360 ■ CHAPTER 20 Communities of Organisms
ECOLOGY
This community has higher
relative species abundance
for white-barked trees than
the community shown below.This community has
higher species richness
than the community
shown above.Figure 20.3
Two measur es o f species di ver sit y
The diversity of an ecological community is determined by relative species abundance and also species
richness. High relative species abundance means that one species dominates the community (so the
community is less diverse), and high species richness means that many species are present in the
community (so it is more diverse).Q1: If relative species abundance increased in the first community, how would the figure look
different from now?Q2: If species richness decreased in the second community, how would the figure look different
from now?Q3: How do relative species abundance and species richness define the species diversity of a
forest community?disproportionately large effect on a commu-
nity, relative to the species’ abundance. There
are few wolves compared to, say, rabbits, yet the
wolves have a stronger effect on the commu-
nity. Keystone species in other communities
include prairie dogs in the American west-
ern plains, hummingbird pollinators in the
Sonoran Desert, and sea stars in intertidal
waters (Figure 20.4).
Keystone species are often recognized only
when they go missing and their disappearance
results in dramatic changes to the rest of the
community. And that is exactly what happened
in Yellowstone in the 1920s: the mighty gray
wolf disappeared. Or, to be accurate, the mighty
gray wolf was exterminated.richness of species had changed in Yellowstone
during the previous century. The relative species
abundance had changed because aspen trees and
willows were in decline, elk and coyote numbers
had increased, and bison and beaver populations
had decreased. Many of the common inhabi-
tants of Yellowstone had changed in abundance;
therefore, the relative species abundance of this
community had changed.
The species richness had also changed
because early in the twentieth century, a
keystone species, the gray wolf, had gone
missing. The total number of species in the
community had gone down by only one, but
that seemingly small change had a big impact.
A keystone species is a species that has a