Transcript The Keystone Predator Hypothesis - Cal State LA

Predation
Key Topics
• Types of predation.
• Effects of predation on prey populations
and communities.
• The Refuge Theory.
• The keystone Predator Theory.
Types of Predation
• Herbivory - eating plant tissue.
– Browsers
– grazers
• Carnivory - eating of animal tissue.
– Predation.
– Parasitiods
Some Effects of Predation
• Set distribution limits of prey.
• Set carrying capacity.
• Influence age/size frequencies within prey
populations.
• Influence diversity of the community.
Moose and Wolves on Isle Royale
After wolves discover a moose population on an isolated island,
the moose population attains a lower, more stable carrying capacity.
The Malthusian Dilemma.
• Because population growth potential is geometric and
world is finite, there is a struggle for existence.
• Darwin reasoned that struggle should be most intense
among most similar organisms - hence competition
should predominate.
• The competitive exclusion principal maintains that no
two species could indefinitely occupy the same niche,
i.e. have the same resource use.
• Therefore, two related questions have driven much of
the work in benthic ecology:
– How do natural enemies coexist?
– How can so many similar species coexist? (Why does not
competition eliminate similar species?)
Two possible answers
• The Refuge Theory is one hypothetical
answer to the question of how
predators and prey can coexist
indefinitely.
• The Keystone Predator Theory is one
hypothetical answer to how similar
species (potential competitors) can
coexist.
The Refuge Hypothesis
• In all stable predator - prey systems, prey attain
an age/size refuge or a spatial refuge.
• “Refuged” prey produce young that replenish
their population and provide food for predators.
• Refuges depend on adaptive limitations of the
predators, e.g. intolerance of exposure at low
tide, limited prey handling abilities.
• In the following examples, “intertidal zonation” is
attributed to spatial refuges.
The Case of Whelks and Barnacles,
Connell 1970
• Location San Juan Island, Washington.
• Predators, three species of whelks, Thais
emarginata, T. lamellsoa, T. canaliculata.
• Prey, three species of barnacles, Chthamalus
dalli, Balanus glandula, Semibalanus cariosus.
• Larvae of all three species occur over a broad
range of shore levels, but adults show restricted
vertical zonation.
• Observations and experiments indicate that
Balanus adults reside in a spatial refuge,
Semibalanus attains and age/size refuge.
Whelks
The whelk -barnacle system
on two Continents.
In contrast to Scotland,
where competition was the limiting factor,
Washington shores had intense predation.
Barnacle zones are different types
of prey refuges.
Connell’s Evidence
• Whelks do not forage on the highest shore levels.
• Balanus adults survive well in the upper zone above the
reach of the whelks.
• Balanus can survive in the lower, Semibalanus zone only
if protected from whelks. (Cage experiment)
• In the lower zone, large, adult Semibalanus survive
attacks of whelks. Smaller barnacles, Semibalanus
juveniles and Balanus adults, are preferentially eaten
over other prey.
• Therefore, the upper zone is a spatial refuge for
Balanus, the lower zone is an age-size refuge for
Semibalanus.
The Mytilus-Pisaster interaction,
Paine 1974,1976
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Site Olympic Peninsula, Washington State
Prey, the sea mussel Mytilus californianus.
The predator Pisaster ochraceus.
Pisaster extirpates mussel on lower shores
levels, promoting diversity.
• Mytilus attains a spatial refuge on upper
shore levels, and, infrequently, an age-size
refuge on lower shore levels.
Predation by Sea Stars
Pisaster ochraceous
Paine and “The
System”
Photograph of the “Glacier Experiment”
After sea stars were removed from foreground area,
mussels took over lower shore levels.
Apparent spatial and age/size refugia.
Field survey results showing
size dependent predation
Statement of Keystone
Predator Hypothesis
• Keystone predators are characteristically large, or
numerous consumers that prey an assemblage of
competing species.
• Mortality on a dominant competitor species keeps
its numbers in check, freeing resources for
subordinate species.
• Therefore, keystones promote coexistence of
competitors at lower trophic levels.
• Experimental removal of the predator leads to
transformation of the community. Certain
competitors take over the limiting resource.
Significance of Keystone
Predator Hypothesis
• Arose in the context of the Malthusian
dilemma.
• An example of the importance of indirect
effects in complex food webs.
• Explains how predators can enhance the
diversity of a community.
Indirect effects in food webs
• Pisaster indirectly
affects barnacles
(Balanus spp.) and
algae by preferential
preying on their
principal, superior
competitor, mussels
(Mytilus californianus),
and by preying on other
consumers of the
barnacles.
Photograph of the “Glacier”
Diagram of the “Glacier Experiment”
Contradictions of the Refuge Hypothesis:
Panulirus-Mytilus Interaction.
The predator
Panulirus sp. the spiny lobster
The prey
Mytilus sp. the mussel
Time lapse photography rig
in the intertidal zone at low tide, Catalina Is.
Time Lapse Sequence
Lobster Exclosure Experiments
on Red Algae “Turfs”
The the lobster exclosures, mussels survive and crowd out
the algae, supporting the keystone predator hypothesis.
Lobster exclosure experiment in a mussel bed.
Lobster predation within
mussel beds affects size
frequency distributions
without significantly affecting
percent covers of mussels.
Marsh 1986 and Wootton
1993 present other
examples of predator effects
within mussel zones.
Some mussel beds fall completely within
the foraging range of the highly mobile lobsters.
See Menge and Lubchenco 1981 for discussion
Absolute age size refuges:
rarely, if ever, attained.
Conclusion
• The Keystone predator hypothesis is
supported by the fidnings about the
Panulirus-Mytilus interaction
• While the refugia concept fits for some
predator prey systems, it does not apply to
all, e.g. the Panulirus-Mytilus interaction
• Rather than invoking refuges, the most recent
formulations of theory look at balances
between rates of prey input (recruitment) and
loss (predation) to explain co-existence.