Biology Reference
In-Depth Information
in the world: the California-Mexico coastal region; the Chilean Matorral; the Cape region of
South Africa (also called the fynbos); and, of course, the Mediterranean region itself. The
Mediterranean region, which offers one of the world's most benevolent climates for humans,
is full of plants with narrow ranges because habitat conditions there are often extremely var-
ied. In all five regions, characterized by winter rains and hot, dry summers, fire is an import-
ant part of the ecosystem, and the interaction of fire, climate, and different soils, many of
them poor, generates a hotbed of habitat extremism.
The soils of Madagascar, New Caledonia, and other ancient islands that formed part of
Gondwanaland often contain concentrations of heavy metals. Soils contaminated with heavy
metals create a challenge for plants—much more so than for birds or mammals—but also
present an extraordinary opportunity. By overcoming nutritional and water constraints of un-
usual soil environments, an evolving plant genotype escapes the customary tangled mass of
vegetation and colonizes what may be virtually free space, with the benefit of reduced com-
petition. For example, in the area around Cape Town, South Africa, the naturally rare plants
are not a random bunch biologically. They are for the most part species that are killed by
fire but whose seeds germinate in the burned-over soil and, like our jack pine, have short
dispersal distances. South African biologists believe that this biology predisposes lineages to
produce rare species. George Schatz, an expert on the flora of Madagascar, estimates that a
considerable number of that island's 10,000 endemic species are strongly associated with a
specific substrate, just as the Kirtland's warbler is to its sandy soils.
A final category of extreme habitat specialization in plants is that made up of climate
refugees. These are species with a long history that have become “trapped” in refugia, a term
biologists use to indicate a location of an isolated or relict population of a once more wide-
spread plant species that persists today only in pockets where the microclimate still resembles
what the plants experienced when they evolved. Worldwide, this group is of huge import-
ance and includes many species. The widely planted dawn redwood (genus
Metasequoia
) is
today represented by only 6,000 individuals in nature but was once one of the most common
and widespread trees in forests across much of the Northern Hemisphere. The same is true
for cycads, a group exemplified by rarities that we know as houseplants such as
Zamia
and
those in the genus
Cycas
. Cycads are ancient palmlike seed plants that were extremely wide-
spread in the age of the dinosaurs. More than 20 percent of the nearly 300 cycads are ranked
as critically endangered or endangered. Although cycads live a long time, they reproduce in-
frequently and now have such small populations that further habitat destruction or theft for
the plant trade puts some species at great risk of extinction.
The key point, I realized, is that substrates have a huge effect on plants, which typically
cannot disperse their seeds far from the parent soil, in contrast with the dispersal capabilities
of many animals. Thus globally, either unique or particularly inhospitable substrates, com-
bined with widely varying climatic conditions of rainfall and temperature, yield tens of thou-
