Increasingly we are made aware of the threats to biodiversity, without necessarily knowing what we can do about it. As gardeners we can make a substantial contribution to maintaining and increasing biodiversity. Those with a biological background will be familiar with the concept of interactions between soil, the microbes, the four layers of vegetation, and animals--the invertebrates and the vertebrates. All these affect the biological sphere, or biosphere, and all are collectively affected by changing climatic conditions. How we treat our patch has a dramatic effect on that biosphere.
For those less familiar, we know that some soils are ‘heavy’—these tend to contain more clay and they hold more water. Others are free draining and will contain more sand or grit. The soil in your garden affects what type of plant will grow there, and basic good gardening includes the addition of organic matter regularly to improve the plant growth. Organic matter can alter acidity (pH), drainage, available nutrients, and temperature. These all affect the extent to which bacteria, fungi, invertebrates and vertebrates thrive in the soil and the extent of their interactions with the plants and with each other.
As for the plants, some are woody and some are herbaceous (die back at certain times of year). Plants occupy four main layers: trees, large shrubs/small trees, herbaceous grassy areas, and those which are on the ground. Different creatures live in each layer, some are exclusive to one layer while others move happily between the ground and the treetops.
The more of these layers we can include in our gardens, the greater the variety of homes or ‘habitats’ we provide for different beasties. Differing groups of wildlife won’t all have the same tastes and needs for homes and foods. Some birds need trees to nest in, others need rough grassland. Some it seems will nest any old place. Some eat butterflies and others eat seeds. Some will eat everything. The same is true for insects—some live on trees and some live on or in the soil at or below ground level. So the more layers of vegetation we can supply, the more likely we are to attract a greater variety of insects and birds, which increases the biological diversity, or biodiversity for short.
Our understanding of the interactions within the soils and between soil life and plants is growing fast, and the more we learn the more astonishing are the degrees of complexity that we find. Some bacteria and fungi (collectively known as microbes) attack plants and cause disease, but equally there are many that attack and control the populations of those ‘bad’ microbes. Plants appear to have methods of providing the nutrients that the ‘good’ microbes require to thrive, and they in turn help to make nutrients available in a form that plants can digest. These interactions are widespread in many soils, and organic matter will provide nutrients to feed both microbes and plants as it breaks down. Chemical fertilizers appear to have a harmful effect on many microbes.
In addition to supplying a range of living quarters, providing an ongoing range of foodstuffs for our garden residents and visitors will help increase their desire to live in or visit our gardens because it will resemble an all-night corner shop. For example, if they can feed themselves and their young over a long period within your garden or local area (which may include birdfeeders), they will multiply. As gardeners we can meet this need by providing flowers for as much of the year as possible for the insects, and that suits us well as that gives us colour year around. Think of bulbs (which can flower spring – autumn); early flowering shrubs, grasses, and herbaceous; late spring flowering trees, shrubs, herbaceous; then throughout summer/autumn all of the aforementioned as well as annuals. Flowers are a food source for many insects. Plentiful insect numbers attract birds. Autumn is a good time for fruits, nuts, and berries. And of course many wild beasties all up and down the biosphere will spot our prize vegetables and make free at various times throughout the year.
We are omnivores which means we can eat lots of different things. But we wouldn’t do well if we had a diet of just bark and tree leaves. On the other end of the spectrum there are animals and insects that can only consume a very limited diet, possibly just one thing, and they are called ‘specialists’. They cannot adapt to eat other things. Lots of other animals who may be specialists or generalists (eat a variety of things) may eat the specialists, but if the specialists’ foodstuff disappears then they will die out and those other animals that feed on them will just have to look elsewhere. Those who are omnivores, or generalists will be fine but others (specialists) will be badly affected. If a few specialists start to die out, other specialists who depend on them are likely to die out, as will more specialists—this is a food chain. Omnivores may also die simply for lack of enough food, though they’ll eat other things when available. Because food chains tend to crisscross they are better described as a foodweb. So the loss of a single species might have a small effect which we don’t notice or a very large effect in a given area leaving us wondering why for example we no longer hear songbirds in such numbers.
So why should we care about biodiversity?
After all, we are putting in all the work and while it’s great to see and hear all the animals in the garden there are definitely times when we wouldn’t miss the aphids, beetles, voles, pigeons, raccoons, etc who have no concept of proper sharing. Why not blitz them so that the plants we cultivate grow and thrive?
The term biodiversity refers to the range of all the living things, and in areas where there is a good range a balance is achieved so that there are no single winners to the cost of everything else. Populations of wild things will fluctuate but with a good balance what goes up will normally go down again. As gardeners we need to tread with caution when zapping a population which seems out of control as by destroying one insect we may destroy a lot more in collateral damage. If the aphids get quite bad on some plants, try moving your peanut feeders when the aphids are partying, and you will find that blue tits will soon find the feeders and also clear the aphids while they are there—they are happy to take a hint. If we zap, perhaps we also take out a specialist that feeds on them and that may break a link in a particular food chain which resonates throughout the web from soil to tree, microbes to birds. If we zap, perhaps we affect our own gut microbes when we inhale or ingest fungicides and insecticides and upset our inner biodiversity.
An area of all one kind of plant, known as monoculture, will have a limited variety of wildlife associated with it. If the owner were to burn off a hillside of heather every few years, as happens on the grouse moors, or cuts all the garden heather back (both activities designed to stimulate new shoots and maintain compact growth) the biodiversity of that habitat would take time to recover and re-establish in balance. Monoculture cropping of acres and acres of farmland with little addition of organic matter has the same effect. Year on year, adding in the effect of regular fungicide and herbicide use, the recovery is slower and not so complete, lowering levels of biodiversity.
How many of us have seen a plant brought from another country settle into a garden or wild area, find itself very happy, and take off like a rocket, spreading so vigorously that it blocks out all the other vegetation around? Most of these plants have been introduced to our gardens because they are lovely plants and didn’t pose a problem in their own country of origin. Most likely they were controlled in their spread because something ate them or they were kept them in check by other means. Many of the plants we have introduced have behaved well and not spread unduly, but others have expanded their range to outside our gardens and have had no control imposed upon them by predator or soil conditions or climate. These particular introductions have become a real menace around the world as they colonize large areas so thickly that they blot out the native plants and animals in those areas. The effect on biodiversity locally can be devastating. Non-indigenous invasive organisms pose the second greatest cause of the loss of biodiversity globally—second only to habitat destruction. Unfortunately, we cannot foresee how a plant will behave when placed in a new environment.
As gardeners what can we do?
Some countries have laws against planting non-native garden plants into the wild, so for solid legal reasons we should resist any desires to ‘brighten up the countryside’. Smuggling plants however attractive from other countries is illegal or unethical for the same good reasons.
In the UK one mistake of the past has had major effects on biodiversity, the introduction of Rhododendron ponticum, which escaped into the wild and now covers 100,000 hectares in the UK, including 53,000 hectares in Scotland. Whilst lovely in flower, it makes a full cover and exudes toxins both of which kill off all the native ground cover where it is established. It spreads vegetatively and by seed, and is very difficult and costly to remove; and the toxins in the leaf litter create another level of challenge. Additionally, it is known to carry Phytophthera ramorum which kills larches and many other plants. Altogether R. ponticum has created a comprehensive onslaught on the biodiversity of the areas it has colonized so readily, and the cost of that is high.
Is it worth it?
Would not the annual expenditure of £2 billion (as estimated by DEFRA) to tackle invasives in the UK be better spent on something else? What looks pretty to your eye in the short term may cost others their livelihood in the longer term. We need to observe the effects of what we plant and take responsibility for the results.
With so many plants being sent around the world we are spreading both diseases and potentially invasive species. Responsible nurseries and horticulture establishments are working hard to test and highlight those plants that pose problems, and they refuse to supply them. By considering their advice and restricting our purchases to UK grown plants we would show we care about our local biodiversity, and take the precautionary principle seriously. And the phenomenon of climate change which can both increase and decrease ranges of plant and animal species supports the case for increased caution.