ENV 380 – Environmental Ecology: Synthesis and Applications
Lecture 14: Parasitism, Mutualism and Co-evolution
10 October 2002 (Dr. Hallett)
Last Day review: What is “Disease” -
Caused by human pathogens
A major limiting factor to human population growth thru history
Describe the relationship of human ecology and disease
Provide examples of that interaction
Provide possible projections for the future
Case studies: Smallpox and Bubonic Plague (“Black Death”)
Human diseases have been on the increase worldwide (Pimental Table 3)
Obscure diseases and “new” diseases are emerging at a rapid rate (Pimental Table 2)
Why is this the case?
Rapid expansion of the human population, humans living in crowded areas cause easy spread,
monocultures in general and in particular are susceptible, thus disease is density dependent.
Reasons why high density populations are more susceptible to affects of disease:
Proximity enhances Transmission
Rapid transmission may increase Virulence
High density hosts are often weakened by malnutrition or other physiological stress
Vector transmission vs. indirect transmission vs. direct transmission
Example of vector transmission: Trypanosoma and Plasmodium
Example of indirect transmission: Schistosomes
Why has there been resurgence in human disease?
Increased population means poorer sanitation
Increased conflict leads to increased refugees, poorer living conditions
Increased drug resistance
Increase contact with native animals in natural habitat
Changes in human behavior
Increased water, air and soil pollution
Global Changes and Disease?
AIDS: Robert May.
How, when and where infectious parasites move in their non-human hosts.
AIDS outbreak in early 1980’s.
Parallels between the movement of successful parasites in nature and the epidemic spread of
human pathogens.
“We took some of our animal models of the shelf and made some adjustments” for complex social
organization of humans.
Predicted the behavioral patterns on the spread of the disease.
The network of the interactions that lead to transmission are as powerful a force and the virulence
of the pathogen.
Relatively small numbers of promiscuous individuals could spark rapid spread.
Seems obvious now, but wasn’t at the time. Led to more cost-effective approaches to prevention.
Review of Parasitism
One organism feeding on another, where the prey or host is rarely killed outright.
Like predation it is a (- , +) interspecific interaction.
(When parasite kill host it is called parasitoid, which is another type of predator/prey interaction.)
Killing the host eliminates a parasite’s habitat – not a good evolutionary strategy.
(When a parasite kills the host it is called parasitoid; this is a type of predator/prey interaction.)
The two species live together for some time
Evolutionary implications.
Transmission from host to host is the key to the parasite’s existence, thus an evolutionarily critical
step.
This is highly dependent on both the density of hosts and the distance between parasite and
potential hosts (just like predation)
II. Mutualism: a brief review, and a classic paper in coevolution.
A positive, reciprocal relationship in which both species survival, growth or fitness is enhanced.
Usually a reciprocal exploitation than a cooperative effort.
Examples:
A. Mycorrhizae. A fungus that becomes associated with roots of trees.
Mutualism: fungus gets carbohydrates from plant photosynthesis and plant gets increased uptake
of nutrients and water through fungus,
B. Wasps and figs. Complex, obligatory relationship. 900 species of tropical figs. Most have
their own pollinating wasp species. Wasps lay eggs in the developing seeds. Larvae feed on seeds
while they develop inside flower. 44-77 % seed mortality, but they pollinate the figs.
C. Janzen Ant Acacia mutualism
ants derive shelter and a complete diet for all stages of development
Ants protect plants from herbivores.
Neither ants nor acacias can survive without each other. Obligatory mutualism.
In temperate
D. Coral reefs – coral and zooxanthelle algae, coral gives N to algae and coral controls release of
organic compounds from algae getting nutrients from photosynthesis, crabs and shrimp protect
coral complex from starfish and receive lipids and shelter
E. Coevolution - the reciprocal evolution of two or more interacting populations. (Ehrlich and
Raven 1964)
Humans in Africa and honeyguides (also a brood parasite) have coevolved for 20,000 years?
Mutualism and communication developed over time,
Most butterflies feed only on one or a few plants of a single plant family.
Why? biochemicals.
Specific species or groups of species breached the chemical defenses, or else evolved ways of
coping with them as they were evolving.
The effect of a predator on the evolution of a prey species and vice versa, offers another example
of coevolution.
The Red Queen Hypothesis (Van Valen 1973)
A species must continually evolve to survive in a world full of other evolving species.