Energy Flow Through Ecosystems

Energy flow through ecosystems

Trophic interactions

•Trophic level is at which level an organism “feeds” at

•Movement of energy through ecosystems

•Effects of one trophic level on the others

Food chains and trophic levels

Trophic levels

•99.9% of biomass on earth’s surface is plants

–Vast majority of production is plants

–Standing crop or biomass can be measured

•In terms of energy storage, animals are not particularly important

–But they are important in moving energy through an ecosystem

–Particularly decomposers (only about 20% eaten alive)

Secondary production

•The rate of accumulation of biomass by heterotrophs

–Animals that derive energy either from organic compounds in the environment or from other organisms

•In contrast to primary production

–The rate of accumulation of energy in organic molecules by photosynthesis (autotrophs) or chemical reactions (chemoautotrophs)

Community properties

•Gross Primary Productivity (GPP) - Rate of biomass production

–Used as an indication of the rate of solar energy conversion to chemical energy

–Net Primary Productivity (NPP) - Energy left after respiration

Productivity varies between ecosystems

Laws of Thermodynamics

•The total energy of an isolated system is constant, although within that system energy may change its form

–Energy can be neither created nor destroyed

•In any energy transfer, some energy is lost in the form of heat

–No energy transformation is 100% efficient

Efficiency of energy transfer

•Once energy is stored in a chemical bond, where can it go?

–Converted into new animal

–Decomposed

–Washed away

•Energy lost at every stage (heat)

•What influences efficiencies?

Ingestion efficiency: ingested/available

•Prey and plant defenses prevent total consumption

•Spatial and temporal refugia reduce ingestion efficiencies

•Predator-predator interactions (e.g. territorially) result in saturating predation pressures

–Reduced ingestion efficiencies

Assimilation efficiency: assimilated/ingested

•Food quality (lignin, tannins)

•Feeding and digestion “tools”

–Teeth

–Gut length

–Enzymes

Production efficiency: growth/assimilated

•Huge range

–< 5% for birds and mammals

–> 40% for many insects

•Endotherms vs ectotherms differ

•Reproductive allocation

•Body size (heat loss)

Ecological pyramids

•Due to Second Law of Thermodynamics, food chains often form a pyramid

–10% Rule

•100 kg clover

–10 kg rabbit

»1 kg fox

Flow of energy in ecosystems

•Trophic level interactions

–Trophic cascade is a process by which effects exerted at an upper level flow down to influence two or more lower levels

–Top-down effects is when effects flow down

–Bottom-up effects is when effect flows up through a trophic chain

Top-down, Bottom-Up

•What controls community structure?

–Top carnivore?

–Nutrient supply rate?

Trophic cascades

•Little doubt that top-down and bottom-up processes are both occurring

–Lynx/hare/plant study

•But how far into the food chain do they have influence?

–Can changes in N supply alter carnivore abundance? Secondary carnivores?

–Can removing a predator alter plant biomass?

Trophic cascades

•Relatively young field, dominated by aquatic ecologists

•An indirect mutualism between non-adjacent levels in a food chain

–an enemy of my enemy is my friend

Trophic cascade with 3 levels

Trophic cascade with 4 trophic levels

Top-down bottom-up

•Adding P increases phytoplankton abundance in lakes

–Evidence that primary production limited by nutrients

•What happens when you remove a top-predator from a system?

–Keystone predators

Bottom up effects

Experimental data