189
Earthworms leave castings that
make them valuable natural fertilizers.
They not only transform the soil for
themselves but also help plants to grow.See also: Ecological niches 50–51 ■ The ecosystem 134–137 ■ Organisms and
their environment 166 ■ The ecological guild 176–177ORGANISMS IN A CHANGING ENVIRONMENT
Earthworms are highly effective
niche constructors, constantly
transforming the soil in which they
live. They break down vegetable
and mineral matter into particles
small enough for plants to ingest.
The worm casts they secrete are
five times richer in usable nitrogen,
have seven times the concentration
of phosphates, and are about 11
times richer in potassium than the
surrounding soil.
Similarly, microscopic diatoms
living in seafloor sediments secrete
chemicals that bind and stabilize
the sand. In Canada’s Bay of Fundy,
for example, the changes diatoms
make to the physical state of the
seabed allow other organisms, such
as mud shrimp, to colonize it.British biologists Nancy Harrison
and Michael Whitehouse have also
suggested that when birds form
mixed-species flocks—as many do
outside of the breeding season—
they are altering their relationship
with competitors to find more food
resources and gain more protection
from predators. The complex social
environment they create modifies
their own ecology and behavior.
In his explanation of niche
construction, Odling-Smee pointed
to ancient cyanobacteria, which
produced oxygen as a by-product of
photosynthesis more than 2 billion
years ago. This was a key factor
in the Great Oxygenation Event,
which changed the composition
of Earth’s atmosphere and oceans,
massively modifying our planet's
environment. The oxygen boost
helped create the conditions for the
evolution of much more complex life
forms—including humans. ■Ecosystem engineers
Niche constructors have been
described as “ecosystem
engineers,” a term coined in
1994 by scientists Clive Jones,
John Lawton, and Moshe
Shachak. They outlined two
kinds of ecosystem engineers.
The first, allogenic ecosystem
engineers, change physical
materials. Take, for example,
beavers building dams,
woodpeckers excavating
nest holes, and people mining
for gravel; these activities
modify the availability of
resources for other species.
When woodpeckers abandon
their holes, smaller birds and
other animals move in. If water
floods a gravel pit, ducks and
dragonflies can colonize it.
Other ecosystem engineers
are autogenic, which means
that simply by growing, they
provide new habitats for other
plants and animals. A mature
oak tree, for example, is a
suitable environment for a
broader range of insects, birds,
and small mammals than an
oak sapling. Likewise, a coral
reef provides homes for more
fish and crustaceans as it
grows larger.A European Starling in Arizona,
US, takes advantage of a hole
abandoned by a Gila Woodpecker
to make its own nest.US_188-189_Niche_constructions.indd 189 12/11/18 6:25 PM