0198566123.pdf

ISLAND SPECIES NUMBERS AND ISARS: WHAT HAVE WE LEARNT? 97

Species–energy theory—a step towards a more

complete island species richness model?

The term species-energy theoryappears to have
been coined by Wright (1983), who modified the
ISAR approach by replacing island area with a
measure of the total energy available to a particu-
lar trophic level. He did so both in a graphical
model equivalent to Fig 4.1 and operationally in
the regression of island species numbers. His
approach was thus very similar to that later
adopted by Triantis et al. (2003) in the choros
model, i.e. he replaced area in the species–area
regression with the product of energy and island
area. As with area, available energy does not esti-
mate directly the variety of resource types present
on an island, but it is likely to be correlated with it.
Wright argued that, particularly for sets of islands
of variable per-unit-area resource productivity, the
species–energy model should work better than the
original.
Wright provided two empirical demonstrations,
first for angiosperms on 24 islands worldwide,

ranging in size from Jamaica (12 000 km^2 ) to the

island continent of Australia (7 705 000 km^2 ), and

second for land (including freshwater) bird species

on an overlapping set of 28 islands, varying from

Sumba (Indonesia) (11 000 km^2 ) to Australia. For

angiosperms he used total actual evapotranspira-

tion (total AET), a commonly used, although not

ideal, parameter (see O’Brien 1993, Field et al. 2005),

which estimates the amount of water used to meet

the environmental energy demand, and found

that the model S123(total AET)0.62explained 70%

of the variation in the logarithm of species number.

For birds, he used, total net primary production

(total NPP), and the model S358(total NPP)0.47

accounted for 80% of the variation in log species

number. In both cases, the original species–area

model explained significantly less variance.

Wright’s data were drawn from a set of islands

ranging from the equator (e.g. New Guinea) to the

arctic island of Spitsbergen (78 N), and thus were a

far more climatically and biogeographically hetero-

geneous set than would normally be considered.

They were thus well chosen to demonstrate how

Immigration/Extinction Dynamics

(MacArthur/Wilson Model)

Speciation

Island area

Species richness

Small islandt-1 t-2

effect

(stochasticfactors)

Figure 4.6Might the island species–area relationship be scale-dependent? This model, taken from Lomolino (2000c), Fig. 6, examines the idea
of thresholds in an arithmetic plot of species richness versus area. Starting at the left-hand end, there is little change in species number until a
critical threshold is reached, but beyond area t
1, species number increases rapidly, as a function of the immigration/extinction dynamics of the
EMIB. With islands larger than area t
2, species number shows a further increase, as it is afforced by in situspeciation. The ‘small-island effect’
is shown by many island data sets, but evidence for the existence of the second threshold and the upward curve towards the right-hand end of
the plot is lacking. See discussion in text.