Agriculture Reference
In-Depth Information
Wasp
Beetle
Owl
Caterpillar
Aphid
Mouse
Squirrel
Leaf
Berry
Nut
Bark
Figure 3.7
A simple woodland food web showing how species interconnect. The bottom level is made up of
plants (primary producers). Both primary consumers such as mice and secondary consumers such as predatory
beetles may feed on several different food sources. Removal of any species in the food web can have an affect
on one or more other species.
Succession
rose bushes), this will have an even greater impact
on the number of birds because their biomass is
comparatively smaller.
In practice, much of the biomass lost as waste and
dead organic matter is recycled by a whole group
of organisms called
decomposers
and
detritivores
which feed on dead organic matter (see Chapter 13).
As well as the flow of biomass, there is also a loss of
energy
through the system (Figure 3.6b). The process
of photosynthesis (see Chapter 9) enables the plant
to convert sunlight energy into chemical energy,
which is stored in the biomass of the plant. As the
plant is eaten by primary consumers, approximately
90% of the energy trapped in the leaf is lost either by
respiration, or by heat in the consumer's body, by
waste which is excreted by the consumer and by
death of organisms. Since matter and energy are
lost from the food chain at each trophic level, this
explains why only a limited number of levels are
possible. Eventually a point is reached where there
is insufficent biomass and energy to sustain a further
level of organisms (which is why large carnivores are
comparatively rare).
Gardens are highly productive and can produce a large
amount of plant biomass, and the greater the biomass,
the more consumers it will support. A densely planted
border, for example, is estimated to produce 0.5-2.0
kg of biomass per square metre per year. Therefore
in a garden the aim is to provide as rich a variety of
organisms as possible, that is, a high biodiversity at
every level together with as much plant biomass as
can be achieved to maintain a healthy and functioning
ecosystem.
Communities of plants and animals change with time.
Within the same habitat, the species composition
will change, as will the number of individuals within
each species. The process of change is known as
'succession' (Figure 3.8).
Primary succession
starts
from uncolonized rock, for example, where plants
move in to colonize the area.
Secondary succession
results from distubance of an existing habitat, for
example, tree and shrub removal, or burning off
vegetation. In this case, existing plants present as
seeds in the soil recolonize the area. This kind of
succession is common in Britain and Ireland where
most land surfaces have been covered by vegetation
at one time or another.
Succession involves a characteristic sequence of plant
types which each change the environment in some
way enabling the next group of plants to become
more successful. The first species to establish make
up the '
pioneer
' community. For example:
X
in a cleared woodland this might well be mosses,
lichen, ferns and fungi
X
in the garden these would be ephemeral and
annual weeds.
As these pioneer species remove water from
and consolidate the soil, they themselves die and
contribute to soil formation (see p. 143). Larger
herbaceous plants such as bracken, willow herb,
foxgloves and tall grasses can then become
established and outcompete the pioneer species for
light, water and nutrients, taking over. Often these
early colonizing species are characterized by being
able to:
