August 12, 2020
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How is the deep sea so diverse?  The struggle is real for late 1900s ecologists

The year is 1967.  A
paper is published that causes pandemonium in the scientific community.  The discovery causes a fundamental shift in
thinking divergent from long-held beliefs.  And like the discovery that the Earth does
indeed revolve around the sun and not the other way around, the discovery of
exceptional diversity of life on the deep-sea floor radically changed the way
we view the natural world.

Prior to 1967, the environmental extremes of the deep were
thought to limit life. The deep sea is dark (can’t-see-your-hand-in-front-of-your-face
dark), cold (only-four-degrees-above-freezing cold), and under an extreme
amount of pressure (one-elephant-on-each-square-inch-of-your-body
pressure).  This suite of factors should
make survival challenging, and thus for a century, scientists assumed the deep
sea was biologically a desolate wasteland.  Even after the discoveries of animals living
at extreme depths in the late 1800s, Victorian scientists expected that there could
not be a diverse array of animals surviving in the deep sea.  Enter Robert Hessler and Howard Sanders who in
1967 used newly developed sampling devices to discover that the deep sea is shockingly
diverse, and perhaps just as diverse as tropical shallow-water habitats. 

A lonely rattail swims over the abyssal seafloor.

Scientists were completely baffled as to how high diversity could
occur in such a bleak place.  They began
to throw out theories, but they were limited by the little data that had been
gathered from a poorly explored deep ocean. 
The scientific publications of this time on deep-sea diversity read like
there were a few people in a room with a whiteboard, writing everything they
remember from their ecological textbooks, talking through each theory, slowly
crossing off possibilities, and working their way down the list.  

Small marine animals called macrofauna — snails, worms, clams, and other creatures no bigger than a pencil eraser — live and feed in the seafloor sediment. In an area the size of a coffee table, there may be more than 300 species of macrofauna in deep-sea sediments. Credit: Craig McClain

Howard Sanders began by writing “Specialization” on the
whiteboard with his paper introducing the Stability-Time Hypothesis in
1968.  He suggested that because the deep
sea is monotonous and predictable (i.e., it is stable), populations have the
evolutionary time to become newly specialized in how they feed. Over time, these
populations become so specialized they evolve into totally new species, eventually
driving diversity up.  Further research
and explorations indicated that the premise of this argument was wrong- the deep
sea is actually not that stable.

Then, Paul Dayton and Robert Hessler walked up to the board
and scratched off the “Specialization” idea with their paper in 1972 entitled “The
role of biological disturbance in maintaining diversity in the deep sea.”  The pair do not argue against the idea that
the deep sea is predictable and stable. 
In fact, they favor the idea… except for the part where they proved that
deep-sea species are actually not
more specialized than shallow water species.

“Specialization” got a strikethrough on the whiteboard, and
Dayton and Hessler wrote “Predation” below it. 
 The duo introduced a specific
type of predation pressure they labelled “biological cropping.”  No, biological cropping is not deep-sea
animals learning agricultural techniques… but a combination of predation and
deposit feeding.  Animals can eat other
animals either intentionally (e.g. hunting down prey) or unintentionally (e.g.
stuffing everything you come across into your mouth and it just so happens that
you get a live one).  This “cropping,” whether
accidental or not, reduces competition by preventing one or a few abundant
species from monopolizing the resource. 
These species get knocked out, allowing far more species to get a piece
of the proverbial pie. Nobody gets sent into extinction by competition.  Dayton and Hessler’s idea is not necessarily that
diversity is driven to be high in the deep sea, just that it is not limited.

Dayton and Hessler’s “Predation” idea never got fully
scratched off the list, but the difficulty of testing the idea and conflicting
results have led many to write large question marks next to it.   Many
other ideas now are situated below “Predation,” including: “Disturbance,” “Patchiness,”
and “Successional Dynamics.”

Ultimately, those of us in the deep-sea scientific community
are still today standing around the dry erase board bouncing many of these same
ideas off each other.  Sometimes we
manage to cross one off the list, or add one, or at least add to our
understanding of the ideas.  One thing is
clear though, we still haven’t gotten it all figured out.  So… anyone have a dry erase marker?