57
See also: Evolution by natural selection 24–31 ■ Ecological niches 50–51
■ Competitive exclusion principle 52–53 ■ Animal ecology 106–113 Yuccas and^
their mothsIn the hot, arid regions
of the Americas, there is
a remarkable mutualistic
relationship between yucca
shrubs and yucca moths. No
other insects pollinate these
plants, and no other plants
host yucca moth caterpillars.
A female yucca moth collects
pollen from the flower of one
yucca plant and deposits it in
the flower of another yucca,
fertilizing the plant as it does
so. The moth then cuts a hole
in the flower’s ovary and lays
an egg; she may lay several
in the same flower. When the
eggs hatch, the caterpillars
feed on the seeds developing
in the flower but do not eat
them all, leaving enough for
the plant to propagate. If too
many eggs are laid in one
flower, the plant sheds it
before the caterpillars hatch –
leaving those insects to
starve. Without these moths,
the yuccas would not pollinate
and would soon die out.
Without the yuccas, the moths
would have nowhere to lay
and nurture their eggs, and
they too would not survive.mutualistic relationship between
acacia trees and ants in eastern
Mexico. His research was one of
the first in-depth studies of such
an interaction. The two partners
were the swollen-thorn acacia and
the acacia ant, which lives in the
bullhorn-shaped thorns of the tree.
He found that queen ants sought
out unoccupied shoots, cut a hole
in one of the swollen thorns, and
laid their eggs, sometimes leaving
the thorn to forage on the tree’s
nectar. Larvae hatching from the
eggs then fed on the acacia’s leaf-
tips, with their rich supplies of
sugars and proteins. The larvae
later metamorphosed into worker
ants. In time, all the tree’s thorns
became occupied, with up to
30,000 ants living in a colony.
Janzen showed that, unless the
acacia ants were present to defend
it, the swollen-thorn acacia lost
the ability to withstand damage
caused by insects that ate its
leaves, stems, flowers, and roots.Without the ants, a tree would be
stripped of its leaves and die within
six months or a year. Because it
could not sustain growth, it was
also likely to be shaded out by
competing trees. Janzen clipped
thorns and cut or burned shoots
to remove ants from trees, and
found that the ants moved back in
when new thorns started to grow.
In return for food and shelter,
the ants provided two services
for the tree: they defended its
foliage from leaf-eating insects
and ate potentially competitive tree
seedlings growing close by. Janzen
described the acacias and their
ants as “obligate mutualists”,
meaning that one species would
die out without the other. If the ants
were removed, the swollen-thorn ❯❯ECOLOGICAL PROCESSES
Ants and their larvae shelter inside
the swollen thorn of an East African
whistling thorn acacia tree. In return
the ants swarm from their nests
to protect the tree from herbivores.US_056-059_Mutualisms.indd 57 12/11/18 6:24 PM