Are these herbivory mechanisms always effective?
Generally herbivory is non-lethal unless
a plant is pre-stressed or just making it. Studies of oak forests indicate that
no more than 8-12% of the vegetation is consumed at any one time.
the question enters here, would these plants be only experiencing limited
herbivory if the compounds we will be discussing did not exist... studies
of island populations which have lost their defense arsenal are threatened
when herbivores are introduced.
The toll we visualize depends on the response of the
Compensation by the plant by:
- herbivory can help
reduce self-shading: sometimes limited herbivory is considered a 'boon'...
maybe not to the species on hand but to competing species. Is it known that
herbivory increases species diversity in an ecosystem by preventing any one
species from taking over. In the long run this may aid all species, as alternating
environmental conditions may favor those systems with higher diversity..
- sometimes the impact
may be greater than we 'see' if the plant responds by mobilization of stored
carbohydrates or nutrients generally from the roots... the impact may not
be felt immediately when using reserves but may affect next years success
or reproductive effort. This compensation may involve one or more of the below...
- plant may attempt
to regain Shoot/root ratio
- plant may increase
rate of photosynthesis
- plant meristems
may produce new tissue to replace lost
however the plant may not be able to compensate
- ring barking/ girdling
or eat off all meristematic buds as with hungry deer or swarms of insects.
The large number
of deer in the Catochins has has major repercussions on the flora.. this
same impact has been seen in parks out west where moose and other large
herbivores without regulation by top predators have overeaten the forests....
- introduce a disease
in the process of herbivory; smaller sap sucking insects may introduce viruses
or bacteria or fungal spores. Larger animals who tear off branches or wound
plants may allow pathogens to enter....
- If the plant is
already stressed by environmental conditions... long droughts, ozone and other
air pollutants weaken plants or if the plant is prediseased. At this point
they may not have the energy to mount a defense and will succumb to additional
- Alien species (
introduced species not native to an area) may be the greatest threat to the
plant kingdom. Most likely the majority of species are adapted to current
herbivores/pathogens. When a new species is introduced and no known defense
exists species can be decimated.
In other cases, alien plant species can outcompete local species and thus
reducing local plant health.
do plants defend themselves against herbivores? On the next page we will
review some morphological
and on the following pages go over