few distinguishable resources, how can so many deep-sea species persist?
(^) In some habitats, the answer is taken to be that there is great temporal variability,
i.e. that continual environmental fluctuations keep the rules of the competition (the
“competitive coefficients”) shifting. No competition reaches its conclusion before the
rules change. In the deep sea, on the other hand, it was once reasonable to think there
could be virtually no variations: salinity is the same within 0.1 PSS temperature varies
within 0.2°C; food fluctuations should be damped by the transit to the seafloor (or
were thought to be in the 1970s); and so on. In the early 1970s, this great “problem”
of deep-sea ecology was the topic of an intense polemic. Scientific disputes are often
the most heated over issues that are fundamentally untestable. Explanations of
regional or inter-habitat differences in diversity, such as the difference between boreal
and tropical forests in number and equitability of tree species, were then being sought
by many ecologists, so that there were several classes of hypotheses to choose among.
Most of them appeared in the discussion of the deep-sea benthos:
(^1) Sanders (e.g. 1968, 1969) proposed that high diversity is maintained
because deep-sea communities, faced with only mild physical variations,
have evolved to ameliorate competition. Character displacement produced
finer and finer division of the resources among competing species, because
even very restricted niches are always present. Evolution of super-
specialists paid off in reduced competition and led to co-existence.
Character displacement does occur in more readily studied, terrestrial
habitats. For example, species of birds competing for food evolve longer
and shorter bills respectively, thus separating the levels at which they feed.
The character “bill length” is said to be displaced. Sanders called this
explanation of high deep-sea diversity the “time-stability hypothesis”. The
great span of time under stable, unchanging conditions has allowed very
refined subdivision of the ecological roles, which he called “biological
accommodation”. Evidence for character displacement in the deep sea is
scanty, however. Determining how four or five species of deposit-feeding,
cirratulid polychaetes might be partitioning the uppermost layer of mud is
extremely difficult.
2 Deep-sea habitats are not so constant as they seem. Large demersal fish
flop about, burying in at times. Sea cucumbers 10 cm wide drag through the
mud, digesting everything along swaths that criss-cross everywhere.
Sediment miners throw up small volcanoes of sediment. Dead whales,
sharks, and pieces of tuna fall (plop) to the seafloor (Smith & Baco 2003),
providing huge doses of food to be distributed locally as macrourid, hagfish,
and amphipod feces. These events create microstructure in the seemingly
homogeneous habitat, and that allows diversity. As the explanation is often
phrased, deep-sea benthos is “a spatial mosaic of patches”, patches in