EXTINCTION & TROPHIC CASCADES
Name: _________________________
Background
If you haven’t heard, we are currently living through one of the greatest extinction
episodes, comparable to the extinctions that are used to segregate the major time
frames in the geologic time scale (i.e. like the extinction of the dinosaurs that forms the
boundary between the Mesozoic and Cenozoic eras. Unlike earlier extinction episodes,
this one is solely the result of human actions (habitat destruction, introduction of exotic
species, over exploitation of food resources, changing global weather patterns,
pollution, etc…). In recognition of our effects, laws like the Endangered Species Act
have been implemented to curb the rate of extinction. In the following thought
experiment, you will learn how the extinction of an organism can disturb the ecosystem
in which they lived.
Objective
1. You should be able to describe and give an example of how an extinction can cause
a trophic cascade.
Procedure
1. Read the following passage from The Song of the Dodo: Island Biogeography in the
Age of Extinction by David Quammen.
Imagine a set of species and relationships in a hypothetical tropical forest. This forest
includes a species of peccary, a species of frog, a species of mosquito, a microbial
parasite, a species of monkey, a species of mango tree, a species of leafcutter ant, a
species of lizard, a species of wasp, and a species of owl – which in this case is a
pygmy, adapted to nesting in small cavities. These species interact with each other in
diverse and incalculable ways, but a few of the interactions can be noted. The peccary
roots for food along the soft banks of forest creeks, and its rooting creates muddy
puddles. The frog and the mosquito both lay eggs in those puddles. As the tadpoles
hatch and grow, they feed on the larvae of the mosquito. The mosquito, as an adult,
carries the microbial parasite in its saliva, and that parasite causes disease in the
monkey. Fortunately, for the monkey, the mosquito population is small – and so the
incidence of the disease is low. The monkey eats a wide variety of plant material, but
it prefers items that are juicy and sweet. Whenever the mango tree comes into fruit,
the monkey gorges on that. The mangos are small enough that sometimes the
monkey swallows them whole, and the woody mango seeds pass through the
monkey’s gut. The leafcutter ant depends on the same tree and no other. The lizard
is slightly less specialized, feeding on ants, flies, and an occasional wasp. The wasp
species builds its nests in the small tree holes originally excavated by woodpeckers.
The pygmy owl, like the wasp, nests in abandoned woodpecker holes.
Now, let’s disturb the ecosystem with a single extinction and postulate what might
happen. The peccary population is killed off, say, by hunters.
Questions
1. As David Quammen suggests, postulate what happens to the individuals that remain
in the ecosystem after the extinction of the peccary. For each species listed, write
how the peccary or some linked event will affect the habitat and life of the species,
and include how the population of this species will be impacted. Will it rise, fall, will
they go extinct themselves, etc…?
Frog
Mosquito
Microbial Parasite
Monkey
Mango Tree
Leafcutter Ant
Lizard
Wasp
Owl
2. Based on your knowledge of the term trophic why are the resulting disruptions
causes by the extinction of a species called a “trophic cascade”.
3. The black-tailed prairie dog lives in the grasslands of North America. Use the internet
to identify three other species that might be affected by the full extinction of this
species. In your answer describe how and why these other species may be
affected.