This question is of fundamental importance to
applications of island thinking to conservation
(Chapter 10).
Pseudoturnover and cryptoturnover
If we are to assess the rate and nature of turnover of
species over time, we have to be able to measure it
accurately. Two forms of error have been described.
Cryptoturnoverrefers to species becoming extinct
and re-immigrating between surveys (or vice
versa), thus depressing turnover rate estimates: a
problem that will increase with the interval
between surveys (Simberloff 1976). A different form
of error, pseudoturnover, occurs when censuses are
incomplete, and information on breeding status
inadequate, leading to species appearing to turn
over when they have actually been residents
throughout, or alternatively when they have never
properly colonized. For both these reasons, esti-
mates of turnover (and of IandE) are dependent on
census interval (Diamond and May 1977; Whittaker
et al. 1989) as well as on the thoroughness of each
survey (Lynch and Johnson 1974; Whittaker et al.
2000).
In assessing turnover, it is important to recognize
the distinction between colonization rates and
immigration rates (Box 4.3) and to realize that it is
exceedingly difficult to measure realimmigration
and extinction rates (Sauer 1969; Abbott 1983). Most
studies of faunal or floral build-up are based on
lists of species found at different points in time. The
rates presented are perforce observed rates of
changes in lists: despite commonly being labelled
immigration and extinction rates (e.g. Whittaker
et al. 1989, 2000), they are really rather different
from the terms as originally defined.
For the extinction of a species to occur, the
species must first be present. Unfortunately, if we
accept that most extinctions are likely to be of the
rarer species, which are hard to detect, it follows
that it will be difficult to determine which have
really colonized in the first place. Only ‘proper’
colonists should be counted in determining the
extinction rate. Examination of the definitions of
terms (Box 4.3) reveals a certain fuzziness at the
centre of this business. The distinction between
immigration and colonization is indeterminate, as
it relies on what constitutes a propagule for a par-
ticular taxon (actually it may vary from species to
species) and by what is meant by ‘relatively lengthy
persistence’. Lynch and Johnson (1974) argue that
persistence (for a colonist) or absence (for an extinc-
tion event) through at least one breeding cycle
should be the minimum requirement. In practice,
most island biogeographical studies have lacked
adequate data on these phenomena, relying
principally on lists of species recorded for different
islands or different points in time, without
adequate knowledge of breeding status, etc. These
problems have plagued island turnover studies,
and have led to some pointed exchanges in the
literature.
The study that prompted Lynch and Johnson’s
critique and which purported to demonstrate
turnover at equilibrium was that of Diamond
(1969), who censused the birds of the Channel
Islands, off the coast of southern California,
50 years on from an earlier compilation. Diamond
found that among the nine islands 20–60% of the
species had turned over. However, the interpreta-
tion of this turnover was disputed by Lynch and
Johnson (1974), who, having first pointed to prob-
lems of pseudoturnover, went on to argue that most
of the thoroughly documented changes in the birds
could be attributed to human influence. This
included the loss of several birds of prey due to pes-
ticide poisoning, and the spread into the islands of
European sparrows and starlings—both part of a
continent-wide process of range expansion. They
concluded that the occurrence of natural turnover
at equilibrium had not been established by the
study. Notwithstanding these data quality prob-
lems, Hunt and Hunt (1974) pointed out an inter-
esting feature in the data for one of the islands,
Santa Barbara: the predatory land birds were repre-
sented by tiny populations, generally just one or
two pairs, and this level of the trophic hierarchy
thus underwent greater fluctuations in species
representation than birds of lower trophic levels.
Nilsson and Nilsson (1985) conducted an interest-
ing experiment by re-surveying the plants of a series100 SPECIES NUMBERS GAMES: THE MACROECOLOGY OF ISLAND BIOTAS