Effect of Herbivores on Plants PDF

Effect of Herbivores on Plants Antoine OHC Leduc Ecology BIOL3005 SQU A plant cannot escape from a herbivore: Only three options exist for the plan Unlike a predator a herbivore does not usually kill the plant on which it feeds Repair...

Effect of Herbivores on Plants Antoine OHC Leduc Ecology BIOL3005 SQU A plant cannot escape from a herbivore: Only three options exist for the plan Unlike a predator a herbivore does not usually kill the plant on which it feeds Repairing the damage costs the plant in nutrients and energy Repeated damage can kill the plant 1. No defense: Defense cost energy that could be spent on growth & reproduction Annuals plants: only live a few weeks/months Need to get through life without being eaten 2. Physical defense: Spines or tough leaves (Cajuero Bravo tree) This is relatively cheap but reduces photosynthesis and growth Cajuero Bravo A plant cannot escape from a herbivore: Only three options exist for the plan 3. Chemical defense: a) Deterrents: strong-smelling chemicals so plant are distasteful and so they are avoided… Garlic, ginger, tomato, cinnamon b) Disrupt digestion: so plant cannot be eaten Tannins and resins c) Toxins: poisons to kill pests/herbivores  Physical defense: a) Spines e.g. Acacia trees Spines are a protection against large herbivores Little insects can eat around the spines Producing spines uses metabolic resources So their abundance on a tree depends on how much grazing Acacias protected from large herbivores by having: ✓ No spines if the plant not getting eaten (SQU campus) ✓ Biggest spines produced from ground upwards to 2m (so keep herbivore away) ✓ Smaller softer (still have photosynthesis) spines above  Structural Defenses Constitutive defense = a defense that is always present  Not all spines are the same: Left vs Right photos… both are acacia plants with different types of spines Some herbivores specialize on acacia… The giraffe has a long neck allowing it to eat the top of the branches, where spines are smaller and/or softer A tough mouth that can withstand damage by the spines The dikdik (small antelope) has a very narrow mouth that allows it to eat the leaves between the spine  Physical defense: b) Hairs against insects Spines and thorns are too big to affect an insect The insect defense is trichomes: ✓ Stiff hairs prevent the insect from reaching the stem or leaves ✓ So it cannot feed on the plant ✓ Hooks are used to trap the insects so dies of starvation – a more aggressive defense ✓ Forsskaolea (around SQU campus) has glass-like silica hooks on leaves ✓ Hairs that secrete a sticky drop of glue to trap and kill insects – another aggressive defense. e.g. Cannabis  Chemical defenses in plants 1. Using Chemical defenses Producing chemical defenses is metabolically expensive ▪ So, the amount of chemical produced depends on: 1. Amount of plant damage 2. Metabolic cost of the chemical (some chemicals are “cheaper” than others) ▪ Chemicals may be concentrated in more valuable tissue like seeds (future plants) and the growing tips - rather than in old leaves (easily replaced) ▪ The amount of chemical is reduced when there are no herbivores around ▪ Increased when herbivores appear ▪ If the plant is stressed (lack of water or sun) the plant cannot produce these chemicals → Chemical defenses are rare in arid region plants  gt 2. Types of Chemical defences: a) Qualitative chemicals are toxins that only need to be produced in small quantities Very toxic and costly to produce Quantity does not matter b/c of toxicity b) Quantitative chemicals make the plant difficult to digest Need to be produced in large quantities  Qualitative Chemicals (toxins) Cyanides and alkaloids = Only a small quantity is needed to kill a herbivore! Disadvantage: ▪ Very expensive to produce ▪ Unstable so has relatively short life (months) Most suitable for: Annual plants are fast growing and only present for a short period Only need small quantities of an unstable chemical to protect against herbivores Deadly nightshade Foxglove  Quantitative Chemicals Tannins and resins disrupt plant digestion in herbivores Tannins attach to the plant proteins and prevent their digestion by destroying the herbivore’s proteases Resins (in conifers) & silica (in grass) form needle-like sharp spikes that cut the gut of the herbivore Disadvantage: ▪ Since they are not toxic… ▪ Quantity is important - too little quantity has no effect ▪ Require time to reach a high concentration ▪ Protection for the plant increases slowly  Quantitative Chemicals Most suitable for: ❖ Perennials and trees that are slow growing and keep their leaves for a long time Problem… Using chemical defenses pose risks: Insects can evolve defenses of a particular plant’s chemical This is a plants many humans like because of their quantitative chemicals (tannins): Tobacco plant Insect problems: Oak trees (perennial) contain tannins Oak trees live 500 years and are deciduous (lose their leaves in the Fall) The tree produces new leaves in the Spring New leaves (in the Spring) are edible but… As the leaves get older (Summer), they are less digestible because: 1. Older leaves have little protein (nutritionally poor) 2. Tannins increase in leaves: damage the digestive enzymes of insects So they cannot digest the leaves Young oak leaves Oak in Winter  Insects have evolved adaptations to overcome these defenses Winter moth appears early in the spring Requires adaptations to survive the cold winter Eats the leaves while they are still young and toxin-free (good) It finishes its life cycle before the leaves become too unpalatable Winter moth To avoid oak’s chemical defences these insects have evolved adaptations to survive cold weather conditions so can start early in the year  Insect problems: Oak trees (perennial) contain tannins ✓ Summer-feeding moths have mechanisms that separate the tannins from the plant proteins so the proteins can be digested They are still faced with the lack of protein in the leaves and are forced to grow very slowly Some grow so slowly that they have to wait through the winter (hibernate) and continue feeding on the spring leaves o Leaf-miners are tiny little moths that live inside an individual leaf and grow very slowly Oak leaf miner