370 ■ CHAPTER 20 Communities of Organisms
ECOLOGY
19481959San FernandoLong BeachSan DiegoRiversideEscondidoSan FernandoSanta BarbaraSanta BarbaraLong BeachSan DiegoRiversideEscondidoBoth species of wasps prey on
the same types of insects that
damage citrus crops. With two
wasp species eating the same
prey, there was less food for
both species.A. lingnanensis
A. chrysomphaliThe wasp Aphytis chrysomphali
was already resident in California
at that time.Figure 20.13
Exploitative competition: a new species moves in
Two di f ferent species of wasps—Aphytis lingnanensis and Aphytis chrysomphali—feed on the same
resource but do not directly compete for access to it.Q1: Which species appears to be the superior competitor?Q2: Why is this example considered exploitative competition?Q3: What would you predict if these species had undergone competitive exclusion? (You will
need to read the next paragraph to answer this question.)There are two main categories of competi-
tion: exploitative and interference. In exploit-
ative competition, species compete indirectly
for shared resources, such as food. In this
case, each species reduces the amount of the
resource that is available for the other species,
but they do not directly interact or come in
contact with each other (Figure 20.13). When
wolves returned and elk populations declined,
beavers had less competition for food, especially
willow trees, and the number of beaver colo-
nies in Yellowstone rose from 1 in 1996 to 12 in- By 2015, Yellowstone had an estimated
100 beaver colonies.
Elk and bears also interact through exploit-
ative competition. Elk eat the leaves and
branches on shrubs, resulting in a decrease
in the number of berries the shrubs produce.
That’s not good for grizzly bears, which love
to eat berries. Knowing of this relationship,
Ripple hypothesized that a decrease in elk
would result in an increase in berry-producing
shrubs, and that bears would be eating more
berries. To test this hypothesis, Ripple,
Beschta, and others spent 2 years analyz-
ing grizzly scat that had been collected in the
park. They compared the percentage of fruit in
current scat to that of scat data that had been