As the weather warms and days lengthen, your attention may be turning to that forgotten patch of your backyard. This week we’ve asked our experts to share the science behind gardening. So grab a trowel and your green thumbs, and dig in.
Whether you live in an urban apartment or a rural homestead, your outdoor area is more than just a private space. Ecologically, a garden is another jigsaw piece in the landscape.
Whatever their size, gardens can contribute to natural functions and processes in the local area, such as regulating water drainage, buffering the damaging effects of strong winds, or providing food and shelter for native wildlife.
Many wildlife species survive in urban areas, but their presence and persistence depend on how specific their food and shelter needs are, how they respond to disturbances, and the quality and quantity of other green spaces in the landscape.
For larger animals, such as birds and mammals, a home garden could become a stepping stone across an otherwise hostile urban landscape. For smaller animals, such as insects, it could be the centre of their home range.
In urban areas, where space is often limited, gardening with pollinators in mind is a simple way to encourage biodiversity in the backyard. And, depending on the surrounding landscape, habitat for pollinators will also be habitat for other animals.
Flowers are just the first step
Flowers produce sugar (nectar) and protein (pollen), the main diet for many adult insects and birds. Unlike other insect groups, native bee larvae develop almost exclusively on pollen collected by their parents, so flowers are essential to grow native bee populations.
There is no single best combination of flowers for wild bees. Many “plants for pollinators” lists available online are based on local experiences and rarely apply to all geographic regions. A general rule of thumb for a pollinator garden is one that produces flowers for most of the year and is built on diversity – monocultures of any single flower type or colour will suit only a very small number of generalist species.
Native plants are an ideal option for attracting native pollinator insects and birds, but many garden exotics, especially herbs, fruit and vegetable plants, are just as popular. Modern hybrid varieties should be chosen carefully, as some are bred for commercial fruit or flower traits (like size or colour), but the flowers lack the nectar or scent cues that attract pollinators looking for food.
Build it and they will come
The structure and design of a garden can determine what wildlife species will visit or make a home. Vertical structure, built from multiple layers of different plant heights, provides more spaces for wildlife to co-exist. Small plants and shrubs provide good shelter for insects and very small birds, while larger trees will attract visits from more mobile birds and mammals.
Large trees with rough or shedding bark that creates lots of cracks and crevices are excellent shelter for insects and small lizards. Trees that produce resins and sap flows, such as conifers, acacias and eucalypts, are also useful for some native bee and wasp species that use resin to seal their nest cells.
Insect hotels can provide homes for insects that usually nest in dead wood. But only a small proportion of the world’s bee species are wood-nesters. About 75% of bee species dig their nests into the ground, usually in sandy, uncompacted soil, preferably on a slope that won’t get waterlogged.
It can be difficult to build all of this into small gardens, but many pollinator insects will have home ranges of a few hundred metres, while birds and mammals can travel much further. So landscape composition can also influence the wildlife potential of an individual garden. A high proportion of paved areas can reduce the number of wild bees or native birds in the neighbourhood. Highly manicured green spaces can also have a negative effect on wild bee species.
Disrupting the food chain
Like any ecosystem, gardens involve an intricate web of life, from the soil microbes underground to the birds in the trees. It’s easy to grab the spray bottle to kill off the dandelions and blow down the flies, but what are the knock-on effects?
Many of the animals and plants we think of as a backyard nuisance often provide services we don’t see. For example, many native wasp and fly species (even blowflies!) are pollinators as adults. And as larvae, they control many of the insect pests we see on our plants, or decompose organic wastes. Small reptiles, like geckoes and skinks, mostly feed on small insects that annoy us, like mosquitoes and midges.
Plants we think of as lawn weeds, particularly dandelions and clover, are a favourite food source for native bees and hoverflies. Aphids and scale insects also produce a sugary substance called honeydew as they suck on plants, which is an important sugar source for some beneficial insects like wasps, bees, ants and hoverflies.
Limiting synthetic chemical use is one of the easiest and cheapest ways to enhance wildlife in gardens. Insecticides can kill beneficial insects, or affect them indirectly by disrupting their metabolism or reproductive cycles. Overuse of herbicides removes important food resources, like dandelions, that pollinators rely on if other flowers are scarce.
There is also the potential for chemicals to work in combination and have a greater impact.
Managing gardens as ecosystems
Many wildlife don’t like regular disturbances, which is why urban areas can be intimidating environments for animals. It can be hard to balance human needs with the habitat needs of wildlife. Many actions that minimise risks for humans can have the opposite effect for wildlife.
For example, pollinators generally prefer open grassy areas to dark forested areas. In urban environments, grassed areas are often mown regularly for human recreational and safety needs. This affects the availability of flowers for pollinators and also affects the persistence of these plant species. Mowing less often and outside peak flowering times can make a big difference for plants and pollinators.
Similarly, large old trees are homes to myriad animals. Unless they pose a very real risk of danger to human lives, pruning overhanging branches can be better for the local ecosystem than removing the whole tree.
Wildlife are rarely deterred by fences, so it is likely that most of the animals you see in your yard are also using your neighbours’ yards. Managing gardens as a collective landscape, rather than individual gardens, can keep wildlife happy while also enhancing neighbourhood communication.