If a species such as Thomson’s gazelle benefits from the grazing effects of wildebeest
due to the increased productivity of the plants, then do the plants themselves
benefit? In other words, what benefits do the plants receive from being grazed and
growing more? In evolutionary terms (see Chapter 3) we have to rephrase this as,
“Does herbivory increase the fitness of individual plants?” In ecological terms one
may ask, “Does the plant grow more after herbivory?”
The studies of lesser snow geese on the saltmarshes of Hudson Bay, which we
have discussed above, are now showing that the grass Puccinelliacomes in different
genotypes ( Jefferies and Gottlieb 1983). Nineteen grazing experiments have shown
that under grazing there is selection for those genotypes that are fast growing. These
types have the ability to take up the extra nitrogen from the goose feces and seem
to outcompete slower-growing genotypes. This is, therefore, an evolutionary benefit
from grazing. Plots where grazing is prevented show that, after 5 years, change to
slower-growing genotypes was only just beginning. The more immediate ecological
benefit from grazing again comes from the addition of nutrients resulting in a
30–50% increase in biomass.
In general there are few studies that show plants increasing their fitness as a result
of herbivory (Belsky 1987). In contrast, we can look at communities of plants and
see that if the majority of plants, such as grasses, can tolerate grazing (i.e. survive
despite herbivory) a few other intolerant species in that community may not survive
due to inadvertent feeding or trampling by large mammals (i.e. apparent competi-
tion rather than true competition between plants). This may be simply a consequence
of grazing and not necessarily an evolutionary advantage for the grass species.
Nevertheless, McNaughton (1986) has argued in opposition to Belsky that grasses
and grasslands have evolved in conjunction with their large mammal herbivores,
especially in Africa. From an evolutionary point of view a grass individual that by
chance evolved an antiherbivore strategy (such as the production of distasteful
chemicals) should be able to spread through the grassland. We have to surmise at
this stage that antiherbivore adaptations are constrained in some way; for example,
it could be that production of distasteful chemicals results in the plant being less
successful, in root competition, or in the uptake of nutrients, as in the example of
lesser snow geese grazing.
On the surface it appears disadvantageous for a grass to grow more as a response
to grazing because it would provide more food and invite further grazing. However,
growth could also be viewed as a damage repair mechanism that is making the best of
a bad situation (i.e. the grass may lose fitness less by growing than by not doing so).
In summary, we know too little about both the ecological and evolutionary con-
sequences of herbivory on plants. We are left with many questions and opposing views,
and more work is needed.Competition, parasitism, and predation are all processes that have negative effects
on a species. However, when they act together they may end up having a beneficial
effect. For example, acorns of English oak (Quercus robur) are parasitized by weevils
and gall wasps, and are eaten by small mammals. Very high mortality rates are imposed
on healthy acorns by small mammals, but parasitized acorns are left alone. While
most of the parasitized acorns also die, some survive and are avoided by the small
mammals. Thus, higher survivorship and hence fitness occurs when the plant is
parasitized (Crawley 1987; Semel and Andersen 1988).158 Chapter 9
9.9.2Do grasses
benefit from grazing?
9.9.3Complex
interactions