ANSWERS TO REVIEW QUESTIONS – CHAPTER 44
1.
Explain the difference between the terms ‘population’, ‘community’ and ‘ecosystem’ (see
also Chapters 42 and 43). (p. 1091)
Populations are aggregations of individual plants or animals, while populations of species assemble
into communities. Assemblages of communities (biotic) interacting with their physical (abiotic)
environments constitute ecosystems. Ecosystems are the basic functional units of ecology and they are
open, dynamic, self-modifying systems.
2.
Explain the difference between the ecological meaning of the terms ‘biomass’, ‘energy’ and
‘productivity’. (pp. 1094–1096)
Biomass is ‘living mass’ and is usually expressed as a weight. For example, the aerial biomass of plant
can be measured as the dry weight of the entire harvested above-ground portion. Each unit (say gram)
of biomass can be expressed as a parcel of energy, usually measured as joules or calories. Biomass can
also be expressed as ‘energy equivalents’, and vice versa. Productivity is the rate at which energy flows
between trophic levels. It sets the upper limit to the food available to the next trophic level. The net
primary productivity of an ecosystem is the portion of biomass available to consumers and
decomposers once the respiratory loss of primary producers has been accounted for. Net primary
productivity of an ecosystem can be measured as the dry weight of biomass synthesised per unit area
per unit time and is expressed as grams per square metre per year (g/m 2/year).
3.
Give an example of a food chain that you might expect to find in an aquatic ecosystem in
your area. (p. 1096)
phytoplankton  copepods  larval fish  mackerel (predatory fish)
4.
Food chains typically have three or four trophic levels.
(a) What is a trophic level? (p. 1091)
A trophic level is a functional grouping of organisms in a community according to their feeding
relationships. For example, the first trophic level in a community includes plants that accumulate
photosynthate, while the second trophic level may consist of herbivores and the third carnivores.
(b) Why are food chains short? (p. 1096)
Food chains or webs describe the dependence of organisms at one trophic level on the availability of a
lower trophic level. For example, herbivores depend on the resources made available at the lower
trophic level of green plants, carnivores depend on the availability of resources available in herbivores,
etc. Food chains are short with only three or four trophic levels. Food chains are short because only a
small proportion of energy entering one trophic level is transferable to the next, higher trophic level.
However, there may be other factors such as logistic constraints, like size, which also limit food chains
to a few trophic levels.
5.
Look at Figure 44.9, which shows common and uncommon patterns in food webs. Suggest
why it is uncommon to find too many omnivores (44.9b) in a system. (pp. 1096–1097)
Omnivores often feed on more than one trophic level but typically feed on species in adjacent trophic
levels. Food webs are typically short and omnivores add complexity and linkages that reduce the
overall efficiency of food webs.
6.
Energy and nutrients move differently through ecosystems. (pp. 1091–1092)
(a) What are these differences?
The sun continually radiates energy and only a small portion is ‘diverted’ and utilised in ecosystems
through net primary production. Most of the constant energy supply that is radiated by the sun is
dissipated and lost as heat. Energy ‘flows’ through ecosystems. Unlike energy, nutrients are not
continually produced. There is a fixed amount of inorganic elements that are essential for life, such as
water, nitrogen and phosphorus, which are available in an ecosystem.
(b) What are the consequences of these differences?
Because nutrients are ‘limiting’ they recycle through systems many times via global and local nutrient
cycles. This contrasts with energy, which is not limiting, and therefore is not conserved through cycling
processes like those that operate for inorganic nutrients.
7.
What effect does land clearing and loss of deep-rooted trees have on the water cycle?
(pp. 1101–1102)
After removal of deep-rooted trees, more moisture drains deep into the soil and feeds into the groundwater pool. The watertable increases and rises through the soil profile towards the surface. In many
areas of Australia the rising watertable brings salt from the subsoil to the soil surface, leading to
salinity problems and land degradation.
8.
In what way(s) have industrialised societies affected the global carbon cycle? (pp. 1102–
1103)
Industrialised societies have affected the global carbon cycle through the mining and combustion of
fossil fuels. This industrial activity has resulted in a carbon dioxide return to the atmosphere greater
than can be cycled. As a result, there is a net increasing concentration of carbon dioxide in the
atmosphere.
9.
Name the processes in the nitrogen cycle that are controlled primarily by soil
microorganisms. (pp. 1104–1106)
Ecosystem function can be limited by the availability of nitrogen. This is because most primary
producers, such as plants, obtain their nitrogen as ammonium or nitrate ions from symbiotic soil
microorganisms or from the soil solution. The processes in the nitrogen cycle that are controlled
primarily by soil microorganisms are the fixation of atmospheric nitrogen, through symbiotic and freeliving microorganisms, the mineralisation of nitrogen in organic matter, and the denitrification of
nitrate ions to gaseous nitrogen.
10. What are the main inputs of phosphorus into a local ecosystem? Where is most of the
phosphorus stored in a eucalypt forest growing on sandy soil low in mineral phosphorus?
(pp. 1106–1107)
The major inputs of phosphorus are from the atmosphere and from decay of organic matter. Most of the
phosphorus in a eucalypt forest is stored within the living biomass of the trees. Phosphorus is made
available to the trees through remineralisation and recycling of plant biomass.