Species Interactions in Agroecology

Species Interactions and Their Role in Agroecosystems

Instructor Details

  • Instructor: Tara Pisani Gareau

  • Course: EESC 3310 Agroecology

Learning Objectives

  • To describe the nature of different species interactions and their role in agroecosystems.

  • To provide agroecological examples of species interactions.

  • To develop an understanding of crop ecology and how agroecosystems are designed to enhance interactions that positively affect sustainable yield.

Types of Species Interactions

  • Interference: When organisms change the environment through direct or indirect interactions.

  • Removal Competition: Interference (-, -) where both organisms compete for limited resources.

  • Parasitism: (+, -) interaction where one organism benefits at the expense of the host.

  • Herbivory: (+, -) where animals consume plants.

  • Addition Epiphytism: (+, 0/+) where one organism grows on another, possibly enhancing its benefit without harming the host.

  • Mutualism: (+, +) where both species benefit from the interaction.

  • Allelopathy: (0/+, -) chemical interactions where plants release substances that affect the growth of neighboring plants.

Details of Competition

Competition (−, −)

  • Definition: Occurs when two organisms remove a resource from an environment that is insufficient to meet both organisms' needs.

  • Types:

    • Intra-specific Competition: Competition among individuals of the same species.

    • Inter-specific Competition: Competition between different species.

Effects of Intra-specific Competition on Plants

  • Average size of plants decreases as density increases.

  • Increased size inequality among individuals.

  • Alterations in phenology, possibly impacting blooming or fruiting times.

  • Higher death rates within populations due to stress.

Effects of Intra-specific Competition on Animals

  • Decrease in birth rates with increasing density leading to higher overall mortality rates.

Inter-specific Competition Examples

  • Monoculture: Growing a single crop species.

  • Additive Intercrop: Adding other species without replacing the main crop.

  • Substitutive Intercrop: Replacing part of one crop with another to maintain yield and improve resource use efficiency.

Land Equivalent Ratio (LER)

  • Definition: A measure comparing the productivity of the intercropped areas versus monoculture.

  • Formulas:

    • LER = ext{sum of the fractions of the intercropped yields divided by the sole-crop yields}

  • Interpretations:

    • LER > 1: Yield advantage of polyculture compared to monoculture.

    • LER = 1: Equivalent yields in monoculture and polyculture.

    • LER < 1: Yield disadvantage for polyculture.

Yield Data from Polyculture vs. Monoculture

Corn-Bean-Squash Polyculture Study in Tabasco, Mexico

  • Low Density Monoculture:

    • Corn density: 40,000 plants/ha

    • Corn yield: 1,150 kg/ha

    • Bean density: 64,000 plants/ha

    • Bean yield: 740 kg/ha

    • Squash density: 1,875 plants/ha

    • Squash yield: 250 kg/ha

    • LER low density: 1.97

  • High Density Monoculture:

    • Corn density: 66,000 plants/ha

    • Corn yield: 1,230 kg/ha

    • Bean density: 100,000 plants/ha

    • Bean yield: 610 kg/ha

    • Squash density: 7,500 plants/ha

    • Squash yield: 430 kg/ha

    • LER high density: 1.77

Herbivory (+, −)

  • Definition: Refers to the consumption of plants by animals, which leads to:

    • Reduced photosynthetic capacity.

    • Decrease in organic material (OM) return to the soil.

    • Potential reductions in yield.

Effects of Simulated Herbivory on Yield (Cerkal et al. 2009)

  • Winter Wheat:

    • 0%: 10.03a

    • 25%: 10.27a

    • 50%: 10.05a

    • 75%: 10.16a

  • Spring Barley:

    • 0%: 8.33a

    • 25%: 7.60a

    • 50%: 7.42a

    • 75%: 7.51a

  • Maize:

    • 0%: 14.32a

    • 25%: 13.70ab

    • 50%: 13.36ab

    • 75%: 12.22b

Plant Defenses Against Herbivory

  • Physical Defenses:

    • Spines, trichomes, thick leaves.

  • Chemical Defenses:

    • Qualitative Compounds: For specific herbivores (e.g., cyanide, caffeine, nicotine).

    • Quantitative Compounds: Affect many herbivores (e.g., tannins).

Specific Examples of Herbivory Defenses

  • Bottle Gourd (Lagenaria siceraria): High trichome density.

  • Cucumber (Cucumis sativa): Low trichome density.

  • Source for examples: Kaur and Kariyat 2023.

Beneficial Herbivory (+, +)

  • Involves holistic management strategies:

    • Grazing when grass reaches approximately 12 inches for optimal root health and regeneration.

    • High stock density (400-800 animals/acre) for a short duration enhances plant growth by creating favorable environmental conditions (higher CO2 levels).

  • This practice allows plants to effectively close stomata and reduce excessive transpiration, promoting growth and resilience.

  • Reference: Allan Savory's Holistic Management.

Parasitism (+, −)

  • Overview: A parasitism interaction where the parasite uses a living host for energy, which may not always lead to the host's death.

  • Parasites:**

    • Generally small and specialized, leading to high speciation and adaptive radiation.

  • Example: Dodder (Cuscuta gronovii) – an obligate parasite that attaches to host plants and absorbs nutrients through haustoria.

Livestock Parasites

  • Common parasitic group: Roundworms in cattle, sheep, and goats.

  • Significant impact on:

    • Younger animals have a higher worm burden.

    • Moist pastures host a greater abundance of roundworms.

  • Management strategies include pasture rotation and mixed animal grazing to control parasite populations.

Parasitoids and Insect Pest Regulation

  • Utilize natural enemies (parasitoids) to regulate pest populations, improving agroecosystem health.

Predation (+, −)

  • Highlighted through a case study with American Kestrels in Michigan cherry orchards, where nest box installations led to increased pest control and significant economic benefit.

  • Study findings:

    • Kestrels received $84 to $357 of sweet cherries saved for every dollar spent on nest boxes.

    • In vineyard research, it was noted that nest box treatment enhanced beet armyworm control.

Allelopathy (0/+, -)

  • Definition: When plants release chemicals into the environment that can inhibit or stimulate other organisms.

  • Example: Black Walnut and Butternut release juglone, a compound toxic to certain species.

Impact of Allelopathic Weeds on Crop Growth

  • Studies demonstrate that allelopathic weeds like bittergrass can inhibit crop growth by affecting various growth metrics such as plant height, number of leaves, and overall yield.

Symbiotic Relationships

Types of Symbiosis

  • Parasitism (+, -): One organism benefits at the host's expense.

  • Commensalism (0, +): One species benefits while the other is unaffected.

  • Mutualism (+, +): Both species benefit from their interaction.

Commensalism (0, +) Examples

  • Cattle Egrets and Cattle: Egrets feed on insects disturbed by cattle.

  • Epiphytes on Trees: They grow on trees benefiting from light and space without harming the tree.

  • Shade Trees and Coffee: Shade trees enhance coffee growth through providing a suitable microenvironment.

Mutualism (+, +) Examples

  • Pollinator-Plants Relationships:

    • Bees collect pollen and nectar, facilitating pollination while benefiting from the nutrition.

    • Increased yields even in self-pollinating plants when pollinators are present.

  • Rhizobia and Legumes: Bacteria fix nitrogen in exchange for carbohydrates from the host plant, optimized by the structure of root nodules with leghemoglobin serving to create an ideal environment for nitrogen fixation.

Mutualism in Other Contexts

  • Observations of wood ants reducing seed consumption by 38% show positive interactions between ants and yucca plants (+/+), leading to improved growth conditions.

Mycorrhizal Fungi and Plants (+, +)

  • Importance: Mycorrhizal fungi help improve nutrient uptake for plants, enhancing overall plant health and growth.

  • Global Distribution: Research suggests that the distribution of mycorrhizal plants is linked to terrestrial carbon stocks, indicating their ecological and agricultural significance.