When we think of
predators most of us think of large mammals such as wolves or bears. We know
these predators play a role in maintaining a diversity of species in their
ecosystem. We also know that disturbances, both natural and man-made can alter
the prey diversity of the environment. Scientists at Centre National de la
Recherche Scientifique in Paris have studied these interactions and have
determined that predation and disturbance interact to shape species diversity.
However, these scientists did not examine typical mammalian predators; they
used bacteria.
These
scientists used a very small bacterium called Bdellovibrio bacteriovorus as their microscopic predator. This
bacteria feeds on other bacteria by burrowing through the wall of its prey and eating
the victim’s insides. Then, it uses the victim’s body as a home to replicate
itself before moving onto the next prey. The prey the scientists chose for the Bdellovibrio was a bacterium called Pseudomonas fluorescens. This bacteria
is important to the experiment in the way it can diversify into three different
morphs based on its environment. The three morphs were referred to as SM, WS,
and FS mutants.
These
scientists inoculated 36 small bottles of nutrient broth with the same amount
of P. fluorescens. Half of these bottles also received B. bacteriovorus. All 36 bottles were
swirled at 200 revolutions/minute to ensure the environment was the same
throughout, which meant the P. fluorescens diversification could not be
predicted. To introduce disturbance into the experiment, the scientists diluted
at different intensities (100-fold and 1000-fold) and at different frequencies.
These disturbances also make the prey bacteria temporarily resistant to the
predator. The scientists also tested competition by setting up 10 bottles with
two competing morphs of P. fluorescens, half
of which received the predator as well. The experiment was run for 4 days then
the contents of the bottles were plated and diversity was measured.
What
the scientists discovered was that in 17 of 18 of the cultures that did not
have the predator, the SM mutant was observed most with very low diversity. In contrast,
all of the samples with the predator showed increased prey diversity.
Surprisingly, the populations of the predators did not change much throughout
the experiment. They also found that diversification happened faster under low
frequency disturbance. However, they found low diversity occurred in the
presence of high intensity disturbance. The scientists discovered that the FS
mutant was rare, but 10 times more resistant to the predator than the SM mutant.
This meant in the presence of the predator the FS mutant dominated and in the absence
of a predator the SM mutant dominated.
What
the scientists learned from their experiment was that predators are essential
for prey diversity. In the absence of predators, the only selective pressure is
competition which results in one dominant species. With predators present, competition
and predation are present. It is rare for one prey species to be the most
resistant to predation and competition, so prey diversity increases. They
describe concept as a “trade-off” in which each mutant must trade-off between
mutations that increase their resistance to predators or mutations that
increase their resistance to competition. The scientists found their results to
be in close agreement with the Intermediate Disturbance Hypothesis (IDH). The
IDH predicts that higher species diversity occurs with intermediate frequency
and intensity of disturbance as shown in the figure below.
The findings of this experiment are
important because they support the theory that predators play a central role in
maintaining diversity of the food web below them and that this concept in
conserved across predators of all sizes. The relationship between predation,
competition, and disturbance observed in this experiment is also important in
understanding the dynamics of diversity in an ecosystem.
