Eco Ch 13 Parasitism

Symbionts

Organisms that live in or on other
organisms. More than half of Earth’s species are these.

parasite

consumes the tissues or body
fluids of the organism on which it lives,

usually have a
higher reproductive rate than their hosts.

Pathogens

parasites that cause
diseases

Macroparasites

Large species such as
arthropods and worms.

Microparasites

Microscopic, such as
bacteria.

Ectoparasites

live on the outer body
surface of the host

Endoparasites

live inside their hosts,
within cells or tissues or in the
alimentary canal

Pros and Cons:
Ectoparasites

an disperse more easily, but
are more exposed to predators, parasites,
and parasitoids.

Pros and Cons:

Endoparasites

protected from the
external environment and have easy access
to food, but can be attacked by the host’s
immune system. have evolved various
mechanisms for dispersal (including in
feces), complex life cycles, and enslavement
of hosts

Host defenses

1. Protective outer coverings (e.g., skin,
exoskeletons).
2. Immune system:
• Vertebrates have “memory cells” that can
recognize microparasites from previous
exposures.
• Plants use nonspecific immune responses such
as antimicrobial and antifungal compounds (we
harvest these for pharmaceuticals!)

Biochemical defenses

Ex) fungal and bacterial parasites require iron;
Vertebrates have a protein called transferrin
that removes iron from blood serum and
stores it so bacterial and fungal endoparasites
cannot get it.

Defensive symbionts

Fungal symbionts in some plant leaves protect
them from pathogen attacks.
Evidence is mounting that bacterial symbionts
in the human digestive tract can protect us
against pathogens and may be heritable.

Biological Control

Importation of a pest's
natural enemies to a new locale where they
do not occur naturally. Relies on predation,
parasitism, herbivory, or other natural
mechanisms

gene-for-gene interactions

resistance
genes to specific parasite genotypes

Trade-offs

A trait that improves host
defenses or parasite counterdefenses may
reduce some other aspect of growth,
survival, or reproduction

Dynamics and Spread of Diseases

1) Number of susceptible individuals (S),
infected individuals (I), and recovered
and immune individuals (R).
2) Host and pathogen genotypes.
3) Factors influencing spread (e.g., latent
period, vertical transmission)

S

Number of susceptible individuals

I

infected individuals

R

recovered
and immune individuals

β (beta)

transmission coefficient, how effectively
the disease spreads

I increases when

the disease is transmitted
successfully and decreases when infected
individuals die or recover

m

death and recovery
rate

threshold density

A disease will establish and spread when the
number of susceptible individuals exceeds this

ST = m/beta

Ecosystem engineer

an organism that modifies, creates or destroys
habitat and directly or indirectly modulates the
availability of resources to other species

Parasite vs Host specificity

Parasites specialize, but hosts harbor a
variety of species

Host-parasite relationships often drive

adaptive evolution of both species
(defenses and counterdefenses)