Vertebrates and Aquatic Food Webs

Key Definitions

  • Secondary Production: Production of biomass by heterotrophic organisms (consumers).

  • Trophic Level: Position in a food web based on feeding relationships.

  • Piscivore: Fish that eats other fish.

  • Planktivore: Organism that eats plankton.

  • Herbivore: Organism that consumes plants or algae.

  • Detritivore: Organism that consumes dead organic matter.

  • Omnivore: Organism that consumes both plant and animal material.

  • Autotrophic: Produces its own food through photosynthesis.

  • Allochthonous: Organic matter originating from outside the system.

  • Autochthonous: Organic matter produced within the system.

  • Trophic Cascade: Indirect effects of predators on lower trophic levels.

  • Biomanipulation: Management strategy that alters food webs to improve ecosystem conditions.

  • Optimal Foraging: Behavior that maximizes energy gain relative to effort.

Aquatic Vertebrates

Fishes

  • Approximately 24,000 species; dominant aquatic vertebrates.

  • Major predators in aquatic ecosystems.

  • Utilize various sensory adaptations:
      - Chemoreception: Chemical detection.
      - Electroreception: Detect electrical signals – movement of prey.

Diadromous Fish

  • Fish that migrate between freshwater and saltwater:
      - Anadromous: Spawn in freshwater but live in the ocean.
      - Catadromous: Spawn in the ocean but live in freshwater.

Ecological Roles of Fish

  • Planktivores: Consume zooplankton; dominant in eutrophic lakes where nutrient levels are high.

  • Piscivores: Consume other fish; found predominantly in oligotrophic lakes characterized by lower nutrient levels and clearer waters.

  • Herbivores: Consume algae and aquatic plants; abundant in lakes rich in macrophytes; found in mesotrophic lakes with moderate nutrient levels.

  • Detritivores: Consume organic matter; abundant in hypereutrophic lakes with high levels of organic matter from nutrient runoff.

Native vs Invasive Species

  • Native Species: Contribute to stable food webs and ecological balance.
      - Examples: Sailfin molly, Eastern mosquitofish, Bluefin killifish, Bluegill, Florida gar, Largemouth bass, Brown bullhead.

  • Invasive Species: Disrupt trophic structure and competition.
      - Examples in Everglades: Mayan cichlid, Snakehead, Peacock bass, Walking catfish, Asian swamp eel.

Amphibians (Tetrapods)

  • Semi-Aquatic Vertebrates: Life cycle includes an aquatic larval stage and are sensitive to environmental changes.

  • Native Amphibians: Pig frog, Greater siren.

  • Invasive Amphibians: Cuban treefrog, Cane toad.

Reptiles

  • Includes aquatic and semi-aquatic species like alligators (top predators), turtles, and aquatic snakes.

  • Invasive reptiles (e.g., Burmese python) can significantly alter food webs.

  • Native reptiles include American alligator and Florida softshell turtle.

Biomass Pyramid Structure of Trophic Levels

  • Primary Producers: Algae, cyanobacteria, and plants.

  • Primary Consumers: Zooplankton and benthic invertebrates.

  • Secondary Consumers: Primarily planktivorous fish.

  • Tertiary Consumers: Primarily piscivorous fish.

Energy Transfer

  • Approximately 1% of energy from one trophic level is transferred to the next.

  • Energy availability decreases with each trophic level.

  • Secondary Production: The generation of biomass by heterotrophic organisms (from primary consumers to tertiary consumers) within an ecosystem. Refers to the conversion of energy obtained from consuming organic material into new biomass.

  • Difficult to measure directly; assessed through sampling and extrapolating population densities to infer total biomass production.

  • Production:biomass ratio varies widely across ecosystems:
      - Higher in streams compared to lakes due to dynamic water flow,
      - Higher in tropical regions compared to temperate regions due to consistent warmth and sunlight promoting year-round growth.

Grazing Food Chain (Green Channel)

  • Energy flow: Phytoplankton → Zooplankton → Fish.
      - The grazing food chain is critical; starts with phytoplankton (primary producers) who utilize sunlight for photosynthesis.
      - Zooplankton: Small organisms such as copepods, krill, and rotifers that graze on phytoplankton, forming essential links in the food web.
      - Energy is transferred to fish, especially planktivorous species relying on zooplankton, demonstrating efficiency in promoting higher biomass accumulation and supporting larger predators.

Microbial Loop

  • Flow: Dissolved Organic Matter (DOM) → Bacteria → Protozoa → Zooplankton.
      - Starts with DOM from the decomposition of plants and animals.
      - Bacteria consume DOM, recycling nutrients back into the water column utilized by phytoplankton.
      - Protozoa graze on bacteria, transferring energy from microbial biomass back to zooplankton, integrating with the grazing food chain.
      - Essential in nutrient-poor waters, maintaining productivity by recirculating nutrients.

Brown Channel (Detrital Pathway)

  • Energy flow: Dead organic matter → Microbes → Detritivores → Predators.
      - Dead organic matter accumulates from decayed material, providing energy and nutrients.
      - Microbes decompose this organic matter, recycling nutrients.
      - Detritivores: Organisms such as crustaceans and worms feed on decomposed matter, converting it into biomass for higher-level predators.
      - Vital in high organic input systems (like river deltas), enhancing biodiversity and supporting various trophic interactions.

Green vs Brown Channels

  • Green Channel: Focused on living plant/algal consumption, crucial for economically important fish and primary producers.

  • Brown Channel: Highlights the importance of dead organic matter for nutrient regeneration, sustaining primary production and ecosystem health.

  • Both channels illustrate the complexity of energy transfer and nutrient cycling, emphasizing balanced management strategies for both living and decomposed materials.

Herbivory (Green Energy Channel)

  • Consumption of Algae:
      - Zooplankton (e.g., Daphnia) and planktivorous fish (e.g., gizzard shad).

  • Consumption of Plants:
      - Herbivores include fish, snails, crayfish, manatees, and birds.

Detritivory (Brown Energy Channel)

  • Detritivores consume:
      - Autochthonous Organic Matter: Produced within the system.
      - Allochthonous Organic Matter: External inputs.

Detritus Characteristics

  • Generally low nutritional quality and high refractory material.

  • Nutritional value can increase through conditioning by bacteria and fungi.

Predation

Predator Strategies

  • Sit-and-Wait Predators: Ambush prey.

  • Active Predators: Actively search for prey.

  • Use a variety of cues for hunting: Visual, Chemical, and Tactile cues.

Optimal Foraging

  • Key Principles:
      - Predators maximize energy gain relative to time spent.
      - Less selective when food is scarce; more selective when food is abundant.
      - Balance between large and small prey, and move to areas with higher prey density.

Size-Selective Predation

  • Predators select prey based on size:
      - Larger predators consume larger prey.
      - This selection controls prey size distribution, impacting overall food web structure.
      - Implications: Large zooplankton are generally more efficient grazers; their removal can lead to algal blooms.

Omnivory

  • Omnivores: Organisms feeding on both:
      - Green channel (plants/algae).
      - Brown channel (detritus).

  • Diet Changes:
      - Varies with life stage and food availability.

Stable Isotopes

  • Used to track food sources:
      - ¹⁵N increases with trophic level, indicating higher consumer levels.
      - ¹³C helps identify carbon sources of the food.

Food Webs vs Food Chains

  • Food webs are complex networks showcasing multiple feeding relationships, contrasting with linear food chains.

Bottom-Up vs Top-Down Control

  • Bottom-Up Control: Regulated by abiotic factors, such as nutrients and light, influencing primary production and energy availability for higher trophic levels.

  • Top-Down Control: Controlled by predation dynamics, where predators regulate lower trophic levels, potentially causing trophic cascades.

Trophic Cascades

  • 3-Level Food Chain: No piscivores present; leads to high abundance of planktivores, smaller zooplankton, and increased phytoplankton (algal blooms).

  • 4-Level Food Chain: Presence of piscivores reduces planktivores, increases zooplankton populations, decreases phytoplankton, leading to clearer water.

  • Biomanipulation Strategy: Involves adding piscivores to reduce planktivores, resulting in increased larger zooplankton and enhanced grazing on algae for improved water clarity.

Limitations of Biomanipulation

  • May not always be effective because:
      - Food webs include omnivory.
      - Detrital pathways are essential to ecosystems.
      - Some organisms (e.g., cyanobacteria) are inedible.

Overall Concepts

  • Vertebrates occupy multiple trophic levels in food webs.

  • Food webs consist of:
      - Green (herbivory pathways)
      - Brown (detrital pathways)

  • Energy flow through trophic levels is inefficient.

  • Predation shapes aquatic community structure.

  • Bottom-up and top-down forces play crucial roles in regulating ecosystems.

  • The complexity of food webs limits simple management strategies.