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(a)
o
n
R
More species because
greater range of resources
(larger
R
)
R
(b)
More species because
each is m
o
re specialized
(smaller
n
)
(c)
More species because
each overlaps more with
its neighbors (larger
o
)
(d)
More species because
resource axis is more fully
exploited (community
more fully saturated)
Fig.
10.3
A simple model of species richness. Each species utilizes a portion
n
of the
available resources (
R
), overlapping w ith adjacent species by amount
o
. (From Begon et al.,
2006, after MacArthur, 1972.)
Drawing the lines between alpha, beta and gamma richness is to some extent a matter of choice.
In Gering's study of beetle species richness on trees in Ohio and Indiana, USA, alpha richness was
calculated as the average number of beetles identifi ed per site in Figure 10.4. Beta richness is then
the difference between alpha richness and richness for the region as a whole (gamma). Beta rich-
ness itself had two components at increasing scales - the average beetle richness for each of three
different sites within an ecoregion (ecoregions contrast in their glaciation history, topography and
soil type), or the average for each of two ecoregions present in the region as a whole. Figure 10.4
shows how regional beetle species richness is partitioned between the local site scale (alpha), the
between-sites scale (beta
1
) and the between-ecoregions scale (beta
2
). Evidently, designation of a
nature reserve in a single site would safeguard fewer than 50% of the beetles. Addition of extra
sites would bring the percentage up to about 70%, but addition of another ecoregion would be
necessary to safeguard most species in the region.
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