Keystone Species

Keystone Species
Consumers (predators, herbivores,
parasites, and diseases) that
maintain food-web diversity.
Two species/island
7 Islands: 2-fold
body size
difference (males)
competition for
St Maarten:
Greater overlap?
Schall, J.J. 1992. Oecologia 92:58-64.
Malaria: Insect-borne infection
• Initially (?) Intestinal parasite of reptiles
• Became infection of blood
Accessible to biting insects
• Vector-borne parasite with complex life cycle
Saurian malaria
Infection reduces a
female’s clutch size:
Fitness cost of infection
Significant in 4 of 5 years
Infected males: Less
capable of evicting
intruders on feeding
Fitness Cost of
Correct for increase in
clutch size as female
size increases
Report: Loss of 2
eggs/year due to
Spatial distribution
Smaller lizard not
found at lower
elevation: exclusion
Smaller lizard common
in central hills: 2
species coexist
Biting sand-fly found
in central hills: vector
of saurian malaria
Malaria, “keystone predator”
Central hills: malaria infects 30 – 46% of the
otherwise competitively dominant species
Infects only 3.6% of A. watsii, smaller species
Differential fitness cost of disease (parasite)
maintains host diversity
Generalized keystone predator
Keystone predator’s diet in twoprey environment:
Dietary frequency of more
common prey exceeds that
species’ environmental frequency
Dietary versus environmental frequency
Keystone predators

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