BIOL305 - Lecture 5: Keystone Species & Trophic Cascade

Who coined the term keystone species?

Robert Paine (1966, 69)

Why do many describe keystone species do not fit the original definition or intent?

they often interchange inappropriately with other terms

Who coined the term trophic cascade?

Robert Paine

What did the effects of predation cause?

a series of nested strong interactions that start at upper trophic levels and cascaded downward across lower trophic levels

The concept of a trophic cascade moves beyond mere species protection to what?

focusing on the functional integrity of species interactions

What is a trophic cascade an important concept for?

conservation of community and ecosystem function

• broadly used in marine protected areas

• provides support for reintroduction of predator species

Trophic cascade defined:

Indirect species interactions that originate with predators and spread downward through food webs

What is top-down forcing?

indirectly effects proceed downwards across successfully lower trophic levels

What is the importance of the idea of trophic cascades?

count of the words ‘trophic cascades’ in titles, abstracts, and keywords in ecosystem types

Wolves in Yellowstone National Park

• When Yellowstone was established in 1872, grey wolf was considered competition for game like elk and deer

• In 1914, U.S. Congress appropriated funds to destroy wolves (and other animals) inside the park deemed "injurious to agriculture"

• At that time, wolf numbers in the Yellowstone area (and more broadly) had already been decimated (hunted, trapped, poisoned)

• Park rangers and government hunters eradicated the remaining population within Yellowstone by 1926

When Yellowstone was established in 1872, grey wolf was considered competition for game like elk and deer, resulting in:

In 1914, U.S. Congress appropriated funds to destroy wolves (and other animals) inside the park deemed "injurious to agriculture"

At that time, wolf numbers in the Yellowstone area (and more broadly) had already been decimated (hunted, trapped, poisoned), resulting in:

Park rangers and government hunters eradicated the remaining population within Yellowstone by 1926

For the next ~70 years in Yellowstone:

there were no resident wolves in the Yellowstone area

Wolf reintroduction to Yellowstone efforts:

In 1995-97, *41 wolves (including elements of several packs) were relocated from Canada to Yellowstone National Park

As of 2026, how many wolves and packs are within Yellowstone National Park?

84 wolves and 7-8 packs

What is the Greater Yellowstone Ecosystem?

the surrounding area of Yellowstone: national forests, parks, reservations, etc.

Why does the Greater Yellowstone Ecosystem exist?

Yellowstone (natural preserved areas) is just not big enough to support all its species

Wolf recovery and Trophic cascade in Yellowstone

• Focus on woody plant structure - willows and aspen

• Wolves were extirpated for ~70 years, plant growth was suppressed by elk browsing

• Following wolf reintroduction, willow and aspen show growth indicative of a 'release' from browsing

• Wolves lower elk numbers and appear change elk behavior

Focus on woody plant structure: what forest plants are essential?

willows and aspen

Since wolves were extirpated for ~70 years, what happened to plant growth?

suppressed by elk browsing (led efforts to reduce elk)

Following wolf reintroduction, what did willow and aspen growth show (increase in size and density)?

a 'ecological release' from browsing

What is 'ecological release’?

predators control aspects of certain organisms (elk to willows)

Wolves do wolves do to elk?

lower elk numbers and appear change elk behavior

How did wolves appear change elk behavior?

by creating a ‘landscape of fear” where elk actively avoided areas with wolf packs, releasing willows from elk

What was Wolf reintroduction was partly justified by?

the idea of reversing a Trophic Cascade

However, reintroduction of wolves to Yellowstone was an equivalent perturbation to?

removing them in the first place