Lecture 14: 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.

  • Use 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 - commonly dominate in eutrophic lakes, where nutrient levels are high, and there is abundant zooplankton for them to consume

  • 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 with large amount of macrophytes (plants) - mesotrophic lakes: intermediate-stage water bodies with moderate nutrient levels (nitrogen and phosphorus), moderate biological productivity, and generally clear water with good oxygen levels in the surface layer

  • Detritivores: consume organic matter - abundant in hypereutrophic lakes: high levels of OM due to nutrient runoff

Native vs Invasive Species

  • Native species: Contribute to stable food webs and ecological balance.

    • Native fish of the Everglades include:

      • Sailfin molly

      • Eastern mosquitofish

      • Bluefin killifish

      • Bluegill

      • Florida gar

      • Largemouth bass

      • Brown bullhead

  • Invasive species: Disrupt trophic structure and competition; examples include invasive fish in the Everglades.

    • Some invasive fish of the Everglades include:

      • Mayan cichlid

      • Snakehead

      • Peacock bass

      • Walking catfish

      • Asian swamp eel

Amphibians (Tetrapods)

  • Semi-aquatic vertebrates: Life cycle includes an aquatic larval stage.

  • Sensitive to environmental changes, often acting as indicators of ecosystem health.

  • Can include both native and invasive species.

    • Native amphibians: Pig frog, Greater siren

    • Invasive amphibians: Cuban treefrog, Cane toad

Reptiles

  • Includes aquatic and semi-aquatic species; examples include:
        - Alligators: Considered top predators.
        - Turtles
        - Aquatic snakes

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

  • Some native reptiles include the American Alligator, and the Florida Softshell turtle.

Biomass Pyramid

Structure of Trophic Levels

  • Primary producers: Consist of algae, cyanobacteria, and plants.

  • Primary consumers: Include zooplankton and benthic invertebrates.

  • Secondary consumers: Mainly planktivorous fish.

  • Tertiary consumers: Mainly piscivorous fish.

Energy Transfer

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

  • Energy availability decreases with each trophic level.

Secondary Production

  • refers to the generation of biomass by heterotrophic organisms (starting from primary consumers to tertiary consumers) within an ecosystem; the conversion of energy obtained from consuming organic material into new biomass

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

  • Production:biomass ratio varies widely across ecosystems, higher in:
        - Streams compared to lakes; attributed to the more dynamic flow of water in streams, which facilitates higher nutrient cycling and supports more rapid growth and turnover of consumer organisms
        - Tropical regions compared to temperate regions; consistent warmth and high levels of sunlight in tropical areas promote year-round growth and reproduction of organisms, leading to greater secondary production

Grazing Food Chain (Green Channel)

  • Energy flow: Phytoplankton → zooplankton → fish.

  - The grazing food chain is a critical pathway in aquatic ecosystems, where energy captured by phytoplankton, the primary producers, flows through various trophic levels. Phytoplankton, which include microscopic algae and cyanobacteria, utilize sunlight to photosynthesize, converting inorganic nutrients into organic biomass. This process supports the foundation of the aquatic food web.
   
  - Zooplankton, primarily small, drifting organisms such as copepods, krill, and rotifers, graze on phytoplankton, forming a vital link between primary producers and higher trophic levels. The abundance and diversity of zooplankton can significantly influence the productivity of phytoplankton populations, demonstrating the interconnectedness of these organisms.
   
  - The energy stored in zooplankton is subsequently transferred to fish, many of which are planktivorous species that rely on zooplankton for nourishment. This direct energy flow from producers to primary consumers highlight the efficiency of this pathway in promoting higher biomass accumulation in fish populations, thus supporting larger predators and a more complex food web.

Microbial Loop

  • Dissolved Organic Matter (DOM) → bacteria → protozoa → zooplankton.

  - The microbial loop is a dynamic process that starts with the production of dissolved organic matter (DOM), which consists of organic compounds resulting from the decomposition of plants and animal materials. DOM plays a crucial role in sustaining microbial populations in aquatic environments.

  - Bacteria are the primary consumers of DOM, breaking down these organic compounds, and converting them into microbial biomass. This conversion is essential for the recycling of nutrients, as bacteria release inorganic nutrients back into the water column, which can then be utilized by phytoplankton, thus supporting primary production.
   
  - Protozoa, small eukaryotic organisms, graze on bacteria, and their populations are heavily influenced by the availability of microbial biomass. The energy from these protozoan populations is then transferred to zooplankton, re-establishing the link to higher trophic levels and further integrating the microbial loop with the grazing food chain. This loop is vital in nutrient-poor waters, where it helps maintain productivity by recirculating essential nutrients that would otherwise be lost from the ecosystem.

Brown Channel (Detrital Pathway)

  • Dead organic matter → microbes → detritivores → predators.

  - The brown channel, also known as the detrital pathway, highlights the ecological importance of dead organic matter (DOM) as a source of energy and nutrients within aquatic ecosystems. This channel begins with the accumulation of organic debris such as decaying plant material, dead organisms, and fecal matter.

  - Microbes, including bacteria and fungi, colonize this dead organic matter, breaking it down through decomposition. This process not only recycles nutrients but also transforms complex organic compounds into simpler forms that can be utilized by other organisms.
   
  - The detritivores, such as certain crustaceans, worms, and insect larvae, feed on this decomposed organic matter, playing a crucial role in further breaking it down and facilitating nutrient cycling within the ecosystem. As they consume organic materials, detritivores convert this dead matter into biomass, which can then be transferred to higher-level predators.
   
  - This pathway is particularly important in systems with high organic inputs, such as river deltas and wetlands, where organic matter accumulation enhances biodiversity and supports various trophic interactions. The energy flow from dead organic matter through microbes and detritivores not only sustains a diverse array of organisms but also contributes to overall ecosystem productivity.

Green vs Brown Channels

  • Green channel: Based on living plant/algal consumption.
       
      - The green channel focuses on the direct consumption of living plant and algal material, emphasizing the importance of photosynthetic organisms in energy capture and biomass production. This channel is pivotal for sustaining economically important fish species and supports primary producers that dominate various aquatic environments.

  • Brown channel: Based on detritus consumption.
       
      - In contrast, the brown channel highlights the significance of dead organic matter and the recycling processes it supports. Through the decomposition of organic matter, this channel is crucial for nutrient regeneration, which sustains both primary production and contributes to the overall health of the ecosystem.

  • Both channels are essential to the overall food web structure, showcasing the complexity of energy transfer and nutrient cycling in aquatic ecosystems. The interplay between the green and brown channels reinforces the resilience and productivity of these environments, underlining the necessity for balancing management strategies that support both living and decomposed organic 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
        - 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
        - Fungi

Predation

Predator Strategies

  • Sit-and-wait predators: Ambush prey.

  • Active predators: Actively search for prey.

  • Use various cues for hunting:
        - Visual cues
        - Chemical cues
        - Tactile cues.

Optimal Foraging

  • Key Principles:
        - Predators maximize energy gain relative to time spent.
        - Less selective when food is scarce and 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, leading to:
        - Larger predators consume larger prey.
        - This selection controls prey size distribution, impacting overall food web structure.

Implications of Size-Selective Predation

  • Large zooplankton generally serve as more efficient grazers.

  • Removal of large zooplankton can lead to increases in algae, altering aquatic ecosystems.

Omnivory

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

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

Stable Isotopes

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

Food Webs (Not Food Chains)

  • Food webs are complex networks where organisms exhibit multiple feeding relationships.

  • Energy flows through several distinct pathways within these webs rather than straight linear food chains.

Bottom-Up vs Top-Down Control

Bottom-Up Control

  • Controlled by abiotic factors including:
        - Nutrients
        - Light

  • This factor determines primary production, influencing energy availability for higher trophic levels.

Top-Down Control

  • Controlled by predation dynamics where predators regulate lower trophic levels.

  • Can lead to phenomena known as trophic cascades.

Trophic Cascades (Very Important)

3-Level Food Chain

  • No piscivores present.

  • High abundance of planktivores.

  • Smaller zooplankton populations observed.

  • Resulting increase in phytoplankton (leading to algal blooms).

4-Level Food Chain

  • Presence of piscivores affects dynamics.

  • Reduced planktivores lead to larger zooplankton populations.

  • Phytoplankton levels decrease, resulting in clearer water.

Biomanipulation

  • Strategy: Adding piscivores to reduce planktivores.

  • Leads to increases in larger zooplankton, enhancing grazing on algae,

  • Resulting effect: Improved water clarity.

Limitations of Biomanipulation

  • May not always be effective because:
        - Food webs are complex and include omnivory.
        - Detrital pathways are essential to many ecosystems.
        - Some organisms, like cyanobacteria, may be inedible.

Overall Concepts

  • Vertebrates can occupy multiple trophic levels within food webs.

  • Food webs feature both:
        - Green (herbivory pathways)
        - Brown (detrital pathways)

  • Energy flow through trophic levels is inefficient.

  • Predation shapes aquatic community structure.

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

  • The complexity of food webs limits simple management strategies.