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Summary: 2261
SPECIAL FEATURE
Latitudinal Gradients1
In 1772, J. R. Forster sailed with Captain Cook in search of the great southern continent
hypothesized by geographers to be necessary to ``balance'' the world. During the three-year trip
they ranged from below the Antarctic Circle to the southern tropics, and Forster sampled the
flora throughout the Pacific. On his return, he published his account of the voyage in which he
noted that different islands support a variable number of species, which he attributed primarily
to climate (tropical islands support more species than Antarctic islands), and secondarily to island
``circumference'' (large islands support more species than small ones). Thus, we have what is
probably the first documented geographical pattern in ecology, as well as an explanation for it
that differs from some modern explanations only by lacking an evolutionary component. Of
course, we now refer to the first part of the pattern as the ``latitudinal diversity gradient,'' and
the second as a ``speciesarea relationship.''
The study of geographic patterns faces two fundamental challenges. First, ecological data across
very large scales are not easy to generate. For example, to understand diversity gradients, we
need global estimates of species richness for particular taxonomic groups. Given our poor state
of knowledge about the diversity of most taxa, we are mostly restricted to the study of a handful
of groups or, alternatively, to the geographic regions where we have reasonable databases of
species distributions. If this were not problem enough, to explain gradients we also need a wide
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