppy prey – tubificid worms
and fly larvae
Eurasian oystercatcher
Search image – this image has dashes arranged
as L, as T, and plus signs – can you find the Ts?
Bluegill sunfish
Effects on populations
• Population regulation refers to the tendency of a
population to decrease in size when above a particular
level, and to increase in size when below that level.
Population regulation can only occur as a result of
one or more density dependent processes acting on
birth or death rates.
• Population abundance is determined by the
combined effects of all factors and processes that
influence population size, whether they are density
dependent or density independent.
Predator effect on individual prey
Mink
Muskrat
Arctic Ground Squirrel –
Predator population is self-limited
Red Grouse in Heather –
Predator population is self-limited
Predator Switching Regulates Prey Population
Bank vole
Tawny Owl
Cinnabar Moth and Caterpillar on
Ragwort Tansy
Snowshoe hare and Lynx
Lynx
Ruffed Grouse
Snowshoe hare
Sea Otter
Sea Urchin
Kelp
Forest
Sea Otter eating Sea Urchin
in Kelp Forest
Comparison of kelp and urchin biomass with and
without sea otters
Sea Urchin Barren
Kelp forest ecsystems with and without
sea otters
Plant Resource Defense
• Qualitative defense - highly toxic substances,
small doses of which can kill predators
• high nutrient environment/fast growth (high
turnover in plants) - use toxins (plant
secondary compounds) that often require N,
expensive to make (must be replaced often),
but can be made rapidly - cyanide compounds,
cardiac glycosides, alkaloids - small molecules
Plant Resource Defense
• Quantitative defense - substances that
gradually build up inside an herbivore as it eats
and prevent digestion of food
• low nutrient environment/slow growth (low
turnover in plants) - primarily use carbon
structures - wood, cellulose, lignin, tannins large molecules - makes plant hard or
unpleasant to eat (woodiness, silica), but plants
are slow to make these defenses
Evolutionary “Arms” Races
Monarch and milkweed
Evolutionary “Arms” Races
Evolutionary “Arms” Races
California garter snake
Pacific newt
Other Plant Defenses Include:
• mechanical defenses - plant thorns and spines deter
many vertebrate herbivores, but may not help much
against invertebrate herbivores
• failure to attract predators - plants somehow avoid
making chemicals which attract predators
• reproductive inhibition - some plants such as firs
(Abies) have insect hormone derivatives which if
digested, prevent successful metamorphosis of insect
juveniles
• masting - the synchronous production of very large
numbers of progeny (seeds) by trees of one species in
certain years
Eurasian Jay with Acorn
Masting
Masting
Fagus sylvaticus – European Beech
Dipterocarp distribution
Dipterocarp trees
Beech seeds and boring moth
Lyme’s disease life cycle
Masting and Human Health
- Lyme’s Disease
Induced Defenses
• Another aspect of plant defenses is that plants
do not always have tissues loaded with
defensive chemicals - in many plants,
defensive chemicals are only produced when
they are needed, usually after the plant has
experienced some herbivory - this is an
induced defense
Impact of Herbivores Is Not
Uniformly Experienced
Aphids attacking Alfalfa
Spotted Alfalfa Aphid
Induced defenses in Birch Trees
Induced defenses in Birch Trees
Induced defenses in Birch Trees
Rubus prickles
Acacia depanolobium
Plant defenses are developed at a cost
to fitness when:
1. Organisms evolve more defenses if they are exposed
to much damage and fewer defenses if cost of defense
is high
2. More defenses are allocated within an organism to
valuable tissues that are at risk
3. Defense mechanisms are reduced when enemies are
absent and increased when plants are attacked mostly true for chemicals not structures
4. Defense mechanisms are costly and cannot be
maintained if plants are severely stressed by
environmental factors