COMMUNITY ECOLOGY: INTERACTIONSBETWEEN POPULATIONS

Introduction to population interactions

Species living together form a

community, with many links

between species

Types of interactions: beneficial (+),

detrimental (-), neutral (o)

Species A B

Competition - - Mutualism + +

Parasitism,

predation,

herbivory

+ -

Commensalism + o

Amensalism - 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))

• a12 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 ismore tolerant toenvironmentalstress?

Which species isthe bettercompetitor?

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 throughcompetition, mutualism,parasitism, predation, herbivory, commensalism,and amensalism

Competition reduces the growth rate of bothpopulations

Competition restricts a population to a “realizedniche”

Predation increases the growth rate of predator,decreases growth rateof prey

Prey species evolve to limit predation