Download General Ecology: Lecture 12

General Ecology: Lecture 16
November 10, 2006
NOTE: Chapter 17 covers both parasitism and mutualism, two types of interactions shaped by
coevolution. I will focus on parasitism and will cover mutualism only to introduce it conceptually and
survey the different types of mutualistic relationships.
I.
II.
Basics of parasitism
A.
Defined: This is the condition whereby two organisms live closely together and one
derives its nourishment (or other resources) at the expense of the other.
B.
Impacts of parasites on individuals
1.
Several examples
C.
Impacts of parasites on ecosystems
1.
Perspective of early ecologists…
2.
Aldo Leopold helped changed that perspective (1933).
a.
Numerous studies show impact of parasites on population dynamics.

Parasites affect: birth rates, death rates, growth rates, sexual behavior
and mating success, susceptibility to predation
Characteristics of parasites
A.
Types of parasites
1.
Examples of parasites among varying kingdoms, domains and phyla.
B.
Location on or within host
1.
Ectoparasites: live on the body surface, but may burrow in.
a.
Examples: ticks, fleas, gill parasites of fish.
2.
Endoparasites: live within the body.
a.
Most have special tissues wherein they reside, such as the liver, kidney,
spleen, intestine or even the brain.
b.
Parasitoids are a special case.

III.
Description of a parasitoid life cycle.
C.
Life history variations
1.
Entire life as a parasite vs. part of their life cycle that is free-living.
2.
Some are very host-specific, while others may find a variety of hosts suitable.
3.
Many require more than one host to complete their life cycle.
4.
Parasites have a variety of innovative modes of transmission.
D.
Characterization by size
1.
Microparasites: include viruses, bacteria and protists
2.
Macroparasites: all others (including the vast array of invertebrate parasites)
Hosts as habitat
A.
The variety of tissue locations reflects the various needs and strategies of the parasite.
1.
Example 1: Snail parasite in gonadal tissue.
2.
Example 2: Tapeworms in digestive tract
3.
Example 3: Schistosoma in capillary bed of intestine
4.
Example 4: The malaria parasite Plasmodium spp. inside liver cells
B.
Host entry and exit occurs via many mechanisms
1.
Host entry
a.
Via food and drink (tape worms and liver flukes)
b.
Via burrowing into their host.(Schistosoma)
c.
Via an insect bite

Malaria (mosquito)

Lyme disease (ticks)

Many parasites have intermediate arthropod hosts…
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2.
IV.
V.
d.
Via other body passageways (Candiru)
Host exit
a.
Via the feces (see Schistosoma, next section)
b.
Burrowing out of host (Guinea worms, bot flies)
c.
Via an insect bite (malaria)
Life cycles
A.
Life cycles involving only a single host (sometimes called “direct transmission” but
this term is confusing as we shall see…)
1.
The parasite has only one host type; may be briefly leave host during transfer.
2.
Mechanisms of host to host transfer
a.
Example 1: Lice are transferred directly,
b.
Example 2: Rabies is transmitted via the bite of an infected animal
c.
Example 3: Bot flies are transmitted via a mosquito vector

Female bot flies lay eggs on mosquitoes. When a mosquito bearing
the bot fly larvae reaches its mammalian host (including humans),
the larvae burrow in beneath the skin. (See earlier for bot fly escape!)

The mosquito in this example is a vector. Vectors, often insects,
carry the parasite from one host to another but the parasite does not
reside within or truly infect the vector.

Do not confuse cases where insects act only as vectors with
cases where the vector is a secondary host, such as malaria.
B.
Life cycles involving secondary hosts (Why did I pick this example?)
1.
Example: Schistosoma mansoni, the blood fluke
a.
Mature flukes: Adults live within the capillaries of the human intestine.
The human host is the “final” or “definitive” host.

The adults reproduce sexually within the human host.

The female lives within groove of male and lays hundreds of
eggs per day.

Since sexual reproduction occurs in the human host, the humans are
considered to be the final or “definitive” host.
b.
Eggs with spikes burrow out of the blood vessel and into the intestine,
where they are incorporated into the feces.
c.
If feces are washed into the water, the egg hatches into a ciliated, freeswimming larva (the miracidium) that burrows into its snail host.
d.
The larvae reproduce asexually in the snail

The snail is considered to be the “intermediate” host. Why?
e.
Another larval stage (the cercarium) that burrows out of its host (with the
aid of enzymes) and survives in the water.
f.
When it locates a human host, it burrows into its skin and travels via its
bloodstream to the capillaries of the intestines.
Dynamics of transmission (briefly)
A.
Single host parasites tends to do well at high population densities of the host. Why?
B.
Multiple host parasites: success linked to the effectiveness of transfer between host species
1.
Advantages of multiple hosts
a.
If one host is scarce, the parasite can persist in the other host
b.
Often, both asexual and sexual reproduction occurs during its life cycle.

Why is this an advantage?
2.
Disadvantages of multiple hosts
a.
Must transfer between hosts to complete life cycle.

How could Schistosomiasis transmission be disrupted?
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
VI.
VII.
Loss of one or the other host will eventually cause a crash of the
parasite population.

Example: Swamps, mosquitoes and malaria
3.
Parasites have evolved a variety of mechanisms to insure transfer to hosts
a.
Example 1: Prey to predator.
b.
Example 2: The brainworm of a deer (via snails in grass)
c.
Example 3: One species of liver fluke (Dicrocoelium dendriticum) has an
ant as an intermediate host. The infected ant’s behavior is altered in a
remarkable way by the parasite...
d.
NOTE: Other examples in text. See III.B. for general mechanisms
Host responses to parasitism (briefly)
A.
Biochemical
1.
Inflammation
2.
Immune response
a.
In some cases, hosts can become resistant to the parasite (Schistosomiasis)
B.
Abnormal growths
1.
Cysts may form around the invading parasite (Plant galls [Fig. 17.10])
C.
Sterility
1.
Example of parasite within snail gonad tissue
2.
Also see text for nematode parasite in fungus-eating flies.
D.
Behavioral changes
1.
Example 1: Ant parasitized by liver fluke
2.
Example 2: Killfish infected with a particular trematode (fluke relative)
Mutualism
A.
Defined
1.
A relationship between two species in which both benefit
B.
A brief survey of mutualistic relationships
1.
Obligate symbiotic mutualism: a permanent and obligatory relationship where it
is sometimes difficult to tell where one organism ends and the other begins
a.
Example: coral [Fig. 17.19 plus photos]

Importance of this relationship

Location of protist symbionts (zooxanthellae) in coral animal

What the coral provides its zooxanthellae

What the zooxanthellae provide to the coral
2.
Obligate non-symbiotic mutualism: two organisms live physically separate lives,
but cannot survive without each other
a.
Example: Pollination (some cases)
3.
Non-obligatory (facultative) mutualism
a.
Example: Seed dispersal (some cases)
4.
Defensive mutualism
a.
Example: Alkaloid-producing fungus that lives within grass
Study questions
1.
Define parasitism, using an example to illustrate your answer.
2.
Briefly list several types of impacts a parasite might have on its host’s body and life cycle.
3.
How might parasites impact populations and ecosystems?
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4.
Which Domains/Kingdoms of organisms contain parasites? Provide examples to illustrate the
breath of parasitism among living (and not quite living) beings.
5.
Define the following: a. ectoparasite; b. endoparasite; c. microparasite; d. macroparasite; e.
parasitoid.
6.
What determines where on/in a host the parasite will reside? Provide several examples, explaining
the advantage of each particular location to the particular parasite.
7.
Describe several ways a parasite may enter its host, providing an example for each. Be sure you
know the examples from lecture.
8.
Describe several ways a parasite might exit its host, providing an example for each. Be sure you
know the examples from lecture.
9.
What is meant by “direct transmission” of parasites? How many hosts do parasites with direct
transmission have? Do these parasites always spend their entire life within their host?
10.
What is a vector? Why is a mosquito considered to be a vector when it transports the eggs/larvae
of bot flies, but a host when it is involved in the transmission of malaria?
11.
Describe the life cycle of Schistosoma, a parasite with two hosts. Your description should include:
a.
The sequential passage of S. mansoni from location to location, be it inside an organism or
traveling in water.
b.
How it gets into and out of each host (either it or its eggs)
c.
Where it is located within its human host
d.
In which host it sexually reproduces
e.
In which host it asexually reproduces
f.
The physical appearance of each life stage
g.
Which host is considered “intermediate” and which “final” (or “definitive”), and what
these terms mean.
12.
Why do species with only a single host tend to be more effective at high host densities, whereas
this may not be as crucial for those with multiple hosts?
13.
Describe the major advantages, as well as the major disadvantages, of having a parasitic life cycle
with more than one host?
14.
Describe some mechanisms whereby a parasite insures its transfer between different hosts.
15.
Describe several ways that hosts respond to parasites, providing examples for each. Also, be sure
you are familiar with the examples provided in lecture. Don’t forget that there are examples in the
previous lecture.
16.
What is mutualism?
17.
List and define the different types of mutualism described in class and provide an example of
each. Be sure you are familiar with the examples provided in lecture.
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