Ecology and garden wildlife - Principles of Horticulture - page 39

Strawberries Aphids Ladybirds

(a)

Strawberries Aphids Ladybirds

detritivores and decomposers

3

Solar energy Recycling of nutrients

Heat Excretion and death

(b)

Ladybirds
(secondary consumer)

3rd trophic level

Aphids
(primary consumer)

2nd trophic level

Strawberries
(primary producer)

1st trophic level

(c)

Figure 3.6 Food chains: (a) a simple food chain found in a garden - the strawberry is the primary producer, the
aphids the primary consumer and the ladybirds the secondary consumer; (b) fl ow of biomass and energy through
the food chain illustrated in (a) - biomass and energy (heat and chemical energy) are lost from the food chain
and nutrients are recycled by detritivores and decomposers; (c) an ecological pyramid representing the loss of
biomass and energy at each trophic leveld

more. A garden example could be a rose bush (the
primary producer) on which aphids feed (the primary
consumer) which in turn are eaten by birds (the
secondary consumer). In practice, the rose bush may
support several primary consumers including, for
example, a fungus causing black spot or caterpillar
larvae or slugs some of which themselves may be
consumed by a range of secondary consumers,
such as hedgehogs. In this way food chains are
interconnected, forming complex food webs
(Figure 3.7). Because of this interdependency, removal
of any one of the organisms in a food web can have
a profound effect on the whole ecosystem. So in a
garden, for example, if we remove the aphids from
the rose bush completely, we could also be reducing
the numbers of birds.

Biomass and energy flow
The weight or volume of living plant and animal
material in an ecosystem is called its biomass . The
flow of biomass in a food chain as organisms feed
on each other can be represented as a pyramid
(Figure 3.6c). At the base is the primary producer
(a rose), resting on this is the primary consumer
(aphids), and above this is the secondary consumer
(birds). Each level in the pyramid is termed a trophic
level . The pyramid shape reflects the loss of matter
from the food chain in that whenever biological
material is consumed at each level some matter is
lost as waste, or through death of organisms and also
through release of gases and water in respiration (see
Chapter 9). In our rose example, this means that the
total biomass of rose material must always be more
than the biomass of aphids which feed on it which,
in turn, is always more than the biomass of the birds
which feed on the aphids. In this way, if we reduce
the rose biomass available in the garden (say by
reducing the size of the rose bush or planting fewer

A primary producer manufactures its own
food from simple molecules (e.g. green plants
photosynthesizing). A consumer feeds on living
organisms.