Energy flow through ecosystems
Trophic interactions
•Movement of energy through ecosystems
•Effects of one trophic level on the others
•Requires understanding of both living and nonliving components
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)
Some things
•99.9% of biomass on earth’s surface is plants
–Vast majority of production also plants
•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)
Community properties
•Primary Productivity - Rate of biomass production
–Used as an indication of the rate of solar energy conversion to chemical energy
–Net Primary Productivity - Energy left after respiration
Food chains and trophic levels
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
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
•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
How do they work?
•May be a simple numerical response
•If top predators are killed (e.g. winter fish kill)
–Lower INGESTION EFFICIENCY
–More herbivores left alive
•Higher INGESTION EFFICIENCY
–Fewer plants left
Example: Old-field community
•Moran et al. 1996. Top-down cascade from a bitrophic predator in an old-field community. Ecology 77:2219-2229
•What happens to plant biomass if you remove the top predator in an abandoned hay field?
–12 plots in field
–Add mantids to 6, none in others
–Put up trap around edge to collect immigrants and emmigrants
–Measured carnivore, herbivore, and plant biomass over the summer
Results
•Add mantids and the spiders and other carnivores left in higher numbers
–50% less non-mantid carnivore biomass
•Add mantids and get 35% reduction in herbivore biomass
•Add mantids and get 25% increase in plant biomass
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