152 SCALE AND ISLAND ECOLOGICAL THEORY: TOWARDS A NEW SYNTHESIS
Box 6.1 Implications of anthropogenic changes in richness of oceanic islands for island theory
In Chapter 5 we considered how introductions
of exotic species to oceanic islands provided
inadvertent experiments in island assembly
structure. Here we consider the implications of
anthropogenic impacts on island species number.
Saxet al. (2002) took the approach of estimating
historical richness prior to human-induced
extinctions and colonization events, and
comparing these values with richness subsequent
to these processes. They did so for vascular plants
and for land birds on 11 and 21 oceanic
islands/archipelagos, respectively. As shown in the
figure, the number of naturalized exotic plant
species greatly exceeds the number of extinctions
(panel A), whereas for birds, exotic additions
approximately match the number of extinctions
from the avifaunas (panel C). Hence, in
comparing current with historic richness
estimates, humans have led to an approximate
doubling in size of the floras (panel B), but little
net change for birds (panel D). In terms of actual
numbers, the number of extinctions varies
between 0 and 71 for plants, and 1 and 64 for
birds: in both cases, the Hawaiian islands
providing the highest value.
These results necessarily involve a number of
caveats concerning data quality, lumping of
islands within archipelagos, and so forth. But,
they are intriguing if viewed through the lens of
the equilibrium paradigm. Taking a simplistic line
of reasoning, we might argue as follows. The
floras of the oceanic islands have approximately
doubled in size, as naturalizations have vastly
exceeded extinctions. Hence, the islands must
originally have been below their intrinsic capacity
for richness, i.e. they were non-equilibrial. The
birds, in contrast, have seen a balance of
extinctions and establishments, and must thus
have been in equilibrium historically, and have
now re-established their equilibria.
How might we explain away the apparent
mismatch between the plant data and the
equilibrium condition? Many of the islands in the
study once possessed extensive areas of forest
that have been cleared for agriculture and
settlements. The removal of forest cover and its
replacement by predominantly low-staturevegetation has provided ideal conditions for the
establishment of numerous species of
(predominantly) herbaceous weeds, which have a
much smaller modular size than the pre-existing
tree flora, i.e. many more species can be fitted in.
Moreover, the bombardment of the islands by
introduced plants effectively means that the
immigration rate has been radically altered,
and in many cases, continues to be. For example,
there is no sign of a slowing down of the pace of
introductions and establishment of exotic species
in the Galápagos (S. Henderson, personal
communication). It is thus evident that we have
simultaneously altered both immigration rate and
carrying capacity. Hence, the enrichment of the
floras may indicate not previous non-equilibrium,
but that we are currently en routeto some new
anthropogenically determined enhanced
equilibrium point.
Unfortunately, if we are to allow this argument
to stand for plants then the same arguments
should apply to birds, in which case their numbers
should also have gone up. We now have to
defend the lackof change in bird richness. Birds
differ in having a much higher proportion of
losses of native species (from a variety of causes,
e.g. Blackburn et al. 2004) than the case for
plants, and it turns out that the lost species have
on average been replaced by exotics. As discussed
in Chapter 5, not all introductions of birds to
oceanic islands are successful, so perhaps for birds
we see an equilibrium condition being
re-established despite the hike upwards in
immigration events. This in turn would imply that
on average, the intrinsic carrying capacity of the
island is determined by the resource base it
represents rather than by the particular
vegetation cover.
Have we squared the circle? Perhaps, but if so,
it is by special pleading, and we have ended up
with an argument that indicates that a substantial
alteration to the immigration rate and to
vegetation cover has had great impact on plant
numbers but not on bird richness. Moreover,
there is a great deal more to understanding
the factors governing anthropogenic changes
to island species numbers, whether taking a