Agriculture Reference
In-Depth Information
However, if the garden is managed to allow for further
stages in succession to establish, for example
X
by allowing some weeds to cover the bare soil
X
by cutting parts of the lawn higher and less
frequently
then it will support a greater biodiversity so long as it
is not neglected, resulting in a climax vegetation.
Succession
is a sequence of changes in the
composition of plants and animals in an area
over time. The
pioneer
community is the first
in the sequence and the
climax
community is
found at the end.
Interactions between organisms
Within a garden there is constant interaction between
all the organisms in the ecosystem. Sometimes
the relationships are beneficial to both partners
(
mutualism
). An example is the nitrogen-fixing
bacteria which live in the nodules of some plant roots
(Figure 3.11). The plant roots provide a home for
the bacteria and sugars to utilize, while the bacteria
trap nitrogen gas in the atmosphere and convert it
to nitrates, a form of nitrogen which the plant can
use (see p. 168). Other examples of mutualisms are
mycorrhiza (an association between plant roots and
fungi) which take the place of root hairs in many
plants and lichens (an association between fungi and
cyanobacteria) (see Figure 4.14).
In other relationships, the pairing is beneficial to
one partner but has no effect on the other - for
example, climbing plants and
epiphytic
ferns which
Figure 3.12
Epiphytic ferns growing on a tree branch
live on trees giving them greater access to light for
photosyntheis (Figure 3.12).
Predation
is a relationship which is harmful to one
of the partners and beneficial to the other, often
where one partner (the predator) consumes all or part
of the other individual (the prey or host). Predation
includes
herbivores
which feed on plants (e.g.
aphids, vine weevils, slugs, rabbits, deer) (Figure
3.13);
carnivores
which feed on animal tissue (e.g.
birds feeding on aphids, nematodes eating bacteria,
hedgehogs eating slugs); and
parasites
in which the
predator has a very close but detrimental relationship
with the host often living inside the host's tissue (e.g.
viruses, fungal diseases, some biological controls)
(see p. 195).
Some plants such as mistletoe (
Viscum album
) can
manufacture their own food through photosynthesis
but extract water and mineral nutrients from the
host so are only partly parasitic
(hemiparasite
)
(Figure 3.14).
Predators and their prey follow closely linked
population cycles (predator-prey cycle) which
increase and decrease. When there is abundant
prey, there is plenty of food and predator numbers
increase. This leads to a reduction in the amount of
prey which then leads to a reduction in the number of
predators as their food supply shrinks. In a balanced
ecosystem, the prey population is never completely
removed, some prey individuals are able to hide
Figure 3.11
Example of mutualistic relationships -
root nodules in a legume (source: Wikimedia
Commons)
