As October unfolds, a sprinkling of crispy leaves gathers beneath trees, spreading autumnal blankets of yellow and orange across the ground. Together with twigs, tree seeds and bits of fallen bark, these leaves are a layer of life that ecologists refer to as ‘leaf litter’. Thousands of species live among the leaf litter, some simply hiding out from cold weather, others actively processing and breaking down the organic material.
Millipedes, woodlice, earthworms, and nematodes eat and digest decomposing plant matter, breaking it up into smaller pieces. Then, tandem forces of bacteria and fungi decompose these and convert them into valuable nutrients. Communities of these detritivores and decomposers together recycle the apparatus of photosynthesis, ensuring the availability of vital nutrients such as nitrogen and phosphorus for trees and other plants to resume another season of growth next year.
And while all this is going on, leaf litter also provides essential habitat for a plethora of overwintering critters.
There are about 1,500 different species of moth in Ireland. Their annual life cycles means that this year’s caterpillars need suitable habitats to spend the winter, and leaf litter is crucial for many of these species.
Fox moth caterpillars, for example, are a hairy dark caterpillar better known as ‘hairy mollies’. They spend the winter in diapause — a state of dormancy akin to hibernation in mammals. For this, they must hide away in leaf litter before pupating next spring. Many species even disguise their cocoons and chrysalis as dried leaves to blend in with the real leaves.
Garden tiger-moths and elephant hawk-moth caterpillars also need leaf litter to survive the winter, though rather than hide out there as caterpillars, they overwinter in cocoons. In March or April, their pupation complete, garden tiger moths will emerge as stunningly dark brown, white and bright orange moths. Unless, of course, some humans have come along and cleared away the fallen leaves, blitzing the pupae’s winter cover with leaf blowers and shredders. To state the obvious, those that don’t survive will never get the opportunity to create a next generation.
This loss of overwintering habitat is one of the factors contributing to a drop in the diversity and abundance of butterflies and moths in recent decades. It is estimated that over half of moth species in temperate regions rely on leaf litter to complete their lifecycle, hence many perish when winter leaves are cleared.
And let’s not forget that birds, frogs, bats, badgers, and hedgehogs all feed on insects and their larvae too.
In winter, song thrushes, robins, blackbirds, and sparrows are among the birds who find essential winter sustenance among fallen leaves. They forage among the crinkling layers of fallen leaves in search of protein rich eggs, larvae, caterpillars and other insect life to eat. Removing the fallen leaves from your garden removes this winter foraging, and consequently there will be fewer birds.
There has been a welcome surge of interest in biodiversity in the last few years. There is growing recognition of how native trees are far more valuable to invertebrate biodiversity than non-native tree species. A nascent enthusiasm for meadow gardens and pollinator patches is emerging among gardeners, parks departments and community groups all over the country.
The successes of the All-Ireland Pollinator Plan have been made huge difference in both awareness and actions for biodiversity across Ireland. Its no longer unusual to see manicured lawns being converted to wildflower areas, and some city councils have adopted policies to stop using herbicide sprays.
While all this is welcome progress, there is a still a long way to go. And one of the areas where we still have a collective blind spot is for the needs of wild species in winter, which includes recognising the ecological value of leaving fallen leaves in situ. Each of us can help by leaving leaves wherever possible in the garden, as well as postponing pruning and allowing the tussle of dead plants that offer nooks for eggs, chrysalis and other larvae.
But at scale, there is a need for local authorities, landscapers and park departments across the country to adopt policies that support overwintering invertebrates too. Leaf litter, dead wood, old plants with hollow stems, and log piles are crucial not only for many hundreds of moth species, but also for many kinds of butterfly, solitary bee, shield bug, hoverfly, ladybird and more.
Leaving fallen leaves in situ offers many benefits. Not only do they provide crucial wintering habitat and foraging resources for many invertebrates and birds; they are also a natural mulch that helps suppress weeds while fertilising the soil as they break down. In terrestrial ecosystems, leaf litter is the main source of nutrients returning to the soil. The trees benefit from the return of their nutrients, recycled by communities of soil biota and mycorrhizal activity. Trees need these nutrients to stay in good health and, as a consequence, are better able to maintain resilience to the many pests and diseases that might otherwise get the upper hand.
It is true that a thick layer of fallen leaves will smother green lawns beneath the trees. But this is perhaps another reason to reevaluate our outdated aesthetic attachment to large swathes of short, green, tidily maintained grassy lawns that offer little of worth to wildlife.
For those who might still need more time to acclimatise their perspective, fallen leaves can be raked up in to bands and then left to break down, minimising the impact on lawns. Providing signage to explain the changes in leaf management can help bring people on board, most will be delighted that positive action for biodiversity is being taken.
