Having dealt with the processes occurring in the natural habitat and in
horticulture, it remains to mention some of the factors working against
a diverse habitat. The main stresses to ecosystems in Britain and other
parts of Western Europe are acidity, excess nutrients, high water tables
and heavy metals.
Research suggests that in the last 25 years in Holland, for example, there
has been a 25 per cent decrease in woodland species attributable to the
increased acidity in the air. The same survey indicated species losses of
50 per cent, 6 per cent and 5 per cent in lakes due to excess nutrients,
high water table and air acidity respectively.
|Figure 3.5 Chimney
Plants are resilient organisms, but stresses imposed on habitats such
as those near large towns, those in the wind-path of polluted air,
those watered by rivers receiving industrial effluent, and agricultural
fertilizers and pesticides are likely to lose indigenous species. The rapid
increase in annual temperatures attributed to 'the greenhouse effect' is
likely to change wild plant communities in as important a way as the
environmental stresses mentioned above.
The eff ects of specific abiotic factors
(pollutants) on plants
Continuing increase in soil acidity reduces vital mycorrhizal
activity, causes leaching of nutrients such as magnesium and calcium,
and leaves phosphate in an insoluble form. In soils formed over
limestone and chalk, the effects of acid rain are much less damaging.
levels in water and soils (especially from fertilizers
and farm silage) encourage increase in algae and a corresponding loss
of dissolved oxygen. This process (called eutrophication
) has a serious
effect on plant and animal survival. It is seen most strikingly when fish
in rivers and lakes are killed in this way.
may be released into the air or into rivers as by-products
of chemical industries and the burning of fossil fuels. Cadmium, lead
and mercury are three commonly discharged elements. While plants are
more tolerant of these substances than animals, there is a slow increase
within the plant cells, and more importantly the levels of chemicals
increase dramatically as the plants are eaten and the chemicals move up
the food chains (see also DDT).
Recent legislation has led to a greater awareness of pesticide
effects on the environment. However, herbicide and insecticide leaching
through sandy soils into watercourses continues to be a threat if
application of the chemicals occurs near watercourses. A herbicide such
as MCPA can kill algae, aquatic plants and fish.
A high water table
can have a marked effect on a habitat if the effect is
prolonged. The anaerobic conditions produced often lead to root death
in all but the aquatic species present in the plant community.
Monitoring abiotic factors
There is constant monitoring by Government agencies for the factors
mentioned above. This is especially so in National Parks, National
Nature Reserves (NNRs
) , and Sites of Special Scientific Interest
). Environmental scientists and laboratories have a range of
techniques for assessing levels of these factors. Chemical tests for
common nutrient substances and for pH can be performed in the field.
More sophisticated analysis is required for heavy metals and pesticides.
A common five day test for water quality called 'Biological Oxygen
) enables a confident assessment of a water sample’s
pollution level. An unpolluted sample would register at about 3 mg of
oxygen per unit volume per day whereas a sample polluted by fertilizers
could be about 50 mg of oxygen per unit volume per day.
Plant modifications to extreme conditions
A survey of plants worldwide shows what impressive structural and
physiological modifications they possess to survive in demanding
habitats. A few examples of British species are described here.
Marram grass (Ammophila arenaria
) living on sand dunes controls
water-loss by means of leaf lamina which in cross-section is shown to be
rolled up. It also possesses extremely long roots.
The yellow water lily (Nupar lutea
) shows the following modifications:
leaves with a thin cuticle (but with numerous stomata), large flat leaves
and a stem with air sacs.
The coastal habitats provide many examples of species which must
conserve water in salty, windy coastal conditions. Glasswort (Salicornia
) is a succulent with greatly reduced leaves which have a thick
cuticle, and its stomata remain closed most of the day. The species is
able to extract water from the seawater and is tolerant to internal salt
concentrations that would kill most plant species (see plasmolysis). An interesting halophyte is the sugar beet plant which was bred in
France from a native coastal species. It is the only crop grown in Europe
that receives salt (sodium chloride) as part of its fertilizer requirements.