ECOLOGICAL RESPONSES TO EMPTY NICHE SPACE 175Species
impoverishmentContinental
resourcesSpecies richnessIsland
resourcesSpecies
abundanceDensity compensationFigure 7.1Density compensation in islands. Island isolation results in species impoverishment (lower obtuse triangle), which promotes the
acquisition of available resources by the species present on the islands, allowing an increase in their density (upper obtuse triangle). From Olesen
et al. (2002), slightly modified.
Table 7.1Niche enlargement and density inflation in tit species (great tit Parus major, blue tit P. caeruleus, coal tit P. aterand crested tit
P. cristatus) in matching habitat gradients on Corsica compared with the nearby French mainland. Values are breeding pairs/10 ha. Habitats
follow a gradient of increasing structure complexity. Each species occupies on average more habitats and has higher population sizes on
Corsica than on the nearby mainland. The crested tit (P. cristatus) is not present on Corsica (from Blondel and Aronson 1999)
Species Low Medium High Low High Old
matorral matorral matorral coppice coppice oakwood
Mainland
Great tit 2.2 3.1 3.2
Blue tit 11.5
Coal tit 0.2
Crested tit 1.8
Total mean 2.2 3.1 16.7
Corsica
Great tit 1.6 1.7 2.5 3.6 2.6 4.7
Blue tit 0.5 0.2 3.3 7.9 14.2
Coal tit 1.2 2.1 4.1
Total mean 1.6 2.2 2.7 8.1 12.6 23.0
mainland has four. Comparable habitats have more
tit species in Corsica than in the mainland, and the
Corsican population densities of all tit species are
greater.
Several other mechanisms have been suggested
that may influence density patterns in addition to
those mentioned (e.g. see MacArthur et al. 1972,
Wright 1980). It is hard, therefore, to generalize on
the combination of factors involved: they are spe-
cific to particular species on particular islands. It is
sufficient for present purposes to note that main-
land and island species densities can vary substan-
tially. The phenomena of ecological release and
density inflation do not apply universally to all