Predation and Herbivory Notes

Predation

• Predation: An ecological interaction where one organism (the predator) consumes another organism (the prey), resulting in a transfer of energy and nutrients. This interaction drives significant evolutionary and ecological dynamics.

• Carnivores, omnivores, and herbivores:

  • Carnivores primarily consume meat through predation, exhibiting specialized hunting and feeding strategies.

  • Omnivores have a mixed diet of both plants and animals, employing diverse foraging techniques.

  • Herbivores focus on consuming plants.

• Evolutionary arms races and mutual dependence:

  • Predator-prey interactions lead to evolutionary $arms \ races$, where each species evolves adaptations to enhance either predation or avoidance.

  • This co-evolution results in mutual dependence and can create limit cycles, where populations oscillate in response to each other.

• Population oscillations:

  • Predator and prey populations often exhibit cyclic oscillations. A classic example is the snowshoe hare and lynx, where the hare population fluctuations drive corresponding changes in the lynx population.

• Keystone predators:

  • Predator-prey interactions maintain species-diverse communities by preventing competitive exclusion.

  • Keystone predators play a critical role in controlling dominant species, thus allowing other species to thrive.

Herbivory

• Herbivory: The process where animals consume plants or plant parts. It significantly influences plant survival, reproduction, and distribution.

• Escape strategies:

  • Plants use various escape strategies such as spatial and temporal refuges to avoid herbivores.

  • Spatial refuges involve growing in locations inaccessible to herbivores, while temporal refuges include life cycle timing.

• Tolerance:

  • Tolerance refers to a plant's ability to withstand herbivore damage and involves increased photosynthesis, compensatory growth, and strategic resource allocation after damage.

• Defenses:

  • Plants employ both constitutive (always present) and induced (activated in response to attack) defenses to protect themselves.

Plant Defenses

• Structural defenses:

  • Physical barriers like thorns, spines, prickles, and trichomes deter herbivores.

  • Sclerophylly, or the hardening of leaves, also reduces palatability.

• Chemical defenses:

  • Plants produce secondary metabolites such as alkaloids, terpenoids, steroids, and phenolics, which have toxic or repellent effects on herbivores.

• Induced defenses:

  • Phenotypic plasticity allows plants to increase resistance after herbivore attack, enhancing their survival.

• Volatile compounds (VOCs):

  • VOCs mediate communication between plants, warning nearby plants of herbivore attack and inducing defensive responses.

• Indirect defenses:

  • Plants attract natural enemies of herbivores by providing food or shelter, thereby reducing herbivore pressure.

Plant Populations and Grazing

• Grazing influences plant distribution and abundance by affecting plant survival and reproduction rates.

• Selective grazing:

  • Selective grazing shapes plant communities, favoring herbivore-resistant species and altering community composition.

• Pest Pressure Hypothesis:

  • The hypothesis explains that common species are more vulnerable to herbivores, promoting diversity by preventing any single species from dominating.

• Competition Pressure Hypothesis:

  • Reduced competition among plants increases species diversity, allowing less competitive species to thrive in the absence of dominant competitors.

• Keystone herbivores:

  • Keystone herbivores such as elephants maintain habitat diversity by preventing woody vegetation from dominating, thus creating space for a variety of plant species.

Key Points

• Predator-prey interactions and herbivory have major community effects, influencing species abundance, distribution, and community structure.

• Plant defenses can be constitutive or induced, providing plants with a range of strategies to protect themselves from herbivores.

• The Pest Pressure Hypothesis explains plant diversity in tropical forests by highlighting the role of herbivores in preventing competitive exclusion.

• Grazing can increase biodiversity by suppressing dominant plants, allowing subordinate species to coexist.

• Ecosystem engineers, including keystone species, regulate plant and animal communities, maintaining ecosystem functions and stability.