Ecosystems

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) plant 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 question.

The importance of plants as energy producers
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 ecosystem.

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.

Succession
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 water lake.

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 community'. In felled woodland, these may well be mosses, lichens, ferns and fungi. In contrast, a drained pond will probably have Sphagnum moss, 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.

Food chains
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 in 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

Decomposers
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.

Biomass
At any one time in a habitat, the amounts of living plant and animal tissue (biomass) 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'.

Countryside management 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 managed area.

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.

Garden considerations
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.

Companion planting
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 uptake.
• 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.