10
Herbivory in Terrestrial Ecosystems
Martin Oesterheld and Samuel J. McNaughton
Introduction
.. . . . I marked all the seedlings of our native weeds as
they came up, and out of 357 no less than 295 were
destroyed, chiefly by slugs and insects. If turf which has
long been mown (and the case would be the same with
turf closely browsed by quadrupeds) be let to grow, the
more vigorous plants gradually kill the less vigorous ....
Within the last ten years, large spaces have been enclosed
[so that cattle could not enter] and self-sown firs are
springing up in multitudes . . . we see that cattle absolutely determine the existence of fir."
Darwin, The Origin of Species
The study of herbivory in relation to ecosystem science has been closely related to the methodological
procedures outlined by Darwin in these lines. Ecologists have attempted to quantify herbivory in a
variety of ways and systems, much like Darwin
quantified consumption of weed seedlings. They
have tried to simulate herbivory through defoliation, mowing, fertilization, and artificial trampling,
much like Darwin used a mown turf as a substitute
for a lawn grazed by quadrupeds. Finally, they have
relied on herbivore exclusion by fencing or poisoning, much like Darwin did to assess the effect of
cattle on fir populations.
In this chapter, we review the major approaches
to quantifying and simulating herbivory and to assessing its effects at the ecosystem level. Considering a general model of energy flow through an
ecosystem (see Chapter 2), we are concerned with
two major fluxes and an interaction between them:
the quantification of consumption, the quantification of primary productivity in systems under the
effect of herbivores, and the effect of consumption
on primary productivity .
Consumption
Consumption, or herbivory, is the rate at which energy or materials enter the herbivore compartment
of an ecosystem. It is expressed as a velocity in
absolute terms or as a proportion of primary productivity, plant biomass, leaf area, or other ecosystem attributes. Differences among plant-herbivore
systems determine the existence of different methodologies. However, there are essentially two approaches: one is based on an estimate of what the
animals have consumed given their density and
mean individual performance, whereas the other is
based on estimates of missing plant biomass. Both
approaches have advantages and disadvantages and
they only partially overlap in terms of the objectives they are prepared to fulfill.
Animal-Based Methods
This approach determines consumption as the product of two variables: herbivore density and individual consumption. The latter is estimated from mean
individual performance (growth, respiration) and
the amount of plant biomass or energy needed to
meet the corresponding animal requirements. This
latter variable is actually a composite of two other
complex variables: the conversion factor between
assimilated energy and animal performance, which
is closely related to body size, and the conversion
factor between consumed and assimilated energy
151
Herbivory in Terrestrial Ecosystems
Martin Oesterheld and Samuel J. McNaughton
Introduction
.. . . . I marked all the seedlings of our native weeds as
they came up, and out of 357 no less than 295 were
destroyed, chiefly by slugs and insects. If turf which has
long been mown (and the case would be the same with
turf closely browsed by quadrupeds) be let to grow, the
more vigorous plants gradually kill the less vigorous ....
Within the last ten years, large spaces have been enclosed
[so that cattle could not enter] and self-sown firs are
springing up in multitudes . . . we see that cattle absolutely determine the existence of fir."
Darwin, The Origin of Species
The study of herbivory in relation to ecosystem science has been closely related to the methodological
procedures outlined by Darwin in these lines. Ecologists have attempted to quantify herbivory in a
variety of ways and systems, much like Darwin
quantified consumption of weed seedlings. They
have tried to simulate herbivory through defoliation, mowing, fertilization, and artificial trampling,
much like Darwin used a mown turf as a substitute
for a lawn grazed by quadrupeds. Finally, they have
relied on herbivore exclusion by fencing or poisoning, much like Darwin did to assess the effect of
cattle on fir populations.
In this chapter, we review the major approaches
to quantifying and simulating herbivory and to assessing its effects at the ecosystem level. Considering a general model of energy flow through an
ecosystem (see Chapter 2), we are concerned with
two major fluxes and an interaction between them:
the quantification of consumption, the quantification of primary productivity in systems under the
effect of herbivores, and the effect of consumption
on primary productivity .
Consumption
Consumption, or herbivory, is the rate at which energy or materials enter the herbivore compartment
of an ecosystem. It is expressed as a velocity in
absolute terms or as a proportion of primary productivity, plant biomass, leaf area, or other ecosystem attributes. Differences among plant-herbivore
systems determine the existence of different methodologies. However, there are essentially two approaches: one is based on an estimate of what the
animals have consumed given their density and
mean individual performance, whereas the other is
based on estimates of missing plant biomass. Both
approaches have advantages and disadvantages and
they only partially overlap in terms of the objectives they are prepared to fulfill.
Animal-Based Methods
This approach determines consumption as the product of two variables: herbivore density and individual consumption. The latter is estimated from mean
individual performance (growth, respiration) and
the amount of plant biomass or energy needed to
meet the corresponding animal requirements. This
latter variable is actually a composite of two other
complex variables: the conversion factor between
assimilated energy and animal performance, which
is closely related to body size, and the conversion
factor between consumed and assimilated energy
151