RHS Journals
The Garden
March 2008
Plants: foundation of the garden ecosystem
In the third part of his series on gardens as ecosystems, Ken Thompson explains how plant choice and management controls the productivity of the system, and the number and range of other organisms it can support.
Images: Neil Hepworth
All terrestrial ecosystems, whether natural ones or those that are managed by humans (such as gardens), are powered by the sun. Plants capture the sun’s energy by photosynthesis, using it to grow: they are the primary producers.
The organic matter they synthesise is the foundation of the ecosystem, providing food for all the other organisms. Herbivores (which will be covered next month) eat living plants; decomposers (see part two, February ) consume dead material, and predators (see part five in May) feed on other animals, be they herbivores or other predators.
While gardeners can do little about the amount of sun their garden receives, we can have a huge influence on how much of the sun’s energy is captured by our garden, and both the amount and the type of plant material it produces. Gardens can actually provide more food – whether in the form of leaves (living or dead), pollen, nectar, berries or wood – and in greater variety than natural ecosystems, and can therefore support increased biodiversity. The varied forms and contents of gardens is among their great strengths: whatever their size, all gardens have their part to play.
Fertility and growth
A basic difference between gardens is in fertility, which is determined primarily by nutrients and water. Gardens in fact vary less in this respect than more natural habitats, as a nutrient-rich soil is (and has always been) the aim of most gardeners. However dry your garden might be if left to itself, it can be modified by irrigation; if too wet, its drainage can be improved. Thus, most gardens have plenty of water and nutrients, so are ideally suited to fast-growing plants.
Fertiliser advertisements emphasise how much faster and bigger your plants will grow if you buy the product. Bigger is seen to equal better, and certainly there are places (the vegetable plot, for example) where this is generally true. But gardeners should think carefully about the type of garden that they actually want. Which plants we choose to grow ultimately controls the ecosystem that will establish in a garden, as plant selection governs what sorts of wildlife a garden can support.
Many garden plants naturally grow quite slowly, including many ‘choice’ species such as alpines and several dwarf or evergreen shrubs. These respond only a little, if at all, to extra fertiliser. Lawns and fast-growing plants need more mowing and pruning in a fertile garden, where weeds will also grow faster, and herbaceous plants need more staking. Fast-growing plants may shrug off the attentions of slugs and aphids – but an abundance of soft, juicy foliage attracts more plant-eaters in the first place.
Garden management
Important as soil fertility is, however, the way a garden is cared for has even bigger implications for garden eco-systems. Soil fertility may govern how fast plants grow in your garden, but your management controls how much and what types of vegetation it holds. It is this that is the most crucial factor in determining the size and diversity of the animal community in your garden.
Consider, at one extreme, a gardener who grows mainly bulbs, annuals and bedding plants. These never get too big, so there is rarely much to eat, and in winter there is nothing at all, which also means nowhere to hide. In the real world in which animals evolved, bare ground is a rare phenomenon, confined to brief periods after disturbances such as fires, floods or landslips. A garden of mainly alpines will also fail to sustain much of an ecosystem: although such plants are permanent, they tend to be smaller, and less palatable, than annuals.
It is important not to look at gardens in isolation from each other – wildlife does not. Even hot, bare soil or gravel provides good conditions for some animals. These may well be rare or unusual species unable to gain a foothold in better-vegetated neighbouring plots. Well-planted gardens with a diversity of trees, shrubs and herbaceous species will support larger and more varied ecosystems, but even bulbs, annuals and alpines provide good seasonal nectar sources for visiting pollinators from other gardens.
Bear in mind many garden pesticides are non-selective, and are as toxic to beneficial insects as they are to the ‘pests’ they are designed to target (more on this subject next month).
Native and alien plants
Much has been written on the relative value of native versus alien plants for wildlife in gardens, particularly from those with an ideological attachment to growing native plants. The evidence does not provide much support for the idea that native plants have any particular value for native wildlife over some exotic species. This is mainly because most garden invertebrates are in fact predators, parasites, pollinators or decomposers, and none of these much care whether plants are native or alien.
Far better to concentrate on growing lots of plants (in terms of both quantity and variety) and on providing those things that native plants do not do as well, such as late-autumn flowers. Where possible, choose single-flowered cultivars over doubles because the extra petals of double flowers are usually formed from the mutated organs that once produced nectar and pollen.
Gardens do not have to be unkempt to benefit wildlife; even plots with few plants will support some animal species. However, as primary producers, plants form the bedrock of garden ecosystems. Remember this when choosing species for the garden, but be aware that the way in which you grow and manage plants can be just as important in maximising their value to wildlife.
Continued >