Agriculture Reference
In-Depth Information
However, because of the assumption that the respiration rate of a single leaf increases
in proportion to nitrogen concentration, this model shows a curious behavior in that
under higher levels of nitrogen absorption from the soil, the entire plant stand will die.
Box 4.4 Herbivory
Herbivory refers to the consumption of living plant material by invertebrate
and vertebrate animals. There is an extraordinary variety of modes of her-
bivory, from the sucking of sap to the consumption of leaves and seeds. There
also are strong contrasts in losses to herbivores in terrestrial versus aquatic
ecosystems. For example, leaf consumption by herbivorous animals in terres-
trial ecosystems is usually less than 5% of net primary production, in strong
contrast to aquatic systems, where herbivory is usually greater than 50% of
net primary production (Cyr and Pace 1993).
To make sense of this situation we have to consider why plants defend
against herbivore losses at all. The basic answer is that the more expensive
the cost of constructing the systems for primary production, the more likely
are additional investments in their defense against loss to herbivores or dis-
ease. The leaves of terrestrial plants and the various ancillary structures
such as roots and transport systems that sustain photosynthetic function are
relatively “expensive” to construct and maintain. Terrestrial plants make
substantial investments in systems for primary production that are only
recovered over fairly long time periods, and hence ancillary investments in
defense can ensure returns on investment in the photosynthetic function of
their leaves.
In contrast to terrestrial leaves, the costs associated with constructing and
maintaining net primary production are much less in aquatic systems. Aquatic
plants need not invest in structures for the uptake and transport of water. They
can utilize buoyancy to offset the force of gravity that imposes structural costs
on terrestrial plants. They can absorb nutrients from the surrounding water
directly with no need of root systems. In short, the investments in systems for
primary production required of aquatic plants are much lower than those in
terrestrial plants, generally too low to justify diverting resources to defense.
It is advantageous to produce more individuals, even if many will be lost to
herbivory, to simply outgrow the risk posed by herbivory.
On the other hand, there is no doubt that terrestrial plants invest in a variety
of defenses against herbivory. A significant part of net primary production is
allocated to plant defenses, which are usually divided into several types:
1. Physical defenses
-
Hard or fibrous tissues resistant to herbivore attack (Lusk et al. 2010)
-
Thorns and stinging hairs that deter herbivores
(continued)
 
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