ENV2201 Module 4 – Ecology, land degradation and the conservation of resources 1 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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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.
ENV2201 Module 4 – Ecology, land degradation and the conservation of resources
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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.