Biology Now, 2e

Of Wolves and Trees ■ 359

The idea of wolves protecting aspens initially

seems nonsensical. How would meat-eating

predators protect trees? They wouldn’t—at

least not directly. Ripple surmised that wolves

in Yellowstone might have had an indirect effect

on the community. As an ecologist, Ripple had

long studied ecological communities, asso-

ciations of species that live in the same area.

Communities vary in size and complexity, from

a small group of microorganisms in a tempo-

rary pool of water to the whole of Yellowstone

Park, home to an estimated 322 species of

birds, 67 species of mammals, 1,349 species of

plants, and an uncounted number of insects

(Figure 20.2).

Ripple knew that an ecological community is

characterized by the diversity of species that live

there, and that diversity is governed by two things:

the relative species abundance (how common

one species is when compared to another) and the

species richness (the total number of different

species that live in the community; Figure 20.3).

Ripple also knew that communities are subject

to constant change, and that something must

have changed in Yellowstone.

Ecological communities change naturally

as a result of interactions between and among

species, and as a result of interactions between

species and their physical environment. Ripple

knew that both the relative abundance and the

William Ripple is director of the Trophic Cascades

Program at Oregon State University. There, he

leads a research project investigating how gray

wolves affect other species in the Yellowstone

ecosystem.

WILLIAM RIPPLE

A community of protists

and small invertebrate

animals lives in a tree hole.

A community of plants, animals, and

microorganisms lives in an aspen woodland.

A community of

microorganisms lives

in the gut of an elk.

Bacterium

Parasite

Worm

Fly larva

Amoeba

Mosquito larva

Figure 20.2

Ecological communities come in all sizes

Smaller communities can be nested within a larger community. This aspen woodland community contains the smaller communities

of a temporary pool of water in a tree hole and an elk’s gut, among others.

Q1: List another species that is part of this community.

Q2: Of which community could this aspen woodland be a smaller part?

Q3: Which other small communities could be found within this larger community?