IB Biology Topic B4.2.9: Adaptations of Herbivores for Feeding on Plants and of Plants for Resisting Herbivory Notes

Herbivory: A Feeding Arms Race Between Plants and the Animals That Eat Them

1. Herbivores are animals that feed on plants.
2. Plant material is tough, fibrous and often chemically defended, so herbivores have evolved specialised feeding structures and digestive systems to exploit it.
3. Plants, in turn, have evolved defences to reduce the damage, driving a long-running arms race.

Herbivore Feeding Adaptations: Grinding, Digesting and Piercing

1. Broad, ridged grinding teeth
   1. Mammalian herbivores have wide molars with ridged surfaces that grind fibrous leaves and grasses into a pulp.
   2. A side-to-side jaw action shears the plant tissue and breaks open plant cell walls.
2. Symbiotic gut microorganisms to digest cellulose
   1. Animals cannot make the enzyme cellulase, so they cannot digest the cellulose in plant cell walls on their own.
   2. Many herbivores host symbiotic bacteria and other microorganisms in the gut that release cellulase and break cellulose down for them.
   3. Ruminants like cattle house these microbes in an enlarged stomach chamber, giving both partners access to the energy in the plant.
3. Specialised insect mouthparts
   1. Chewing herbivores such as beetles use jaw-like mouthparts to bite off and chew pieces of leaf.
   2. Sap-feeders such as aphids use tubular mouthparts to pierce the plant and reach phloem sieve tubes.
   3. They then feed on the nutrient-rich sap flowing through the phloem.

Example

• An aphid uses its sharp tubular mouthparts to draw sap from a plant for energy and growth.

Plant Defences: Physical Barriers and Chemical Weapons

1. Physical defences
   1. Thorns and spines are sharp structures that injure or deter a feeding animal.
   2. A tough, waxy cuticle makes leaves hard to bite and gives insects little grip.
   3. Silica deposits in grass leaves wear down herbivore teeth and make the tissue abrasive to chew.

Example

• The tree nettle (Urtica ferox) has stinging hairs that inject irritating chemicals into any animal that brushes against it.

Chemical Defences: Secondary Metabolites Poison or Repel

1. Plants make secondary metabolites, chemicals that are not needed for basic metabolism but act as toxins or deterrents.
2. This contrasts with primary metabolites such as proteins and carbohydrates, which the plant needs to live and grow.
3. Many secondary metabolites, such as alkaloids, are toxic enough that herbivores avoid the plant entirely.

Counter-Adaptations: How Herbivores Beat Plant Chemistry

1. Detoxification enzymes
   1. Some herbivores make enzymes that break down or neutralise plant toxins.
   2. Certain insects metabolise the alkaloids of their host plant and stay unharmed.
2. Specialised feeding and co-evolution
   1. Many herbivores feed on just one type of plant, a pattern called plant-herbivore specificity.
   2. The plant and herbivore then co-evolve, each gaining new traits in response to the other.