Biology Reference
In-Depth Information
are often unimodal or positive relationships, but absence of, or negative
correlations occur as well. At regional scales (200-4000 km), an absence
of, or negative correlations are about as common as unimodal or positive
relationships for plants, and somewhat more common for animals. Also, as
pointed out by Rohde (
1998a
), an element of circularity may be involved
when considering productivity: increased productivity may well be the
result of greater diversity and not vice versa. Thus, highly diverse coral
reefs are highly productive, but they have evolved in low productivity
seas (Rohde
1998a
). Experiments have shown that increasing biodiversity
by a factor of two or three increases productivity by the same factor
(Kareiva
1994
).
Spatial heterogeneity
A very well supported example for the effect of spatial heterogeneity on
diversity was recently provided by Rahbeck and Graves (
2001
). They
analysed the geographic ranges of 2869 species of birds breeding in South
America, which represent almost one third of the world's bird fauna.
Factors considered were the influence of climate, quadrat area, ecosystem
diversity, and topography. The analysis was conducted at 10 spatial scales
(about 12 300 to 1 225 000 km
2
quadrat area). Regional variability in
species richness correlated best with topography, precipitation, topogra-
phy
latitude, ecosystem diversity, and cloud cover. Ranking of these
factors depended on the scale used for analysis. Direct measurement of
ambient energy (mean and maximum temperature) were of ancillary
importance. In toto, humid montane regions near the equator have the
greatest diversity. The authors conclude that a synergism between climate
and coarse-scale topographic heterogeneity ultimately controls terrestrial
diversity from the poles to the equator. It must not be forgotten, how-
ever, that temperature - although not giving good correlations with
diversity as such - is the primary factor affecting climate.
Distinct latitudinal gradients in marine plankton show that increasing
diversity towards the tropics does not depend on an increase in hetero-
geneity. Indeed, there is no general trend of increasing heterogeneity
towards low latitudes. Exceptions are the presence of a greater variety of
habitats in tropical mountain ranges, because there we have tropical
habitats at the base, to cold habitats near the peaks (e.g., K¨ rner
2000
).
Indeed, South America is the habitat where the effect of spatial hetero-
geneity should be most marked, because the Andes extending along the
continent provide an increasingly wide range of habitats towards the
