(4/24) Energy Flow and Trophic Structure

Introduction

• Previous lectures covered individuals, populations, species, and communities.
• Ecosystems are communities plus the abiotic conditions that surround them.
• Ecosystem science is largely concerned with energy flow and cycling of matter. Matter cycling to be covered in the next lecture.
• Energy flow starts with solar radiation.
  • Some sunlight is reflected back into the atmosphere.
  • Some is absorbed, increasing kinetic energy and temperature, which affects biochemical reaction rates.
  • Some is absorbed by chlorophyll in plants.
• Plants convert carbon dioxide and water into carbohydrate using sunlight, providing energy for herbivores.
• Herbivores transform these molecules, which can then be used by carnivores.
• Plants lose leaves and stems, animals defecate, shed skin/hair/feathers, and eventually die. Non-living organic matter provides energy for decomposers.
• Decomposers break down molecules and return nutrients to the soil and carbon to the atmosphere.
• Nutrients can then be taken up by plants to start the process over.
• Energy does not cycle through an ecosystem; it flows through and exits as it is lost through cellular function.
• A natural environment can be viewed as a system that absorbs, transforms, and stores energy.
• Physical, chemical, and biological structures and processes are inseparable in this view of an ecosystem.

Trophic Structure

• The Players:
  • Abiotic Environment: sunlight, soil, climate, temperature, local weather conditions, water, and nutrients.
    • Nutrients are elements and molecules that an organism takes from the environment (e.g., potassium, zinc, ammonium for plants).
    • Plants get carbon from the air in the form of CO_2.
    • Consumers get their nutrients from plants and other organisms.
  • Primary Producers: Plants, bacteria, and algae (autotrophs) that produce their own food through photosynthesis.
    • Some bacteria convert inorganic compounds to food in extreme environments.
    • Created chemical energy is used for:
      • Maintenance and respiratory costs
      • Growth and reproduction
  • Consumers: Heterotrophs that feed on other organisms to obtain energy for maintenance, growth, and reproduction.
    • Herbivores: Primary consumers
    • Carnivores: Secondary, tertiary, or quaternary consumers; primary or secondary carnivores, etc.
    • Decomposers (Detritivores): Get energy from consuming dead organisms or their wastes.
• Trophic level: Organisms that obtain their energy from the same type of source are in the same trophic level (e.g., primary consumer vs. secondary consumer).
• Food chain
  • Connects trophic levels in an ecosystem and describes how energy flows between trophic levels.
  • A food chain is essentially a list of who eats whom in an ecosystem.
  • At higher levels, many food chains merge.
  • All food chains end up going into the decomposer food chain eventually.
  • Most animals eat more than one type of food and may be represented at different trophic levels in different food chains, which makes the idea of a food chain a bit over-simplified.
• Food web
  • A compact way of summarizing energy flow and documenting the complex trophic interactions that occur in communities.
  • Food webs are highly interwoven with several links through each level and possibly through each individual.
  • Food webs cannot be understood solely on the basis of their direct interactions.
    • Example: A predator may reduce competition between prey species by keeping numbers low (as in Keystone species).
    • Prey, by their presence or absence, may impact predator/predator competition.

Energy Flow

• Draw a diagram of energy transfer from sun to plant to rabbit to fox, accounting for all energy transfer.
• Specifics
  • On average, < 1% of sunlight striking Earth is captured by plants.
  • Plants take in CO_2 from the atmosphere and convert it (and water) to sugar by photosynthesis.
  • Gross Primary Production (GPP) is the total amount of carbohydrates created by plants.
  • Photosynthesis equation: 12H2O + 6CO2 + \text{solar energy} \rightarrow C6H{12}O6 + 6O2 + 6H_2O
    • Notice the uptake of carbon in the form of CO_2. This is the only way in which plants get carbon. They do not take up carbon from the soil.
  • Energy is stored in the chemical bonds of carbohydrate. What happens to it?
    • It may be stored as starch.
    • It may be used for growth of plant material.
    • It may be used for reproduction.
    • It may be used for plant maintenance.
  • All of these options require the functioning of cells. And the cells use carbohydrate to function. This is cellular respiration and is the opposite of photosynthesis.
  • Cellular respiration equation: C6H{12}O6 + 6O2 + 6H2O \rightarrow 6CO2 + 12H_2O + \text{Energy}
    • Notice that CO_2 is given off during cellular respiration.
  • Every time a cell respires, energy is released from the bonds of the carbohydrates and is used to drive cellular function. Because this process is not 100% efficient, some of the energy is lost from the system and is not used by the organism or by the others in the community. This is true for all cells.

Ecological Efficiency

• Ecological efficiency is the percent energy transferred between trophic levels.
• Typical ecological efficiency is between 10-20%, with most research reporting closer to 10%.
• Approximately 50% of a plant’s energy usage is in the form of cellular respiration only (the remainder goes to storage and building bulk plant matter). That means that energy that is not used for respiration becomes available energy for herbivores. (Keep in mind that not all plant matter is available to herbivores due to various defense mechanisms and availability. See previous notes on interspecies exploitation.)
• Net Primary Production (NPP) is GPP minus that energy used for respiration. In other words, NPP is the amount of matter in plants that is plant material and that may be available to herbivores.
• Depending on the ecosystem, somewhere between 10 and 70% of the NPP is harvested by animals.
• Pyramid of Productivity shows that productivity (total net biomass production) is highest at the bottom, or first, trophic level, and decreases dramatically as you move up through the higher trophic levels.
  • What conclusions can we draw from this?
• What is your moral of the story?