Charles Darwin, The Formation of Vegetable Mould Through the Action of Worms (1881)
I recently decided to up sticks from my home in the U.K. and moved to New Zealand. This group of islands has been cut off from the rest of the world for 80 million years and is a wondrous place of evolutionary isolation where spectacular things happen. Within the last six hundred years, there were still moa roaming the forests—flightless birds standing 3.6 meters in height, themselves preyed on by one of the largest known raptors, the Haast’s eagle. Whereas New Zealand’s North Island is warm and geothermally active, the climate of the South Island is cooler, being dominated by the sea and the striking topography of the Southern Alps, the vast spine of mountains that run the island’s length. Along the South Island’s West Coast, native forests look almost prehistoric, dense with tree ferns and towering podocarps (a large family of conifers). These curious trees once covered the ancient southern supercontinent of Gondwana, and it’s reasonable to assume some areas have had continuous forest cover since New Zealand broke away into isolation. The term used to describe this area is “temperate rainforest,” owing to the mild climate and the incredible volume of rain—over nine meters per year—dumped by clouds before they pass over the mountains.
Today I have traveled to one of my favorite places on the West Coast, a small coastal village named Punakaiki, to photograph the mysterious, tiny creatures of the litter layer in the native bush. I first took up the hobby of photographing small animals, known as macrophotography, during the first pandemic lockdown in the U.K., and it’s become one of my favorite ways to communicate the beauty and importance of soil invertebrates to people. My drive along the winding coastal road to Punakaiki provided spectacular views of sea-battered cliffs leading into a tall, continuous ridge of dense, lush forest-covered mountain, reminiscent of the island jungle in Jurassic Park. To the left of the ever-snaking road, crescendos of waves smashed into rocky outcrops, sending clouds of sea mist rolling up the rocky coastline and into long banks of lush tree ferns. With such scenery, it was hard to stay focused on the road.
This dramatic landscape is reflected in the place’s name, which is thought to be a misspelling of Punangairi, meaning “pools of stormy water” in te reo Māori (the language of the indigenous people of Aotearoa, New Zealand), referring to the dramatic coastal blowholes, which tourists flock to in droves. As I park and begin my walk along the gravel track that follows the winding greenish-blue Pororari River, camera in hand, I look up at the gigantic podocarps towering above. High up in the emerald canopy, kererū—iridescent New Zealand wood pigeons—feast on the swollen orange-red fruits of the mighty kahikatea and the yew-like fruit of the miro trees. Tonight, their flightless cousins, the kiwi, will navigate trailing vines and buttress roots in the dark as they rummage through the litter and deep peaty humus below in search of invertebrates to eat: the same invertebrates that I now hope to immortalize with my camera. As wonderful as the charismatic animals above the ground are, I’ve discovered that the life in the soil here is just as amazing.
The forests in Punakaiki have diverse leaf litter, much of it falling from the hardwoods that dominate the understory, since the taller softwoods are evergreen and therefore tend to hold on to their leaves, not dropping them all at once. After five or so minutes of walking, I spot an accessible way into the dense forest to the right of the path, and duck inside to rummage through the uppermost layer of the soil: the litter (a name that describes the leaves and detritus that scatter the ground like litter). It’s here, among the decomposing leaves, the rotting carcasses of insects, birds, and mammals, and the decaying branches and logs strewn about the forest floor, that we shall start this book’s descent through the soil layers, or the “pedosphere.” Litter is home to an astonishing variety of creatures, some working together and others competing, all trying to unlock the precious carbon and nutrients hidden away within the dead material. The litter layer is a fragile ecosystem, kept in balance by the work of diverse hands. Nowhere is this more apparent than here in the unexplored ancient temperate rainforests of New Zealand, where every upturned leaf is likely to yield an undescribed invertebrate species. This is as close to paradise as it gets for a soil ecologist and macrophotographer like me.
The litter layer is the surface layer of fallen leaves, branches, twigs, and logs, which are consumed and decomposed by soil organisms, releasing organic carbon and nutrients to the soil below. Soil scientists refer to the litter as the L-, F-, or H-horizon (based on how decomposed it is), and don’t consider it technically part of the soil, as it has little to no mineral (sand, silt, or clay) content. Of course, nothing is ever neatly arranged in nature, and so beneath this surface litter layer there is also another, underground type of litter, consisting of dead plant roots and the organic compounds that have been secreted by still-living roots as the plants go in search of food. Life within the soil largely depends on these surface and underground inputs of organic carbon from plants, which lock away (or “fix”) carbon dioxide from the atmosphere during photosynthesis. Some of this carbon is turned into simple sugars to fuel the plants’ metabolism, but a great deal of it is combined with minerals taken from the soil through their roots and turned into the complex organic compounds that make up leaves, wood, and other plant tissues. Forest ecosystems are a major store of carbon: it’s been estimated that the world’s forests store roughly 45 percent of the planet’s terrestrial carbon stocks. Estimated at 861 billion tons of carbon, this is a mass equivalent to roughly 2.4 Mount Everests. Of this, 44 percent is thought to be in the soil, while 42 percent is locked away in living trees, 8 percent is in deadwood, and 5 percent is stored in leaf litter. The process of carbon and nutrients moving between organisms, such as through grazing on litter (and each other through predation), underpins the soil food web. Its importance cannot be overstated; ultimately, the activity of the soil food web (the complex, interconnected community of soil organisms) supports all terrestrial life on our planet. Without properly functioning soil, life as we know it cannot survive.
The soil food web and the interactions between different species are a finely tuned machine, and the ongoing health of the system depends on the resilience of the biological communities it contains. The greater the biodiversity, the more adaptable it might be to environmental changes (such as those caused by human activity). As a rule, natural habitats with varied plant life have greater soil biodiversity, and are therefore more resilient than monoculture plantations such as forests grown for timber, or cropland sown for agriculture. This is exemplified in the mixed podocarp forests here at Punakaiki, where a single forest patch may contain eight or more tree species as well as supporting a wealth of other flora. The litter-dwelling creatures in this forest are equally varied, as staggeringly diverse as the forest itself. Walking through this ancient environment reminds me just how abundant and varied life is on our planet.
As I clamber over the thick, sweeping vines of supplejack that trail down from the canopy to make a path through the forest, a dense carpet of brown leaf litter rustles beneath my feet. My boots stir up leaves in a wave of rich amber hues, causing countless unseen tiny creatures to leap and crawl to safety. These are the inhabitants of the litter layer: woodlice, mites, springtails, and myriad other bizarre invertebrates. Many of these animals are no bigger than a pinhead, almost impossible to see with the naked eye. But with a hand lens, my powerful macrophotography camera equipment, and a little patience, I can begin to explore an entire world that many would otherwise walk past without a second thought.
Finding a suitable clearing among the vines, I kneel on the wet leaves and feel the cold damp of the forest floor begin to soak into the fabric on my knees. It’s a small price to pay for the opportunity to observe the wondrous creatures that call this patch of ground home. I peel back a clump of wet leaves and a cacophony of activity erupts, as hundreds of suddenly disturbed “land hoppers” fling themselves in all directions, seeking safety. These small shrimp-like amphipods are native to Australia (now introduced around the world) and live exclusively on land, in wet litter, such as here on New Zealand’s West Coast.
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