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THE more seismic activity there is on the sea floor, the more species cluster
around the hydrothermal vents that the activity produces. These fields of vents
could one day help ecologists understand why some environments support greater
biodiversity than others.

Hydrothermal vents shoot water, heated by hot or molten rock and carrying
minerals and hydrogen sulphide, into the cold ocean depths. The warmth and
nutrients create isolated communities of giant tube worms, vent mussels and
other animals.

Cindy Van Dover of the College of William and Mary in Virginia wondered if
more densely packed vents might create greater species diversity, since this
would increase the chances that the larvae of creatures could reach a new vent
if the one they were living on died. 鈥淭he larvae have a very high chance of
going extinct before they get to the next vent,鈥� says Van Dover.

Tectonic plates move apart or 鈥渟pread鈥� at mid-ocean ridges. Rapidly spreading
ridges create more vents than slow-spreading ones, so Van Dover鈥檚 team carried
out a series of piloted submersible dives on different ridges and collected
dozens of samples from mussel beds near vents. Samples from the southern part of
the East Pacific Rise, which is spreading at a rate of 160 millimetres per year,
yielded more than 50 different species larger than 0.25 millimetres. By
contrast, vents from the slower spreading northern East Pacific Rise had between
30 and 40 different larger species, and the Mid-Atlantic Ridge, where the plates
creep apart at 20 millimetres a year, 22 species.

In addition to reducing the distance between individual vents, more active
fields could promote diversity by injecting more energy into the system.
Ecologists who want to know whether this is generally true of ecosystems might
find vents a useful tool, Van Dover says. Although they are hard to get to,
vents are at least free of variables such as weather that complicate experiments
on land.