RHS Journals: The Garden January 2008

Greater than the sum of their parts

A varied habitat is essential to help wildlife adapt to a changing climate. In the first of his new series, Ken Thompson delves into the fascinating and often largely ignored ecosystem that is your garden.

Today, ecosystems are the stuff of newspaper headlines, up there with other ecological buzzwords such as sustainability and biodiversity.

It is now an established fact: our gardens (regardless of their content, and thanks in part to their diversity) are, when combined together, an important ecosystem. But what exactly is an ecosystem? There are almost as many definitions as there are ecologists, but they boil down to some-thing like this: an interdependent and dynamic system of living organisms, considered together with their physical and geographical environment.

The key words are ‘interdependent’ and ‘dynamic’. Interdependent, because everything in your garden depends on everything else, via a bewildering web of interactions. Those aphids you dislike are food for a hoverfly larva, which is in turn attacked by a parasitic wasp. When this carelessly gets eaten by a spider, the spider may become food for a fledgling blue tit, which then eventually is eaten by a sparrowhawk.

Your garden, like it or not, is home to an astonishing variety of creatures that spend 24 hours a day eating plants and each other, mostly without ever being noticed. Studies have shown that, even in gardens not specifically managed for wildlife, about 2,000 species of insect alone may visit, or be resident, over a number of years.

The food chain

It is not helpful to arbitrarily pigeonhole animals in your garden into ‘trophic levels’ (the position that they occupy in the food chain). For example, a spider or ground beetle may be a top predator one minute and food for a bird or hedgehog the next. Many creatures play varying roles in the ecosystem at different stages of their life cycle; most adult hoverflies eat pollen or nectar, but their larvae eat aphids as well as decomposing organic matter or even flower bulbs.

Unsurprisingly, this complex, volatile system is rarely the same two days in a row, and certainly no two years are the same. In the only garden that we know a great deal about (thanks to zoologist Jennifer Owen’s heroic 30-year study of the wildlife in her Leicester garden), out of 94 different species of hoverfly caught the commonest species one year would seldom be the most frequent the next. For example, Episyrphus balteatus often invades gardens in large numbers in July and August, where its larvae are fond of cabbage aphids. Jennifer trapped 1,467 individuals of E. balteatus in 1997, but only four in 1998. Another species turned up in 1972, then not again until 1992; after that it was caught every year.

I would love to claim that we understand these fluctuations, and in a general sense we do: no species remains unusually abundant, because it either runs out of food or is overtaken by rising numbers of predators. However, in such a complex system, predicting how any small part will behave is effectively impossible.

Every year, my two pear trees fall victim to pear midge and I get no fruit at all. Just twice, in the past 16 years, something has depressed the midge numbers and I have had magnificent crops (which were much appreciated by the local squirrels). I suspect that in those two years, some predator or parasite of pear midges was unusually common; but even that ‘explanation’ is, in reality, only something else that needs explaining. In a world where the average research project runs for just three years, I suspect the conundrum of my pears will remain unsolved.

Non-intervention

Every now and then, one species or another becomes abundant or troublesome enough to attract our attention, and sometimes it is difficult to fight the impulse to ‘do something’. Every spring, aphids colonise young growth on my blackcurrants, and equally predictably (as long as I can resist the temptation to wade in), ladybirds will turn up, lay their eggs and eventually their larvae eat most of the pests.

Nature’s way

Sometimes you do not even need to wait for the ladybirds. Rose aphids often build up huge populations on garden roses in the spring and then, just when it looks like they are on course to take over the world, they decamp to their summer homes of scabious and teasel. Because gardens are so dynamic and ever-changing, many pest ‘problems’, just like many of the aches and pains we grumble about to our doctors, are self-correcting. Nor does intervention always have the desired effect.

In the March 2007 (p181) issue of The Garden, Val Bourne wrote about an experience with non-systemic insecticides and aphids. She sprayed the insecticide inside a glasshouse, which of course killed any aphids it came into contact with, as well as aphid predators. Any aphids hiding in nooks and crannies, or under leaves, survived, and went onto produce a bumper crop of aphids within two weeks.

So far, so much like everywhere else. All ecosystems are dynamic, interacting systems with (like the serenely gliding swan) much more going on out of sight than we can ever be aware of. In other respects, however, gardens are unusual. Most natural or semi-natural ecosystems are relatively unvarying places, each with a narrow and predictable range of flora, taxonomically and structurally. A heathland, or an oak wood, however far you look, is always pretty much the same. If a herbivore does not like the taste of oak leaves, or living in deep shade, then it will not be found living in an oak wood.

Even those ecosystems with a greater diversity of plants, such as chalk grassland, are structurally monotonous. There may be many plants in a chalk pasture, but they are united by their tolerance of heavy grazing (hence they are short) and low-nutrient soils (thus slow growing). Gardens are different. Because humans are in command (most of the time), we can grow what we like, and because we have the flora of the temperate world to choose from, we jolly well do. When Lawrence Johnston finished his garden at Hidcote Manor, it contained 15,000 different plants, or about 10 times as many as the entire native British flora (but Hidcote is unusual for a garden, and British flora is unusually limited compared with mainland Europe).

Network of gardens

This makes garden ecosystems extremely variable and interesting places. They may provide homes not only for animals that like oak woodland or chalk grassland, but also everything else in between.

What is more, gardens are perfectly suited to the large number of animals that prefer different things at different stages of their life cycle. Many insects like a sunny, sheltered spot to search for food or a mate, but their larvae may live in water, or in rotting vegetation (compost heaps are ideal). There must be hundreds of thousands of gardens where all three habitats can be found within a few metres, and millions more where they are found in neighbouring gardens.

Gardeners also go to enormous lengths to provide a generous and continuing supply of some resources (flowers, for example) that are much less abundant – and often only sporadically – in natural ecosystems. Given this bountiful cornucopia, it is hardly surprising that gardens provide homes for a large fraction of the British fauna. For groups of species that exploit the resources that gardens provide in particular abundance (such as flowers for adult hoverflies and aphids for their larvae), no nature reserve can hold a candle to a single well-managed garden. The value for native wildlife of the total gardened area within the UK is literally incalculable.

It is also worth emphasising two things that are often overlooked or misunderstood. First, gardens that are managed for aesthetics, rather than for wildlife, nevertheless make a major contribution to Britain’s biodiversity. During the 30 years of her study, Jennifer Owen identified 1,999 insect species in her garden (without doubt only a fraction of the actual number that occur there), despite her making few concessions to wildlife beyond a bird box or two. Second, it is the total garden network, and not any particular garden, that is so important for wildlife. No garden is too small to provide some benefit for wildlife, even if many animals that visit it are actually resident in neighbouring gardens.

Gardening with wildlife in mind, as I hope to show you, is an entirely natural, even unavoidable activity. All garden inhabitants are more interesting alive than dead – and taking an interest in them should make their lives, and yours, a little more agreeable.

Ken Thompson is a Senior Lecturer at the University of Sheffield.

Read more facts about UK gardens