Agriculture Reference
In-Depth Information
rhizosphere, the zone of soil affected by the growth and maintenance of plant roots (Swift
et al., 1979; Coleman et al., 1983). The description condenses the high diversity and com-
plexity within soils to a manageable level by defining the web in terms of functional
groups of organisms that share similar prey and predators, feeding modes, life-history
attributes, and habitat preferences (
Table 4.2
; Moore et al., 1988). At the base of the web are
plant roots, labile (C:N ratio < 30:1) and resistant (C:N ratio > 30:1) forms of detritus, and an
inorganic nitrogen source. The plant roots and their by-products and the different forms
of detritus are utilized by microbes and invertebrate consumers, which are then consumed
by different predators.
The energy flux food web description displays the amounts of biomass or population sizes
of the functional groups and quantifies the rates of feeding or energy fluxes within the
connectedness description (
Figure 4.2
)
. More often than not, the energy flux description
depicts elemental flows, usually C or N, inferred from biomass estimates and models of
trophic interactions.
Table 4.3
provides the physiological and life-history parameters and
biomass estimates used to estimate the elemental flows (O'Neill, 1969; Hunt et al., 1987; de
Ruiter, Moore, et al., 1993).
The model starts with estimates of the biomasses of the individual functional groups.
The biomass estimates are obtained from direct or indirect estimates of population sizes
(e.g., morphometric conversions) obtained from field samples, preferably at several times
to capture temporal dynamics or to estimate steady states or long-term averages (Moore
et al., 1996).
Feeding rates are estimated using the model presented by Hunt et al. (1987). The
model assumes that at the steady-state biomass any new biomass produced within a pop-
ulation is offset by biomass that is lost from the population (i.e., inputs equal outputs at
steady state). Consumed matter is divided into a fraction that is immobilized into con-
sumer biomass (assimilation) and a fraction that is excreted to the environment as feces
and unconsumed. Of the assimilated fraction, material either is incorporated into new
biomass as growth and reproduction (production) or is mineralized as inorganic material
(
Figure 4.3
)
. Notice that matter is conserved and, per the second law of thermodynamics,
that not all assimilated matter is converted to new biomass. Hence, feeding rates
F
are
estimated as follows:
F
= (
dB
+
M
)/
ap
(4.1)
where
F
is the feeding rate (biomass time
−1
),
d
is the specific death rate (time
−1
) of the con-
sumer,
B
(biomass estimated from field samples) is the population size of the consumer,
M
is the death rate due to predators (biomass time
−1
), and
a
and
p
are the assimilation (per-
centage expressed as proportions) and production (percentage expressed as proportions)
efficiencies, respectively. For consumers that feed on multiple prey types, the fluxes are
weighted by the predators' feeding preferences for the respective prey.
The estimation procedure starts with top predators since the death due to predation is
zero. The fluxes from prey to the top predators serve as the estimates of the prey's death due
to predation. Hence, the process moves downward through the prey to the basal resources
with fluxes to each prey taking into account the biomass lost to predation. To construct a
dynamic version, we simply need to take into account changes in the biomasses over an
interval of time
t
by adding Δ
B/t
to the numerator of
Equation 4.1
(O'Neill, 1969).
Search WWH ::
Custom Search