Planktic foraminifera reveal differentiated relations with
the physical environment which characterise the individual species.
Only G. rubescens and G. tenella, and G. calida and G. siphonifera
do not show clear differences in their adaptations. This may
either suggest to include each pair in one taxonomic category,
or the existence of unknown ecologic niches. These four species
require taxonomic and biologic research.
Most species are widespread but are rare (relative abundance
<1%) over at least 50% of their biogeographic range. More
than half of the species does not exceed 5% in relative abundance
in any sample. As a consequence, the number of counted specimens
(494 ± 180) and the resultant counting error is close
to the limits for reliable interpretations of much of the data.
The present counting methods do not allow to characterise species
at the fringes of their biogeographic range and to exploit their
paleoceanographic potential in these areas.
For some species evidence emerged for reasons of their endemism
in certain ocean basins. Species adapted to high salinity may
be excluded from the Indian Ocean or may be rare because they
exist at the margin of their ecologic range. Short-term changes
in the biogeographic ranges of these species may relate to environmental
change. On longer timescales migration may also result from evolutionary
change in the adaptations of species.
The individual adaptations can also shed light on large-scale
differentiation in planktic foraminifera. The limit between high
latitude and low latitude assemblages in planktic foraminifera
appears gradual when observed against sea surface water temperature
and other parameters. In plots of relative abundances vs. vertical
temperature gradients, a well defined biogeographic boundary
becomes evident in regions where the vertical temperature gradient
between the sea surface and 200 m is about 6 °C in summer.
The adaptations of individual species can be exloited in paleoenvironmental
analyses with planktic foraminifera as tracers. It will be subject
to future tests to include species in such analyses which are
sensitive to certain parameters, and to reduce the noise introduced
by unsensitive species. New applications seem to be possible
in the reconstruction of vertical temperature and density gradients.
A number of species can characterise mainly winter conditions.
They are probably applicable in mapping polar to subtropical
water masses, and some high latitude species (e.g. T. quinqueloba)
may indicate areas of intermediate or deep water formation. Semiquantitative
analyses are also possible when relative abundances of species
exclude certain intervals of conditions, e.g. when a species'
relative abundance is only exceeded above a certain salinity.
Any application of planktic foraminifera, however, needs to consider
the multivariate character of their adaptations. For most species
the biological background of these adaptations is unknown.