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ENV2201 Module 4 – Ecology, land degradation and the conservation of resources

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Module 4 – Ecology, land degradation and the

conservation of resources

Module overview

This module introduces ecosystems, the components of ecosystems, and ecological

succession, followed by the major types of soil and water degradation common in Australia,

as well as appropriate conservation practices. Ecological changes associated with resource

degradation are also discussed.


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Objectives

On completion of this module, students will be able to

• describe the difference between open and closed ecosystems

• describe the roles of producers and consumers in ecosystems

• explain how energy and nutrients flow through ecosystems

• depict and explain the nitrogen, carbon and phosphorus cycles

• explain the processes of ‘bioconcentration’ and ‘biomagnification’ and the role they

play in ecosystem degradation

• describe the processes of ecosystem change in response to disturbance

• define and discuss soil ‘erosion’ and its key drivers

• describe how rainfall drop size and the terminal velocity of raindrops influence

erosion

• describe and explain methods used to control of soil erosion

• explain the difference between salinity and sodicity

• describe the processes involved in (a) dryland and (b) irrigation salinity

• describe and explain methods used to address soil salinity and sodicity

• explain the process of soil acidification and its impact on plant growth

• describe the major ways of overcoming issues associated with soil acidity

• describe the major impacts that dams can have on stream ecology

• describe how mining may impact stream water quality and ecology

• explain the concept of ecological succession and its relevance to forestry

management.


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Introduction

The previous module introduced the concept of resource management. However, to manage

resources appropriately requires an understanding of the nature of the resource, including its

relationships with both non-living and living systems. The study of the connections between

living organisms, including humans, and their physical environment is called ecology.

Ecology helps us to understand our relationship with the natural world. It teaches us about

the ecosystem services provided by the natural world and about how the way we use Earth's

resources impacts the environment. These impacts can be both beneficial and detrimental;

they also have societal consequences, both in our own lifetimes and for future generations.

Ecology is often broken into three separate, but interlinked, elements: plant ecology, animal

ecology and human ecology. While these specialities are important, we will be concerned in

this module with general ecological principles and concentrate on the interactions of the

major systems.

Degraded land is defined as land which has lost some or all of its value for human use

(McTainsh & Boughton 1993). Land may be degraded through natural processes, although

the vast majority of degradation is either human induced or human accelerated. In general,

natural processes of degradation are very slow, occurring over many hundreds or thousands

of years. However, human induced degradation can occur over periods ranging from

instantaneous to tens of years.

4.1 Open and closed ecosystems

The ecosystem concept pulls together the physical environment and the related biological

community as they interact both with themselves and with each other in a complex web of

relationships. Ecosystems can be either ‘open’ or ‘closed’ systems depending on whether

there are interactions (or transfers) outside of the web of relationships studied. The nature of

such relationships is normally space- or time-dependent and the nature of the system is often

affected by the scale at which it is observed. Thus, at a large scale, the earth’s biosphere

may be considered a closed system because there is little interaction with systems outside

the biosphere. At the other end of the scale, the recycling of nutrients through the breakdown

of organic matter by bacteria in the soil is considered an open system. In this case, nutrients

move in complex organic molecules through living organisms (plants and animals); once

‘used’ they are broken down by bacteria into their elemental forms and move into the abiotic

(non-living) environment, either to be taken up again by plants and converted back into

organic molecules or to be transported in solution or suspension by water moving through the

environment.


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4.2 The holistic (ecological) view of natural systems

Ecology is essentially a holistic science. Holism views all living things (biota) as parts of a

system in which individual organisms interact with each other and with the physical (abiotic)

environment. Based on this view, several themes are apparent within ecology:

Everything in the environment is related to everything else. Ecosystems are complex

dynamic systems; hence, change in one part of the system will instigate a series of changes

in other parts of the system. For example, construction of a dam to enable water storage and

regulation of streamflow for the purposes of irrigation, urban supply or hydroelectric power

generation, will alter river flows and flooding patterns. This may in turn affect riverine fish

populations, wetland inundation cycles, the fertility of floodplain soils, groundwater recharge,

estuarine productivity, and the breeding cycles of marine species.

Complexity is, in part, responsible for the stability of most ecosystems. The more food

chains there are in an ecosystem, the more cross-connecting links there are among the food

chains and the more chances there are for the ecosystem to compensate for changes

imposed on it.

Humankind’s activities tend to reduce the complexity of ecosystems. For example,

planting a field to only one crop—a monoculture such as wheat—and managing it by the

addition of pesticides for weed or insect pest control significantly reduces the complexity of

the system. Similarly, engineering works to straighten a stream for whatever reason, may

significantly simplify, if not destroy, the stream ecosystem.

No species encounters, in any given habitat, the optimum conditions for all its

functions. Thus, populations of species tend to be greater in areas where most conditions

are most favourable. Humans, similarly, tend to concentrate locations where they see their

environmental (and socio-economic) needs as being especially well met (e.g. cities, coastal

areas).

Although living organisms react to all environmental factors in their location, there

frequently occurs a discrepant factor which has controlling power through its excess

or deficiency. For example, deficiency of a nutritional requirement will produce disease or

death, as may the presence of toxic levels of the same nutrient. This is often termed a

‘limiting factor’, particularly when it is present in quantities outside the organism’s ‘limits to

tolerance’ (terms we will discuss in more detail below).

Some resources do not renew themselves because they are the result of processes

that occur over extremely long timeframes. Thus, soil, coal and mineral resources are all

considered to be non-renewable.


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Environmental change occurs more rapidly than organic biological evolution.

Environmental change can occur over the space of a few years (or less) under human

influence. However, the evolution of species frequently requires several million years. Thus,

when environmental change occurs, affected species may be forced to migrate to more

hospitable habitat or face extinction.

A species’ distribution is geographically limited by the environmental conditions it can

withstand (i.e. its limits to tolerance). Modifications of the environment can lead to either

adaptation, a shift in the species’ geographical distribution or extinction. Similarly, for the

human species; there is a possibility that human tolerance to the changes we are inducing on

the environment may eventually be exceeded, and that we may also become extinct.

In any ecosystem, one species of plant or animal usually dominates the ecosystem.

Thus, in a forest, it is often the largest tree species that dominates and disproportionally

influences the habitat of other organisms in the system. However, the dominance of humans

in nearly all ecosystems is now so all pervading that we are in the process of altering the

habitat for every living creature, including ourselves.

4.3 The components of ecosystems

All ecosystems consist of both abiotic (non-living) and biotic (living) components. To

understand the interactions between these components more easily, they are generally

subdivided into five major sub-groups. The abiotic component includes:

● abiotic resources: includes the energy and inorganic substances needed by organisms

to live (e.g. water, nitrogen, carbon dioxide)

● abiotic conditions: the substrate and/or medium in which organisms live (e.g. air, water,

soil) and the ambient conditions (e.g. temperature, water availability,

currents).

The biotic component includes:

● primary producers: autotrophs (organisms that manufacture their own organic

constituents from abiotic resources e.g. algae, some bacteria, plants)

● secondary consumers: heterotrophic organisms that feed on other living organisms to obtain

organic material (e.g. most animals)

● decomposers and detritivores: heterotrophs that breakdown or feed on dead organic compounds.


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4.3.1 Tolerance ranges

Many organisms are unable to survive under abiotic conditions outside the range to which

they have historically been exposed and are adapted. For example, plant and animal species

which have evolved and are adapted to living in hot, tropical regions would be unlikely to

survive in the cold conditions found in arctic regions; while those adapted to living in wet

regions are generally unable to survive in arid areas. Similarly, animals which have particular

(specialised) food requirements—for example, for a particular plant group, such as koalas for

eucalypt leaves—will not survive in areas where that resource either does not grow or has

been removed. Hence, all organisms require the availability of particular resources and

particular ranges of abiotic conditions to survive. The maximum and minimum limits of an

abiotic resource or condition between which an organism is able to survive and reproduce

represent the tolerance range of that organism. Figure 1 shows the different (species-

specific) temperature tolerance ranges for the activity (survival) of trout and frog eggs. Note

that the activity (growth, maturation) of the eggs is not constant throughout the tolerance

range but increases towards some optimum temperature. This optimum is also not

necessarily in the middle of the tolerance range.

Figure 1 The limits of tolerance (adapted from Brum et al. 1994, p. 938). For each physical factor,

organisms have minimum and maximum tolerance limits.


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Tolerance ranges:

● are typically different for each abiotic factor that influences survival;

● contain an optimum value;

● may be affected by interactions with other factors which may influence the maximum,

minimum and optimum levels of the factor (e.g. water availability may affect the levels of

nutrients required to produce maximum plant growth);

● may be different for different life stages of the organism (e.g. adult trout survive

temperatures well above the tolerance range for their eggs); and

● may differ slightly from population to population within a species (e.g. members of a

population in a cold area may be better able to survive under cold conditions than a

population of the same species which has acclimatised to warmer conditions).

4.3.2 Limiting factors

As abiotic factors fluctuate, the growth, survival or reproductive capability of organisms within

an environment also vary according to their tolerance for particular conditions. As the limits of

tolerance for individual factors approach either the maximum or minimum limit of the

tolerance range, those factors take on a greater importance than others in determining the

productivity and survival of the organism. Hence, a single factor may limit an organism’s

activity if it exceeds the organism’s tolerance limit for that factor. These are termed limiting

factors as they impose some constraint on the organism by affecting its activity, health and

distribution. In this case, improving the availability of other resources or factors does not

improve the activity of the organism. Only by improving the availability of the limiting factor

will activity increase. However, activity will only increase to the level constrained by the next

most limiting factor. Only by overcoming all limiting factors will activity be maximised. This

phenomenon is known as Liebig’s Law of the Minimum and is commonly encountered in

ecological systems.

A common example which may help you to visualise the process is that of a plant growing in

an infertile soil which is lacking a range of nutrients (Figure 2). In this example, the nutrient

most limiting plant growth is iron, closely followed by nitrogen. Hence, where nitrogen (or any

other nutrient) is added without adding iron, no additional growth occurs. However, where

iron is added alone, growth increases until it is limited by the availability of nitrogen. At this

point, additional growth requires the addition of nitrogen. Maximum growth produced where

both iron and nitrogen are added will then be a function of the next most limiting factor (e.g.

other nutrients, water, sunlight).


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Figure 2 The limits of growth by nutrient availability

4.4 Energy flows and nutrient cycles

We discuss energy and nutrient flows through ecosystems in some detail in the lectures for

this module.

In summary, energy flows through ecosystems but is not recycled and eventually lost as heat.

Light energy drives the photosynthetic processes in plants by which elemental nutrients—the

most important of which are carbon, oxygen, hydrogen, nitrogen and phosphorous—are

converted to complex organic molecules which form the basis of living things. Unlike energy,

nutrients are constantly recycled through and between the biotic and abiotic elements of

ecosystems.

These are basic and critical functions of ecosystems and disruption of these cycles can throw

out the balance of natural systems. While we can use our understanding of ecosystem

function to our advantage (e.g. in agricultural production systems), our activities can also lead

to significant pollution problems, as we discuss in the lectures and as will also become

evident in future modules.


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4.4.1 Pollutants in ecosystems

Pollution problems can result from excess quantities of particular naturally occurring elements

(e.g. plant nutrients such as nitrogen and phosphorous) in ecosystems which can cause

excessive plant growth (e.g. toxic algal blooms in waterways due to excess phosphorous).

Pollution problems can also result when compounds which do not naturally or commonly

occur in environments are released as a result of industrial processes, often with significant

detrimental consequences for living organisms which have no/limited tolerance for these. The

toxic nature of such materials can vary. Some will cause instant death; others will not be

immediately toxic, but may become so above certain threshold concentrations. Heavy metals

(e.g. mercury, lead), polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane

(the insecticide DDT) are examples of persistent organic pollutants (POPs) which can

bioaccumulate/bioconcentrate in the fatty tissue of living organisms because they are not

able to be broken down by normal digestive/cellular processes. When organisms at lower

trophic levels of a food chain are eaten by those at higher levels, these persistent pollutants

further accumulate and increase in concentration. This biomagnification process can result in

increasingly toxic levels of these substances at higher trophic levels. This is known to result

in population collapse in top order carnivores such as fish-eating eagles. It also has

significant implications for human health where our diet includes species at the top of the

food chain (e.g. tuna), where these come from contaminated environments.

4.5 Ecological succession

We also discuss the concept of ecological succession in the lectures for this module.

‘Succession’ describes the changes that we can observe when living organisms start to

occupy/colonise an area of bare soil/substrate. Such areas may be newly formed substrates

(e.g. volcanic lava flows, surfaces exposed by retreating glaciers) which have not previously

supported living organisms/ecosystems or areas disturbed by the actions of humans (e.g.

clearing, mining, agriculture) or natural forces (e.g. floods, fire, landslides). Succession

involves a progression from an assortment of usually short-lived short-statured colonising

plant species (usually species with broad ecological tolerances) through various interim

stages to a relatively stable system of interacting species.

Previous thinking viewed succession as a transition to a predictable end state or ‘climax’

community; current understanding is that succession can follow different trajectories towards

a number of alternative states, depending on the combination of factors (including

subsequent disturbances) operating in that location and through time.


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An understanding of the roles of disturbance and ecological succession is critical to

assessing the ecological impacts of land use development and to planning successful

restoration projects to redress land degradation.

4.6 Erosion

The word ‘erosion’ is derived from the Latin erosis meaning ‘to gnaw away’. In general, soil

erosion implies the physical removal of topsoil by raindrops, water flow, wind and gravitation.

The detached solid material is referred to as ‘sediment’ and its deposition in another place,

when the energy of the erosion causing agent is diminished, is described as ‘sedimentation’.

Geological erosion is a natural process and some of the most fertile valleys of the world have

developed from this process, including the Nile Valley in Egypt and, more locally, Condamine

floodplains (‘Darling Downs’) of southern Queensland. In this situation, there tends to be a

balance between soil loss and soil formation, creating a mature soil balance. However,

human activity may rapidly accelerate erosion through activities such as the clearing of

forests; overgrazing of pastures; cultivation; mining or urban development. In such instances

case, soil losses of 300 tonnes/ha/yr (≈ 3cm depth of topsoil) are not uncommon, and in

some areas rates of soil loss exceed rates of soil development/deposition by some 50 times

(NLWRA 2001; Campbell 2008).

Accelerated soil erosion occurs at many times the rate of natural erosion and causes

significant disruption to plant life, animal well-being, and possibly to entire human

communities. Massive changes in the environment including land clearing, intensive

cultivation, or overgrazing can result in accelerated soil erosion. Most of these situations are

due directly or indirectly to human decisions and actions. Rates of soil erosion accelerated

over much of Australia throughout the 20th century due to land use associated with food

production and settlement. For example, a joint Commonwealth-state survey of human

induced soil erosion in the late 1970s found that two-thirds of Australian cropland was

seriously affected. The study also found that one-third of non-arid and over half of arid

grazing land showed signs of severe erosion. Experimental data indicate that rates of

erosion may have increased by 10 to 100 times in many Australian agricultural areas

compared to the previous 100 years. While long-term natural rates of erosion are estimated

to be well below 1 t/ha/yr, rates in agricultural areas generally fall between 0.5 t/ha/yr (in

pasture with good groundcover) and 10 t/ha/yr (in summer cropland). Rates of 10–50 t/ha/yr

and higher have been recorded immediately following clearing at a number of sites in

Australia. Erosion rates peak soon after land is cleared; however, renewed phases of

accelerated erosion are likely to coincide with further environmental pressures which

dramatically reduce vegetation cover, particularly drought and fire. Severe wind storms and

floods following such events can subsequently move very large volumes of soil.


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The most serious feature of soil erosion is the displacement and loss of soil. The most

vulnerable part of the soil profile is the surface horizon which is rich in organic matter and

plant nutrients and allows for the development and maintenance of soil structure. The newly

exposed subsoil horizons typically contain very little organic matter and are less likely to have

a structure that readily allows water and air entry. Hence, water tends to be shed from such

eroded soils and it is difficult for oxygen to enter and carbon dioxide to escape. A sharp

reduction in these characteristics due to erosion results in the soil being much less suitable

as a medium for plant growth and much less fertile, hence much less productive.

The displacement and sediment transport processes associated with soil erosion can also

cause serious disruption. Floods can result in water courses becoming silted and greatly

altered, and sediments can become deposited on highways, roads and urban communities.

Streams in erosion areas are often laden with sediments, and may silt up and develop highly

irregular flows patterns. Similarly, farm dams subject to significant levels of siltation will have

reduced water storage capacity and value over time. Soil erosion also degrades the

landscape, reducing the biodiversity value and amenity of areas such as creeks and rivers.

Greater emphasis is usually given to soil erosion than to other processes in land degradation

for a number of reasons. Soil is effectively a non-renewable resource which cannot be

replaced by fertilisers or other additions. The surface soil lost is the most fertile part of the

profile and such losses affect all soil fertility parameters. Of particular importance is the loss

of soil depth in shallow soils and the removal of organic rich surface soil. This will in turn also

affect local plant and animal species. Sedimentation also has serious effects on reservoirs

and is a major source of pollution and eutrophication in wetland systems, such as streams

and lakes.

The costs of soil erosion include loss of agricultural productivity (largely from loss of nutrients

and organic matter), and losses associated with sediment load, including reduced water

quality and damage to infrastructure such as roads and dams. Losses in soil depth lead to

reduced productivity via less plant available water and reduced root penetration. However,

these costs are generally poorly quantified except for specific sites, and reported values vary

widely. On the other hand, the off-site effects of erosion are often easier to calculate as they

involve specific actions. For example, the off-site effects of soil erosion in Queensland on

water treatment and road maintenance during 1989 was estimated to be $31 million

(LWRRDC 1995).

The majority of erosion occurs due to the action of either water or wind. In Queensland, water

erosion is considered the most serious and widespread form affecting some 90% of cultivated

land. Wind erosion is generally only significant in arid to semi-arid regions.


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4.6.1 Forms of water erosion

Water erosion is caused by overland flow (i.e. runoff). The amount of erosion caused by

water is directly proportional to the soil’s tendency to break down or disperse in water (i.e. the

dispersivity of the soil), and is inversely proportional to the soil’s capacity for infiltration. The

infiltration rate is closely related to the size and distribution of pores in a soil, which in turn are

dependent on soil aggregation and texture (Module 5). Obviously, the higher the rate of

infiltration, the less ponding and overland flow will occur (and vice versa). Stable soil

aggregates resist dispersion and erosion because they have a larger size and mass and

require a higher overland flow rate before becoming entrained in the flowing water. Because

they are larger, they also maintain a higher infiltration rate. The stability of aggregates is a

result of both physical and chemical processes. The level of exchangeable sodium (i.e.

sodicity) and the amount of organic matter in the soil have a major influence on the soil’s

chemical properties and its behaviour when wetted. Pressures created by shrinking and

swelling (due to drying and wetting, respectively) are also important.

Sheet erosion

Erosion processes, which result in the removal of a fairly uniform layer of soil and a

significant absence of rills (small channels), create a condition commonly known as ‘sheet’

erosion, which results from the impact of raindrops on a sloping soil. When soil particles are

splashed into the air by the impact of raindrops, more sediment falls downslope of the initial

impact point than above it due to the sloping surface. Thus, the net movement of material is

downslope. Sheet (or more accurately ‘splash’) erosion occurs predominantly on freshly

cultivated bare soils and bare soils which have naturally friable or self-mulching surfaces

which do not form a crust. Due to high infiltration rates in well-structured soils, the extent of

sheet erosion tends to reach serious proportions only when such soils become saturated.

This form of erosion is most common on unprotected (i.e. lacking vegetative cover/mulch) low

grade (<4%) slopes.

Rill erosion

Rilling commonly occurs on bare soil and is therefore generally associated with arable land.

The depth of rills is in the order of a few centimetres (compared to the dimension of gullies

which are generally an order of magnitude larger) and rills are generally regarded as those

erosion channels which can be treated (filled and restored) by normal cultivation practices.

Rills normally form due to the minor concentration of overland flows. As the water

concentrates into localised areas, the volume of flow in the area increases and hence the

velocity of water flow increases. As the velocity increases, soil particles become entrained in

the flow.


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Gully erosion

Gully erosion is a fairly complex process and is influenced by many factors, such as soil type;

soil management; geomorphology; land form; slope; land use; and the characteristics of the

catchment. Gullies are commonly one to several metres deep with width often ranging up to

10 m. Gullies usually require earthmoving equipment to fill them and special soil conservation

practices to prevent them forming/washing out again. Alternatively, the specific area (gully)

may be stabilised and sown to pasture for use as a waterway with limited grazing.

Gully formation and development may result from a number of processes

● channel scouring from rills;

● the gully head may migrate uphill by waterfall action and create headward erosion—

subsoil dispersion, undercutting, and slumping of topsoil at the gully head accelerates the

process in the case of dispersible soils; and

● the channel widens and deepens as the gully lengthens uphill—the catchment area

above the gully decreases and the area on either side contributes to gully branching.

Climatic factors, particularly the frequency of intense summer thunderstorms, are of great

importance in determining the amount of gully formation and development. Dispersible soils

are also much more likely than other soils to form a massive network of gullies. However,

with slumping, the gully walls may reach a slope sufficiently stable to allow vegetation to grow

and to begin to stabilise the system.

Tunnel erosion

Soils with dispersible subsoils are also prone to tunnel erosion. If the subsoil has a high

exchangeable sodium percentage, it is likely to be unstable and dispersible in water. When

such soils are saturated and an outlet is available, any weakness, crack or channel may allow

underground movement of dispersed clay downhill. In this case, the dispersed clay is washed

out of the outlet resulting in the formation of ‘tunnels’. These gradually get bigger, eventually

joining up and causing slumping of the topsoil inwards and the resumption of more normal

gully erosion. Underground telecommunication cables or drainage pipes laid in dispersible

soils have been found to contribute significantly to tunnel erosion in some areas.

Tunnel erosion can also occur when water moves down through a permeable layer and then

encounters an impermeable layer. Lateral movement of water above the impermeable layer

tends to take place, emerging via an outlet lower down the slope or in the side of a gully. With

relatively rapid flow, fine clay material is then transported towards the outlet and a tunnel

formed. This is an important cause of tunnelling in earth dams built using dispersible soil

materials, and can lead to failure of dam walls.


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Landslip erosion

Landslip is the mass movement of earth material (soil and rock) down a slope. Such landslips

can be damaging to agricultural productivity and dangerous in urban developments. The

landslides are generally shallow in depth but may be complex. The rupture zones are often in

plastic weathered shale and clay overlying beds of sandstone. Rain percolates through the

weathered surface material and forms perched aquifers on the clay beds. The excess

moisture causes a reduction in the shear strength of the clay and subsequent failure of the

slope. Most landslides can be attributed to the removal of natural vegetation, particularly

trees, on susceptible slopes.

Critical factors for slope stability include the dip direction of potential failure surfaces; shear

strengths of the rocks; groundwater conditions; and the type and performance of clays on

shear surfaces. Tracks used by animals or vehicles may tend to concentrate water and

enhance the probability of landslips. In central Queensland coal mining operations, damage

due to blasting has also contributed to landslips.

4.6.2 Factors influencing water erosion

Erodibility

When exposed to rain of the same erosivity, different soils will exhibit different rates of

erosion. The vulnerability or susceptibility of the soil to erosion is called the ‘erodibility’. The

erodibility of soils is dependent mainly upon their inherent properties but also upon the way in

which they are managed. Many investigations have been directed to understanding the

relationship between the physical properties of soil and erodibility. However, much of the

work has been inconclusive, principally because different soils rank differently in respect of

the detachment and/or transport phases. For example, fine sand particles are more easily

detached than clay size particles, but clay particles are more easily transported than sand

particles. Nevertheless, some general statements can be made. Soils with large, water-stable

aggregates, separated by plenty of large pore spaces (Module 5), infiltrate water rapidly and

are resistant to erosion. Soils with fine aggregates, or aggregates which break down when

wet (i.e. unstable aggregates), usually erode easily as water entry is slow, the surface

puddles easily, and the pore spaces are small. Sandy soils are generally less erodible on

gently sloping land, but the reverse is true on steep slopes. Some duplex soils (soil with

complex horizonation) with high sodium levels are extremely fragile and may erode, even

when uncleared but lightly grazed.

Management also influences the extent of erosion. The main point is to avoid a situation

where bare soil is exposed to intense rainfall, as occurs particularly in summer in the northern


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regions of Australia. When soils are dry and deeply cracked, rainfall is absorbed rapidly, but

when they are wet, falls of less than 20 mm may result in run-off and erosion.

Erosivity

As inferred above, the intensity of rainfall is one of the major factors influencing soil erosion.

‘Erosivity’ is the potential ability of rain to cause erosion. The larger the diameter of a

raindrop, the greater the terminal velocity of the drop and the higher the impact (or splash)

energy. Drizzle, and virtually all rainfall that occurs during the winter months in southern

Australia, consists of small raindrops with a terminal velocity of less than 6 m/s. Under these

conditions, water erosion is slight and generally of little significance. However, rainfall in the

summer months (predominantly in northern Australia), particularly that resulting from tropical

storms, has large drops and a terminal velocity of 8–9 m/s. Rain falling with this terminal

velocity on a bare soil surface for a sustained period (>30 min) has a high probability of

causing water erosion.

The energy of raindrops striking bare soil results in the physical disruption of soil aggregates,

compaction of the soil surface, and the splashing of soil particles into the air. When the soil is

dry and bare, the raindrop impact breaks up the soil aggregates and forces the soil down and

to the side leaving a small crater. As the soil becomes wetter during the rainfall event, it loses

cohesion, the aggregates break-up more easily, and the displacement of soil in the splash

increases. The clay and silt size particles detached from the aggregates are washed and

compressed down the surface pores, increasing the density of the surface soil and reducing

infiltration. The effect of rainfall intensity is illustrated in Table 1with data from a farming

region in southern New South Wales.

Table 1 The effect of rainfall intensity on run-off and erosion for a cultivated farm (NSW)

Period Total rainfall in

period (mm)

Amount (mm) of

rainfall in excess of

50 mm/h

Run-off

(%)

Soil loss

(kg/ha)

11 months

(Jan–Nov)

725 25 0.5 9

3 months

(Dec–Feb)

325 147 25.0 490


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Groundcover and vegetation

Groundcover and vegetation act as a buffer, so that most of the energy of falling rain is

dissipated before it hits the soil. Living vegetation, or stubble and residues from crops and

other plants, provide effective buffering to raindrops. Soil erosion is uncommon in uncleared

well-vegetated areas but will occur occasionally in timbered country if the ground surface is

bare due to over-grazing or fire. Soil aggregates that are high in organic carbon are generally

stable and unlikely to be detached or removed. With more stable aggregates, a high

infiltration rate is maintained and there is little or no run-off. An indication of the effect of

groundcover on runoff and erosion is provided in Table 2 for a farming region in southern

New South Wales.

Table 2 The effect of soil surface condition on run-off and erosion

Soil surface condition Run-off (%) Soil loss

(kg/ha)

Bare fallow (cultivated) 42 25 000

Wheat stubble 51 12 000

Pasture 25 500

4.6.3 Forms of wind erosion

Wind erosion is fairly widespread in the semi-arid and arid regions of Australia, particularly in

southern Australia where winter rainfall predominates and summers are hot and dry. Many of

these regions are used for grazing, and are of low economic value. In these situations, the

main factor in reducing wind erosion is grazing land management. Wind erosion is also

significant where light textured soils (e.g. Mallee soils) are used for cropping in South

Australia, western Victoria and to a limited extent in New South Wales. Wind erosion is also

of concern along coastal beaches, particularly along the eastern coast of Australia.

There are several basic mechanisms involved in the movement of soil particles by wind:

● Surface creep – the movement of large particles (too heavy to be lifted into the air) in

response to horizontal wind action. In this case, the horizontal movement of the wind

causes friction over the soil surface, moving soil particles and aggregates along the

surface.

● Saltation – the bouncing of particles across the soil surface by the action of wind. This

type of movement dislodges other particles and breaks up soil crumbs, thus exposing


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them to movement. Sand grains and particles moving in saltation are commonly 0.2 to

0.35 mm in diameter.

● Suspension – the movement of suspended soil particles (dust) in general air currents.

These small particles (diameter < 0.02 mm) can be carried great distances in dust storms

(e.g. Australian soil has been found on the ski slopes of New Zealand). Dust particles are

kept suspended by the upward component of turbulent wind, which is about one fifth of

the horizontal velocity.

Scalding

Duplex soils, characterised by a clay horizon overlain by a loose sandy surface horizon, may

develop localised scalds in semi-arid regions such as western Queensland. In these cases,

vegetation fails to grow in the scalded area and the surface soil becomes prone to severe

wind erosion. As the surface soil is removed, the weakly developed subsurface aggregates

tend to disperse, and the clay forms a surface seal, significantly reducing water entry. The

resulting hard impermeable surface is of low fertility and highly resistant to colonisation by

plants and the area tends to present as a bare scalded area (i.e. a ‘clay pan’).

Windsheeting

Windsheeting occurs on deep sandy soils (such as desert loams) which typically show little

change in profile with increasing depth. Wind in excess of 22 km/h at a height of 300 mm

above the ground level will cause erosion of dry bare soil particularly if the surface soil is

unprotected and the soil particles are detachable. Although considerable quantities of soil

may be removed, the new surface has a ‘swept’ rather than the ‘polished’ impermeable

appearance of a scald. Permeability (water entry) does not decline greatly and revegetation is

readily accomplished.

Moving beach dunes and mining of coastal sands create special problems of wind erosion.

The problem is usually initiated and accelerated with the removal of vegetation and intense

vehicular traffic. However, extensive areas of sand-mined beaches on hind-dunes have been

stabilised using protective wind barriers and revegetation. In the case of beaches and tourist

resorts, co-operation between local government authorities and beach protection groups

have been successful in effectively reducing soil erosion.

Severe wind erosion is usually due to overgrazing (removing vegetation); excessive traffic

(vehicular, livestock or pedestrian); overclearing; and cultivation. In remedying the situation,

the first step is to remove or greatly reduce the impact of the causative agent. Subsequent

efforts should be directed at maximising water entry and storage in the soil. Improved storage

of water in the soil and possible entrapment of windblown seeds can encourage revegetation.

However, it is seldom practical or economic to re-introduce plant species in arid and semi-

arid regions, particularly over wide areas. In these areas, prevention is the preferred option.


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4.7 Degradation of water resources

Competition for limited water resources (both surface and groundwater) exists between

agricultural, mining, industrial, municipal, environmental and other users. As demand for

water of high quality has increased, there has been increasing pressure to allocate water to

new uses and users. Because of Australia’s poor and unreliable rainfall and heavy reliance

on a few perennial rivers, it is not possible to absorb the levels of industrial, municipal and

agricultural pollutants that might be possible in more water abundant regions.

Agricultural land use and rural development can have a suite of adverse effects on water

quality and freshwater ecosystems. Evidence of river system degradation includes the

erosion of banks and channel structures; high turbidity; the occurrence of blue-green algal

blooms; damage to riparian vegetation; and the presence of exotic aquatic species.

Problematic in themselves, these are also the obvious symptoms of more subtle changes

occurring in the structure and ecological processes operating within riparian/river systems

(Arthington 1995).

4.8 Degradation of soil resources

Australian soils are generally regarded as fragile and susceptible to degradation, especially

when subjected to repetitive mechanical disturbance (Hunt 1980) and cannot be considered

to be a renewable resource in any practical sense. Australian landscapes are ancient and

highly weathered (McKenzie et al. 2004) and the rate of soil formation in Australia is often

referred to as relatively slow (SCERA 1989); however, recent analysis reveals that these

differ little from soil formation rates elsewhere in the world (Stockman et al. 2014). Despite

differences in climate or parent material, ‘global’ soil production rates reportedly average 114

± 11 mm/thousand years for shallow soils, exponentially decreasing with increasing soil

thickness (Stockman et al. 2014). Given the slow rate of soil formation, any soil degradation

risks the productive potential of land and may produce serious long-term environmental

problems. Despite widespread awareness of soils as an important non-renewable resource,

the degradation of soil resources through inappropriate management remains significant.

Recent estimates placed the cost of soil and land degradation at around 12 per cent of GDP

(in 2010–11, this was $5.7 billion), annually (Australian Bureau of Statistics 2012). Although

the following discussion concentrates on the rural implications of these problems,

degradation issues also arise through urban activities such as the inappropriate clearing or

location of residential areas, erosion of parkland and development sites, and the use of

inappropriate sizes or locations of road culverts.


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4.8.1 Salinity and sodicity

Salt affected soils may be saline (excess of soluble salts, mainly NaCl) and/or sodic (enough

exchangeable Na to affect the soil properties). Large areas of saline soils, and very large

areas of sodic soils, are found naturally in Australia due to the low rainfall and hence, low

leaching of soluble salts from the soil profile. The salt input generally comes from the soil

parent material and/or from rainfall. Both salinity and sodicity typically increase in the subsoil

due to the reduced leaching. Soil salinity is a major environmental problem in many areas of

Australia where the effects of both dryland and irrigation related soil salinity have made vast

tracts of land unsuitable for agriculture. Estimates suggest that up to 5 million hectares of

agricultural land have been affected.

4.8.2 Acidity

Soil acidification occurs as soils become increasing acid (i.e. below a pH level of 7.0) to the

point where plant growth and crop yields decline. Soils become acid because of prolonged

recurrent leaching, coupled with the input of various acids. Additions of acids to soils include

nitric and sulphuric acids (formed by bacterial oxidation of ammonium and sulphur in the soil)

and carbonic acid (formed by absorption of CO2 in rainwater or through the action of roots

and microbes). Around 35 million hectares are affected nationally in Australia while there is

the potential for more than 55 million hectares of moderately or slightly acidic land (pH 4.9–

6.0) to degrade to highly acidic conditions (pH<4.8) (AACM International Pty Limited 1995).

Soils may acidify naturally with leaching. Thus, highly permeable soils located in high rainfall

areas tend to have a low pH and may be naturally acidic. However, soils can also acidify

rapidly in conjunction with many agricultural activities including the addition of nitrogen and

sulphur-based fertilisers; the use of subterranean clover pastures; the continued removal of

alkaline plant and animal produce or waste; and the build-up of organic acids from increased

levels of soil organic matter. The main areas affected in Australia are thought to be the humid

coastal regions (due to leaching) and the pastoral areas of southern Australia (due to organic

matter build up in fertilised leguminous pastures).

Soil acidity at pH of less than 5.5 can inhibit the growth of sensitive plant species. However,

the major effects of low pH on productivity are due to associated effects, such as:

• prevention of nodulation by legumes;

• reduction in the presence of beneficial soil micro-organisms;

• release of toxic levels of aluminium and manganese;

• increased level of cadmium uptake by crops and pastures which can be harmful for

stock and human consumption; and


20

• reduced availability of some nutrients for plant uptake (e.g. phosphorus,

molybdenum).

Many soils require no management of soil acidity as their pH remains between 5.5 and 8.5, at

which the growth of most plants is unaffected. However, where growth is affected, it is

important to determine if this is due to the soil pH per se or one of the associated effects

outlined above. If low pH has affected nutrient availability, which in turn affects plant growth, it

may be more economic to apply fertiliser to directly overcome the nutrient deficiency than to

correct the soil pH. However, where there is a toxicity (e.g. aluminium), or soil pH is directly

responsible for the observed plant response, the most common method of rectifying soil

acidity is to apply a lime-based product (e.g. crushed limestone, dolomite).


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4.8.3 The concept and practice of conservation

Conservation basically means preventing the wasteful use of a resource. In this course, we

are particularly interested in the conservation of the earth’s natural resources which include

air, water, soil, minerals, fauna and flora.

One of the earliest attempts within the conservation community to grapple with the meaning

of conservation can be found in The World Conservation Strategy published by the

International Union for Conservation of Nature and Natural Resources (IUCN) in 1980. In this

document, the main objectives listed for conservation are

• To maintain essential ecological processes and life support systems

• To preserve genetic diversity, and

• To ensure the sustainable utilization of species and ecosystems

Although most people have a basic understanding of what conservation is, there are some

issues with defining what conservation does and does not include. Part of this confusion is

the difference between conservation and preservation. Conservation seeks the ethical use,

allocation and protection of natural resources by humans. Preservation, on the other hand,

seeks protection of nature from use by humans (US National Park Service n.d.).

Figure 3 The overlap between conservation and preservation

https://portals.iucn.org/library/sites/library/files/documents/WCS-004.pdf


22

Another part of the confusion is the myriad of elements or organisms in the natural

environment that can be conserved. One way of dealing with that, is to categorise the

resources, for example into soil, water, air, fauna and flora, amongst others.

The practice of conservation has evolved since the 1980 strategy quoted above. Evidence-

based conservation practices have become very important to ensure that we are adhering to

proven, best-practice conservation policies and practices. The Australian Government

Department, the Department of Environment and Energy gives extensive advice on

conservation agreements that are made between the Australian Government Environment

Minister and another person, aimed at the protection and conservation of biodiversity in an

area of land or sea (DEE 2019). Conservation thus occurs over many different geographical

scales, from small-scale conservation by individual landholders to large scale and strategic

conservation attempts by national governments. Examples of conservation agreements under

the EPBC Act includes

• Conservation Agreement for the protection and conservation of Lowland rainforest of

subtropical Australia at Palmview, Queensland

• Conservation Agreement for the protection and conservation of the Grey-headed

Flying-fox (Pteropus poliocephalus) at Batemans Bay NSW

• The protection and conservation of areas of State Forest separating the Tasmanian

Wilderness World Heritage Area from adjoining wood production coupes

• Low-impact aquaculture activities in relation to the Great Sandy Regional marine

aquaculture plan

• The development of the Edmondson Park Precinct under the EPBC Act

• Growling Grass Frog conservation management plan for proposed Donnybrook-

Hume Highway interchange, Kalkallo, Victoria by Biosis research (DEE 2019).

Soil conservation on agricultural land

Soil conservation includes actions to both prevent and control soil erosion. The old saying

‘prevention is better than cure’ applies both practically and economically to soil conservation.

However, soil conservation is influenced by socio-economic conditions within the community

including, particularly, public awareness of the risks associated with soil erosion, and is

therefore often a complex topic. In many cases, it is difficult to distinguish precisely between

prevention and cure with regard to soil erosion. In general terms, persuading farmers,

developers, community leaders and others to preserve the stability of the environment and to

prevent or minimise erosion is one of the main activities in prevention. Accordingly,

prevention of soil erosion should be one of the cardinal considerations at the planning stages

of projects and developments.

http://www.environment.gov.au/protection/environment-assessments/conservation-agreements


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On individual farms, soil erosion can be prevented by:

• responsible land use;

• the retention of strategic areas of trees and vegetation on steep slopes and along

water courses; and

• the retention of stubble on cropping land.

Trees growing along rivers and streams also strengthen and protect the banks against

erosion. Indiscriminate clearing along rivers has contributed to serious bank erosion with

subsequent loss of extensive areas of adjacent fertile alluvial land.

Control measures are often used where soil erosion has already caused serious damage and

considerable engineering structures or work programmes are required to arrest the erosion

and restore the environment to a stable condition. Actions, including those undertaken for

prevention, might include:

● provision of graded banks and grassed waterways;

● installation of contour banks at regular intervals to intercept run-off water and divert it

at slow speed to a water disposal system such as a grassed waterway;

● correct location of farm dams;

● using land according to its capability (i.e. avoid cultivating steep land, overgrazing,

and overclearing);

● retention of stubble and practising stubble mulch farming;

● halting cultivation and grazing of badly eroding areas so that they can be revegetated

with appropriate species; and

● relocation of sediment by earthmoving equipment. Such a programme may be costly

and would need to allow for a stabilisation programme, probably including

revegetation with appropriate species before the next storm season.

Contour banks (or graded banks) are often built at intervals across slopes (i.e. on the

contour) to intercept run-off water and divert it at slow speed to a waterway or other water

disposal system. All ploughing, cultivation and planting operations are carried out parallel to

the contour between the banks. The length of contour banks is normally <500 m and the

grade along the bank <3% to prevent scouring. The interval between contour banks depends

on the slope and the amount of expected run-off. Hence, rainfall intensity is a critical

consideration.


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Strip cropping consists of strips of closely growing pastures alternating with cultivated crops

on the contour with (>2% slope) or without (<2% slope) contour banks. The strips are aligned

at right angles to the general flow of water. The growing crop and pasture will cause run-off

water to spread evenly across the paddock. Strips should be no wider than 100 m and no

narrower than 20 m and should fit in with practical farming considerations such as the width

of cultivation equipment. On the Darling Downs, alternating strips may consist of winter and

summer crops, with stubble retained from the previous season’s crop.

Soil conservation on non-agricultural land

Removal of trees and other vegetation and subsequent land shaping during urban

construction can result in surface erosion. Sediment removed by urban development

damages land downslope, blocks storm water drains, and may be eventually deposited into

streams and harbours. Sediments cause turbidity in natural water bodies, reduce their

recreational value, and damage aquatic life and the function of aquatic ecosystems.

Increased flooding may follow the reduction in stream capacity caused by deposited

sediment. Urban sediments erode from a variety of sources, including:

• building allotments;

• roads and associated surface drains;

• easements for overhead wires, sewerage or underground cables;

• waste disposal areas and quarries continuously exposed while in use;

• recreation areas, particularly those subject to intensive pedestrian or vehicular traffic;

• hobby-farm subdivisions;

• streams through and below urban areas; and

• roads, easements or other developments may divert run-off into new channels,

subjecting these to erosion.


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List of references

AACM International Pty Ltd 1995, Social and economic feasibility of ameliorating soil

acidification – a national review, LWRRDC, Canberra.

Australian Bureau of Statistics 2012, Year Book Australia, 2012 (no 1301.0), Australian

Bureau of Statistics (ABS), Canberra ACT:

<http://www.abs.gov.au/ausstats/[email protected]/Lookup/1301.0Main+Features22012>.

Arthington, AH 1995, ‘State of the rivers flowing through the areas of irrigated cotton within

New South Wales including the border rivers shared with Queensland’, Proc. 12th IAA

Conference, Tamworth.

Brum, G, McKane, L & Karp, G 1994, Ecosystems and communities. In Biology: exploring

life. Brisbane Qld, Australia: Wiley & Sons.

Campbell A (2008), Managing Australia's Soils: A Policy Discussion Paper. Prepared for the

National Committee on Soil and Terrain (NCST) through the Natural Resource Management

Ministerial Council (NRMMC).

Department of Environment and Energy

Hunt, JS 1980, Structural stability of Mallee soils under cultivation, Journal of the Soil

Conservation Service of NSW, vol. 36, pp. 16–22.

Land and Water Resources Research and Development Corporation 1995, Data Sheets on

Natural Resource Issues, Occasional Paper 06/95, Canberra.

McKenzie, N, Jacquier, D, Isbell, R & Brown, K 2004, Australian soils and landscapes. An

Illustrated Compendium.CSIRO Publishing, Collingwood.

McTainsh, G & Boughton, WC 1993, Land degradation processes in Australia, Longman

Cheshire, Melbourne.

National Parks Service N.d., Conservation vs Preservation, viewed 29 March 2019, <

https://www.nps.gov/>.

NLWRA (2001). Australian Agriculture Assessment 2001. National Land & Water Resources

Audit, Land & Water Australia, Canberra.

Standing Committee on Environment, Recreation and the Arts 1989, The effectiveness of

land degradation policies and programs, Report of the House of Representatives, AGPS,

Canberra.

https://www.nps.gov/


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Stockman, U, Minasny, B & McBratney, AB 2014, How fast does soil grow? Geoderma vol.

216, pp. 48–61.

Stuart, S 2017, What is conservation?, Synchronicity Earth – Bring Conservation to Life,

viewed 29 March 2019, < https://www.synchronicityearth.org/thinking-about-conservation/>.

Sutherland, WJ, Dicks, LV, Ockendon, N & Smith, RK 2018, (eds.), What works in

conservation, 2nd edn, Open Book Publishers, Cambridge.

https://www.synchronicityearth.org/thinking-about-conservation/

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