Generally speaking, warm tropical ecosystems are richer in
species than cold temperate ecosystems at high latitudes (see
Gaston and Williams, 1996 ,
for general discussion). A similar pattern is seen for highertaxonomic groups (genera, families). Various hypotheses
(
e.g. , environmental patchiness, solar
energy, productivity; see
Blackburn and Gaston, 1996 )
have been raised to explain these patterns. For example, it isassumed that warm, moist, tropical environments, with long
day-lengths provide organisms with more resources for growth andreproduction than harsh environments with low energy resources
(
Hunter, 2002 ). When environmental
conditions favor the growth and reproduction of primaryproducers (
e.g. , aquatic algae, corals,
terrestrial flora) then these may support large numbers ofsecondary consumers, such as small herbivores, which also
support a more numerous and diverse fauna of predators. Incontrast, the development of primary producers in colder
temperate ecosystems is constrained by seasonal changes insunlight and temperature. Consequently, these ecosystems may
support a less diverse biota of secondary consumers andpredators.
Recently, (
Allen
et al. 2002 ) developed a model for the effect of ambient
temperature on metabolism, and hence generation time andspeciation rates, and used this model to explain the latitudinal
gradient in biodiversity. However, these authors also noted thatthe principles that underlie these spatial pattern of
biodiversity are still not well understood.
Species and ecosystem diversity is also known to vary with
altitude
Walter (1985) and
Gaston and Williams (1996:
214-215) . Mountainous environments, also called
orobiomes , are subdivided vertically into
altitudinal belts, such as montane, alpine and nival, that havequite different
ecosystems .
Climatic conditions at higher elevations(
e.g. , low temperatures, high aridity) can
create environments where relatively few species cansurvive. Similarly, in oceans and freshwaters there are usually
fewer species as one moves to increasing depths below thesurface. However, in the oceans there may be a rise in species
richness close to the seabed, which is associated with anincrease in ecosystem heterogeneity.
By mapping spatial gradients in biodiversity we can also
identify areas of special conservation interest. Conservationbiologists are interested in areas that have a high proportion
of
endemic species ,
i.e. , species whose distributions are
naturally restricted to a limited area. It is obviouslyimportant to conserve these areas because much of their flora
and fauna, and therefore the ecosystems so-formed, are foundnowhere else. Areas of high endemism are also often associated
with high
species richness (see
Gaston and Spicer, 1998 for
references).
Some conservation biologists have focused their attention on
areas that have high levels of endemism (and hence diversity)that are also experiencing a high rate of loss of ecosystems;
these regions are
biodiversity
hotspots . Because biodiversity hotspots are
characterized by localized concentrations of biodiversity underthreat, they represent priorities for conservation action (
Sechrest
et al. ,
2002 ). A
terrestrial biodiversity
hotspot is defined quantitatively as an area that has at
least 0.5%, or 1,500 of the world's ca. 300,000 species of greenplants (
Viridiplantae ), and that has lost at
least 70% of its primary vegetation (
Myers
et al. , 2000 ;
Conservation International,
2002 ).
Marine biodiversity
hotspots are quantitatively defined based on measurements
of relative endemism of multiple taxa (species of corals,snails, lobsters, fishes) within a region and the relative level
of threat to that region (
Roberts
et al. , 2002 ). According to this
approach, the Philippine archipelago and the islands of Bioko,Sao Tome, Principe and Annobon in the eastern Atlantic Gulf of
Guinea are ranked as two of the most threatened marinebiodiversity hotspot regions.
Conservation biologists may also be interested in
biodiversity coldspots ; these are
areas that have relatively low biological diversity but alsoinclude threatened ecosystems (
Kareiva
and Marvier, 2003 ). Although a biodiversity coldspot is
low in species richness, it can also be important to conserve,as it may be the only location where a rare species is found.
Extreme physical environments (low or high temperatures orpressures, or unusual chemical composition) inhabited by just
one or two specially adapted species are coldspots that
warrant conservation because they represent uniqueenvironments that are biologically and physically
interesting. For further discussion on spatial gradients inbiodiversity and associated conservation practices see the
related modules on
"Where is the world's
biodiversity?" and
"Conservation
Planning at a Regional Scale."