1. Video: Some Animals are More Equal than Others. Trophic Cascades and Keystone Species

Please start by watching the video below. You can also view it at the HHMI Biointeractive website.

2. Interactive Reading: Trophic Cascades

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[h]Interactive reading: Trophic Cascades

[i]

[q labels = “top”]Let’s start with some of the basic questions addressed in this video. The first of these is what determines the population size of each ________ in a particular ____________. Along these same lines is a question specifically about herbivores: why don’t herbivores expand to the point where they’ve consumed all of the ____________ that provide the energy that supports the ecosystem?

[l]habitat

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]producers

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]species

[fx] No. Please try again.

[f*] Great!

 

[q]Earlier in this unit about ecology, we studied the diagram below. In this diagram, each level is called a [hangman] level. As energy moves from one level to the next, approximately [hangman] percent of the usable energy is lost. As a result, there’s very little energy available to support the top level tertiary consumers (also sometimes called secondary [hangman]) at the top of this energy pyramid. But the question we looked at in the previous card still remains: what keeps the primary consumers from eating all the [hangman]?

[c]trophic

[c]90

[c]carnivores

[c]producers

[q labels = “top”]The Green World Hypothesis is illustrated below. This idea suggests that what keeps the plant eating ____________ at level 2 from consuming all of the ecological ____________ at level 1 is the effect of ________________ exerted by the carnivores at level 3. If carnivores were removed, then the number of primary consumers would ______________, which reduces the amount of biomass produced by the ____________ at level 1.

[l]increase

[fx] No. Please try again.

[f*] Good!

[l]producers

[fx] No. Please try again.

[f*] Excellent!

[l]herbivores

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]predation

[fx] No, that’s not correct. Please try again.

[f*] Good!

 

[q]A key test of the Green World Hypothesis was carried out by Robert Paine off of the rocky coast of Washington state. In the system that Paine studied, the sea star was the top predator, feeding on a variety of other animals in the intertidal zone. In Paine’s experiment, he removed the sea stars from the study area. As we’ll see, this had a dramatic effect on the other animals in the intertidal zone. These animals include

  • chitons: a type of mollusk that grazes on algae growing on rocks.
  • limpets: a member of the snail family, and another grazer
  • mussels: relatives of clams. These animals attach themselves to the intertidal rocks, and then filter their food from ocean water.
  • barnacles: highly specialized crustaceans (members of the crab family) that also filter their food from ocean water.
  • sea snails: predators that can drill holes through the shells of mussels and barnacles.

Study the diagram above, and click “continue” when you’re ready .

 

[q] Which line below represents the effect of removing sea stars from the intertidal ecosystem?
[textentry single_char=”true”]

 

[c*] B

[f] Exactly. What Robert Paine found was that by removing sea stars, the overall diversity of the intertidal zone decreased.

[c] Enter word

[c] *

[f] No. Here’s a hint. When Paine removed the sea stars, it allowed the population of one species  to explode in numbers. This species was then able to outcompete the other species on the rocks. What would be the effect of this on other species?

[q multiple_choice=”true”]See if you remember this from the video. When Paine removed sea stars from his experimental plot, the species that grew out of control, crowding out all of the other species on the rocks was the

[c]gooseneck barnacles

[f]No. It wasn’t the gooseneck barnacles.

[c]chitons

[f]No, it wasn’t the chitons.

[c*]mussels

[f]Yes. The mussels were the species whose numbers expanded when the sea stars were removed.

[q]Because of the sea star’s essential role in maintaining the biological structure of its community, Paine named it the community’s [hangman] species.

[c]keystone

[/qwiz]

 

Paine called the overall effect demonstrated in his seastar study a trophic cascade. Trophic cascades are community-wide changes that indirectly result from the addition or removal of one member of a biological community. The “indirect” part is important. For example, imagine that an invasive plant species moves into and overgrows an area. In this case, the subsequent reduction in species diversity is not a trophic cascade. That’s because it’s a direct effect. In the sea star example that we looked at above, sea star removal lead to mussels overgrowing the intertidal zone. The mussels, not the sea stars, are directly responsible for the reduction in species diversity.

Let’s look at some other examples of trophic cascades.

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[h]Trophic Cascades: Case Studies

[i]

[q labels= “top”]The offshore waters from California to Alaska, are capable of supporting huge forests of a type of seaweed called kelp. Sea urchins eat the kelp. Sea otters eat the sea urchins. Drag the words “increases” or “decreases” in a way that captures how this system works.

 

[l]increases

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]decreases

[fx] No, that’s not correct. Please try again.

[f*] Correct!

 

[q]During the 1800s, otters were hunted and harvested by humans for their fur. The resulting decline in otter numbers led to an explosion in the number of [hangman]. Expansion of this herbivore led to a decline in the population of [hangman]. This removed habitat for dozens of other species of fish and invertebrates. In this system, the otters played the role of a [hangman] species.

[c]urchins

[c]kelp

[c]keystone

[q labels = “top”]In the 1990s, a decline in the population of seals and sea lions off of the coast of Alaska led orcas (killer whales) to start preying on otters. The diagram below shows the resulting food chain. Use what you learned in the video and what you know about ecology to label each curved arrow as “increases” or “decreases.”

 

[l]increases

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]decreases

[fx] No. Please try again.

[f*] Great!

[q]When orcas began to prey on otters, the otters could no longer control the [hangman]. These led to destruction of the [hangman]. Removal of the otters, the [hangman] species in this system, caused the entire system to collapse.

[c]urchins

[c]kelp

[c]keystone

[q]The indirect effects by which a [hangman] species suppresses the population of another member of the community, increasing the diversity of the system as a whole, is called a [hangman] [hangman].

[c]keystone

[c]trophic

[c]cascade

[/qwiz]

Deepening your learning

The reduction in populations of top level predators is having negative effects on ecosystems and biological diversity around the planet. Click the following link for an annotated review of this problem by James Estes (one of the two scientists whose work was highlighted above). Updated, for sync, 8/10/20

Links

  1. Ecological Succession (the next tutorial in this Community Ecology Module)
  2. Community Ecology Main Menu