As another wildlife documentary draws to a close, I can’t help looking out of the window at our back garden: the patio, the lawn, the garden wall strewn with ivy, the flowers nodding their heads gently in the evening breeze.
It all seems so quiet and mundane compared to the wondrous tropical ecosystem I’ve just witnessed on the screen. But first impressions can be misleading….
Our gardens support an intricate web of life that, in its own way, is every bit as complex the cloud forests of Borneo, the African savannah or the Amazon rain forest. And for the species that share our little piece of Ireland with us the life and death struggle for survival is every bit as fierce.
Your garden: a wildlife haven
Over the last several decades our gardens have become increasingly important refuges for wildlife, especially in and around our towns and cities. As human populations continue to rise, and urban areas sprawl to accommodate them, they encroach on the surrounding countryside. Many of our native species are adaptable, and adjust well to life in and around our towns and cities.
Gardens offer wildlife an alternative habitat and, for some species, like hedgehogs, badgers, rabbits, squirrels and foxes, a potentially vital thoroughfare linking areas of countryside that would otherwise become isolated.
All ecosystems, our gardens included, are defined by the species that live in them, by the relationships between those species, and by their interactions with each other and their physical environment. Outside your back door, right now, the wheel of life is turning as the plants, animals and fungi that make up your resident garden community compete for a finite pool of resources. Even a cursory examination of the life in your garden will soon reveal it to be far from mundane.
But surely there can’t be that many species in our gardens, can there?
It is surprisingly difficult to unearth concrete data on what must be one of our most accessible wildlife habitats. Formal scientific study on the ecology of a typical garden is scarce, which means we’re forced largely to rely on personal observations and records kept and published (in print and online) by amateur naturalists and interested householders.
One particularly comprehensive record of an average urban garden in central England maintained during the 1970s and ’80s recorded over 1,700 distinct species – and that specifically excluded species too small and difficult to collect or accurately identify. That study extended the accepted geographical range of a number of insect species, added eight species of Ichneumonidae (solitary parasitic wasps) and five other species of wasps (Serphidae) to the official British species list, and identified two species thought to be new to science.
While not all gardens are necessarily so rich in terms of species diversity it is clear that there is much more going on in the average garden than initially meets the eye. But why should our gardens contain such an array of life?
Part of the answer to that question lies in the very act of gardening itself.
In nature plant communities tend to progress through a series of stages known as successions, until they become what ecologists call a “climax community” containing relatively few dominant species. Left to its own devices a disused field, for example, will first be colonised by rapidly growing annual wildflowers (the “weeds” commonly found in our gardens). These will in time give way to herbaceous perennials; which will in turn be followed by shrubs; small, fast-growing trees; and finally will develop into mature woodland. Our gardening activities interfere with this natural process of succession. By weeding, planting, feeding, watering and pruning our gardens we actively maintain a contrived diversity of plant life that is often much higher than the garden would naturally support.
A 3D habitat mosaic
Gardening also tends to introduce a structural diversity rarely found in such small areas in nature. A typical mature garden will have a variety of small trees or large shrubs; extensive beds of vegetation, including flowers, ferns, vegetables and of course weeds; open areas of lawn; a patio area and often a small pond or other water feature. Our gardens are a three dimensional patchwork of varying habitats, and that patchiness encourages diversity by offering suitable conditions for a greater variety of species in a confined area.
Where these habitats meet there are areas of overlap, or ecotones, which typically exhibit higher species diversity than neighbouring habitats – a phenomenon known as the “edge effect”. Thus the diversity of plant life, the patchy nature of the garden environment and our continued interference with the natural process of succession results in the garden harbouring a far greater range of species than you might at first expect.
But the number of species is only the beginning of the story. A little time spent examining your garden closely will reveal a whole new world, where species interact and compete with each other in a complex biological system, employing a wide range of strategies and counter-strategies to enhance their chances of survival.
The struggle for garden survival
Plant species form the foundation of almost every ecosystem on the planet, and your garden is no exception. All of the energy that sustains the species in your garden comes from the sun.
Plants capture that energy through photosynthesis and lock it into complex chemical compounds within their tissues. Animals and bacteria that feed on this plant matter digest some of these compounds and release some of that stored energy, using some to function, and assimilating some into the consumers’ own body tissue.
In turn these organisms are preyed on by others, which are preyed on by others and so on up the many food chains that make up your garden ecosystem.
Most species in the garden will be involved in multiple food chains, and together these chains, and the links between them, form a complex food web that describes the flow of energy through the ecosystem. Scientists have established that only about ten percent of available energy passes from each level of a food chain (known as a trophic level) to the next; the rest is either lost as heat or is locked into indigestible compounds within the prey organism’s body. This means that there is an ever-decreasing pool of available energy as you move up the levels of the food chain – which generally means that each successive level can support less species and fewer individuals than the level below it.
These basic rules apply to the energy flow through any ecosystem, but of course the system is far more subtle and complex than that. Individual species can occupy multiple trophic levels in different food chains within the web, and can vary their feeding habits depending on other variables within the ecosystem, including physical conditions, like temperature and rainfall, and biological conditions, such as predation pressures or nutrient levels. Many organisms, and particularly the insects (of which there are a huge number in our gardens), can occupy different trophic levels in different food chains during different phases of their life cycles.
Winners, losers, enemies, friends
Slugs and snails are a familiar, if rarely welcome sight in most gardens. These terrestrial molluscs use their rasping tongues to graze on a variety of plant species, their nocturnal foraging causing damage to several individuals but rarely killing them outright. The relationship between the slug and the plant is an antagonistic one, where the slug or snail benefits directly at the expense of the plant. Grazing is one form of antagonistic relationship common in the garden, the others are predation and parasitism. Predators like the Wolf Spider, Ladybird and Blue Tit kill and eat numerous individual prey animals, while parasites feed on a single individual of the host species to the detriment and sometimes the death of that individual, as is common with the larvae of ichneumon wasps that develop from eggs laid by the adults within the larval stages of other insects.
Mutualism is at the other extreme, and results in both species benefiting from the interaction between them. Such relationships are much more common than you might at first imagine, and occur everywhere that the selfish pursuit of one species results in a positive, if unintended, effect on the other. A typical example of this in the garden is where bees, hoverflies and other insects visit a flower. They do so to plunder its store of nectar and pollen for food… but the adaptations of the plant mean that the insect acts as an inadvertent courier for the plant’s pollen, helping it to reproduce. Similarly birds that eat the berries and fruit from our garden shrubs and trees are involved in a mutualistic relationship with the plant, where they benefit from the food, and in return provide a convenient dispersal mechanism, spreading the plant’s seeds over a wide geographical area.
Other mutualistic relationships occur at a much more pervasive level than that. Many animals rely on bacteria in their digestive system to break down elements of their food, and numerous plant species have bacterial associations in their root systems that help them to take up nutrients from the soil. At its extreme mutualism can result in a species becoming totally dependant on the presence of the other for its survival, a condition known as obligative symbiosis.
Between these two extremes there are commensal relationships, where one species benefits and the other is unaffected by the interaction. In the garden, for example, when aphids feed on the sap of a strong, healthy rose plant the aphids benefit from the food, but have a negligible impact on the plant itself. As always in biological systems though things are not so clear-cut. These relationships are in a constant state of flux as physical and biological variables within the ecosystem change. Lets assume for a moment that ladybirds and their larvae prey heavily on the resident aphid population, and that for some reason the ladybird population drops off. The resulting explosion in aphid numbers could shift the relationship between the aphids and the rose plant to be a more antagonistic one. Similarly, aphids feeding on a smaller, weaker plant may have a detrimental affect on the health of the plant, and the relationship again becomes antagonistic.
The garden offers abundant examples of specific adaptations evolved to help species exploit their particular niche in the ecosystem. Woodlice, terrestrial crustaceans more closely related to crabs and lobster than to their garden neighbours, breathe using gills that they must keep moist at all times. To conserve water, woodlice restrict their activity to the hours of darkness and retire to cool, damp retreats in the heat of the day.
Some species in the garden have developed chemical weapons for defence and attack: bees, wasps, ants and beetles boast a host of stings and chemical sprays used both to overcome prey and to defend against aggressors. Ladybirds and other beetle and bug species, along with the previously mentioned woodlice, can exude a pungent secretion to dissuade would-be-predators. Many of these species have in tandem evolved bold warning colourations so that predators soon learn to avoid them. Still other species, like certain members of the hoverfly family, mimic the colouration and pattern of more dangerous or unpalatable creatures and “piggy-back” on their successful adaptation to their own advantage. The social bees, wasps and ants, all common in the garden, live in colonies ruled by one or more queens. These colonies work collectively to meet the challenges of survival in the garden ecosystem.
And it’s not only animals that are specifically adapted to their niche in the environment. Plants also have countless adaptations to improve their tolerance of desiccation, to maximise their access to light, to increase their chances of pollination, to improve their dispersal, to access the nutrients in the soil and to defend against attack.
The average Irish garden is very much alive, and, for me at least, is every bit as unique and fascinating as any other natural community. So the next time you’re watching the wonders of a tropical ecosystem unfold on your television screen, take a look out of your window… there could easily be just as much going on quite literally in your own back yard!