One of the biggest mysteries in biology centers - perhaps surprisingly - around one of its
smallest organisms - single-celled ocean-dwelling plankton: specifically, why are there so many
different species?
Welcome to MinuteEarth.
In most instances, when similar species live in the exact same place and compete for the
exact same resources, only one species succeeds - and the other ones either go extinct or
have to try something different.
In California, for example, when almost identical plants known as tarweeds and rosinweeds start
growing on the same rocky hillside, the rosinweeds will take over and the tarweeds will die off.
And in the American northeast, different bird species that once competed for the same resources
in the same spruce trees now don’t; the Cape May warbler has come to dominate the
valuable top of the tree while other warbler species have had to carve out a different
living lower in the tree.
In these situations, a little tiny difference - like rosinweeds' marginally-quicker growth
in shallow soil,- gives them a slight advantage again and again and again.
It’s enough to result in one species coming out on top.
These KOs are backed up by math; we’ve figured out equations that simulate how matchups between
similar species will play out, and in almost every case the models agree that there’s
going to be a winner, and that winner will take all.
​​Over time, one species will apply its small advantage again and again - and become
the champion.
Plankton seem like they should follow this winner-take-all rule, too.
After all, most plankton species compete for the same resources within the same thin surface
layer of the ocean: one species should come to dominate, and the others should scram.
But instead, thousands of very similar plankton species all seem to coexist in relative harmony.
What gives?
One possibility is that the models are wrong - at least for plankton - because they don’t
account for the unpredictable conditions in which plankton live.
Wind and waves may be mixing up the water enough - and regularly enough - that no one
plankton species has enough time to exploit whatever tiny advantage they have and outcompete
their rivals.
Perhaps every disturbance simply puts all the competitors back on nearly-equal footing
- just like when tarweeds and rosinweeds first start growing on that rocky hillside.
The second possibility is that the models are right, but we’re using them wrong.
When scientists collect plankton from the ocean, they often use what’s called a plankton
net - basically a stocking attached to a bottle.
When they pull this contraption through the water, they may actually be sampling several
different micrometer-thick micro-environments - each with a slightly different combination
of resources.
So while it appears that several species of plankton are coexisting peacefully, perhaps
what we’re actually seeing is a jumbled up collection of species that - in their natural
state - actually dominate their own tiny, just-different-enough micro-environments.
It’s as if we counted all the birds in the spruce tree together and said “these trees
are shared by many similar species of warblers” without realizing they specialize in different
parts of the tree.
Recently, though, many researchers have come to favor a third possibility: the model is
right but occasionally spits out something weird.
It turns out that when you model five or more species competing for three or more resources,
the entire system can occasionally get caught in a chaotic loop, and no clear winner ever
emerges.
It seems like this is the case for certain groups of cave-dwelling bats; maybe the coexistence
of thousands of species of plankton is the result of that chaos, too.
One thing is for sure: as much as we know about the world, to truly understand its inhabitants
- even those as seemingly simple as single-celled sea-dwellers - we’ve got a plank-ton more
to learn.
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