M. amboinensis(average weight 149 g) or a repre-
sentative of the Reinwardtoenasuperspecies (aver-
age weight 297 g). This suggests that a certain
degree of niche difference, signified by a weight
ratio of 1.5–2.0 between members of the same guild,
may be necessary to enable coexistence (Diamond
1975 a). Diamond undertook a more formal analysis
of the species combinations to ensure that they
were indeed non-random, and thus evidence for
competitive effects in island assembly. There are 4
Bismarck cuckoo-dove species (represented by the
letters AMNR), so 15 possible combinations, but
only 6 combinations were actually observed on the
26 islands having one or more cuckoo-dove species.
With such a small sample size relative to the num-
ber of permutations, it is not expected that each
of the 15 possible combinations would occur. The
analyses thus relied upon a modelling exercise
designed to test whether the empirical data exhibited
a significant degree of departure from a random
processes of assembly.
The interpretation of the combinational patterns
begins with the incidence rules, which determine
that certain species combinations are ‘forbidden’
(i.e. do not occur). The second form of rule was
termed a compatibility rule. The allopatric
M. mackinlayiandM. nigrirostrisexemplify this rule,
as they are so ecologically similar in their resource
requirements (if not in their incidence characteris-
tics) that they are presumed to be unable to co-occur
other than on the most temporary basis. Diamond
postulated that the two are probably the product of
fairly recent speciation from within a former super-
species. Thus the combinations MN, MNR, AMN,
and AMNR cannot (i.e. do not) occur. The final rule
type,combination rules, in effect mops up the
unknowns. For instance, Diamond calculated on
the basis of the incidence functions that the combi-
nation AMR should occur frequently on islands
with a medium species number. It is allowed by
the compatibility rules which were deduced, yet it
was not found to occur within the data set. This112 COMMUNITY ASSEMBLY AND DYNAMICS
147 ° E 150 ° E 153 ° EKey0 °3 ° S6 ° SBismarck SeaM. mackinlayi
M. nigrirostris
NeitherNew
GuineaNew BritainFigure 5.2Checkerboard distribution of cuckoo-doves (Macropygia) in the Bismarck region. Of those islands for which data are available,
most have one, but no island has both species. (Modified from Diamond, J. M. (1975) In Ecology and evolution of communities(ed. M. L. Cody
and J. M. Diamond), Fig. 20. Copyright © by the President and Fellows of Harvard College. Reprinted by permission of Harvard University Press.)