This term brings emphasis to both the community of living organisms
and to their non-living environment. Examples of an ecosystem are
a wood, a meadow, a chalk hillside, a shoreline and a pond. Implicit
within this term (unlike the terms habitat, niche, and biome) is the idea
of a whole integrated system, involving both the living (biotic
and animal species, and the non-living (abiotic
) units such as soil and
climate, all reacting together within the ecosystem.
Ecosystems can be described in terms of their energy flow, showing
how much light is stored (or lost) within the system as plant products
such as starch (in the plant) or as organic matter (in the soil). Several
other systems such as carbon, nitrogen, and sulphur cycles and water
conservation may also be presented as features of the ecosystem in
The importance of plants as energy producers
|Figure 3.3 Mature woodland
Energy perspectives are relevant to the ecosystem concept mentioned
above. The process of photosynthesis enables a plant to retain, as
chemical energy, approximately 1 per cent of the sun’s radiant energy
falling on the particular leaf’s surface. As the plant is consumed by
primary consumers, approximately 90 per cent of the leaf energy is
lost from the biomass, either by respiration in the primary
consumer, by heat radiation from the primary consumer’s body or as
dead organic matter excreted by the primary consumer. This organic
matter, when incorporated in the soil, remains usefully within the
The relative levels of the total biomass as against the total organic
matter in an ecosystem are an important feature. This balance can be
markedly affected by physical factors such as soil type, by climatic
factors such as temperature, rainfall, and humidity, and also influenced
by the management system operating in that ecosystem. For example,
a temperate woodland on 'heavy' soil with 750 mm annual rainfall will
maintain a relatively large soil organic matter content, permitting good
nutrient retention, good water retention and resisting soil erosion even
under extreme weather conditions. For these reasons, the ecosystem
is seen to be relatively stable. On the other hand, a tropical forest
on a sandy soil with 3000 mm rainfall will have a much smaller soil
organic matter reserve, with most of its carbon compounds being used
in the living plants and animal tissues. As a consequence, nutrient and
moisture retention and resistance to soil erosion are usually low; serious
habitat loss can result when wind damage or human interference occurs.
For temperate horticulturists, the main lesson to keep in mind is that
high levels of soil organic matter are usually highly desirable, especially
in sandy soils that readily lose organic matter.
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. This process of change is known as
'succession'. Two types of succession are recognized.
- Primary succession is seen in a situation of uncolonized rock
or exposed subsoil. Sand dunes, disused quarries and landslide
locations are examples. Primary succession runs in parallel with the
development of soils or peat. It can be seen that
plant and animal species from outside the new habitat will be the ones
involved in colonization.
The term ‘sere
' is often used instead of 'succession' when referring
to a particular habitat. Lithosere
refers to a succession beginning with
uncolonized rock, psammosere
to one beginning with sand (often in the
form of sand dunes).
- Secondary succession is seen where a bare habitat is formed after
vegetation has been burnt, or chopped down, or covered over with
flood silt deposit. In this situation, there will often be plant seeds
and animals which survive under the barren surface, which are able
to begin colonization again by bringing topsoil, or at least some of
its associated beneficial bacteria and other micro-organisms, to the
surface. This kind of succession is the more common type in the
British Isles. A hydrosere refers to succession occurring in a fresh
Influences on succession can come in two ways. ‘Allogenic succession
occurs when the stimulus for species change is an external one. For
example, a habitat may have occasional flooding (or visits from grazing
animals) which influence species change. In contrast, ‘autogenic
succession' occurs when the stimulus for change is an internal one. For
example, a gradual change in pH (or increased levels of organic matter)
may lead to the species change.
Stages in succession
Referring now to secondary succession, there is commonly observed
a characteristic sequence of plant types as a succession proceeds.
The first species to establish are aptly called the ‘pioneer
In felled woodland, these may well be mosses, lichens, ferns and
fungi. In contrast, a drained pond will probably have Sphagnum
reeds and rushes, which are adapted to the wetter habitat.
The second succession stage will see plants such as grasses, foxgloves
and willow herb taking over in the ex-woodland area. Grasses and
sedges are the most common examples seen in the drained pond. Such
early colonizing species are sometimes referred to as opportunistic
They often show similar characteristics to horticultural weeds, having an extended seed germination period, rapid plant
establishment, short time to maturity, and considerable seed production.
They quickly cover over the previously bare ground.
The third succession stage involves larger plants, which, over a period
of about five years, gradually reduce the opportunists' dominance.
Honeysuckle, elder and bramble are often species that appear in
ex-woodland, whilst willows and alder occupy a similar position in the
drained pond. The term ‘competitive
' is applied to such species.
The fourth stage introduces tree species that have the potential to
achieve considerable heights. It may well happen that both the exwoodland
and the drained pond situation have the same tree species such
as birch, oak and beech. These are described as climax
species, and will
dominate the habitat for a long time, so long as it remains undisturbed
by natural or human forces. Within the climax community there often
remain some specimens of the preceding succession stages, but they are
now held in check by the ever-larger trees.
This short discussion of succession has emphasized the plant members
of the community. As succession progresses along the four stages
described, there is usually an increase in biodiversity (an increase in
numbers of plant species). It should also be borne in mind that for every
plant species there will be several animal species dependent on it for
food, and thus succession brings biodiversity in the plant, animal, fungal
and bacterial realms. Not only is there an increase in species numbers
in climax associations, but the food webs described below are also more
complex, including important rotting organisms such as fungi which
break down ageing and fallen trees.
Charles Darwin is said to have told a story about a village with a large
number of old ladies. This village produced higher yields of hay than
the nearby villages. Darwin reasoned that the old ladies kept more cats
than other people and that these cats caught more field mice which were
important predators of wild bees. Since these bees were essential for the
pollination of red clover (and clover improved the yield of hay), Darwin
concluded that food chains were the answer to the superior hay yield. He
was also highlighting the fact that inter-relationships between plants and
animals can be quite complicated.
At any one time in a habitat, there will be a combination of animals
associated with the plant community. A first example is a commercial
crop, the strawberry, where the situation is relatively simple. The
strawberry is the main source of energy for the other organisms, and is
referred to, along with any weeds present, as the primary producer
that habitat. Any pest (e.g. aphid) or disease (e.g. mildew) feeding on the
strawberries is termed a primary producer
, whilst a ladybird eating
the aphid is called a secondary consumer
. A habitat may include also
tertiary and even quaternary consumers.
Food chains and webs
Any combination of species such as the above is referred to as a food chain and each stage within a food chain is called a trophic
level. In the strawberry, this could be represented as:
|strawberry → aphids → ladybird
In the soil, the following food chain might occur:
|Primula root → vine weevil → predatory beetle
In the pond habitat, a food chain could be:
|green algae → Daphnia crustacean → minnow fish.
Within any production horticulture crop, there will be comparable food chains to the ones described above. It is normally observed
that in a monoculture such as strawberry, there will be a relatively short period of time (up to 5 years) for a complex food chain to
develop (involving several species within each trophic level). However, in a long-term stable habitat, such as oak wood land or a mature
garden growing perennials, there will be many plant species (primary producers), allowing many food chains to occur simultaneously.
Furthermore, primary consumer species, e.g. caterpillars and pigeons, may be eating from several different plant types, whilst secondary
consumers such as predatory beetles and tits will be devouring a range of primary consumers on several plant species. In this way, a
more complex, interconnected community is developed, called a food web
(see Figure 3.4).
|Figure 3.4 Food web in a woodland habitat
At this point, the whole group of organisms involved in the recycling
of dead organic matter (called decomposers or detritovores) should
be mentioned in relation to the food-web concept. The organic matter derived from dead plants and animals of all
kinds is digested by a succession of species: large animals by crows,
large trees by bracket fungi, small insects by ants, roots and fallen
leaves by earthworms, mammal and bird faeces by dung beetles, etc.
Subsequently, progressively smaller organic particles are consumed
by millipedes, springtails, mites, nematodes, fungi and bacteria, to
eventually create the organic molecules of humus that are so vital a
source of nutrients, and a means of soil stability in most plant growth
situations. It can thus be seen that although decomposers do not
normally link directly to the food web they are often eaten by secondary
consumers. They also are extremely important in supplying inorganic
nutrients to the primary producer plant community.
At any one time in a habitat, the amounts of living plant and animal
) can be measured or estimated. In production
horticulture, it is clearly desirable to have as close to 100 per cent
of this biomass in the form of the primary producer (crop), with as
little primary consumer (pest or disease) as possible present. On
the other hand, in a natural woodland habitat, the primary producer
would represent approximately 85 per cent of the biomass, the
primary consumer 3 per cent, the secondary consumer 0.1 per cent
and the decomposers 12 per cent. This weight relationship between
different trophic levels in a habitat (particularly the first three) is often
summarized in graphical form as the 'pyramid of species'.
utilizes these succession and food-web
principles when attempting to strike a balance between the production
of species diversity and the maintenance of an acceptably orderly
Succession to the climax stage is often quite rapid, occurring within
20 years from the occurrence of the bare habitat. Once established, a
climax community of plants and animals in a natural habitat will usually
remain quite stable for many years.
When contemplating the distribution of our favourite species in the
garden (ranging from tiny annuals to large trees), a thought may be
given to their position in the succession process back in the natural
habitat of their country of origin. Some will be species
commonly seen to colonize bare habitats. Most garden species will fall
into the middle stages of succession. A few, whether they are trees,
climbers, or low-light-requirement annuals or perennials, will be species
of the climax succession. The garden contains plant species which
compete in their native habitat. The artificial inter-planting of such
species from different parts of the world (the situation found in almost
all gardens), may give rise to unexpected results as this competition
continues year after year. Such experiences are part of the joys, and the
heartaches of gardening.
An increasingly common practice in some areas of horticulture (usually
in small-scale situations) is the deliberate establishment of two or more
plant species in close proximity, with the intention of deriving some
cultural benefit from their association. Such a situation may seem at first
sight to encourage competition rather than mutual benefit. Supporters
of companion planting reply that plant and animal species in the natural
world show more evidence of mutual cooperation than of competition.
Some experimental results have given support to the practice, but most
evidence remains anecdotal. It should be stated, however, that whilst most commercial horticulturalists producers in Western Europe grow
blocks of a single species, in many other parts of the world two or three
different species are inter-planted as a regular practice.Biological mechanisms are quoted in support of companion planting:
- Nitrogen fixation: Legumes such as beans convert atmospheric
nitrogen to useful plant nitrogenous substances by means
of Rhizobium bacteria in their root nodules. Beans inter-planted with
maize are claimed to improve maize growth by increasing its nitrogen
- Pest suppression: Some plant species are claimed to deter pests and
diseases. Onions, sage, and rosemary release chemicals that mask the
carrot crop’s odour, thus deterring the most serious pest (carrot fly) from
infesting the carrot crop. African marigolds (Tagetes) deter glasshouse
whitefly and soil-borne nematodes by means of the chemical thiophene.
Wormwood (Artemisia) releases methyl jasmonate as vapour that
reduces caterpillar feeding, and stimulates plants to resist diseases such
as rusts. Chives and garlic reduce aphid attacks.
- Beneficial habitats: Some plant species present a useful refuge
for beneficial insects such as ladybirds, lacewings and
hoverflies. In this way, companion planting may preserve a sufficient
level of these predators and parasites to effectively counter pest
infestations. The following examples may be given: Carrots attract
lacewings; yarrow (Achillea), ladybirds; goldenrod (Solidago), small
parasitic wasps; poached-egg plant (Limnanthes), hoverflies. In
addition, some plant species can be considered as traps for important
pests. Aphids are attracted to nasturtiums, flea beetles to radishes, thus
keeping the pests away from a plant such as cabbage.
- Spacial aspects: A pest or disease specific to a plant species will
spread more slowly if the distance between individual plants is
increased. Companion planting achieves this goal. For example,
potatoes inter-planted with cabbages will be less likely to suffer from
potato blight disease. The cabbages similarly would be less likely to
be attacked by cabbage aphid.