- 0 +

- Competition

Amensalism Predation, parasitism

0 Amensalism

Commensalism

+ Predation, parasitism

Commensalism

Mutualism

Types of interactions

Consumer-Resource Interactions

• All life forms are both consumers and victims of consumers.

• Consumer-resource interactions organize biological communities into consumer chains (food chains):– consumers benefit at the expense of their

resources– populations are controlled from below by

resources and from above by consumers– The relative importance of top-down

versus bottom up control of populations is an important focus of ecological research

Some Definitions

• Predators catch individuals and consume them, removing them from the prey population.

• Parasites consume parts of a living prey organism, or host:– parasites may be external or internal– a parasite may negatively affect the

host but does not directly remove it from the population

More Definitions

Parasitoids consume the living tissues of their hosts, eventually killing them:– parasitoids combine traits of parasites and

predators

Herbivores eat whole plants or parts of plants:– may act as predators (eating whole plants) or as

parasites (eating parts of plants):•grazers eat grasses and herbaceous

vegetation•browsers eat woody vegetation

Predation

Theory: Lotka-Volterra Equations.

P = Predator population sizeV = Prey resource

dV/dt = rV – σPV

dV/dt = 0 P=r/σGeometric increase of prey (resource) in absence of predator; subtract predation, where σ is catching efficiency.

dP/dt = βVP – qP

dP/dt = 0 V=q/βGeometric decrease of predators in absence of prey;Predation loss of prey corrected for assimilation efficiency, or " β ".

Solution:Limit cycles (periodic solutions) such that

• Species can coexist, but• Random walk to extinction, • No interaction of prey with food supply

(& no time lags),• Predator mortality independent of prey density.

Testing the theory -- Gause’s Paramecia

Testing the theory -- Huffaker’s oranges

Case studies – Opuntia and Cactoblastis

Cactoblastis chronology• 1839 Opuntia stricta in pot to Australia f/Texas or

Florida• 1900 10,000,000 acres• 1925 60,000,000 acres (i.e. area twice size NC)

increasing at 1,000,000 acres per year. Too dense to walk, 3-6' high. Sheep would not eat, horses could not traverse.

• Cactoblastis - northern Argentina2750 eggs in 1925; 2x106 eggs out in 19 locations

• 1930-31 Opuntia ravaged, mostly back to grass• 1932-33 Opuntia recovered some• 1935-40 Cactoblastis recovered and expanded• >1940 Only scattered Opuntia plants remained

Klamath weed and Chrysolina quadrigemina

L-V Assumptions

• Growth of victim population is limited only by predation (exponential growth)

• Predator is a specialist on victim (starves in absence of victims)

• Individual predators can consume an infinite number of victims

• Predator and victim encounter one other randomly in a homogeneous environment

Carrying capacity

Functional response

Keystone predators – Piaster and Mytillus