Pisaster Disaster
PSI AP Biology
Name: _________________________
Objective
Students will analyze Robert Paine’s exclusion experiment and the resulting keystone species
hypothesis.
Standard
Essential Knowledge:
2D1c : The stability of populations, communities, and ecosystems is affected by interactions with biotic
and abiotic factors.
4B3 : Interactions between and within populations influence patterns of species distribution and
abundance.
Materials
Every student needs: Activity worksheet
Procedure
1. Read the worksheet.
2. Answer the Analysis and Application questions.
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PSI AP Biology
Emergence of Organic Molecules
Pisaster ochraceus
A keystone species is a species that has a large effect on a community relative to its abundance.
Keystone species increase species richness, the number of different species present in a community.
Robert Paine was the first person to study and term the phrase “keystone species”. He reached his
keystone species hypothesis through the following chain of reasoning.
Predators keep prey
populations below their
carrying capacity.
Competitive exclusion is low in
populations below carrying
capacity.
If competitive exclusion is
reduced, then the number of
species that coexist is increased.
Therefore, some predators must
increase species richness.
Paine studied the rocky, intertidal zone of the Washington coast. In particular, his first keystone species
experiment included Pisaster ochraceus, also known as a seastar.
Pisaster ochraceus Food Web
Seastar (Pisaster ochraceus)
Snail
Chitons
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Limpets
Bivalves
PSI AP Biology
Acorn Barnacles
Gooseneck barnacles
Emergence of Organic Molecules
Paine was the first person to do an exclusion experiment, in which a species is completely removed from
an environment. Below is a summary of his experiment.
Hypothesis
•Some species play a larger role than expected in a
community.
Experiment
•Created two study sites: an experimental site and a
control site.
•Control site: nothing was removed.
•Experimental site: Pisaster was removed. Paine visited
the site every week and removed any Pisaster that had
moved into the area during his absence.
•Both sites were studied for several years.
Results
•Control site: diversity of invertebrate species remained
constant at 15
•Experimental site: diversity of invertebrates dropped
from 15 to 8
•After five years, the experimental site was dominated by
two species: the mussel, M. californianus and the gooseneck barnacle, P. polymerus.
Conclusions
•The limiting resource in rocky, intertidal zones is space.
•Pisaster is a keystone species.
•When a keystone species is removed, the entire
community collapses.
•Keystone species increase the species richness of a
community by reducing competitive exclusion.
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PSI AP Biology
Emergence of Organic Molecules
Analysis
1. Competitive exclusion is a principle stating that two species competing for the same resource
cannot coexist. Competitive exclusion resulted when Pisaster was removed. For what resource
were the remaining species competing?
2. Species richness is the number of species present in a community. What was the species
richness of both the control and the experimental sites before and after Paine’s experiment?
Control Site
Before:
After:
Experimental Site
Before:
After:
3. Create a graph that illustrate the information in question #2.
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PSI AP Biology
Emergence of Organic Molecules
Application
1. Elephants are a keystone species in African grasslands. African grasslands are a diverse
ecosystem that supports grasses, trees, herbivores, carnivores, and decomposers. Elephants
maintain this diversity by feeding on and destroying trees. What do you think might happen if
elephants were removed from the ecosystem?
2. Food webs are organized ways to illustrate the different food chains in an ecosystem. Each level
of a food web is called a trophic level. The transfer of energy in an ecosystem flows upwards
from one trophic level to another. Unfortunately, not all of the energy is transferred between
trophic levels. The percentage that is successfully transferred is called the ecological efficiency.
On average, ecological efficiency is 10% from one trophic level to another.
Look back at the food web for Pisaster on the western coast to answer the following questions.
a. How many trophic levels are present?
b. If 10,000 grams of biomass is located on the bottom trophic level in a specific study site,
what biomass of Pisaster is present?
3. Humans are often viewed as a keystone species, except that we reduce species richness when
we are added to ecosystems instead of when we are removed from ecosystems. Using the
deforestation of the rainforest as an example, design an exclusion experiment that could
provide evidence for the link between human behavior and a loss in species richness.
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PSI AP Biology
Emergence of Organic Molecules