growth of algae or macrophytes and the abundance of fish and invertebrates.
Chemical analysis of water samples allows the level of eutrophication of different
water bodies to be compared (see Jones and Reynolds 1996 for the main
methods). The Secchi disk measures water clarity, which can be important for
birds feeding underwater and is a measure of eutrophication. This is a disk 30 cm
across with alternate black and white quarters that can be either bought or made.
It is submerged using a calibrated line with the depth recorded at which the white
and black can no longer be distinguished. It is then submerged slightly more
and raised until the quarters can be distinguished. The mean of the two measures
is used.
11.4 Vegetation
The type and structure of vegetation is important to birds in providing nest sites,
roost locations, refuge from predators, acting as food for herbivorous birds
or providing herbivores for carnivorous birds, and, by its structure, enabling or
constraining foraging.
11.4.1Mapping of broad habitat types
Mapped data on the distribution of vegetation communities is valuable for many
types of studies. Maps are usually required for large areas, so detailed descriptions
of small quadrats over the entire area are not practical. Instead, the researcher
may make detailed descriptions of a few representative samples of particular
habitat types in order to identify their defining characteristics, such as tree
density or species composition. It is essential to devise and document precise
definitions, for example, when does savannah become grassland or woodland.
Without precise definitions it is impossible to relate to other studies or repeat to
document changes. With this information it is then often possible to walk
around the study area mapping the boundaries of patches of particular habitats.
Recording habitat edges using a GPS makes this much easier. Having an aerial
photograph or high resolution satellite image of the study area is also useful. If a
print of the image is taken into the field it is then often the case that habitat type
boundaries visible on the ground can also be identified on the image and this can
save time in mapping. This is a valuable approach even when differences in the
appearance of habitats on the image are too subtle or affected by topography and
lighting to allow habitat mapping using the image alone. If rigorous comparison
between ground surveys with aerial photographs or satellite images indicates that
the latter can be reliably interpreted, then it may be possible to map habitat types
over a huge area using remote sensing.
Vegetation| 257