Agriculture Reference
In-Depth Information
Competition
Organisms compete for shared resources such
as light, moisture, nutrients and space, reducing
their growth and their potential for reproduction.
Competition can be by
exploitation
of the shared
resource - for example, when two plants compete
for water in a dry environment, the more successful
organism is the one which is able to survive on less.
It can also be by
interference
where one organism
directly affects the uptake of the resource by the
other, as in trees and shrubs such as
Rhododendron
ponticum
which produce chemicals which prevent
seeds germinating beneath their canopies (see
Fig. 1.10b). Organisms overcome the effects of
competition by moving to new sites through seed and
fruit dispersal in the case of plants, or by seeking new
territories in the case of some animals.
well-structured and fertilized soil enables deep root
penetration, reduce competition by rooting at different
depths, allowing plants to be grown closer together.
Competition can also be
interspecific
, between two
different species. When designing a mixed border it is
important to allow for the future size of the plants and
place them accordingly so they do not compete with
each other for resources (see 'niche' above).
Interspecific competition
is between individuals
of different species.
Intraspecific competition
is
between individuals of the same species.
Where competition is removed completely growth can
take place unchecked. A single plant growing in isolation
with no competition is as unusual in horticulture as it
is in nature. However, specimen plants such as leeks,
marrows and potatoes, lovingly reared by enthusiasts
looking for prizes in local shows, grow to enormous
sizes when freed from competition. In landscaping,
specimen plants are placed away from the influence of
others so that they not only stand out and act as a focal
point, but also can attain perfection of form.
Mutualism
is a relationship between two
organisms which is beneficial to both.
Predation
is a relationship which is harmful to one partner
(the host) and beneficial to the other (the
predator).
Parasitism
is an extreme form of
predation where the predator often lives within
the host.
Gardening for wildlife
When we think of gardening for wildife it is very
easy to just imagine a garden full of flowers and the
three 'B's - birds, butterflies and bees - and certainly
we want to encourage them. But a healthy garden
which is good for wildlife contains much more. It
supports a wide range of interdependent organisms,
as we have seen above, most of which we never
see but on which many more visible organisms
depend. Invertebrates (organisms which have no
internal skeleton) include not just bees and butterflies
but other insects such as flies and lacewings,
beetles, spiders, snails and slugs, earthworms and
microscopic worms, which all have an important
role in food chains. We have 2,400 native moth
species, an important food source for bats, but only
60 butterflies which are regularly seen in Britain and
Ireland. By providing suitable environments within
the garden and managing it appropriately, we can
create the right conditions for complex food webs to
become established, thereby increasing biodiversity.
Fundamentally, it is the primary producers, the plants,
which hold the key. If there is a good range of these
and plenty of them, the ecosystem's food chains and
webs will be well supported.
In addition, the presence of decomposers and
detritivores in a healthy soil will cycle organic matter
through the ecosystem. To support the widest
The degree of competition between plants as they
grow will depend very much on the spacings between
them, whether this is in the garden border, in the
vegetable plot or in the greenhouse. Competition
can be
intraspecific
, that is, between individuals of
the same species, and this will be the case in plants
grown in a
monoculture
,
where only one species is
present.
For example, if carrots are sown too close
together, their roots will be small. Sometimes this
can be an advantage, as in carrots grown for canning,
closer spacings are used deliberately. Small, even-
sized carrots may be specified by supermarkets
so spacings can be designed to achieve this.
Usually though, close planting will be detrimental.
Overcrowded plants may be more susceptible to
fungal diseases due to poor air circulation, they may
grow leggy with weak stems or seed production
may be reduced if shaded and competing for light.
Growth may be poor if roots are competing for soil,
water and nutrients. Three ways by which gardeners
overcome these problems as plants grow is by
transplanting seedlings from trays into pots, increasing
the spacing of pot plants in greenhouses and hoeing
out or 'thinning' a proportion of young vegetable
seedlings from a densely sown row. Crops grown in
deep bed systems, in which a one metre depth of
