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| Despite two centuries of exploration, our understanding of factors
determining the distribution of life on Earth is in many ways still in
its infancy. Much of the disagreement about governing processes of
variation in species richness may be the result of differences in our
perception of species-richness patterns. Until recently, most studies
of large-scale species-richness patterns assumed implicitly that
patterns and mechanisms were scale invariant. Illustrated with examples
and a quantitative analysis of published data on altitudinal gradients
of species richness (n = 204), this review discusses how scale effects
(extent and grain size) can influence our perception of patterns and
processes. For example, a hump-shaped altitudinal species-richness
pattern is the most typical (c. 50%), with a monotonic decreasing
pattern (c. 25%) also frequently reported, but the relative
distribution of patterns changes readily with spatial grain and extent.
If we are to attribute relative impact to various factors influencing
species richness and distribution and to decide at which point along a
spatial and temporal continuum they act, we should not ask only how
results vary as a function of scale but also search for consistent
patterns in these scale effects. The review concludes with suggestions
of potential routes for future analytical exploration of
species-richness patterns. | |
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