Energy Flow

Energy Flow

Introduction

  • The ecological roles of organisms depend on their trophic interactions, which include what they eat and what eats them.

  • These interactions influence the movement of energy and nutrients through an ecosystem.

Feeding Relationships

  • Each feeding category, or trophic level, is defined by the number of feeding steps away from autotrophs.

Trophic Levels

  • 1st Trophic Level:

    • Composed of autotrophs or primary producers, primarily green plants.

    • Autotrophs generate chemical energy from sunlight or inorganic chemical compounds.

    • Responsible for most of the dead organic matter in an ecosystem.

  • 2nd Trophic Level:

    • Comprises herbivores that consume autotrophs.

  • 3rd and Higher Trophic Levels:

    • Consist of carnivores that consume animals from the level below.

Trophic Levels in a Desert Ecosystem

  • Fourth Level (Tertiary Consumers): Secondary carnivores.

  • Third Level (Secondary Consumers): Primary carnivores.

  • Second Level (Primary Consumers): Herbivores.

  • First Level (Primary Producers): Autotrophs (e.g., green plants).

  • Detritivores: Organisms that consume detritus (dead organic matter).

  • Detritus: Refers to dead organic matter.

Energy Inputs in Ecosystems

  • Much of the detritus in aquatic ecosystems comes from terrestrial organic matter.

  • These external energy inputs are termed allochthonous inputs.

  • Energy generated by autotrophs within the system is referred to as autochthonous energy.

Importance of Allochthonous Inputs

  • Example: In Bear Brook, New Hampshire, it accounts for 99.8% of its energy as allochthonous inputs.

  • In contrast, nearby Mirror Lake derives almost 80% of its energy budget from autochthonous energy.

Food Webs

  • A food web is a diagram showing connections between organisms and their food sources.

  • It qualitatively illustrates how energy moves from one component to another within an ecosystem.

Complexity of Food Webs

  • Food webs can depict complex relationships, including:

    • Great horned owls

    • Golden eagles

    • Coyotes

    • Different species of snakes

    • Rodents and insects

  • Example species include the Red-tailed hawk and various ground squirrels.

Static vs Dynamic Nature of Food Webs

  • Food webs provide a static description of energy flow and trophic interactions.

  • Actual trophic interactions can change over time:

    • Some organisms alter their feeding patterns as they age.

    • An example of this is frogs, which transition from omnivorous aquatic tadpoles to carnivorous adults.

Stability of Food Webs

  • The stability of ecosystems is assessed by measuring population changes over time.

  • Ecosystem responses to species loss or gain are closely associated with the stability of food webs.

Energy Flow Between Trophic Levels

  • Autotrophs have various defenses against herbivory:

    • Examples include secondary compounds and physical defenses like spines.

  • Plants in resource-poor environments tend to exhibit stronger defenses than those in resource-rich environments.

Herbivores and Digestion

  • Some herbivores possess mutualistic symbionts that aid in cellulose digestion.

  • Ruminants (e.g., cattle, deer, camels) have specialized guts housing bacteria that break down cellulose, leading to higher assimilation efficiencies compared to other herbivores.

Trophic Cascades

  • Trophic Cascade: A series of interactions where predation by a top carnivore (4th trophic level) leads to a decline in 3rd level carnivores, which then allows 2nd level herbivores to increase, consequently reducing primary producers.

Determining Trophic Levels

  • The number of trophic levels in an ecosystem may change due to:

    • The addition or loss of a top predator.

    • Loss of a predator in the middle of the food chain.

    • Changes in the food preferences of omnivores.

Toxins in Food Webs

  • Biomagnification: Refers to the process where toxin concentration increases in animals at higher trophic levels as they consume prey with existing increases of toxins.

  • Bioaccumulation: Individual organisms accumulate toxins over time, leading to higher concentrations in top predators.

Example of Biomagnification

  • Diagram showcases various fish species (e.g., common carp, grass carp, northern pike, and cormorants) along with measured mercury concentrations in micrograms per kilogram (μg/kg).

    • Mercury concentration data shows that higher trophic levels such as carnivores can have concentrations exceeding 4,000 μg/kg.

Historical Context of Toxin Awareness

  • The dangers of bioaccumulation and biomagnification were notably emphasized by Rachel Carson in her work Silent Spring (1962).

  • Carson highlighted the adverse impacts of pesticides, particularly DDT, on non-target bird and mammal species.

    • This work garnered significant public awareness about environmental toxins and their propagation through food webs.