integrated lec 23 Flashcards
trophic levels
Definition: Hierarchical levels in an ecosystem representing the flow of energy and nutrients.
Key Trophic Levels:
Primary Producers: Plants, algae, and photosynthetic organisms.
Primary Consumers: Herbivores that consume plants.
Secondary Consumers: Carnivores that eat herbivores.
Tertiary Consumers: Carnivores that eat other carnivores.
Decomposers: Organisms that break down dead organic matter, recycling nutrients.
Concept: Biomass decreases with increasing trophic levels due to energy loss.
Food Chains vs. Food Webs
Food Chain: A linear sequence of energy transfer between trophic levels.
Food Web: A more complex network of interconnected food chains showing all interactions within an ecosystem.
Example:
Parasitoid-herbivore-plant network (Costa Rica).
Plant-pollinator network (Greenland).
Direct vs. Indirect Effects
Direct Effects: Direct interactions between two species (e.g., predation, competition).
Indirect Effects: Occur when one species affects another through a third intermediary species.
Example: Exploitative competition where two species compete for the same resource.
Trophic Cascades
Definition: Interactions at one trophic level cause cascading effects on other levels.
The Green World Hypothesis: Carnivores suppress herbivores, preventing overgrazing and maintaining plant abundance.
Example: Hairston, Smith, and Slobodkin (HSS, 1960) proposed that predators indirectly benefit plants by controlling herbivores.
Experimental Evidence:
Caribbean Islands (Spiller & Schoener, 1992):
Removing lizards increased herbivorous beetles, reducing plant biomass.
Top-Down vs. Bottom-Up Control
Top-Down Control: Predators regulate population sizes by suppressing prey abundance.
Example: Removing lizards increased herbivore populations, reducing plant biomass.
Bottom-Up Control: Population sizes are limited by resource availability.
Test: Resource addition (e.g., fertilization experiments).
How does top-down control differ from bottom-up control in regulating populations?
Top-Down Control: Predators or higher trophic levels regulate population sizes of lower trophic levels. Example: Wolves controlling deer populations.
Bottom-Up Control: Availability of resources (e.g., nutrients, food, habitat) regulates population sizes. Example: Plant growth limiting herbivore populations.
Key Difference: Top-down focuses on predation pressure; bottom-up emphasizes resource availability.
Plant-Herbivore Interactions
Plant Defenses Against Herbivory:
Structural defenses: Thorns, tough leaves.
Chemical defenses: Secondary metabolites like alkaloids, tannins, and toxins.
Example: Milkweeds exude toxic sap when damaged.
Herbivore Counter-Adaptations:
Generalists avoid highly toxic plants.
Specialists evolve mechanisms to overcome or utilize plant defenses.
Example: Monarch caterpillars sequester milkweed toxins for their own defense.
Coevolution and Arms Races
Definition: Reciprocal evolutionary adaptations between interacting species.
Examples:
Plants evolve toxins; herbivores evolve mechanisms to detoxify or utilize these chemicals.
Monarch caterpillars use milkweed toxins to become toxic themselves.
Importance: Drives diversification in both plants and herbivores.
Key Takeaway: The plant-herbivore “interface” is a major driver of terrestrial biodiversity.
Vertebrate Herbivory
Challenges:
Plants are nutritionally poor and often toxic.
Vertebrate herbivores must process large quantities of plant material.
Solutions:
Mixed diets reduce the impact of any one toxin.
Symbiotic microbes aid digestion in specialized fermenting chambers (e.g., rumen or cecum).
Trophic Cascades:
Predators indirectly benefit plants by controlling herbivores
Explain the Green World Hypothesis with an example of a trophic cascade.
Green World Hypothesis: Herbivore populations are kept in check by predators, preventing overgrazing and allowing plants to thrive.
Example of Trophic Cascade:
Yellowstone Wolves: Reintroduction of wolves reduced elk populations, relieving grazing pressure on vegetation. This led to forest regrowth, benefiting species like beavers and birds.
Key Idea: Predators indirectly support plant biomass by controlling herbivores.
What are the primary producers in ecosystems?
A: Plants and other photosynthetic organisms that convert sunlight into chemical energy.
Define primary consumers.
A: Herbivores that feed on primary producers.
What are secondary consumers?
A: Carnivores that eat herbivores.
Why does biomass decrease at higher trophic levels?
A: Energy is lost at each level due to metabolic processes, resulting in a pyramid-shaped biomass distribution.
