Geoscience Reference
In-Depth Information
absorbed by chlorophyll, the efficiency of total radiation
is only about 1 per cent. In water bodies much radiation
is absorbed by the water and its impurities, so efficiencies
are much lower, in the range 0·1-0·2 per cent on average.
In attempting to understand ecosystems, biogeog-
raphers frequently subdivide organisms into trophic levels ,
where trophic level is defined as 'the level at which an
organism feeds in food chains or food webs'. It gives the
level or stage at which food energy passes from one
organism to another. In Wytham Wood insects, caterpil-
lars, earthworms and mites are primary consumers,
whilst owls, weasels and titmice are tertiary consumers.
Note that titmice can also be primary consumers and
secondary consumers, and weasels can also be quaternary
consumers. A species thus often occupies different trophic
levels within the same ecosystem.
One limitation of such energy flow diagrams is that
they display average conditions only; they do not show
variations over time. There are great seasonal changes in
the feed value of deciduous trees and shrubs throughout
the year. Leaf and bud growth leads to large seasonal
populations of caterpillars and insects. Earthworms are
most active in spring. Populations of birds and mammals
also vary during their breeding seasons, and large
interannual variations can occur, largely depending on
climatic conditions. There are large interannual variations
in the populations of insects, small animals, small birds
and greenery. Human influence could also be important,
as timber is harvested from the wood each year. This
could have large effects on the ecosystem, especially if the
rate of harvesting exceeds the rate of primary production.
Our knowledge of the production ecology of eco-
systems has increased enormously since the start of the
International Biological Programme (IBP) in the 1960s.
This worldwide programme to learn more about the
world's biomass was based on the theme ' the biological
basis of productivity and human welfare '. The aim was to
solve the fundamental equations of production ecology:
GPP =
NPP + R(A)
NPP =
GPP - R(A)
NEP =
NPP - R(H)
=
GPP - (R(A) + R(H))
=
B
where GPP is gross primary production, NPP is net
primary production, NEP is net ecosystem production,
R(A) is autotroph respiration, R(H) is heterotroph
respiration and B is biomass. In terms of the importance
of global biomes, Table 21.3 shows the data for NPP above
ground and below ground based on biomass harvests.
Some NPP is not available to harvest, due to consumption
by herbivores, root exudation, transfer to mycorrhizae,
and volatile emissions. Table 21.4 shows where global
biomass occurs.
Table 21.5 shows additional information on leaf area
index (LAI) the area of leaves per unit area of ground. It
ranges from highs of about 23 in some swamps and
marshes down to about 0.5 in polar deserts. LAI has
become an important factor in explaining differences in
productivity both within and between biomes, as the leaf
is the organ of photosynthesis. The length of time a plant
can photosynthesize is equally important (evergreen v .
deciduous). This is the index LAD or leaf area duration,
not shown here. The percentage of NPP consumed by
herbivores is also shown. There are very real differences
Table 21.3 NPP of the world's biomes
Above ground
Below ground
Below ground
Total NPP
Total NPP
Biome
NPP g m -2 yr -1
NPP g m -2 yr -1
NPP % total
g m -2 yr -1
Pg C yr -1
Tropical forest
1400
1100
0.44
2500
21.9
Temperate forest
950
600
0.39
1550
8.1
Boreal forest
230
150
0.39
380
2.6
Mediterranean shrubland
500
500
0.50
1000
1.4
Tropical savanna
540
540
0.50
1080
14.9
Temperate grassland
250
500
0.67
750
5.6
Desert
150
100
0.40
250
3.5
Arctic tundra
80
100
0.57
180
3.9
Crops
530
80
0.13
610
4.1
Total
66.0
Source: Data from Saugier et al. (2001).
Note: Pg = peta grams = 10 15 g.
 
 
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