Q.Explain amensalism and the anti-herbivory defences shown by plants against grazing animals.

[!TLDR] Amensalism harms one species while the other is unaffected (e.g. a tree shading smaller plants); plants defend against herbivory via physical (thorns) and chemical (toxins) means. …

Amensalism is a biotic interaction in which one species is harmed or inhibited by another's presence, while the second species is completely unaffected and gains no benefit from causing that harm. A common example is a large tree whose dense canopy shades out smaller plants beneath it, depriving them of sunlight — the smaller plants suffer while the large tree is unaffected by their presence. A chemical example is a species releasing an allelopathic substance that inhibits nearby plant growth without itself gaining any advantage.

Because plants cannot flee grazing herbivores, many have evolved defences against being eaten:

• Physical defences: thorns, spines, tough or hairy leaf surfaces, and thick cuticles that deter feeding or make consumption physically difficult.
• Chemical defences: secondary metabolites such as alkaloids and tannins that are toxic, bitter, or otherwise unpalatable, reducing digestibility or actively poisoning herbivores that attempt to feed, discouraging further grazing. …