Native animals and the formation of healthy soils part of Vignettes:Vignette Collection
Soil is an important resource on which most of the world's production depends. Soil is not an inert medium; it is a living and breathing ecosystem made up of water, air and organic material. Soil provides the structure for plants to grow in, habitat for a host of small animals, and a store of water and nutrients. Like all important resources, soil is not limitless. But where does soil come from? Soil comes from the slow breakdown of rocks over time. In most environments, except for perhaps tropical rainforests, this rate of formation is extremely slow. In fact, it is fairly true to say that the rates of soil formation are orders of magnitude less than current rates of soil erosion. In other words, we are losing more soil than we are producing and this cannot be sustainable in the long run. A lot of factors contribute to soil loss; poor farming practices, often combined with drought, excessive clearing of forests, overgrazing, burning, the list goes on. The conversion of marginal grazing land to cropland often leads to increased erosion and the loss of productive topsoil. Nowhere is this more apparent than in the world's semi-arid drylands, where suitable soil for farming is scarce, and where rainfall is both low and highly variable. In the middle of the 20th century a soil scientist Hans Jenny outlined his ideas on the main factors responsible for soil formation. These are parent material, climate, topography, biota and time (Jenny 1941). In comparison to some of the other soil forming factors such as climate and parent material, there is still relatively little known about the importance of organisms in soils. Most soil scientist recognize, however, that organisms play a significant role in soil formation. If you take a close look at even a small cup full of soil under a microscope you will see that it is teeming with life. It will contain millions of bacteria, fungi and protozoa, thousands of arthropods and nematodes, and even the occasional earthworm if you're lucky. We have known about the importance of earthworms for building healthy soils ever since Charles Darwin published his famous book 'The formation of vegetable mould through the action of worms, with observations on their habits' in 1881. Research over the past couple of years has shed some light on the roles played by animals in soil formation and their importance in developing healthy soils. Invertebrates such as ants, termites and beetles move huge quantities of soil to the surface when they construct their underground tunnels, burrows and storage galleries. This soil is often high in carbon and nitrogen, nutrients that are critically important for plant growth. The soil is not well compacted, doesn't really stick together well, and so it is easily blown around by wind and moved by water. This redistribution can result in soil loss, but it also brings soil into contact with plants and helps to build up soil profiles. In fact, the movement of soil by some ants can results in the buildup of about 1 cm of soil every ten years, or a metre every thousand years (Eldridge and Pickard 1995). Small native mammals such as rodents, bandicoots and even echidnas move substantial amounts of soil when they forage in the soil. In semi-arid Australia the mass of soil contained in the mounds and pits constructed by a range of soil disturbing animals ranges from 0.1 to 6 tonnes per hectare. Even seabirds such as shearwaters move huge amounts of soil, more than 10 tonnes per hectare per year when they construct their burrows, and birds such as the lyrebird and brush turkey that inhabit wet forest can move up to 200 tonnes per hectare per year when they are foraging for food (Eldridge and James 2009). These very high rates of soil movement suggest that animals have substantial effect on surface soils and help to build up rich soil profiles. But soil formation is about more than just moving soil around. The effect of this soil movement and surface disturbance is to create opportunities for plants which often leads to the restoration of degraded lands. Soil disturbances are important because the small pits and depressions created by animals become traps for litter. The pits also catch water, and studies worldwide have shown that pits trap not only more water, but that the water remains in the pits for longer. With more water and more organic matter, these small depressions soon become perfect sites for microbes, which quickly break down the litter, releasing nutrients like carbon and nitrogen. The depressions also catch seed, and with a moister soil with greater levels of nutrients, the plants that germinate generally have greater survival. Some of these plants are those that are preferred food items for the animals that originally constructed the pits, and when they re-excavate the pits to expose the nutrient-rich plant roots, the whole process starts over again. Unfortunately, many of the native soil disturbing animals have been lost over large areas of the Australian continent. This means that we have also lost those services that these animals once provided, what we call 'ecosystem services'. Instead, many of the once common native animals such as bilbies and bettongs have been replaced by the invasive and exotic European rabbit. Rabbits also dig in the soil, but unlike native animals, they don't dig enough, and they have many other negative effects on the environment. Some environmental groups are reintroducing locally-extinct species back into environments where they were once plentiful before European settlement. This is likely to have major benefits not just for nature conservation, but also for rebuilding damaged ecosystems. These animals are critically important for building up healthy soils and maintaining diverse plant communities, and probably for other important services that we are only just beginning to realize. Is up to all of us to find ways of protecting and enhancing native animal populations because of the amazing environmental services that they provide.
Ant activity and the development of nutrient-rich soil profiles part of Vignettes:Vignette Collection
Rainfall is one of the most important factors in arid and semi-arid (dryland) environments; it supplements soil moisture storage, stimulates plant germination, and brings new life to dormant vegetation. Rainfall can also be a important trigger of animal activity, causing a flurry of activity as animals such as ants and rodents collect and store seeds, insects pollinate plants, and birds prepare for nesting. Dryland soils are generally nutrient-poor and sometimes eroded. In the semi-arid woodlands of eastern Australia, more than a century of overgrazing by domestic animals has severely damaged surface soils and has lead to reductions in their chemical fertility and biological activity. This means that when it rains, water flow through the soil is restricted, often because the natural pores in the surface have been destroyed through compaction or the surface has a thick, impenetrable seal. Although dryland soils might at first appear to be biologically barren, most are habitat for a wide variety of insects, particularly ants. One common ant on sandy soils is the funnel ant (Aphaenogaster), which tends to occupy sandy soils dominated by white cypress pine (Callitris glaucophylla). Funnel ants are very small, less than a few millimeters across, but they build relatively conspicuous nest entrances which they surround with mounds of soil. The mounds resemble small volcanos (Figure 1), and their large cavities lead down into huge underground colonies. No one knows exactly how big the colonies are nor how many ants they support, but there are thought to be many thousands of ants in any given colony. Funnel ants rarely come out during the middle of the day, though they can sometimes be seen in the early morning and late afternoon carrying sand grains out through the nest entrances. Instead, they spend most of the daylight hours underground, eking out a living by tending fungi that grow on the roots of the white cypress pines. It is rather a mystery why ants build such large volcano-shaped entrances. Their shape may be a mechanism to help them prevent water from entering the nests when sheets of water (overland flow) moves down these gently sloping hillsides. The large nest entrances would certainly deflect shallow flows of water away from the entrances. It is rather strange, however, as the soils on which the nests are constructed are generally very deep and tend to have high rates of water infiltration. After even a few millimeters of rainfall, the ants can be seen busily cleaning out their nests, even during the daytime. They pile up fresh soil outside the nest entrances, effectively 'rebuilding' their volcanos. If you happen to be our a few days after rain the first thing you will notice is that the surface takes on the appearance of a lunar landscape. The next thing you notice is all of the ants. We have been tracking the nest building behavior of funnel ants for the past 15 years and now appreciate the importance of the nests for altering runoff and sediment movement. On gently sloping sandy soils, falls of rain of more than about 5 mm over the period of a day are sufficient to generate overland flow; the movement of a thin layer of water down the soil surface. This water brings with it a huge payload of pine needles as it moves in and around the pine trees and the nest entrances. Pine needles generally occur in large patches under the trees and in places can be many centimeters thick. When the water moves the pine needles down the long low slopes they quickly become trapped against the sides of the volcano-like nest entrances (Figure 2). This just happens to coincide with the time that the ants are out feverishly working to clean out their nests, trying to get rid of wet soil that has fallen off the sides of the nest entrances. Once it stops raining, ant activity speeds up, and soon the litter around the nests is covered by nest soil (Figure 2). Unwittingly, the nest cleaning activities by ants are also helping to build up a fertile soil profile. Some water does however manage to penetrate the nest entrances. Our studies on these soils indicate that rates of water flow can be in excess of 300 mm per hour (Eldridge 1993, 1994). If you take a thin slice of the soil through one of the nests you'll see layer upon layer of pine needles, like layers of cake. Because the pine needles are sandwiched between the soil they quickly begin to break down through the action of fungi. Look closely at the pine needles and you will see the signs of fungal breakdown; long, fluffy white filaments called hyphae. The ecosystem effects of ant activity combined with the movement of litter by overland flow, results in the area around the nests having substantially higher concentrations of carbon, nitrogen and other trace elements (Eldridge and Myers 1998). It's pretty hard to imagine how long this has all been going on, but it's probably been for many hundreds of thousands of years. These tiny ants have a close association with the sandy pine soils. Apart from making them more fertile they also help to move the soil around, a process called bioturbation. As ants create new tunnels and abandon others, the locations of the volcano-like nest entrances change, about once every few months or so. In fact, the mass of soil moved during this process is so large (about 3.4 tonnes per hectare per year) that the ants are responsible for 'turning over' the entire soil profile down to a depth of 30 cm every 200 years or so (Eldridge and Pickard 1994). This surely is an amazing feat earning them the title 'ecological engineers'.