A. Are these herbivory mechanisms always effective?


1. 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.

However 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.

2. The toll we visualize depends on the response of the plant:
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

  
  Sometimes however the plant may not be able to compensate if:

• 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 herbivory.
  
  
• 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.

  How do plants defend themselves against herbivores? On the next page we will review some morphological adaptations and on the following pages go over secondary biochemicals.....