Pisaster Disaster

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.

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.

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 Limpets Bivalves Acorn Barnacles Gooseneck barnacles

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.

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.

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.

How many trophic levels are present?

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.