80 SPECIES NUMBERS GAMES: THE MACROECOLOGY OF ISLAND BIOTAS
Researchers use SACs to estimate and compare
the species richness of different localities
(Magurran 2004). SACs based on design (3)
are frequently used in plant ecological
research to determine an appropriate
minimum plot size to use for sampling
vegetation. As sampling effort is represented by
area, these plots are in fact often called
species–area curves.
Species–area curves
The term species–area curve has been applied to
two very different types of plot of species
number versus area. First, it has been used for
species accumulation curves where the yaxis
represents the cumulativenumber of species and
where the xaxis often (but not in all studies)
represents contiguous areas (as above). Second,
it has been used for plots of the species number
per sample, where the xaxis often (but not in allstudies) represents spatially separated areas, such
as islands. The use of the same term for such
very different phenomena has been and remains
the source of considerable confusion (e.g.
compare Scheiner 2004a,bwith Gray et al.
2004). To save us from future confusion, Gray
et al. (2004) argue for restricting the use of the
term species–area curve to plots of numbers of
species per sample against sample area, but the
dual use of the term seems too deeply
embedded for this proposal to succeed.
Moreover, it is easy to find yourself using the
same acronym for species accumulation and
species–area curves. Our solution here is to
avoid the use of the term species–area curve
when a more precise term can be used: and
we hope that the key distinctions between
different types of analysis can then be more
readily grasped. Just remember in reading
the wider literature to check what type of
analysis is being done under the heading
of species–area curve or species–area
relationship. Scheiner’s (2003) scheme
shown in the figure provides a useful starting
point in appreciating differences between
analyses.
Island species–area relationships: ISARs
Similar to the species–area curve, different authors
may mean different things when they talk about
the species–area relationship (SAR, or sometimes
SPAR). In the present work we attempt to restrict
the use of the term to refer to analyses of the
number of species found on each of a set of
distinct islands or habitat islands in relation to the
area of each unit. Thus, the only permitted
sampling design is the non-nested type (d) in the
graphic. To make this explicit we have added the
term island, to create the acronym ISAR.(Dammerman 1948) colonization of the Krakatau
islands. MacArthur and Wilson linked these
observations to work (by Preston 1948, 1962 and oth-
ers) on the distribution of individuals within an
assemblage, i.e. the Species–Abundance Distribution
(or SAD; Box 4.2), which had established the
prevalence of rare species in virtually all ecological
assemblages.
MacArthur and Wilson’s theory combines all
these elements. In essence, from among the rare
species typical of ecological systems, those
species present in an island system at very low(a) (b)(c) (d)Classification by Scheiner (2003) of different types of sampling
layout used in constructing species–area curves: (a) strictly nested
quadrats, (b) a contiguous grid of quadrats, (c) a regular but non-
continuous grid, and (d) a set of areas of varying size, often islands.