Ecology of Plants: Beneficial Interactions & Herbivory

Facilitation

• Also known as commensalism.
• A (+, o) interaction, where one species benefits and the other is unaffected.
• Examples:
  • Epiphytes: Plants that grow on other plants.
  • Nurse plants and beneficiaries: One plant provides shelter or other benefits to another.
• Facilitation and abiotic stress: Saguaro cacti and Palo Verde trees exemplify how one species can create a more favorable environment for another.

Positive Plant Associations

• Plant clumping often indicates facilitation.
• Examples:
  • Mesquite-shrub clusters in Southern Texas.
  • Live oak-shrub clusters in Central Texas.

Mechanisms of Facilitation

• Direct (Active) Mechanisms:
  • Alterations in the habitat.
  • Modification of soils, leading to changes in nutrient and water availability.
  • Amelioration of microclimate, creating more favorable conditions.
• Indirect (Passive) Mechanisms:
  • Involvement of other species.
  • Enhanced seed dispersal.
  • Protection from herbivores.

Trees as Focal Points for Seed Dispersal

• Trees like Ashe juniper serve as central locations for seed dispersal of plants like Agarita (Berberis trifoliolata).

Transition from Facilitation to Competition

• Facilitation can transition into competition as plants mature.
• Example: Creosote bush and pencil cactus interaction.
  • Birds and rodents deposit seeds of O. leptocaulis in L. tridentata (nurse plant).
  • L. tridentata colonizes open space.
  • O. leptocaulis grows under L. tridentata.
  • O. leptocaulis root system develops superficially above that of L. tridentata; seeds of L. tridentata spread by wind
  • L. tridentata dies; Rodents, wind, and water expose O. leptocaulis roots.
  • O. leptocaulis dies; this demonstrates how initial facilitation can lead to later competition as resources become limited.

Mutualisms

• Mutualisms involve (+, +) interactions, where both species benefit.
• Classification of mutualisms:
  • Based on degree of physical interaction:
    • Direct: Physical contact between partners.
    • Indirect: No direct contact, interaction occurs via another species.
  • Based on degree of closeness:
    • Symbiotic: Partners live closely together.
    • Non-symbiotic: Partners do not live closely together.
  • Based on degree of dependency:
    • Obligate: Each species requires the other for survival.
    • Facultative: The interaction is not required for survival.

Examples of Symbiotic Mutualisms

• Rhizobium-legumes: Bacteria fix nitrogen for plants, plants provide habitat and nutrients.
• Mycorrhizae: Fungi enhance nutrient uptake for plants, plants provide carbohydrates to fungi.
• Lichens: Fungus + alga; Alga provides photosynthates, fungus provides structure and protection.
• Reindeer moss (Cladonia spp.)

Non-symbiotic Mutualisms

• Pollination: Animals transfer pollen between plants.
• Dispersal: Animals disperse seeds.
• Defense: Ants & Bullthorn Acacias
  • Swollen thorns = ant house
  • Food source:
    • Extrafloral Nectar
    • Beltian Bodies
  • Obligate relationship.

Herbivory

• The consumption of plant material by animals.
  • Animals = “predators”
  • Plants = “prey”
• A type of predation (+, -) interaction.

Coevolution of Plants and Herbivores

• Selective forces differ for plants and herbivores.
  • Plants: Selection favors individuals better at deterring or defending against herbivores.
  • Animals: Selection favors individuals that can detect and circumvent plant defenses.
• Coevolution = reciprocal evolution of two parties (animal and plant).
• An evolutionary “arms race”.

Types of Herbivores

• Leaf eaters: grasshoppers, cows, deer, snails.
  • Grazers = grass eaters.
  • Browsers = tree/shrub/forb eaters.
• Metabolite feeders - aphids.
• Belowground feeders - nematodes.
• Seed predators (granivores) - rodents, insects, vertebrates.
• Frugivores - fruit eaters.

Effects of Herbivory on Individual Plants

• Decrease growth and fitness.
• Plants vary in responses.
• Grasses:
  • “Decreasers” = highly palatable species.
  • “Increasers” = less palatable.
• Compensatory photosynthesis
• Herbivory reduces plant growth and reproduction. Average effects for 82 studies (growth) and 24 studies (reproduction).

Herbivory and Plant Distribution

• Example: Haplopappus squarrosus distribution is affected by herbivory.
• Observed frequencies vs. expected frequencies in maritime, coastal transition, and interior zones.

Community-level Effects of Herbivores

• Unpalatable species increase in abundance, leading to diversity decline.
• Overgrazing reduces grass cover.

Moderate Grazing and Plant Community Diversity

• Moderate grazing can increase plant community diversity.
• Example: Yellowstone National Park study.
• Species diversity (H') and species richness at 20 cm and 4 m scales in grazed vs. ungrazed areas.

Herbivory and Ecosystems

• Effect of herbivores on primary productivity varies with vegetation type.
  • 2-3% in deserts.
  • 4-7% in forests.
  • 10-60% in grasslands.
• Aboveground vs. Belowground herbivory has different effects.
• Effects of herbivores on nutrient cycling.

Plant Adaptations to Herbivores: Structural Defenses

• Thorns, spines - good against large herbivores.
• Leaf hairs - good against crawling insects.
• Leaf toughness and silica.

Examples of Structural Defenses

• Solidago trichomes.
• Stinging Nettle (Urtica dioica).

Plant Adaptations to Herbivores: Chemical Defenses

• Secondary Compounds
  • Nitrogen-containing compounds (e.g., alkaloids).
  • Terpenes.
  • Phenolics.
• Constitutive vs. Inducible defenses: Echinacea angustifolia.

Phenolics

• Contain phenol group.
• Water-soluble.
• Simple phenolics: allelochemicals, Furanocourmarins.
• Complex phenolics: Lignins, Flavonoids, Anthocyanins, Flavones, Isoflavonoids (phytoalexins), Tannins.

Alkaloids

• Nitrogen-containing compounds.
• From amino acids.
• Found in 20-30% of plant species.
• Some highly toxic.
• Legal and illegal drugs.

Terpenes

• Lipids from acetyl CoA via the Mevalonic Acid Pathway.
• Monoterpenes: Essential oils, Pyrethroids.
• Polyterpenes: Latex.
• Triterpenes: Steroids.
• Terpenes are the major components of resin in pines.

Evolution of Plant Defense “Strategies”

• Specialist herbivores
  • Evolved mechanisms to detoxify or sequester harmful secondary metabolites (e.g., alkaloids).
  • Often rely on a single food plant and may not be able to eat other plants that differ in their secondary metabolites.
  • In some cases, they use plant chemicals as signals to find host plants or to defend themselves against predators.
• Generalist herbivores
  • Can tolerate a wide range of host plants.
  • Are more sensitive to secondary metabolites but employ more general strategies to cope with secondary metabolites, especially those shared by a wide range of plant species (e.g., tannins)

Quantitative vs. Qualitative Chemicals

• Qualitative Chemicals
  • Low molecular weight.
  • Specific mode of action; often toxic.
  • Easy to detoxify.
• Quantitative Chemicals
  • High molecular weight.
  • General effects on herbivores; digestibility, palatability, etc.
  • Difficult to detoxify.

Plant Apparency Hypothesis

• Feeny (1976)
  • “Apparent” vs. “Unapparent” plants.
  • Apparent plants should use quantitative chemicals for defense.
  • Unapparent plants should use qualitative chemicals for defense.

Resource Availability Hypothesis

• Coley et al. (1985)
  • Fast-growing vs. Slow-growing plants.
  • “Costs” of defense in relation to growth.
  • Fast-growing plants:
    • “Disposable” leaves; invest little in defense; qualitative chemicals.
  • Slow-growing plants:
    • “Expensive” leaves; invest heavily in defense; quantitative chemicals.

How Plants Sense and Respond to Insect Herbivores

• Wounding.
• Insect saliva.
• Damage can increase levels of Jasmonic Acid (JA), Systemin, and Salicylic Acid in plants, which trigger plant defenses.
• Induction and release of volatile organic compounds.

Volatile Compounds, Induced Defense and Communication Between Neighboring Plants

• Different roles of volatile isoprenoids (VIPs) in protecting plants from abiotic and biotic stress factors.
  • (a) Constitutive VIPs have a protective action against oxidant factors (e.g., ozone), high temperature, and other environmental constraints.
  • (b) Biotic stresses: Constitutive VIPs repel dangerous herbivores, thus acting as a direct defense mechanism. Some insects have learned to use VIPs to locate the host plant, a remarkable example of co-evolution.
  • (c) Biotic stresses: The attack by herbivorous insects induces the emission of VIPs that attract natural enemies (e.g., carnivorous insects) as an indirect defense mechanism of the host plant.
  • (d) Biotic stresses and plant communication: Induced emission of VIPs not only attract natural enemies of herbivorous insects but also help attacked plants communicate to neighbors the presence of risk.

Additional Information

• For additional information on plant secondary chemistry, herbivory, defensive strategies, and communication using volatile compounds, see a series of YouTube videos by Dr. Ian Baldwin (Max Planck Institute).