YSM Issue 89.1

FOCUS
evolution
PREDATOR
vs.
PREY
Who’s changing
whom?
by Sonia Wang
art by Ashlyn Oakes
Late October, twilight, Gorton Pond. The
water lies still, reflecting the pitch-black
night sky and the outlines of the fiery
orange autumnal trees bordering the shore.
The pond’s alewife juveniles are preparing for
their migration to sea as their six-month stay
in the freshwater pond comes to an end.
Four miles north of Gorton Pond, Pattagansett
Lake has a similar environment and
population of fishy residents, including alewife
and their predators, the chain pickerels.
But unlike Gorton Pond’s inhabitants, Pattagansett’s
cannot migrate; Pattagansett is a
landlocked lake.
These two bodies of water would have had
almost identical ecologies and fauna a few
hundred years ago. But European colonists in
southern Connecticut built permanent dams,
disconnecting lakes like Pattagansett from the
sea. Within these dammed lakes, landlocked
fish populations have since evolved in
ways that spark the interest of ecologists
and biologists alike. Among these curious
scientists is Yale’s David Post, a professor of
ecology and evolutionary biology.
Over the past decade, Post’s team of
researchers has studied the alewife and chain
pickerel populations in 12 lakes including
Gorton and Pattagansett. The group’s most
recent paper, published in the journal
Nature, showed that changes in the alewife
population can drive diversification and
adaptation in the pickerel population. This is
a departure from ecology’s traditional focus
on the trickle-down impact that predators
have on prey. Rapid predator evolution,
occurring over a short time span, can be
difficult to observe. But the Connecticut
lakes conveniently exhibit isolated predatorprey
systems, enabling the recent Post lab
study and others like it. Research on alewife
and pickerel fish is providing revolutionary
insight into the predator-prey relationship in
reverse.
A split history
Post first began studying alewife in 2004
after he recognized that there are two forms of
alewife that are quite literally defined by their
ecologies: The anadromous form migrates
to the sea, and the landlocked form cannot
migrate and is restricted to its home lake.
The anadromous fish of Gorton Pond
and the landlocked fish of Pattagansett Lake
were at one point the same population of
migrating, anadromous alewife. Anadromous
fish are born in freshwater areas and migrate
to saltwater environments, returning to
freshwater lakes and rivers only later in life
to spawn a new cycle of migration. It was
not until after the construction of manmade
dams that alewife trapped inside Pattagansett
began to evolve in isolation while Gorton
Pond’s anadromous alewife continued to
migrate. The ecological isolation affected a
trend of increasing divergence between the
two populations. Distinctions between the
two types of fish would become apparent
across New England over the next 300 years.
One component of this divergence is
habitat. Any body of water — marine or fresh
— is divided into distinct regions such as the
littoral and pelagic zones, which differ in their
chemical makeup and species composition.
The near-shore littoral zone rings the lake and
contains many unique habitats because of its
numerous distinct species of rooted plants
and animals. Meanwhile, the pelagic zone
is what is considered open water, in which
structure is defined by the thermocline, or the
temperature gradient across varying depths.
Because temperature drastically affects the
amount of oxygen and nutrients dissolved
in the water, the thermocline can impact the
biology of the organisms living in the various
layers of a pelagic habitat.
In every lake observed by Post and his
colleagues, anadromous alewife lived in
both the littoral and pelagic zones during
their freshwater phase, whereas landlocked
alewife lived exclusively in the pelagic zone.
Although the reason for this choice of
12 Yale Scientific Magazine December 2015 www.yalescientific.org