Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 246

80
60
40
20
4.0
3.0
2.0
1.0
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JJASONDJ F MM A JJ A S O N D J F MM A JJ A S O N D J F MM A

1965 1966 1967 1968

Year

Fig. 9.3 Results of a classic large-scale experiment at Hubbard Brook, New Hampshire, USA,
where all the trees in a forested catchment area were cut down (mimicking clear felling by
foresters). The arrow shows when deforestation occurred. The concentrations of nitrate ions
in stream water draining from the deforested catchment (solid line) were considerably
greater than in a neighboring control catchment that remained forested (dashed line). This
happened because deforestation fractured the within-forest nutrient cycling by uncoupling
decomposition losses of nitrogen from its re-uptake by vegetation. (After Likens & Borman,
1975.)

contents, indicative of high requirements for this nutrient. The stochiometric ratio of carbon : ni-
trogen (C : N) in bacteria and fungi is about 10 : 1. Terrestrial plant material, on the other hand, has
considerably higher ratios, ranging from 19 : 1 to 315 : 1 (Enriquez et al., 1993). Put another way,
dead plant material has relatively much lower amounts of nitrogen than the microbes require. This
means that the plant material, when it dies, can support only a limited biomass of decomposer
organisms and the pace of decomposition is slowed. It turns out that there is a critical C : N ratio
in dead plant material of 30 : 1 above which bacteria and fungi are N-limited - under these circum-
stances they withdraw ammonium and nitrate ions from the soil, competing with plants for these
resources (Daufresne & Loreau, 2001). But when the C : N ratio is below 30 : 1, the microbes are
C-limited and the process of decomposition leads to an increase of inorganic N in the soil, which
may in turn increase N-uptake by plants. Stoichiometric knowledge can be used to manipulate the
carbon content of the soil in a way that reduces nitrogen and facilitates the restoration of a native
plant community.

9.2 Food web theory
and human disease
risk

If left untreated Lyme disease can damage the heart and nervous system and lead
to a type of arthritis - tens of thousands of people are affected around the world
each year. The disease is caused by the bacterium Borrelia burgdorferi , which is
carried by ticks in the genus Ixodes . In their 2-year life cycle the ticks pass from egg
to larva to nymph to adult, taking blood meals from a succession of vertebrate
hosts.
Eggs, which generally do not carry the Lyme bacterium, are laid in spring and
after hatching the larva takes a single blood meal from a vertebrate host. If the host
(a small mammal, bird or reptile) carries the bacterium, this may be transmitted to
the larval tick, which then remains infective for the rest of its life (as nymph and
adult). The larval tick drops from its host and molts into an overwintering nymphal
stage. In its second year the nymph seeks another host (small mammal, bird or
human) in spring/early summer for a further single blood meal. This is the really