Download COMMUNITY ECOLOGY: INTERACTIONS BETWEEN POPULATIONS

COMMUNITY ECOLOGY: INTERACTIONS
BETWEEN POPULATIONS
Introduction to population interactions
Species living together form a
community, with many links
between species
Types of interactions: beneficial (+),
detrimental (-), neutral (o)
Species
Competition
Mutualism
Parasitism,
predation,
herbivory
Commensalism
Amensalism
A
B
-
-
+
+
+
+
o
o
-
-
Competition--mutually detrimental: reduces growth rate
of both populations
Occurs when there is a shared, limiting resource, for
example:
plants
animals
water
food
light
water
nutrients
mates
pollen
shelter
space
Types of competition
1) Scramble ("exploitative") competition
organism consumes resource and reduces
availability, but no physical conflict between
competitors
2) Contest ("interference") competition
organism excludes others or actively robs resources;
territoriality
Examples:
Scramble: deer, cattle on grassland; roots
absorbing Pi
Contest: red-wing blackbirds defending space;
allelopathy?
Both at bird feeder: sparrows below feeder, finches
on feeder
Both: tomato horn worms and humans competing
for crop
Effects on population dynamics
Intraspecific competition: arises from a shared, limiting
resource, implies density dependent population
growth:
dN/dt =rN(1-N/K)
• provides selective pressure for evolution of traits that
improve competitive ability
• d oes not change geographical distribution of
population (since replacement of one organism by
another of same species does not change
distribution), in contrast to...
Interspecific competition (between organisms of different
species):
dN1/dt = r1N1 (1- N1/K1 - a21 (N2/K1))
dN2/dt = r2N2 (1- N2/K2 - a12 (N1/K2))
• a 12 and a21 are "competition coefficients" that show
how much one species affects the carrying capacity
for the other
Ecological niche concept
“Niche”: where and how a species lives
There are two ways of estimating a species's niche:
(1) determine all possible conditions experimentally or as
union of field sites = "fundamental niche"
(2) see where the species really occurs = "realized niche"
These differ because of competition
Example in which realized niche ! fundamental niche:
Semibalanus and Chthamalus barnacles
• shows shared, limiting resource (space)
• shows "principle of competitive exclusion": 2
species will not share the same niche indefinitely
• shows evolution focusing on two different life history
strategies: tolerance to environmental stress,
competition
Acorn barnacle (Semibalanus balanoides) Stellate barnacle (Chthamalus stellatus)
Realized niches
Fundamental niches
Which species is
more tolerant to
environmental
stress?
Which species is
the better
competitor?
Predation, Herbivory, Parasitism
Predation--animal kills and eats another animal
Herbivory--animal eats plant
Parasitism--organism (plant, animal, fungus,
bacterium...) absorbs nutrients from, eats another
organism without first killing it
These are clear environmental resistances on the
growth rate of the eaten population and on
carrying capacity
Could you write growth rate equations that take into
account the effect of predation on the prey and on
the predator?
Predation, Herbivory, Parasitism
Predation--animal kills and eats another animal
Herbivory--animal eats plant
Parasitism--organism (plant, animal, fungus,
bacterium...) absorbs nutrients from, eats another
organism without first killing it
These are clear environmental resistances on the
growth rate of the eaten population and on
carrying capacity
Could you write growth rate equations that take into
account the effect of predation on the prey and on
the predator?
If species 1 is prey and species 2 is predator:
dN1/dt = r1N1 (1- N1/K1 - a21 (N2/K1))
dN2/dt = r2N2 (1- N2/K2 + a12 (N1/K2))
Effects of predation on prey
Predation leads to evolution of defense mechanisms:
behavioral, physical, chemical
Behavioral defenses (generally defenses of animals)
running; hiding; nocturnal habit; camouflage
fighting; vigilance (improved by group living)
predator satiation (group is protected when there are too
many for predators to eat)
Physical defenses (animal or plant)
spines, thorns; tough hide, suberized cork; slipperiness
urticating hairs (nettles), insect stingers, etc.
Chemical defenses (mainly plant, some animal)
poisons, photosensitizers, digestion inhibitors, insect
development inhibitors; alarm compounds (corn, grazed
by caterpillars, releases terpenoids that attract parasitic
wasps)
Advertisement (protective coloration)
When organisms are distasteful, poisonous, it is valuable
to advertise
Monarch butterfly eats milkweed alkaloids to taste bad to
predator birds—the distinctive color pattern
advertises the fact
Mexican palm (Reinhardtia gracilis) has ragged leaves,
suggesting insect predation, suggesting defense
induction--herbivory is reduced in the wild, but not
in the lab when leaf shape obscured
Mimicry (protective coloration of another species)
synergistic (Müllerian) mimicry: both species defended
parasitic (Batesian) mimicry: one species a cheater
(but too many mimics spoils the defense for all)
Mimicry (protective coloration of another species)
synergistic (Müllerian) mimicry: both species defended
parasitic (Batesian) mimicry: one species a cheater
(but too many mimics spoils the defense for all)
Summary
In a community, populations can interact through
competition, mutualism,
parasitism, predation, herbivory, commensalism,
and amensalism
Competition reduces the growth rate of both
populations
Competition restricts a population to a “realized
niche”
Predation increases the growth rate of predator,
decreases growth rate
of prey
Prey species evolve to limit predation