different phases of a succession that recurs regionally, and generally with
greater fidelity along isobaths. Fast-growing, mobile species
(“opportunists”) can take advantage of empty spots. Whole successions are
possible, from early colonists through to permanent “climax” species, or at
least permanent until the next disturbance. Overall, the endless cycle of
disturbance allows co-existence of a large number of forms on a scale of
hundreds of meters. In the deep-sea context, this was suggested by Dayton
and Hessler (1972), who sought alternatives to the time-stability hypothesis.
They saw no evidence of real niche separation among deposit feeders; noted
that species ranges scatter along the depth gradient such that a given species
may live with several different assemblages (which does not suggest tight
biological accommodation); and proposed that population density is too low
to generate much interaction among individuals. Jumars (1975, 1976)
showed that there is indeed small-scale patchiness in deep-sea benthos. Of
course, patchiness could be a mechanism of biological accommodation in
its own right. Rex and Etter (2010) call this “the principal paradigm for
explaining local species coexistence”.
3 It can be shown in mathematical models of competition that competitive
exclusion will not run its course if predators keep competing species from
reaching the population limits set by resources. Given suitably unselective
predators (“croppers”), exclusion never acts and diversity is sustained.
Dayton and Hessler (1972) brought this under the rubric of disturbance, but
really it is different.
4 Rex (1981) and Grassle and Morse-Porteous (1987) invoked an
“intermediate disturbance” hypothesis (attributable to Joseph Connell). That
is, species diversity will be low where disturbance is frequent and violent,
only allowing motile opportunist species to survive, and where there is no
disturbance so that competitive exclusion can run its course. At some
intermediate level of disturbance, the mosaic of patches − ranging from
recently settled opportunists to fully accommodated climax assemblages −
will have its greatest complexity and diversity will be highest overall. This
is, of course, simply a gloss on explanation “2” above.(^) There may be other aspects to the explanation, but probably all of at least this set of
mechanisms are at work to some degree. Grassle and Sanders (1973) offered an
answer to Dayton and Hessler. If you read it these several decades later, you can still
feel the intensity. Work on the general issue continues, for example, the work of Wei
et al. (2010) on the shelf-to-abyss gradient of diversity in the Gulf of Mexico. Rex
and Etter (2010) review the data and the arguments in a useful book, extending the
discussion to meiofauna and megafauna. Many benthic ecologists have moved on to
studies of population biology in individual species and to the interaction of given