Predation and Herbivory Notes

Predator-Prey Interactions

• Predation: One organism (predator) consumes another (prey).

  • Types of Predators:

  • True Predators: Kill their prey immediately after capture. Examples include lions and spiders.

  • Grazers: Attack many prey individuals but remove only a part of each prey. Examples include sheep and leeches.

  • Parasites: Consume parts of their prey (host); usually do not kill the host. Examples include tapeworms and ticks.

  • Parasitoids: Lay eggs in or on another host organism, after which the larvae hatch and consume the host, eventually killing it. Examples include wasps and flies.

• Evolutionary Arms Race: Predators and prey evolve to outcompete each other.

  • Example: Predators evolve better hunting strategies, while prey evolve better escape or defense mechanisms.

• Predator-Prey Population Cycles: Predator and prey populations often show cyclic oscillations (e.g., snowshoe hare and lynx).

  • Factors Influencing Cycles:

  • Food availability for prey.

  • Predation pressure on prey.

  • Environmental conditions.

• Keystone Species: Predators maintain species-diverse communities by reducing competitive exclusion.

  • Examples: Sea otters control sea urchin populations, preventing them from overgrazing kelp forests.

Herbivory Mechanisms to Reduce Impacts

• Escape: Traits to avoid being found by herbivores (spatial or temporal refuges).

  • Examples: Certain plant species growing only on steep cliffs to avoid grazing.

• Spatial refuges: growing in areas inaccessible to herbivores (e.g., cliff edges).

• Temporal refuges: growing leaves when herbivores are rare or inactive (e.g. shrub in Panama leafing during dry season).

• Tolerance: Traits to reduce the negative impact of herbivory on plant fitness (e.g., increased photosynthetic rate).

  • Examples: Plants that can quickly regrow after being grazed upon.

• Defence/Resistance: Traits that deter or repel herbivores or reduce herbivore performance.

  • Examples: Thorns, toxins, and sticky resins.

• Constitutive vs. Induced: Whether defenses are always present or produced in response to attack.

• Direct vs. Indirect: direct defenses affect the herbivore and Indirect defenses involve attracting natural enemies of herbivores.

  • Direct Defenses:

  • Impact herbivore's biology directly (e.g., toxins).

  • Indirect Defenses:

  • Benefit the plant by attracting predators or parasitoids of the herbivores.

Plant Defenses

• Structural Defenses: Physical barriers like thorns, spines, prickles, trichomes and sclerophylly.

  • Examples: Thorns on roses, spines on cacti, and trichomes on leaves.

• Chemical Defenses (Secondary Metabolites): Organic compounds that deter herbivores.

  • Function: Can be toxic, repellent, or reduce digestibility.

• Examples: Alkaloids (e.g., nicotine), terpenoids (e.g., limonene), steroids (e.g., saponins), phenolic compounds (e.g., tannins).

  • Alkaloids: Often toxic and affect the nervous system of herbivores.

  • Terpenoids: Can be repellent or toxic; contribute to plant's scent.

  • Steroids: Interfere with herbivore's hormonal balance.

  • Phenolic Compounds: Reduce digestibility of plant tissues.

• Constitutive Defenses: Always present.

  • Benefit: Provide constant protection.

  • Cost: Can be resource-intensive to produce and maintain.

• Induced Defenses: Produced in response to attack; phenotypic plasticity increases resistance to future attacks.

  • Benefit: Saves resources when herbivore pressure is low.

  • Talking Trees Hypothesis: Damaged plants release volatile compounds to initiate defenses in nearby plants.

  • Function: Alerting neighboring plants to prepare defenses.

Plant Populations and Grazing

• Grazing impacts: Distribution and abundance of plant species are affected by grazing.

  • Overgrazing: Can lead to soil erosion and loss of plant diversity.

• Selective grazing: Can drive the structure of plant communities.

  • Example: If herbivores prefer certain plant species, those species may decline, allowing others to dominate.

• Pest Pressure Hypothesis: Abundance makes a species vulnerable to predation; common plant species have a build-up of specific herbivores.

  • Prediction: Rare species have fewer specialist herbivores compared with common species.

• Competition Pressure Hypothesis: Intense competition results in the exclusion of less competitive species.

Keystone Herbivore Species

• Elephants maintain forest 'bias' (forest clearings).

  • Impact: Promotes habitat diversity and affects the distribution of other species.

Summary

• Plants have different mechanisms (escape, tolerance, avoidance) to reduce the impacts of herbivory.

• Plant defenses may be constitutive or induced.

• The Pest Pressure Hypothesis helps to explain the enormous diversity of plants and animals in tropical forests.

• Suppression of dominant plants (grazing, cutting for hay) can lead to increased biodiversity (flowering meadows).

• Ecosystem engineers (keystone species) create and regulate complex plant and animal communities.